first topic is selection natural selection artificial selection sexual selection then we'll delve into population genetics and the Hardy Weinberg equilibrium we'll spend a lot of time looking at the evidence for evolution that'll lead us to looking at how species diverge into multiple descendant species that's the idea of speciation we'll look at variation in populations and we'll also look at how species become extinct both on a small level single species at a time and mass extinctions in which huge numbers of species disappear usually for geological or even astronomical causes we look at life's branching pattern in history which is the topic of philogyny and then we'll end by looking at the origin of life 7.1 to 7.3 natural artificial and sexual selection in terms of understanding the process of evolution the most important idea to understand is natural selection developed by Charles Darwin in the 1800s Darwin himself built the idea of natural selection upon the foundation of artificial selection so that's where we're going to start artificial selection is also known as selective breeding and in this process breeders Envision a desired trait in an animal or a plant for many generations the breeder select organisms with that trait and the process over time over many generations creates a carefully guarded gene pool with individuals that only have genes for the desired trait here are two examples the plants that are in the Brasa olari family include cauliflower broccoli brussels sprouts Etc they're all the same species The ancestral plant Brasa olari has been bred for specific traits for flower clusters in terms of cauliflower for flower buds in terms of broccoli for lateral buds in terms of Brussels sprouts for kale in terms of leaves but all of these are varieties of the same species that have been bred for specific traits the same is also true of dogs dogs and wolves are all members of the same species canis lupus and all the breeds varieties of dogs the German Shepherds and the schneer and the dochin those are all varieties that have been bred for specific human purposes for protection for hunting or for just looking cute and being really cuddly so how does natural selection work in any population there's variation the kind that matters is inherited variation that's in the genes and that comes about through recombination and mutation in any population the reproduction rate the number of individuals who were born exceeds the survival rate and that's even true of slow breeding populations sometimes that's summarized by the phrase many are born but few survive the survivors have a beneficial trait that gave them some type of Advantage as the generations proceed mutation continues to create new variants and if you repeat steps 1 through four you wind up with adaptation adaptations can include things like the incredible structure of the wing of a bat or the incredible behavioral adaptation which is sonar which is of course combined with structures like the ears of a bat which can hear high frequency sounds they can be camouflage mutations like this satanic leaf gecko or they can be on a molecular level like the way that enzymes fit with substrates you've learned a lot of biology and whenever you see things and you find yourself thinking how could that work how could that be so perfectly put together it was put together by natural selection in in addition to artificial selection and natural selection a third type of selection to know about is sexual selection sexual selection is selection for traits that directly increase reproductive success they create an evolutionary Dynamic that results in sexual dimorphism where there are different phenotypes in males and females one type to know about is called intersexual selection and that's where members of One sex typically the females choose mates of the other sex that are typically males so here's a photograph that I took relatively near to my home this is a female turkey there's a bunch of male turkeys who are displaying to her and they're essentially saying through their movements and through their extension of their tail feathers I'm really attractive you should mate with me and that means that there's been selection for these tail feathers and for features like these Waddles and for the Mal's Behavior peacocks are a bird species where that's been carried to extraordinary lengths and that typically leads to colorful and highly adorned males and choosy females the other kind of selection is called intrasexual selection and that's where competition between males for control of a harm of females or control of a breeding territory leads to a dynamic where the males are aggressive and often really big and strong and that's what we see over here with these elephant seals that live off the coast of Northern California and they're about twice the size of females they're huge aggressive testosterone filled animals selection can change the distribution of phenotypes in a population a variety of ways in any population most characteristics can be represented by a bell curve of continuous variation if you took all the students in your school and you organize them by height most of the people are average height there are a few people who are short and a few people who are tall that's a bell curve of distribution directional selection which is shown over here selects against one of these Extremes in other words here's the original population you're selecting against this extreme over here and so you're pushing the population in this direction in stabilizing selection you are selecting against the two extremes an example of stabilizing selection is birth weight in babies babies that are too small have lower survival babies that are too big have lower survival the babies who survive most are the ones who are of average size the third type is called disruptive selection if you can imagine a scenario where the average phenotype is maladaptive then there would be selection for the two extremes it would kind of split the population into two an important type of directional selection to know about is adaptive melanism adaptive melanism is the darkening of the body within a population in response to the darkening of the environment it occurs within a population as its gene pool evolves in response to Natural Selection so it's not like getting a tan which happens in an individual this is something that happens over evolutionary time as genes for light coloration are selected against and genes for darker coloration are selected for the selective pressure that results in adaptive melanism is typically predation what's shown here in this example is the rock pocket Mouse which lives in the southwest of the United States and what's been documented on a genetic level is that populations of these mice that live on a dark colored substrate have evolved a mutation that results in more melanin production and that mutations spreads throughout the population and results in subpopulations that have the dark coloration we've been talking about various types of selection natural selection is often summarized as survival of the fittest what does it mean to be fit how is evolutionary Fitness measured evolutionary Fitness is not about strength or speed it's the number of Offspring and offspring of offspring that survive to reproduce and the reason why I have this Montage of penguin life is just to indicate that it is something that emerges at every level of the life cycle when the penguins are feeding when they're doing their March Inland when they're breeding when they're juveniles huddling together there's Fitness at every single stage and fitness throughout the life cycle is what enables genes to be passed on to the next Generation to end this discussion of all all types of selection we're going to look at this question explain how the peppered moth demonstrates an evolutionary change in response to environmental change the key point about the peppered moth is that it provides science with a directly observed example of directional selection and adaptive melanism in response to an observed change in the environment the peppered moth comes in two forms there's a peppered form mostly light colored and a dark form and before the Industrial Revolution the Industrial Revolution began with the invention of the steam engine the burning of coal the predominant phenotype was peppered was this one over here so in any population of moths almost all of them were peppered there'd be a few dark moths why is this because that was a form of adaptive coloration the moths were camouflaged as they rested on light colored tree trunks which were often covered by these whitish colored lyans that's the situation before the Industrial Revolution with the Industrial Revolution soot from factories was put into the air it landed on the trees it killed the lyans and darkened the tree trunks and what that did is that it created a selective Advantage for the dark colored moths if you were to do a census of the moths um at the beginning of Industrial Revolution most of them were white only a few were black but as the Industrial Revolution proceeded the mean phenotype shifted from light to dark until in the depths of the Industrial Revolution the majority of the moths were dark colored the interesting thing is that this process was also historically reversed about 1960 s from pollution declined with the development of laws like the Clean Air Act that happened around the world the lyans returned the trunks became lighter the mean phenotype of the peppered moth shifted from dark colored to light colored I have a song about the peppered moth and there's also fast ating stuff related to the history of this because it was embroiled in some controversy but the work of Michael madaras which was done in the early 2000s which replicated some work done by BD kettlewell which was done in the late 1800s or early 1900s established this as a clearly observed case of adaptive melanism phenotypic shift in response to an environmental change your success in AP biology starts here are you struggling with AP Bio with learn biology.com students get the skills and confidence to be a top student and earn fours and fives on the AP Bio exam guaranteed go to learn biology.com to find out how you can master your biology course and crush the AP Bio [Music] exam topic 7.4 to 7.5 population genetics and Hardy Weber what is population genetics population genetics is a study of how genes are distributed in populations and how they change over time key thing that we measure in population genetics is alal frequency in this population over here we have 20 individuals and we're tracking the frequency of the AL big a and little a and we can see that the frequency of little a is 0.5 it makes up 50% of the alals in this gene pool what is a gene pool it is simply all of the alals of all of the genes in a population and in that context Evolution means a change in the genetic makeup of a population over time it's the change of alal frequencies within a gene pool before we get more into the details of population genetics I want to address the biggest misconception I've seen among my students related to population genetics that's the wrong idea that's why it's crossed out that the dominant alil has to be more common than the recessive Al these are different dimensions of biological analysis and a frequency whether an Al is comment or rare has nothing to do on dominance or recessiveness it's based on the advantage or harm an Al confers to its Associated phenotype or random historical factors and here's an example the AL for the condition called acondroplasia is exceedingly rare but it's a dominant Al population genetics is one of the more mathematical parts of the AP curriculum and it's built around two equations p + qal 1 and P ^2 + 2 PQ + Q ^2 = 1 in this system P represents the frequency of the dominant alal and Q represents the frequency of the recessive alal if you load p + Q into a punet square the whole thing makes a lot more sense p+ qal 1 means that for a gene with two alals the frequency of the dominant Al and the frequency of the recessive Al equals all the alal p + Q = 1 P ^2 + 2pq + Q ^2 = 1 what does that mean well first of all here's p and here's P if you do the punet square kind of thing this would be p^ squ the frequency of homozygous dominance p^2 plus the frequency of heterozygotes PQ + PQ = 2pq plus the frequency of recessive individuals q^ sared is all the individuals in a population and you can see that right here here's how you might see this in a sample problem you've identified you've been told that 49% of the mice in a population have the recessive trait of white fur figure out the frequency of PQ p² 2p q and Q ^2 so here's our original setup what we're going to do is q^2 is this value over here so we're just going to put .49 over here from there what we're going to do is we're going to work to figure out that the square root of 049 is 7 then we know that this plus this equals 1 so therefore this the value p has to be. 3 then we know that 2pq has to be42 why. 3 * 7 is. 21 * 2 is42 and p^2 is 93 * 3 is9 if you set up punet Square cross multiplication tables you'll get this right every time the Hardy warberg principle is that the frequencies of Al in gene pools will stay constant unless one of the more following conditions is not met this Hardy Weinberg population is a fictional population that's used to understand what causes evolutionary change in real populations the first is that the population is infinitely large there are no harmful or beneficial Leal there's random mating there's no immigration or immigration and there's no net mutation of one Al to another if any of these five conditions is violated then alal frequencies can change and these are the factors that can cause evolution what are these factors small populations leading to a phenomenon called genetic drift natural selection where certain alals are beneficial and others are harmful sexual selection which is the opposite of random mating gene flow where genes move into a population or out of a population and directional mutation where one AAL has a tendency to mutate into another alil so what's genetic drift and we'll also look at one variant of genetic drift called a population bottleneck genetic drift is random change in alal frequencies in a population's gene pool usually associated with small population size a population bottleneck can cause genetic drift a biotic living or abiotic non-living factor wipes out a large percentage of the individuals in a PO population leaving only a few survivors the survivors alals might not be representative of the AL frequencies in the former larger population and some alals might Disappear Completely the reason why I have a picture of a cheetah here is that cheetahs are extraordinarily genetically uniform it's as if all cheetahs are practically identical twins or at least very close siblings based on genetic analysis it looks like the cheetah underwent some enormous population bottleneck maybe a viral infection or something like that that wiped out almost all of the cheetas down to only a few individuals they repopulated from those few individuals but as a result many Al were lost and they're highly genetically uniform note that in a population bottleneck the survivors didn't have any advantage it's not natural selection they were just lucky another form of genetic drift is the founder effect and this is where a small number of individuals from a large population founds a new population but because of insufficient sampling because it's only a few individuals who leave the larger population the alal frequencies of the gene pool of the founders who found the new population might be different from those in their parent population in this graphic over here here's the original population and the squares and circles represent different alals the founders go off to colonize three islands in this situation over here the blue alil is completely lost and now the red Al is at 100% here there's a representation of the diversity of the former population and in this case the Red alil Is Lost and the frequency of the blue alil is now at 100% what is gene flow and what's its effect on evolving Gene pools gene flow is the movement of alals from one population to another and that can involve the movement of individuals like we see here this beetle is moving from population 2 to population one but it can also involve the movement of gametes and that's easier to imagine in the case of plants where pollen is flying from one field to another what's the effect of gene flow it can change alal frequencies especially in the recipient population and it can diminish differences between adjacent populations mutation is one of the conditions that violates the Hardy Weinberg equilibrium so what's its importance to evolution mutation is the ultimate source of genetic variation within and between populations and if it's directional if alal big a tends to mutate to little a then it can change alal frequencies within a population here's a population genetics illustrative example CLE cell disease is caused by a recessive alil in the gene for hemoglobin it can significantly decrease the quality of life and lifespan yet the alil is in high frequency in certain populations explain CLE cell disease is caused by a point mutation in the gene for hemoglobin it results in the substitution of veine a non-polar amino acid for glutamic acid an acidic amino acid it occurs only in children who are recessive for the alil but heterozygotes can experience a small amount of sickling not enough to generate pain crisis and tissue damage under most circumstances the red blood cells and heterozygotes despite the fact that they don't normally sickle are hostile environments for the plasmodium parasite that causes malaria this is a parasite that's carried by mosquitoes and then lives inside red blood cells so what we have is a phenomenon that's called heterozygote Advantage where big S big S homozygous dominant is selected against why because of malaria big S little s heterozygosity is selected for why because it doesn't have the symptoms of cyel anemia but it provides protection against the malaria parasite and small as small s is selected against because you have CLE cell disease what this map shows is the high correlation between the frequency of the little s alil and the intensity of malaria in various parts of Africa and this same model heterozygote Advantage might apply to genetic diseases like cystic fibrosis and tasac I'm Mr W from learn biology.com where we believe that interaction and feedback is what leads to deep substantial learning we're so sure of that that we provide a money back guarantee that comes with your subscription topic 7.6 to 7.8 evidence of evolution there is abundant evidence to support the theory of evolution for AP Bio the most important type of evidence to know about is the idea of homologous traits homologous features homologous traits are traits that share a common underlying structure and a common embryological origin they show descent with modification from a common ancestor here's the Forum of a human dog bird and whale the bones that make up their forelimbs have been given the same name by anatomist who recognize that they were the same bones you can also see that they develop from the same piece of tissue Embry logically it's because these structures evolved from a common ancestry who had these structures and then natural selection Evolution LED these structures to evolve differently in different lineages homologous features result from an evolutionary pattern that's called adaptive radiation adaptive radiation occurs when one parent species produces several descendants Each of which has unique adaptations and fills a different ecological niche these are some of the finches that live on the Galapagos Islands they are all the descendants of an ancestral Finch that was blown over there from Ecuador a small population of finches and that evolved in different directions what's the connection these are descendants of the same species and their beaks are actually homologous features they descended from a common ancestor they became modified in each of the descendant species in different ways a vestigial structure is a special kind of homology that provides further evidence for evolution a vestigial structure has no apparent function but it was inherited from an ancestor from whom that structure had a function for example whales have no hind limbs but they have a pelvis over here onto which to attach those limbs why do they have that their ancestors possessed Hine limbs which were lost as whales adapted to their aquatic lifestyle we have a coxic a tailbone this is homologous to the Tails of cats and dogs and other vertebrates other mammals we lost it in the course of our primate Evolution into one of the great apes what can be confusing when you're looking at common features to draw evolutionary connections are the difference between homology and analogy analogous features have a similar function but when you look deeply there's a different underlying structure they arise through a different kind of evolution not adaptive radiation but convergent evolution and an example is shown here we have sharks we have iosaur an extinct kind of reptile and we have Dolphins they all have a hydrodynamic form and that's a convergent solution to the challenge of swimming here's a tricky question are the wings of birds and bats analogous or homologous think about the answer then see mine the wings of birds and bats are a convergent solution to the challenge of flying they did not evolve from a common ancestor bats evolved from some ancestral mammal that was probably like a rodent and birds are dinosaurs and they evolve from small land living Dinosaurs the function of the Birdwing the function of the Batwing is similar but the underlying structure the organization of the bones is different the wi thingss are an analogous structure but if you look at this question a little bit differently you get a different answer the four limbs of birds and bats are homologous they're the same bones inherited from a common ancestor so another example of why on the AP Bio exam or in your AP Bio course you need to carefully read The Prompt and think before you answer what are molecular homologies how do they serve as evidence for evolution molecular homologies are hom features but at the molecular level they're molecules that by their structure and monomer sequence indicate common ancestry all vertebrates for example have hemoglobin that has the same structure two alpha chains two beta chains many of the amino acids are the same but as you might expect the closer an animal is to us morphologically the closer its hemoglobin is to us in terms of amino acid sequence so there's only one difference in amino acid sequence between us and gorillas between rees's monkeys and humans there are eight differences mice which are also mammals but not primates there are 27 differences chickens which are yet more distantly related to us have 45 differences and frogs which are the most distantly related have the greatest amount of difference 62 differences pseudo jees are yet another category of evidence for evolution and they're really extraordinary a pseudogene is a nonfunctional gene that's a variant of a functional Gene in a related species an example is the glow pseudo Gene that humans have as well as most of our primate cousins here's the glow pseudogene it has a series of mutations that keep it from working perhaps the mutation is in a broken promoter or perhaps the mutation is in the structural genes but rodents have a functional version of this Gene it enables them to produce the enzymes to syn synthesiz vitamin C but primates can't do it why do we have the remnants of this Gene it's a molecular fastigial feature these pseudogenes only exist because of descent with modification we and rodents share a common ancestor over here that common ancestor had the glow Gene and it was able to produce the enzymes to synthesize vitamin C we primates lost the ability to do that a mutation emerged broke the gene but because we lived in a fruit environment that mutation wasn't selected against what's really fascinating is that we're not the only mammals that have that pseudogene it's also found in guinea pigs and it's also found in bats but if you look at the sequence of those pseudogenes they have different mutations from R pseudo gen so this pseudogene this pseudogene and R pseudogene those all convergent features they're analogous not homologous biology so amazing homologies can take us all the way back to the origin of life there are a couple of features that show that all living things are related what are they DNA used as a genetic material ATP used for energy coupling the same genetic code that's universally used ribosomes used for protein synthesis shared metabolic pathway like glycolysis the KB cycle the electron transport chain and ATP synthesis through CH osmosis this shows that all living things are cousins we all share a common ancestor going back to the origin of Life the homologies that we just discussed go back 3.8 billion years or so to the origin of life there are also more recent ones still ancient that go back 1.8 billion years that indicate that all you can carots are cousins these include the presence of a nucleus mitochondria or organel that are derived from mitochondria the endomembrane system genes that possess introns linear chromosomes as opposed to the circular chromosomes of procaryotes and sexual reproduction involving gamt production and fusion of gametes to form a diploid zygote these are features shared by all ukar they indicate common ancestry we're cousins to all the ukar we're cousins to all life but we're closer cousins to those in our domain ukaria describe how embryological development provides evidence for evolution early embryos of vertebrates look similar here's a fish a reptile a bird and a human during development the embryo differentiates and that leads to the adoption of the body form of the adults of the lineage so bird and human embryos quite similar but we all know what adult birds look like and we all know what adult humans look like the similarity of the embryonic form indicates common ancestry there was a common vertebrate ancestor that gave rise to the lineage of fish reptiles birds and humans descent with modification leads to the elaboration of form in different lineage the different body forms of fish reptiles birds and humans but the embryonic similarity along with fossil evidence shows that all of these organisms are descendants of a common ancestor an additional line of evidence for evolution related to embryos is as follows embryos often show vestigial features such as the tail in humans the Fingal Gill slit in humans that indicate descent with modification from a common ancestor so again embryonic development an entirely separate line of evidence for evolution diverse species share common genes for animal development describe how this provides evidence for evolution this is a mind-blowing example of homology an animal separated by hundreds of million years of evolution shared genes control certain developmental processes here's an example there's a gene that's called isas that's a master switch that turns on eye development in arthropods and vertebrates the genes can be transplanted leading to successful development what do I mean by that here's a mutated version of Tropa that doesn't develop an eye it has a mutant version of the eyeless Gene you can find mice that have similar mutations that also don't develop eyes but if you can take the gene from uh eye developing dropa and implanted it into the eyeless form it'll develop eyes but the amazing thing is that if you take the Gene from an eye developing dropa and put it into an eyess version of a mouse that Mouse will develop an eye in other words this doesn't control the eye the eyes are quite different but it basically says build an eye in a fruit fly it'll build a compound eye that's common to insects and in a mammal it'll build the kind of eye that we have but there's a master switch that's common to all animals that have eyes that control eye devel vment another example of that is homeotic genes these are genes that basically say uh that you should have a head up here and you should have a tail down there and you should have these appendages over here and these are common across all animals as diverse as arthropods and vertebrates and these shared genes again are homologies and they indicate that all animals shared a common ancestor that goes back to the origin of the animal clay about six 100 million years ago what is biogeography how does biogeography provide evidence for evolution biogeography is the study of the geographic distribution of species and varieties if you enjoy looking at birds as I do you know that there are certain species that are found on the west coast of North America and there are others that are found on the East Coast that in of itself is biogeography the pattern of distribution on the surface of the Earth fits the idea that populations first of evolve in one area then spread to adjacent areas where subsequent Evolution occurs in other words the pattern of evolution is not only seen through time it's also seen through space and a great example of biogeography is the fact that the marsupial mammals are mostly but not completely limited to Australia why because Australia is a kind of isolated island continent the only placental mammals that were ever able to get to Australia were the bats which could fly there and a few rodents that were able to reach Australia on floating vegetation mats but other larger land living non-flying placental mammals couldn't get to Australia and hence it's the marsupiales that are the predominant mammals in Australia a fascinating thing connected with the evolution of marul is parallel Evolution where because placental mammals never made it to Australia all of the niches that are filled by placental mammals in North America and Eurasia and Africa are filled by marsupial and those similarities are all convergent so the marsupial mole and the Eastern mole convergent similarities based on the common Niche they're both digging in the soil the sugar glider and the flying squirrel and even top predators the now extinct Tasmanian wolf and the wolf that we find in North America and Eurasia biogeography another entire line of evidence for evolution as we've seen you can find evidence for evolution in the structure of animals in their genes in their distribution over the face of the Earth but you can also find evidence for evolution in time that's what fossils are all about what are fossils how are they evidence for evolution fossils are petrified remains of living things evolution is change over time and fossils demonstrate this change as you go back in time you find different arrays of living things through transitional forms fossils show Descent of modification in a variety of lineages now this example over here is an artistic rendition but it provides the point which is that as you go back in time you don't find modern whales you find the ancestors of whales who have features that indicate that whales evolve from animals that once were mammals that lived on land that over time adapted to an aquatic lifestyle what is relative dating of fossils how does it work it's based on the idea of superposition and the idea is that in rocks that are sedimentary that are laid down layer by layer the younger material is on top of the older layers so this would be younger over here and this would be older down there the deeper layers are older than the fossils in layers closer to the surface that's superp position but but because the Earth is complex and because there are earthquakes and volcanic eruptions there's faulting uplift and inversions that can complicate this analysis but based on the position of fossils within sedimentary strata you can determine which fossils are older than others in addition to relative dating determining which fossil is older than others based on their position in sedimentary strata there's also absolute dating describe how absolute dating of fossils works it's based on the decay of radioactive isotopes that are in fossilized remains or nearby volcanic strata that are interspersed with sedimentary strata the key idea is the idea of Half-Life that's the time it takes for half of a sample of radioactive isotopes to Decay from one element to another so an example carbon 14 decays to nitrogen 14 carbon 14 is radioactive unstable it's a case to nitrogen 14 the half-life is 5,730 years if a sample has 50% carbon 14 and 50% nitrogen 14 then for example if this was a piece of bone then that bone would be 5,730 years old it's one halflife if 1/4 of the carbon 14 is left then it would be two half lives 11,460 years other Isotopes have longer half- lives you can find carbon 14 directly in organic remains but you can also find as I said before radioactive elements that are in volcanic strata and you can use that to date older fossils so that's how we know how long ago the dinosaurs became extinct when the first vertebrates emerged when the first fish emerged so on and so forth that's absolute dating of fossils Evolution can be observed in structures that we see in modern organisms but there's plenty of evidence that Evolution continues to this day a great example of that is shown in this slide how is evolution of resistance to DDT and mosquitoes evidence of continuing Evolution let's talk about what's happening in the slide here's a mosquito this is showing the percent of mortality of um mosquitoes to a 4% solution of DDT exposed to them for one hour DDT is a pesticide kills mosquitoes why do we want to kill mosquitoes mosquitoes can spread a parasite that causes the bloodborne disease malaria and malaria is a devastating disease that causes illness in hundreds of millions of people around the world what happens is that when you spray the pesticide at First It's very effective and it kills most of the mosquitoes but never all of them there's always some mosquitoes that have some inborn resist resistance that survive the initial use of DDT those mosquitoes pass on their genes for resistance the other ones have been wiped out so the mosquitoes that survive have resistance and they pass them on over time within a matter of months you have only about 50% of the mosquitoes are killed why because mosquitoes have a very short generation time so this process is occurring quite rapidly and then after about 16 18 months very few of the mosquitoes are being killed this is observed genetic change there's some Gene that's producing some protein and that Gene is increasing in frequency throughout the population caused by human selection for DDT resistant mosquitoes this is not the only example of the rise of resistance there are parallel instances of evolution of resistance that have been observed in the evolution of antibiotic resistance in bacteria resistance to herbicides and weeds resistance to chemotherapy drugs by cancer cells it's a phenomenally important example of evolution that we can observe in this dayspeciation and Extinction what is the biological species concept and what are some of its limitations the biological species concept defines a species as a group of organisms that can naturally interbreed to produce viable meaning healthy fertile meaning able to reproduce Offspring and which is reproductively isolated from other such groups here is an example of one species there are different reads of dogs but the dogs don't care left to their own devices they'll interbreed and they'll produce viable fertile offspring these two species of ducks by contrast don't interbreed or at least they interbreed to such a small degree that their Gene pools are separate the concept is not perfect and it falters in a couple of cases the first is in the case of closely related species which can often hybridize they can often interbreed but not so much so the gene pools maintain relative separateness extinct or asexual species how could you tell if they could produce fertile or viable Offspring and procaryotic species what are reproductive isolating mechanisms compare and contrast pre and postzygotic forms of isolation reproductive isolating mechanisms are processes behaviors or other traits that keep the gene pools of closely related species separate malards pintails they're both Ducks how do they maintain separate Gene pools there are two kinds of barriers the first are prezygotic isolating mechanisms and they prevent breeding altogether they prevent the formation of a zygote postzygotic barriers exist between species that are close enough to mate and form a zygote but formation of a zygote doesn't ultimately lead to the production of successful individuals viable individuals who can survive and produce Offspring themselves let's look at prezygotic isolating mechanisms the first kind is behavioral different mating rituals or courtship behaviors would keep a female and a male from accepting one another as members of the same species and mating temporal means that they would breed during different times of the day or different seasons one species mates in the winter another mates in the summer they're not going to breath mechanical structural barriers that prevent sperm or pollen from reaching an egg if the flower structure is very different then a pollinator wouldn't be able to bring pollen from one flower to another flower habitat one species lives in the forest another lives in a meadow one species lives in the Uplands another lives in the lowlands they won't meet each other in order to breed and finally GIC the egg won't allow fertilization because because there's some molecular mismatch that would keep the sperm from being able to bind with the egg and induce the kind of changes that lead to fertilization list and describe three postzygotic isolating mechanisms postzygotic barriers can exist between species that are close enough to mate and form a zygote but nevertheless their Gene pools remain separate these can include hybrid inviability the hybrid organisms don't develop there might be hybrid sterility the hybrid offspring are healthy but they can't reproduce that's what's going on between horses and dunkeys which can interbreed to produce mules but the mules themselves are sterile finally there's hybrid breakdown the hybrids are healthy and can reproduce but the Next Generation the f2s are inviable or infertile contrast allopatric and sympatric modes of speciation allopatric speciation involves a geograph graic barrier it's what's shown over here in number one sympatric speciation occurs without a geographical barrier as shown over here in number two in allopatric speciation geographical isolation leads to genetic differentiation which leads to reproductive isolation in stage one a species is spread out over a geographical range there's gene flow between any subpopulation ations in stage two some kind of geographic barrier like this shown at B arises and it splits the species apart so there's no longer gene flow between these two isolated populations in stages 2 and three environmental differences result in different selective pressures which lead to genetic differentiation there's mutation happening and the environment is selecting which variants are going to be more favorable in each different environment and finally when that barrier disappears the two formerly subspecies have become so different that they can no longer interbreed they were geographically separated now they're reproductively isolated by any of the mechanisms that we've just discussed sympatric speciation can occur in animals and plants in Plants there can be sympatric speciation through poly empy this can lead to changes in chromosome numbers causing instant one generation reproductive isolation between the newly emerged species and its parent species one example of sympatric speciation in animals involves the hundreds of species of these fish that are called cichlids that live in Africa's Lake Victoria what is thought is that sexual selection has led to to reproductive isolation between the subspecies that's been experimentally verified it's a very complex story and what I want to encourage you to do is go look at the case study on learn biology.com the link is below here's another mechanism of sympatric speciation in animals adaptation to specific micro habitats can also lead to reproductive isolation and speciation such as the evolution of a variety of Li that inhabit different parts of birds this is kind of astonishing but there are Li that for example live only on the head of an albatross and there are different species that live on the wings there are other species of lice that only live on the heads of parrots While others live on the wings of parrots these are different species and they are in different parts of the bird and you might say well for uh little La well maybe that's like Mountain but that doesn't count as allopatric speciation there's no Geographic barrier all of these lights are just becoming extraordinarily specialized as they adapt to different parts of these birds and these specializations are getting passed on and that's leading to speciation over time so cpatrick speciation in animals because of microhabitat adaptation what is adaptive radiation this is a concept that we dealt with earlier when we talked about the evolution of homologous features adaptive radiation occurs when one parent species produces several descendant species Each of which has unique adaptations and fills a different ecological niche the 14 species of Galapagos finches shown here all of which are descendants of a single South American species are an example this one species arrived at the Galapagos Islands and then radiated into a whole variety of descendants Each of which has different features Each of which has a different ecological niche philogyny is a reflection of adaptive radiation each of the different branches show splitting from a common ancestor that's adaptive radiation on a very large scale finally homologous and vestigial traits which we talked about in the context of evidence for evolution are also a result of adaptive radi it's about splitting from a common ancestor and subsequent descent with modification we're now going to Pivot away from speciation and just look at the evolution of variation and its importance so why is phenotypic variation important for evolution I'm Mr W from learn biology.com where we believe that interaction and feedback is what leads to deep substantial learning we're so sure of that that we provide a money back guarantee that comes with your subscription phenotypic VAR variation is essential for evolution it's the raw material upon which natural selection acts natural selection selects for organisms with phenotypes that confer a selective Advantage remember that the genotype is invisible to Natural Selection what natural selection sees what natural selection acts upon is the phenotype and the result is that individuals with advantageous phenotypes survive and reproduce at higher rates than organisms with less advantageous phenotypes now as those organisms reproduce they're passing along their genotype but that's somewhat of a secondary effect natural selection acts upon phenotypes and note that with no phenotypic variation there can be no natural selection and no adaptation and that's why loss of variation is so dangerous to a species and can often lead to Extinction so here are a couple of examples of the importance of phenotypic variation let's start with this one explain how variations in phospholipid structure can serve an Adaptive function in browsing mammals that Forge in snowy environments phospholipids we remember those from unit one they've got a glycerol with a hydrophilic head and a hydrophobic tail these are fatty acids over here and note that these fatty acids can be saturated or unsaturated and there can be a lot of variation in that in mam anim that brows in the snow there is a tendency for the phospholipid tails that are in the body core to be more saturated and those in the extremities to be more unsaturated why because as the temperature decreases these unsaturated fatty AET tailes keep the membrane fluid so that diffusion can continue to occur so that oxygen can continue to flow from the blood into the cells whereas the phospholipid tails in the body core are more saturated and that provides a correct level of fluidity in that part of the body so there tends to be a gradient of more saturated fatty acid taals in the core more unsaturated fatty acid tailes in the extremities explain how variation in hemoglobin maximizes oxygen absorption in humans and other placental mammals at various life stages let's remember that hemoglobin is the protein that transports oxygen in red blood cells in almost all vertebrates before birth humans and other mammals produce fetal hemoglobin fetal hemoglobin is a variant and it has a higher affinity for oxygen than adult hemoglobin here's the difference all hemoglobins are the same in that they have these two alpha chains they're shown in red over here but adult hemoglobin has beta chains where fetal hemoglobin has gamma chains they're shown in yellow over here because of the higher affinity for oxygen the fetal hemoglobin will absorb it'll create a gradient that will cause oxygen to flow from maternal blood in the placenta over here into fetal blood so here's the gradient and that of course is highly adaptive because that's how the fetus gets oxygen while it's in the uterus but after birth what happens is that developmental genes kick in and slowly the beta form start to be produced more and more until it's completely predominant in the adult whereas the gamma form starts to fall and fall and fall and you can see that that happens even before birth but the transition is pretty complete complete 42 weeks after birth has occurred explain how variation in chlorophyll types increases the efficiency of photosynthesis green plants have two main types of chlorophyll there's chlorophyll a over here and Chlorophyll B over here and the difference comes down to just a functional group chlorophyll a has a methyl group over here whereas Chlorophyll B has a carbonal group Chlorophyll B absorbs more blue light and that's best in indirect light or in Shady environments or Shady situations and chlorophyll a absorbs more red light and it's best in direct light and therefore plants can do a couple of things one is that there are plants that are shade adapted and another plants that are direct light adapted and those will have different proportions of chlorophyll a and Chlorophyll B and then most plants have both and having both types of chlorophyll increases the amount of light energy that plants can use during photosynthesis over the course of a day or throughout the seasons now that we've looked at processes that increase biological variety speciation the development of variation now let's look at the flip side and equally important side of the evolution of life which is the idea of Extinction and the big idea is that Extinction is a normal part of the process of life greater than 99% it might be 99.9% of the species that have ever lived have become extinct let's describe the process that a species goes through as it heads towards Extinction this is called an Extinction Vortex and it starts with a population decline so small isolated populations are the ones that are most vulnerable to Extinction that population decline can be caused by an adverse change in the physical physical environment climate change which has happened throughout the history of Earth or the arrival of a competitive species that reduces the species Fitness and reproductive rate that leads to genetic drift which we discussed in population genetics that results in a loss of genetic diversity that leads to less variability and that loss of variability leads to reduced Fitness there's more genetic uniformity and that reduces the ability to adapt to Environmental change and the result is a smaller population that then goes through this process again and again it's a positive feedback loop it's not a positive thing but it's a positive feedback loop that feeds on itself it accelerates itself leading to Extinction what we just described is the typical normal process of Extinction that's quite different from mass extinction what is a mass extinction a mass extinction is a widespread rapid decrease in Earth's biodiversity often mass extinctions are caused by geological or even astronomical causes though biological causes are possible as well and there have been at least five major Extinction events during the last 600 million years and those are the numbered peaks in this diagram the most recent Peak which occurred 60 million years ago is the Cretaceous Extinction that's the one that wiped out the dinosaurs now you might think that Extinction is all bad mass extinction in particular but there's a connection between mass extinction and adaptive radiation mass extinction leaves vacant a variety of ecological niches and that is an ecology term that we'll deal with in Unit 8 those are ways for a species to make a living and the result is a mass extinction such as that shown at two which wiped out all of this previous existing biological diversity is followed by extensive adaptive radiation in the species that survive and here's one the species at four and it made its way through for whatever reason this mass extinction over here and then it underwent an extensive adaptive radiation and an example of that is the diversification of placental mammals mammals like us that followed the Cretaceous Extinction that wiped out the dinosaurs and again just to connect this to our own lives if that mass extinction 65 million years ago hadn't happened we wouldn't be here talking about mass extinctions it's because our ancestors made it through that mass extinction and then radiated to produce the whales the hippopotamuses the primates the rodents and our group is the primates and here we are how is human activity affecting Extinction rates humans are the cause of what's been called the sixth Extinction here are the five great extinctions that have happened in the past humans are causing a decline in global biodiversity that could rival these previous five mass extinctions how are we doing that human activities that are causing Extinction include destruction and fragmentation of habitat over hunting over harvesting of animals and plants intentionally or unintentionally introducing invasive species into new habitats this is material that we'll look at more closely at the end of AP Bio unit eight are you asking yourself how am I going to get a four or a five on the AP Bio exam it's a good question because it's a hard test but we have a plan for your success go to learn biology.com sign up for a free trial and complete our interactive tutorials and interactive AP Bio exam reviews we guarantee you a four or a five on the AP Bio exam see you on learn biology.com topic 7.9 ph what is philogyny what is a philogenetic tree how are such trees built philogyny means evolutionary history a philogenetic tree also known as an evolutionary tree is a branching diagram that shows evolutionary relationships and philogenetic trees are built using morphological that means structural molecular or genetic evidence this is a philogenetic tree and one thing that I want to emphasize is that that it is a claim that's based on evidence so the claim here is that hippos and whales are more closely related to one another than whales are to Deer why is that because based on the evidence which would be morphological molecular genetic hippos and whales have a more recent common ancestor than either does with deer we'll see the details in the subsequent slides Define the term CLA clade is an incredibly important biology term that's not known to the general public but now you'll know it and you'll be able to explain it to people a CLA is a group of organisms that consists of a common ancestor and all of that ancestor's descendants in this diagram all of the species and all of the numbered groups are Clays so down here the large ground finch that's a seed eating Finch in the galopagos islands that's a CLA all members of a species have a common ancestor all humans have a common ancestor all human beings are a clay the large ground finch and the common Cactus Finch have a common ancestor that's this over here at e so those two species together constitute a clay if you go back in time because there's implied time in any philogenetic tree then all of these four species the Woodpecker Finch the small tree Finch the common Cactus Finch and the large ground finch they comprise a clay that's group number five over here their common ancestor is this species that's over here at C Define shared derived character a shared derived character is a trait that identifies that distinguishes a clay and it evolved in the common ancestor of that clay and it sets it apart from other such clades so for example lungs and four limbs separate this clay all of these organisms that are classified as frogs ards alligators Robbins rats and gorillas that constitutes a clay and they all have this shared derived character going back in time the Shar dve feature vertebral column separates this clay that includes all the salmon through gorilla fur and mamory glands separates the clay that includes rats and gorillas from all the other organisms these are mammals and those are shared derived features of mammals what are nodes and sister groups a node is where two branches in a philogenetic tree diverge and nodes represent the common ancestor of the two diverging lineages so at left letters a through e those are all nodes nodes common ancestor so over here there was a common ancestor that led to the Divergence between this clay over here and this um species over here the green warbler Finch which is not part of the clay that includes includes all of these organisms that have a common ancestor at B Sister groups are The Descendants that split apart from the same node such as the common Cactus Finch over here and the large ground finch those can also be called sister species because that's the taxonomic level at which they're at so the Woodpecker Finch the small tree Finch that clay that's a Sister Clay to this clay that's defined by letter E the common Cactus Finch and large ground fch what is an outgroup an outgroup is a more distantly related group of organisms that's used to determine the evolutionary relationships among the other organisms in the tree which is the ingroup in this philogenetic tree a is the outgroup for this CLA that's over here at number two it's a point of comparison for the ingroup and it's a species or it's some other taxonomic category that's not part of the clade to which all the other organisms in the philogenetic tree belong what's the biggest mistake to avoid in philogenetic analysis the misconception that many students naively have is that vertical closeness on a horizontal tree indicates evolutionary closeness but it doesn't for example the frog and the lizard they're next to one another but they're not particularly closely related the only thing that matters on a philogenetic tree in terms of evolutionary relatedness is the recency of common ancestry so alligators and Robins they have their common ancestor over here rats and gorillas they have their common ancestor over here frogs are right next to lizards but their common ancestor the common ancestor the frog and the lizard is all the way back here and the basic idea is that nodes can rotate so this philogenetic tree over here is correctly drawn it's exactly equivalent to this one over here but you can see that in this one the lizard is no longer next to the Frog right so here's a lizard down here here's the Frog over here and here's a lizard over here and here's the Frog over here it's just about rotation and if you think about the art form called a mobile mobiles all rotate it's a kind of kinetic sculpture and if you can think of phenetic trees as one instantiation of a kinetic sculpture you'll have the right idea about this always look for recency of common ancestry when you're trying to figure out the evolutionary relationships described by a philogenetic tree Define ancestral feature an ancestral feature is a trait that members of a clay share but which is also shared by larger more inclusive clades it doesn't Define the clay so for CLA G which includes rats and gorillas that's the mammal clay an ancestral feature would be claws or nails and why is that because mammals they do have that trait but it's also found in this larger more inclusive clay that runs from lizards down to gorillas over here lungs and four limbs is another ancestral trait of rats and gorillas this mammal CLA it doesn't Define the CLA but everyone in the CLA has to have that because there's a larger more inclusive CLA that has that feature what type of evidence is typically used to construct pho genetic trees of existing species before the 1960s morphological similarities was what biologists used to determine who was more closely related to whom and thereby construct a phylogenetic trees but more recently post 1960 remember Watson or Crick figured out the structure of DNA in 53 nucleotide sequences in DNA and RNA and amino acid sequences in proteins have become the gold standard in determining philogenetic relationship so a DNA analysis would indicate that the common Cactus Finch is more closely related to the large ground finch than to any of the other galopago species that's a hypothesis and you could confirm that through DNA sequencing what are molecular clocks here's how this works for any specific protein or nucleic acid the rate of change that's caused by accumulated mutations is constant over time and if you can calibrate the amount of change in a gene or protein to when species split apart which you can sometimes determine from the fossil record then you can do two things one is you can determine a rate of change in that Gene or protein and you can use that rate to determine when other species split apart what this graph says is it's looking at hemoglobin as a molecular clock based on some fossil evidence it's basically saying that 20 mutations accumulate every 100 million years so therefore if you're looking at the hemoglobin between two species and you notice 40 amuno acid substitutions and you could infer that that occurred more like 230 million years ago this is widely used in evolutionary biology it's the basis for claims that for example humans and chimpanzees had a common ancestor about 6 million or so years ago based on the similarity of DNA between hus and chimps topic 7.13 the origin of life this is one of the coolest topics in biology for AP biology you need to master a very specific body of knowledge the key question to address when trying to understand the origin of life is how did life emerge naturally and this connects to some biology that you've already learned in your AP biology course for example you've learned about the cell theory and the idea that all cells come from pre-existing cells well at some point there had to be a first cell and in fact that moment really marks the emergence of Life how did a cell emerge with the absence of another cell to create it that's one of the Mysteries that we have to contend with in the origin of Life the other one has to do with chemistry the chemistry of life is largely controlled by complex proteins called enzymes enzymes can take simpler substances and make them into more complex substances through processes like dehydration synthesis well how do we get the complex substances that life is based on in the absence of enzymes what are the key steps required to explain the origin of Life the Earth is extremely ancient it's been around for 4.5 billion years and life is thought to have emerged about 3.8 billion years what steps allowed life to emerge the first thing is that Earth needed to become a habitable and more stable Planet the Earth formed from a nebula about 4.5 billion years ago that's beyond the scope of this course but what's important to know is that for the first several hundred million years of Earth's existence it was pummeled by asteroids and comets and all of that needed to come to an end before life could begin to emerge step number two is about chemistry leading to biology biology is based on cells and those cells in terms are based on complex molecules polymers the polymers are built from monomers so how did those monomers emerg that had to happen abiotically in the absence of enzymes monomers had to start to emerge we need to start to have amino acids nucleotides fatty acids Etc in today's Living World monomers get combined into polymers how could that happen in the absence of enzymes there needs to be some kind of abiotic process by which that could happen where we have the abiotic synthesis of polymers from monomers and the formation of vesicles those are going to become the little capsules into which cells will emerge in step four we need to combine those monomers and polymers into those little vesicles to create Proto cells these aren't quite living cells the way that we have them today but they have many of the characteristics ICS of cells and finally we have the emergence of self-replicating cells that would be the last Universal common ancestor over here and that gives rise to the three great domains archa bacteria those two combine that's another story altoe to form the ukar Nots the Miller Yuri experiment which was carried out in the 1950s showed that in an abiotic setup without life you could successfully synthesize amino acids in a simulated early Earth environment now as we'll see some of the details that were involved in the setup of the Miller Yuri experiment were wrong they don't correspond to our current understanding of the geology of the early Earth but it's an important proof of concept so what did Miller Yuri do they created this sterile apparatus with various Chambers and tu beings and the whole thing was sealed off from the outside world and sterile this chamber over here at number two represented the early oceans it was heated by a bunson burner or some other heating device and then the steam circulated through these tubes over here this large area over here is a chamber that represented the early atmosphere and it was loaded with gases that were supposed to be present in the early atmosphere and that includes gases like methane ammonia hydrogen and water vapor those gases were chosen because in the 1950s that was the current understanding of what the early Earth's atmosphere would be there are two important points one is that that was chosen because that was the known atmosphere of some of the gas giant planets like Jupiter and it was thought that that might also be present on the Earth but the most important thing was that no oxygen was present because oxygen was produced by photo synthesis it wasn't present in the atmosphere of the early Earth there were electrodes shown over here at number four by this plus and minus sign and what they did is they produced Sparks which simulated lightning the gases continued to circulate they circulated through this condenser over here and that caused whatever was formed in this chamber here to be caught within this trap this trap could be sampled without contaminating the apparatus and after a certain period of time Miller who was the actual experiment it was Stanley Miller Harold Yuri was his graduate thesis adviser sampled the liquid and they found the presence of amino acids well like I said that didn't necessarily establish that life could be produced in the absence of life but it did establish that some of the monomers of life could be produced in the absence of Life As I said some of the details were wrong but this has proven to be the model for many subsequent experiments where there are inorganic catalysts and different mixes of gases that are changed and in subsequent experiments the nucleotide bases have been produced fatty acids have been produced many of the monomers of Life have been produced so what's the basic idea in the absence of life it's been experimentally established that some of the monomers that make up the chemistry of life could be produced to understand the origin of life you have to understand the origin of heredity it's widely thought that RNA not DNA was the first hereditary molecule why RNA does store genetic information in viruses so it has the capability of being a genetic molecule but Unlike DNA RNA can also act essentially as an enzyme RNA can have catalytic properties properties it does so in ribosomes where the catalytic part of a ribosome is what actually stitches together amino acids to create polypeptides and them proteins we have splies we have micrornas all of which are catalytic acting upon the world DNA with its double helical form is fantastic for storing genetic information RNA because it has both catalytic abilities and information storage capabilities is widely thought to have emerged as the first genetic molecule before there were cells it's thought that there might have been a phase in which there were self-replicating systems of RNA molecules that were subject to Natural Selection that would grow in complexity and that's called the RNA world that led up to the last Universal common ancestor of Life here's how that might have happened in the beginning what we had were inorganic precursor molecules ules it's what we talked about in the preceding slide with the discussion of the Miller Yuri experiment so at some point the sugars the phosphate and the nitrogenous bases that make up RNA would have to emerge those would combine through abiotic inorganic processes to form RNA monomers other abiotic non-enzymatic processes would lead to the next step and in that next step we have RNA polymers that start to emerge they're shown at C rnas can fold up into complex shapes and that's what we start seeing over here at D and those complex shapes can start to have enzymatic properties one of those enzymatic properties could lead systems of rnas it doesn't have to be just one molecule but there could be a system of interacting molecules that could lead the system to replicate itself at some point Point those self-replicating RNA systems would become encapsulated within some kind of lipid bilayer and at that point at letter F we have a Proto cell further natural selection further Evolution would lead to the emergence of the last Universal common ancestor that's the population of organisms that gives rise to both archa and bacteria and ultimately through a fusion of those two domains to ukar and if you can look at this diagram you should pause and test yourself and identify all of the numbered items then you've learned a lot of cellular biology and you're well on the way to being set up for Success on the AP Bio exam just to run through at number one we have a lipid bilayer remember it doesn't have to be a phospholipid bilayer because ARA have a different kind of by layer structure at two we have DNA being the genetic material that's found in all living things at three we have RNA which is used for information transfer and other things at four we have ribosomes which are translating messenger RNA into protein at five we have a membrane channel that's allowing matter and energy to flow in and waste to flow out at six we have complex proteins that are acting as enzymes which are combining monomers into polymers through dehydration synthesis and then pulling part those polymers through hydrolysis and at seven we had ATP synthes which is using a flow of protons across the ATP synthes channel to generate ATP that Universal energy molecule from ADP and phosphate that's the last Universal common ancestor