today what I want to talk about is genes Darwin observed variation in natural populations and he understood that traits could be inherited by Offspring but he didn't know how this happened and of course today we know that the reason is genes genes were discovered in the 1850s by a man named Gregor Mendel who was an Australian Monk and a high school teacher and he experimented with inheritance in pea plants he actually really loved to garden and he did his experiments between 1856 and 1863 and then he published his results in 1866 but in 1887 he became the Abbot of his Monastery and he had to quit his biological research so in essence his his career as a biologist was relatively short and he published only one paper during his life so here are Mendel's characters um he looked at the length of the stem of the pea plant they could be short plants or they could be long and viny um he looked at where the flowers were the flowers could be distributed all the way along the length of the plant or they could be just right at the top he looked at whether the unripe pods were yellow or green he looked at whether the pods were inflated or or pinched when they were ripe he documented the color of the flowers so he had purple and white pea flowers and he looked at the seed interior was it yellow was it green and he looked at whether the peas were round or wrinkled when they were ripe you know and some things are more interesting and easier to understand if you work through them on the board so now what I'd like to do is is talk about how Mendel worked out these inheritance systems so Mendel knew some things as he started his experiments he knew that each individual pea had a parent a mother plant and a father plant that that contributed the Pollock so it's you know he there's inheritance material coming from two individuals and he also he would he defined you know we went through the traits that he defined so he defined these traits and for each trait um he felt like there was a dominant and a recessive version of the trait and the difference is that the dominant version is whenever that genetic copy is present it will be expressed the recessive version of the trait is not expressed if the dominant version is present but if there are two copies of the recessive gene then the trait that is expressed will be the recessive now just for the purposes of convention a a trait is generally assigned a letter and if it's a dominant trait we give it the uppercase letter and if it's the recessive trait we give it a lowercase letter now this is a little piece of vocabulary here and it's kind of difficult because these are odd funny words but they have their Greek Roots so the first one is homozygous and homozygous has the Greek root homo meaning the same and zygous meaning you know genetic material so if you're homozygous you have two copies of the same gene and so you could be homozygous for the dominant trait in which you would have a double uppercase a or you could be homozygous for the recessive trait in which case you would have two lowercase A's now you can be homozygous but you can also be hetero zygous and this is also from the Greek root so hetero means different and zygous again refers to genetic material so this would mean if your heterozygous you have one copy of the dominant trait and you have one copy of the resist recessive trait so a heterozygous organism would be designated with a capital A and a lowercase a and then what Mendel did was he worked out what proportions of homozygous and heterozygous individuals and what types of homozygous individuals you would expect if you crossed any two different pea plants and this is how that works so when we're doing this and I'm going to do a cross between two heterozygous individuals and the way we're going to do this is we're going to make ourselves a box and we're going to put one parent on the top and so this is the dominant copy of the gene this is the recessive copy of the Gene and we're going to put the other parent on the side once again the dominant and the recessive okay so if you have two heterozygous parents there's a couple of there's like four ways you can have offspring so one offspring or or one group of Offspring or one possible result of this pairing are individuals who are homozygous dominant so that's an a here and an a here both uppercase homozygous dominant the other thing you can do is you can get a dominant gene from this parent and a recessive gene from this parent so this organism which is an offspring would be heterozygous same here you can get a dominant gene from this parent a recessive gene from this parent and once again heterozygous descendant and finally you could get two copies of the recessive gene so the ratio of inheritance would be one quarter homozygous dominant one-quarter homozygous recessive and because there's two ways to get a heterozygous offspring half of The Offspring would be predicted to be heterozygous if you start out with two heterozygous parents and I didn't say this when I defined heterozygous but what I want is what I want to remind you now is that because the capital a trait is the dominant trait even though both of these individuals have a recessive gene they're going to look identical to the uh to the to the homozygous dominant trait so you know if you were just looking at how these organisms look you would say that three-quarters of them are going to express the dominant trait and only a quarter of them are going to have the recessive trait and you can work this out yourself for other possible process for instance two homozygous individuals one dominant one recessive or a heterozygous individual with a homozygous recessive or a heterozygous individual with a homo with a homozygous dominant it's kind of fun so what did Mendel actually demonstrate well first of all he did demonstrate that inheritance was a binary system there was information from the mother and information from the father however this was already widely assumed when farmers were doing breeding experiments and developing new breeds and new crop plants during the Agricultural Revolution Mendel also demonstrated that traits could be dominant or recessive and he demonstrated that the ratios of inheritance can be predicted if you know the genetic combinations of both of the parents then those ideas were convincing because he could predict the probability of different numbers of homozygous and heterozygous individuals in genetic crosses however I have to say we now know that Mendel cooked the books by ignoring data points that didn't fit his ideal ratios we learned this when when evolutionary biologists attempted to go back and use Mendel's pea plants um and and basically to reconstruct Mendel's experiments we never get data as good as the data that Mendel reported however in Mendel's defense I would say that today we have a much better understanding of how to use statistics to test hypotheses than Mendel did now I want to talk a little bit about genes and evolution and I'd like to just introduce two useful terms these are genotype which is the genes that an organism possesses and phenotype which is how an organism looks and what its physiology is basically phenotype is the expression of genotype and the point I want to make is that sexual reproduction and genetic mutation are sources of variation for evolutionary change and of the two I am going to really stress sexual reproduction what sexual reproduction does is it puts together new Gene combinations within organisms and it's it's it's possible that genes may not have much effect on reproductive Fitness until they are paired with other genes so it's sort of and this is a way of saying that Evolution works on the phenotype and the phenotype is the expression of all of the genes of an organism so just by putting together new Gene combinations um sexual reproduction will drive evolutionary change the other thing that drives evolutionary change are mistakes in copying DNA or mistakes in the timing of gene expression which cause mutations now most mutations are harmful but every once in a while you get helpful mutations and every once in a while you get a single mutation that that makes a new species an example of this is the Coke's gray tree frog and the gray tree frog species pair now these species were only discovered when we began to do chromosome studies and it turns out that Cope's gray tree frog which was probably the original first species has 24 chromosomes and Gray's tree frog has 48 chromosomes the reason we had to discover these species with chromosome account counts is because to the human eye these frogs look identical and actually if you look around they both both the Cope's great tree frog and the gray tree frog live in College Station um they look identical the only way you can tell them apart is if you record their songs however because they are tree frogs they have songs that human voices can't hear so it's a kind of an interesting thing but in a reproductive event the hypothesis is that there was a there was there was a mistake um during reproduction for a copes gray tree frog that resulted in a new set of tree frogs that has 48 chromosomes instead of the original 24 and you know once this happens they can never back cross you know you cannot mate a Coke's gray tree frog with a gray tree frog because of the mismatch in chromosome numbers and right now you know the only morphological difference that we can distinguish between these two species is the difference in their songs but because they can never cross back with each other in time they may evolve and different and an additional morphological differences to distinguish the two species may develop so let's just step back and think of some of the key points one key point is that Darwin's theory of natural selection reveals how evolutionary change occurs different traits lead to different amounts of reproduction and when those traits can be inherited the population changes to look like the organisms that have the most offspring the other key point is that Darwin did not know that genes were the mechanism of inheritance uh he he he knew that there must be some mechanism of inheritance but he didn't know what that was and Mendel who was a contemporary of Darwin's discovered genes but the importance of Mendel's Discovery wasn't recognized until the early 20th century so far as we know Darwin did not know of the existence of Mendel during his lifetime the other thing to sort of think about and it's kind of a fun idea is that natural selection works on biological entities like populations and species but it may also work on cultural entities and when you're thinking about so what do you need to make natural selection work you need variability so there has to be variation among the organs or the individuals or the things that you're selecting the variation has to be heritable and the variation has to be able to influence the ability of organisms or entities to reproduce and one of the things that's sort of an interesting thought experiment is to is to sort of think about natural selection in the context of businesses for instance you know pizza restaurant franchises business models and economic systems okay now that we know about um genes and we have the basics for natural selection I want to step back and think a little bit about different types of genes and how major evolutionary change can occur and right now I hope everybody is thinking what do you mean by Major evolutionary change and by Major evolutionary change I'm thinking of the types of changes that lead to whole new groups of organisms like vertebrate animals when before there were no vertebrates or dinosaurs when previously there were not dinosaurs in the world or pterodactyls or mammals just a brand new types of organisms that can radiate and become really really important in ecosystems so the first thing to realize is there are two types of genes there are structural genes sometimes these are nicknamed housekeeping genes these are genes that code for proteins and enzymes and basic cell functions changes in the frequency of structural genes can drive evolutionary change within populations and changes in structural genes can sometimes drive speciation events but the thing about a structural gene is because these genes are so essential to the working of the cell that it's hard to to accumulate enough changes in structural genes to drive evolutionary change above the species level because if you change too many components of your structural genes the result is an organism that doesn't function you know individuals that can't even live to reproduce the other type of gene is a regulatory or an informational Gene and in animals these are the Hox genes and hoxes Hox and it is the shorthand for homeobox genes and in animals homeobox genes are the major type of regulatory genes although animals also have mads box genes but they just don't seem to be as important for um for for for development in animals but if you look at plants um you would find a different set of genes and these are called the mads box genes and they appear to be more important in regulating the development of plants so what do these regulatory genes do well they turn whole sets of structural genes on and off and by doing this they control development and you may remember when we were talking about evolutionary change I I describe developmental changes as changes that happen during the growth and the maturation of organisms so you know how organisms change through time are those are developmental changes and you know it's sort of hard for us to imagine this because our development is um you know it's it's relatively slow uh but if you think about it you know we all have part of our life that we spend in utero and in utero we need organs and we need physiological pathways that we don't need as children or adults at birth we suddenly need to exist on our own so we need a digestive system we need lungs um you know we need a working heart we need kidneys we need all of these things that we didn't need in utero because in utero we could depend on the placenta and finally you know when we pass from childhood into adulthood we develop sexual organs um and and you know all of the organs that we need to reproduce that that you may be present in children in a very very nacent state but they're but they're not developed because children don't need them and in our lifetime each one of these transitions is is is is is uh happens because of regulatory or informational genes um okay so what do Hox genes do so Hox genes specify regions of the body so it's it's it's Hox genes that that that make sure that your head is at the front of the body and all of the organs associated with the head or at the front of the body and the tail is at the back um they also specify right and left sides of organisms and they they they specify up which is dorsal and down which is ventral and you know the way to think about this is think about yourself as a fish and if you were a fish swimming through water your backbone would be on the top and your stomach would be on the bottom which is why we call up dorsal and down ventral so the interesting thing about Hox genes is that nearly all animals have the same set of Hox genes particularly vertebrates and arthropods have all of the same families of Hox genes and you can see that here the fly Hawks genes and the parts of the fly body that those genes are expressed within are on the top and then you see the little mouse embryo on the bottom and in both cases you know the Red Hawks genes are associated with the head of the organism um now the difference between arthropods and vertebrates is that vertebrates do have more copies of each Hox Gene and it does seem as if the number you know the more copies you have the Hox genes the more complexity you can have as an organism um but you know as I said the same Hox genes determine the position of the head and tails in both groups and we've done probably most experiment and we probably you know probably the organism that is best understood is the fruit fly particularly in terms of how genes control development of fruit flies and I think one of the most interesting demonstrations of Hox genes is that there is a mutation in normal flies that causes the antenna to develop as little legs and what this means is it means that that evolutionarily the antenna of the fruit fly is you know is derived from a leg so here's a picture of um the families of Hox genes in different groups of animals on the top is a round worm round worms are really simple they don't have very many Hox genes um the next organism down is Drosophila and you can see they're pretty much Drosophila has all of the Hox genes that are present in invertebrates and in mammals um the uh the organism between the Drosophila and the mouse and the little man on the bottom is an early is sort of it's a non-vertebrate relative of the vertebrates and once again the theme Here is that you know the more complex the animal becomes the more copies of the Hox Gene it has mammals have many many copies of the Hox genes so this is kind of an interesting thing and one of the points I want to make is you know when we're thinking about dinosaurs and mammals um is that mammals have very very similar skeletons just about every mammal you look at has 206 bones and seven cervical and cervical refers to neck vertebrae so you know mammals may lose toes and fingers uh but pretty much the only thing that changes um you know aside from you know loss of toes and fingers is that manatees and two-toe sloth have lost some of their cervical vertebrae and there's one mammal the three-toed sloth which is shown here on the left um which actually has developed or gained cervical vertebrae and three-toed sloth can have I mean I guess there was one discovered with 10 cervical vertebrae but the one shown here only has nine so the idea here is mammals have very stereotypical skeletons not so dinosaurs dinosaurs can add bones very very easily you know compared to our seven cervical vertebrae sauropods can have up to 19 and maybe even more than 19 cervical vertebrae um evolutionarily the ceratopsians that's the group that Triceratops belongs to adds a whole new bone the rostral bone to the front of its face and and the ornithischian dinosaurs also added a bone to the bottom of their jaw the predentory bone and and I think that the thing to think about is that the ability of dinosaurs to shuffle bones in their skeletons you know to add more vertebrae to brace the pelvic girdle to add more neck vertebrae to make their necks longer um to to develop whole new bones this seems to be a dinosaur character and and and a lot of the times when we talk about different dinosaur groups we're going to be talking about you know different bones that are specific to to to to a dinosaur group and I think that this you know this suggests to me that that the Hox genes that are controlling dinosaur skeletons and skeletal development in the dinosaurs um are are much more open to mutation and to changes than the Hox genes that are responsible for developing the mammalian skeleton