as I mentioned at the end of last recording this part two of module 2 is going to focus on evolutionary Fitness and the um things that can affect Fitness uh and certain trends that we generally see in evolution uh related to evolutionary Fitness so one major thing that will affect uh evolutionary Fitness or you know impact which alals persist into the Next Generation is gene flow I've already mentioned it a couple times all it means is there is uh individuals from um different populations mating with each other so there is like movement of gametes uh from one population to another so here we have two a hypothetical species of bird and on one side of this mountain all of the birds are uh blue because the h Gene uh is dominant and so they're blue dominant uh and on the other side of the mountain all of the birds are red because the they all have recessive H genes however these birds will migrate and mate with each other as a result there is gene flow from this population to this population and from this population to that population because of that gene flow you will still get the presence of the recessive Al in this population and you'll see you know some uh blue birds pop up over here because of the presence of the dominant alil so bringing this back to Hardy Weinberg right if we were to look at this initial population and all of them were homozygous dominant and we were to make predictions about the Next Generation we would predict 100% of The Offspring would all be homozygous dominant however because this population is not in Hardy Weinberg equilibrium we have a different result where some of the offspring are heterozygous that's because we have individuals from this other population mating over here and there is gene flow between the two populations if gene flow were to stop between the two populations then they would become reproductively isolated and they would start to diverge into becoming two different species so first thing that affects uh Next Generation in terms of what traits are produced uh or or seen is gene flow another concept that connect that can affect uh a and genotype frequency uh in a population is genetic drift genetic drift are random or is caused by random changes that change the frequency of alal in a population um often this means certain harmful alals or some you know evolutionary less desirable traits can persist at a high frequency uh there are two kind of major ways we see genetic drift in nature first is the founder effect with the founder effect there is a small few number of individuals within a population that colonize a new area and just by chance those uh new individuals may have some kind of bias and as a result that population will represent that bias uh similarly with a population bottleneck this is when there's some kind of environmental event that only a few survive so if we look at that like the uh kind of um an example of what that would look like right we have our population here with these red and yellow beans we're going to take this bottle this population of red and yellow beans and we are going to pour a little bit out into this new environment now it just so happens that the small sample that we poured out has a lot more red beans than yellow beans whereas if we look at the original population it's maybe 50/50 red and yellow but if this new population is able to reproduce and colonize this new area we would see a much higher frequency of those red beans not necessarily because the red trait is more desirable than the yellow trait or more fit or makes individuals more fit than the yellow trait but just because our starting population had a much higher frequency of red than yellow it's essentially an idea of like taking too small of a sample size right if you are trying to make conclusions about you know whatever it is and you're like oh well I went around and I pulled uh some people and and they told me that this was their favorite food so this must be everyone's favorite food well if you only asked 10 people there's that's not really a strong that's not really a lot of evidence that's not a strong claim whereas if you asked a billion people what their favorite foods were well you have a much more reasonable sample size then uh and you can make uh more informed claims about whatever it is you're asking them so here we're taking a too small of a sample size and as a result getting skewed or unpredicted uh or uh like unpredictable data because of that and so a Founder Effect right so that's genetic drift the founder effect is when that small sampling happens due to colonization of a new area population bottleneck is when uh that small sampling happens due to some kind of environmental uh disaster up until this point we've talked mainly about qualitative Trad meaning you know like with the butterflies they could be orange or white um but a lot of traits exist on a spectrum those are our uh quantitative traits and natural selection impacts those uh just like it impacts any other trait uh but the way that we kind of observe it is a little bit different because it's you can't really point to individual frequency of genotypes but you can still see changes across the population uh and generally there are three ways that natural selection acts on these quantitative traits there is stabilizing selection directional selection and disruptive selection and we'll talk about each of those on their own slides stabilizing selection reduces variation over time in other words the uh frequency of median traits will increase with each generation this is also called purifying selection so uh if you think about in terms of like probability right those on the very edge will start to disappear and the peak will become higher the example shown here is with birth weight uh individuals born at a really really low weight have a higher incidence of mortality and individuals born at a really really high weight have a high incidents of mortality whereas the mean the average birth weight is much closer to Optimal so over time these traits that lead to mean birth weight are going to be selected for and this population will move toward this Center Point the center line directional selection occurs when one end or the other of the spectrum is more favorable uh so example here uh with Texas Longhorn cattle uh this was a population of wild cattle that started to colonize an area with more Predators as a result the larger the animals with the larger horns were able to better protect their young and therefore over time this population of cattle started to develop very large horns so unlike stabilizing selection where we move toward the center directional selection is either moving to one end or the other with the example of of cattle we have this positive selection toward larger and larger horns and then we have disruptive selection which is sort of the opposite of stabilizing where instead of moving toward the mean we get uh the population kind of separating onto either side of the spectrum and so here uh the example is a species of bird and their uh uh Bill sizes or beak sizes uh these birds eat seeds and in this population where they live there are lots of very soft seeds and then there are lots of very hard seeds and so over time what we start to see are individuals with very wide uh uh bills or beaks are able to crack open the hard seeds whereas individuals with very narrow sharp bills are able to tear through the Flesh of the soft seats whereas individuals with the kind of mediumsized beak aren't aren't really optimized for the hard seeds or the soft seeds so as a result we see an increase in either end of the spectrum and a decrease in the median a uh similar related phenomenon phenomenon is frequency dependent selection where two fairly distinct phenotypes will exist in a population because they benefit each other here we look at a species of of uh scale eating fish and we're talking about these smaller gray fish here right and in this population there are lots of fish whose mouths are either right mouthed meaning they tilt to the right or left mouth meaning they tilt to the left now the more left and right mouth fish there are the better they're able to work together because they can Target their prey from both sides so as long as there's lots of right fish uh sorry uh right mouth fish there will be left mouth fish and vice versa because they work well together whereas a center mouth fish doesn't necessarily work well uh with either these two are kind of optimized uh for hunting prey in their given population so this is frequency dependent uh selection um uh which is you similar to uh disruptive selection where you get kind of extremes on on both sides another example of uh selection for certain traits that is a little counterintuitive or or you know different than natural selection is sexual selection um or non-random mating so when individuals uh have preference on on which mates they choose um of often it's not just about selecting the most evolutionary fit individual that's where sexual selection comes in um and so sexual selection is when mates are chosen for traits that don't necessarily have a uh like an evolutionary Advantage um and this is for the most part how human select mates uh I'll show an example in class but another a really common example of sexual selection is with birds in their uh mating rituals often males with very bright colors are able to attract more females however the bright colored males can also attract Predators so even though these really bright colored birds are able to attract more females it's not a very evolutionary advantageous trait because uh those males are more e more easily attract Predators so this is how we get into that idea of natural selection or how Evolution doesn't always lead to an organism perfectly suited for their environment because certain traits get selected for even though it doesn't make them the best for their particular habitat another example of this is a concept called the heteros Z get's advantage in these populations uh a given uh genotype um or a heterozygous genotype is has some sort of advantage over both the homozygous dominant and homozygous recessive genotypes and as a result both alals will persist in the population a go-to example of this are populations of the world where malaria is a very prevalent cause of uh death um often in those populations there will be a higher frequency of the sickle cell Al and if we look at uh we can look at why that's the case so individuals who are homozygous recessive will have sickle cell and they'll suffer from complications from that disease that often results in Death at a very young age so it doesn't that doesn't have an evolutionary advantage people who are homozygous dominant and and do not have that CLE cell trait are much um more uh vulnerable to malaria and therefore will suffer the ill effects of Malaria whereas individuals that are heterozygous are much less are much more resistant to malaria and their sickle cell trait their CLE cell um symptoms are very very mild if present at all so as a result the heterozygotes have an advantage when it comes to mating but because of that this trait of CLE cell that we you know think of as as bad or undesirable will persist in those populations because it sort of backdoor gives an advantage uh elsewhere so natural selection evolution is not as straightforward as like the fittest survive um there are lots of reasons why a certain certain trait will persist into the Next Generation and why it'll uh increase in frequency over time despite maybe not being the best thing for that uh for individuals of that population to survive and thrive in their habitat