Evolutionary Biology BIOL 3350 Dr Lisa Rapaport Lecture 10 Part 2 Notes I II IV V Evolution in Small Populations Rate of fixation A As the population gets smaller the deviations from HWE become more and more pronounced and the rate of fixation increases Overall effects of genetic drift A An allele can go to fixation or go to extinction just by chance alone B When genetic drift is really strong the changes are not a result of selection but instead they are a result of chance this can make the individuals of the population less able to survive III Promotes population subdivision and can result in speciation A When demes become different enough from each other they become no longer able to interbreed so they become different species B There are many different species of fruit flies on the islands of Hawaii they drifted further and further apart genetically due to genetic drift so they eventually became different species Interaction of selection and drift A Drift is another force like migration that can act against a favored allele and prevent it from going to fixation like we would expect Selection can fail and can lead to effective neutrality A We can mathematically predict when selection will win and when drift will win B Small population size leads alleles to become effectively neutral C Interaction between population size and strength of selection is such that all phenotypes have equal fitness because the population is so small that drift will win out i e natural selection will not be able to increase the frequency of a favorable phenotype D E When the population is really small you see wild fluctuations and the favored allele does not necessarily go to fixation F Selection fails when the population size is sufficiently small so drift takes precedence over selection G Selection can fail if selection is very weak in a small population and thus cannot overcome the force of drift H When drift is very strong it does not matter what the fitness of an allele is it is just random or by chance thus fitness will not be able to increase the frequency of the favored allele I Directional selection the most favorable phenotype will go to fixation if selection rules J Population size is very important when determining if selection or drift will rule VI Interaction of drift with gene flow 1 A Gene flow is countering the effect of drift in this situation B When the 2 lines cross equilibrium is reached so no change occurs C Migration can be really important in small populations because it can counter the force of genetic drift and help small populations maintain genetic variability Forest corridors maintain historical gene flow in a tiger metapopulation in the highlands of central India A These are little islands of habitat now that do not have connections to VII each other B 2 of these large reserves were connected by a forest corridor and 2 others were connected by another forest corridor the gene flow in the tigers that lived in these 4 reserves was approximately the same as the tigers from historical times C The reserves that did not have access to forest corridors had significantly less gene flow and genetic variability VIII Nonrandom mating inbreeding A Inbreeding mating with a close relative more common in a small population because there are less mate options B Some lizard species are able to selfing and produce clones on C themselves Inbreeding increases homozygosity 1 In the case of HWE half of the population is homozygous and half is heterozygous Inbreeding greatly decreases the expected amount of heterozygotes and increases the amount of homozygotes 2 D Example Boxers 1 The white markings on boxers are homozygous recessive so if 2 parents that have white markings mate they will likely produce some completely white completely homozygous recessive puppies 2 Completely or almost completely white boxers have much greater chances of being blind and are much more prone to cancers Genotype frequencies with inbreeding A B If there are no heterozygotes at all P F Inbreeding doesn t change the allele frequencies from one generation to the next but it does change the genotype frequencies Phenotypic consequences of inbreeding A As the inbreeding coefficient increases the proportion of eggs that do B C not hatch increases as well Inbreeding can hasten extinction of the population In this case inbreeding has caused fertility in this population to drastically reduce Bounding Rebounding Panthers Make A Comeback A One species of cougar has become separated from the rest and has consequentially become a different species entirely this different species is located in Florida IX X XI 2 XII XIII A small endangered population is much less likely to be able to get rid of deleterious alleles Limited dispersal capabilities A Some organisms that have limited dispersal capabilities are able to sustain really high levels of inbreeding without many deleterious effects B Naked mole rates have limited dispersal capabilities C Typically one female breeds in the group an she is typically bigger than the other females D She mates with her sons and grandsons E Inbreeding coefficient of 0 8 highest known of undisturbed populations 3