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Lecture 16

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Biology 1001A
Tom Haffie

Lecture 16:  Whether or not mutations are directed toward the needs of the organism: Mutation creates new alleles (it is the only source of new alleles and genetic variation in a population so if there is no mutation in a population then there will be no variation for selection to act and genetic drift couldn’t happen)  Mutation is not “random”, but not directed towards the needs of the organism either -how do mutations affect fitness? It have little effect on fitness! -Of those that affect fitness, most are harmful! -mutation often opposes selection, but also provides raw material for adaptive evolution  General fitness effects of mutations: -Other evolutionary mechanisms that cause HWE to change: -One of non-adaptive evolutionary forces is mutation! Because it is the only source of new alleles. -mutation often opposes selection, but also provides raw material for adaptive evolution. However it is important for selection because selection require genetic variation and mutation is the only source of that raw material!  Why most mutations that affect fitness are harmful:  The general effects of mutation on fitness: recall that most mutations have actually no effect on fitness or very little effect on fitness, but the ones have effect on fitness, they tend to be harmful, ex. When there is a certain kind of protein to be copied and there is mutation, it wont be copied as good as if it was no mutation because the organisms genes are the product of millions and million of years of natural selection that are now well adapted to the environment (ex. Take the laptop and put it apart then try to put it together again, the odds are that it wont be as good as it was again!) thus, there are more odds on having something worse than the previous one with mutations.  Effect of gene flow on genetic variation within a population:  The effect of gene flow (migration) on genetic variation: it has a similar effect to mutation (increase genetic variation)  Effect of gene flow on variation (differences) between populations: Gene flow (migration) moves alleles between populations • Like mutation, can introduce new alleles • Like mutation, often opposes selection (selection-migration balance) • The effect of gene flow (migration) on genetic variation: it has a similar effect to mutation EX. a mouse living in the desert has a pale color that differ from other darker mice. • Even though mutation is the only way crating new alleles, migration can introduce those new alleles to a population because individuals that are migrating bring with them new alleles to that population. gene flow tends to enhance (increase) the amount of genetic variation when it is present in a populations (ex. Isolated populations like a one far in an island where there is no or little gene flow, they tend to be very poor in genetic variation) • -Gene flow also tends to operate in opposition to natural selection because individuals may be coming in from different environment (like mice from sand are pale in color migrate to another environment of mice that are much darker color) this results in some of the individuals in the population being not as well adaptive to the prevalent conditions ((so this is selection migration balance))  How various evolutionary forces interact with (reinforce or oppose) one another: Selection (several kinds) Genetic drift Mutation Gene flow Which have similar effects on genetic variation within a population? Mutation and gene flow On differences between populations?  Reasons why not all living things are perfectly adapted to their environment: • Many factors constrain (limit) selection (see kinds of selection) – Examples? 1-Dominance status of alleles plays a role here in that we cant necessarily weed out every copy of a harmful recessive allele. 2- frequency dependent selection can maintain phenotypes in a population that have low fitness simply by being the most common phenotype in a population. Selection also requires time so in situations whereas an environment is changing very rapidly then the population may not be able to evolve quickly enough to keep up with pace of environmental change. Also selection requires lots of genetic variation in a population so if the mutation rate is so low or genetic drift have outweigh the effects of selection such that there are not a lot genetic variation, then that constrains the ability of selection to favour beneficial alleles. Also trade-offs (compromises) between competing demands (ex. A male fish try to attract females by having colorful shape and doing smooth dances, but this increases the chance by which it could be eaten by predators) – • Non-adaptive evolutionary forces often oppose selection(ex. Harmful mutation, genetic drift migration from other population…), but without these other evolutionary forces, especially mutations, adaptive evolution would stop. Non-random mating and evolution? • If a population begins mating assortatively for a certain trait, but all genotypes have the same fitness, what will happen? – -getotypes AA, Aa, aa – -each genotype mates only with itself SOLUTION: We have 250 AA 500 Aa 250 aa The homozygotes will breed with each other in the same genotype and will get the same homozygote AA and aa BUT the heterozygotes will be some heterozygotes and homozygotes for both AA and aa The offspring from heterozygotes is not goimg to be in HWE because there will no enough heterozygotes when we sum the total number of heterozygotes together in the offspring generation population.! The inbreeding and assortative mating effect is that it increase the homozygotes individuals Disassortative have the exact effect, it increase frequencies of heterozygotes. But have the population evolve? No because the allele frequencies have not changed N
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