Evolution of Populations Chapter 16

I. Populations & gene pools Concepts ◦ a population is a localized group of interbreeding individuals ◦ gene pool is collection of alleles in the population  remember difference between alleles & genes! ◦ allele frequency is how common is that allele in the population  how many A vs. a in whole population

II. Genetic Variation Variation is the raw material for natural selection ◦ there have to be differences within population ◦ some individuals must be more fit than others

III. Sources of Genetic Variation A. Mutation ◦ random changes to DNA  errors in mitosis & meiosis  environmental damage from mutagens B. Gene Shuffling (sexual reproduction) ◦ mixing of alleles  recombination of alleles during meiosis  new arrangements in every offspring as a result of crossing over and independent assortment  new combinations = new phenotypes ◦ spreads variation  offspring inherit traits from parent

IV. Populations Evolve Natural selection acts on phenotypes of individuals ◦ Individuals dies its genes are removed ◦ Individuals reproduce its genes are added ◦ Evolution is the change of frequency of genes in a population’s gene pool. Populations evolve ◦ genetic makeup of population changes over time ◦ favorable traits (greater fitness) become more common

V. Effects of Natural Selection Natural selection can affect the distributions of phenotypes by: ◦ Directional selection ◦ Stabalizing selection ◦ Disruptive selection

A. Directional Selection Individuals at one end of the curve have a higher fitness than the middle or the other end of the curve ◦ Moves toward a more fit trait

B. Disruptive Selection When inidivduals at both ends of the curve have a higher fitness than those in the middle

C. Stabilizing Selection When individuals in the center of the curve have higher fitness

VI. Genetic Drift Effect of chance events ◦ A. founder effect  small group splinters off & starts a new colony ◦ B. Bottleneck effect  some factor (disaster) reduces population to small number & then population recovers & expands again Warbler finch Tree finches Ground finches

Founder Effect: ◦ When a new population is started by only a few individuals  some rare alleles may be at high frequency; others may be missing  skew the gene pool of new population  human populations that started from small group of colonists

VII. Genetic Equilibrium Evolution = change in allele frequencies in a population ◦ hypothetical: what conditions would cause allele frequencies to not change? Hardy-Weinberg Princple ◦ Hypothetical, non-evolving population  preserves allele frequencies ◦ Serves as a model  natural populations rarely in H-W equilibrium  useful model to measure if forces are acting on a population  measuring evolutionary change G.H. Hardy mathematician W. Weinberg physician

Non-evolving Population Population must meet the following conditions 1.very large population size (no genetic drift) 2.no migration (no gene flow in or out) 3.no mutation (no genetic change) 4.random mating (no sexual selection) 5.no natural selection (everyone is equally fit)

VIII. So…what is a species? Biological species concept ◦ defined by Ernst Mayr ◦ population whose members can interbreed & produce viable, fertile offspring ◦ reproductively compatible Western MeadowlarkEastern Meadowlark Distinct species: songs & behaviors are different enough to prevent interbreeding

How and why do new species originate? Species are created by a series of evolutionary processes ◦ populations become isolated  geographically isolated  reproductively isolated ◦ isolated populations evolve independently ◦ Changes in the new populations gene pool

A. Geographic isolation Species occur in different areas ◦ physical barrier ◦ River or mountain range separates the two populations Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel Ammospermophilus spp

B. Behavioral isolation Unique behavioral patterns & rituals isolate species ◦ identifies members of species ◦ attract mates of same species  courtship rituals, mating calls  reproductive isolation Blue footed boobies mate only after a courtship display unique to their species

C. Temporal isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes ◦ reproductive isolation Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

IX. Speciation in Galapagos Finches Founders Arrive Separation of populations Changes in gene pool Reproductive isolation Ecological Competition Continued Evolution