Ch 16 Evolution of Populations

16-2 Evolution as Genetic Change Population Genetics Hardy-Weinberg Principle: When allele frequencies remain constant, a population is in genetic equilibrium. p² + 2pq + q² = 1

16-2 Evolution as Genetic Change Hardy-Weinberg allows us to determine equilibrium frequencies of each genotype in a population. p² + 2pq + q² = 1 or AA + 2Aa + aa = 1 q² = 0.33 q = 0.57 p = 1 – q = 0.43 What is the frequency of the heterozygote (Aa)?

16-2 Evolution as Genetic Change

Genetic Drift Change in allele frequencies due to chance. More pronounced in smaller populations where the chance of losing an allele is greater.

Founder Effect Small sample of a population settles in a location separated from the rest of the population. Alleles that were uncommon in the original population may be common in the new population.

Bottleneck When a population declines to very low number and then rebounds.

Gene Flow Increases genetic variation within a population. Reduces differences between populations.

Nonrandom Mating Promotes inbreeding and could lead to a favoring of homozygous individuals.

Natural Selection Selects individuals best adapted for survival and reproduction.

Stabilizing Selection Eliminates extreme expression of a trait when average expression leads to higher fitness.

Directional Selection Makes an organism more fit.

Disruptive Selection Splits a population into two groups.

16-3 The Process of Speciation Sexual selection- Operates in populations where males and females differ significantly in appearance. Qualities of sexual attractiveness appear to be opposite of qualities that might enhance survival. What do YOU look for in a mate?

Prezygotic Isolation Prevents reproduction by making fertilization unlikely. Prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences. Western Meadowlark

Postzygotic Isolation Fertilization occurs but hybrid offspring can’t develop or reproduce.

Allopatric Speciation Physical barrier separates a population into two or more populations.

Sympatric Speciation Species evolve into new species without physical barrier. Both species live side by side during speciation.

Adaptive Radiation Can occur quickly when a species gives rise to many different species in response to a new habitat or ecological opportunity. Follows large-scale extinction events.

Coevolution Evolution of one species affects evolution of a closely related species. Mutualism Coevolutionary arms race.

Convergent Evolution Unrelated species in different parts of the world evolve similar traits.