Chapter 19

microevolution

the change in allele frequencies in a population over generations

Mechanisms of evolution:
Natural selection
Genetic drift
Gene flow
Nonrandom mating

genetic variation

Differences among individuals in the composition of their genes

Results in differing phenotypes

Only the genetic component of a phenotype is influence by evolution

mutations

result in the formation of new alleles

Many are harmful
Harmful alleles can persist through “heterozygote protection”
Many are neutral
Some are beneficial and are selected

gene pool

All of the alleles for every gene in a given population

Each member receives its genes from its parents

When an individual reproduces, they contribute to the gene pool

fixed

If only one allele exists it is fixed in the population

All individuals are homozygous

polymorphic

If there are two or more alleles: polymorphic

Individuals are homozygous or heterozygous

allele frequency

Number of copies of a specific allele in a population / Total number of all alleles for that gene in a population

genotype frequency

Number of individuals with genotype in a population / Total number of individuals in a population

Hardy-Weinberg Equation

if population is not evolving...
p2 + 2pq +q2 = 1

p2 = genotypic frequency of homozygous dominants

pq = genotypic frequency of heterozygotes

q2 = genotypic frequency of homozygous recessive

directional selection

Individuals at one extreme of phenotypic range have greater reproductive success

stabilizing selection

Favors individuals with intermediate phenotypes and selects against the extremes

disruptive selection

Favors the survival of 2 or more phenotypes

balancing selection

favors the heterozygote over the homozygotes

sexual selection

refers to the evolution of traits that increase an individual’s reproductive success

Often affects males more than females

intrasexual selection

Development of secondary sexual characteristics that aid in mate competition

sexual dimorphism

Intrasexual selection can lead to a profound size difference between the sexes

intersexual selection

Development of secondary sexual characteristics that aid in mate choice

runaway selection

Female choice leads to the development of exaggerated male characteristics that have a survival disadvantage

genetic drift

A change in allele frequencies in a population due to random chance

founder effect

Subset of original population separates and creates a new population…with fewer alleles

bottleneck effect

The rapid reduction in population size due to famine, disease or environmental change  causes change in allele frequency

gene flow

The movement of some individuals from one population to another

This movement of alleles between populations, called gene flow, changes the allele frequencies of the populations

bidirectional migration

Reduces differences in allele frequencies between populations

Increases genetic variation within a population

assortative mating

occurs when individuals of similar phenotypes are more likely to mate

Increases homozygotes and decreases heterozygotes

inbreeding depression

occurs when population size decreases due to habitat destruction

Lowers the mean fitness of the population if homozygous individuals have lower fitness values