mechanisms of microevolution

natural selection

Acts non-randomly on phenotypes of individuals

Changes allelic & genotypic frequencies of populations non-randomly

Always leads to adaptation of population to current environment

genetic drift

genetic frequency changes due to random events

Often occurs in small populations

(like “sampling errors” in statistics)

founder effect

a few founders start new isolated population

founder gene pool differs from original source

small population size leads to more drift

better alleles may be lost!

bottleneck effect

an event drastically cuts population size

gene flow

alleles move in/out of population

tends to add genetic diversity to population

tends to reduce genetic differences between populations

results of gene flow

relative fitness

Fitness is relative to other individuals in the population

“fittest” = best reproductive success

Stabilizing selection

Directional selection

Diversifying selection (Disruptive selection)

Frequency dependent selection

Sexual Selection

different forms of selection

stabilizing selection

an average phenotype is favored.

directional selection

a change in the environment shifts the spectrum of phenotypes observed.

Diversifying (Disruptive) selection

two or more extreme phenotypes are selected for, while the average phenotype is selected against.

Negative frequency-dependent selection

fitness of a phenotype decreases as its frequency increases in population

frequency dependent selection

the fitness of a phenotype depends on how common it is in the population

sexual selection

success based on traits related to obtaining mates (not directly related to environment)

intrasexual selection

individuals of one sex compete directly for mates of the opposite sex.

intersexual selection

also called mate choice, individuals of one sex (usually the females) are choosy in selecting

their mates from the other sex.

sexual dimorphism

observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and in (c) wood ducks. (credit “spiders”: modification of work by “Sanba38”/Wikimedia Commons; credit “duck”: modification of work by Kevin Cole)

heterozygote advantage

selection favors heterozygote over either homozygote, maintaining both alleles