Gene flow is the movement of traits between populations. Mechanism – Migration Keeps neighboring populations similar. Low gene flow = increased chance populations evolve into different species.

Gene Pool All genes in a population Relative frequency = number of times an allele occurs in a gene pool

A normal distribution graphs as a bell-shaped curve. most fall in the middle decreases towards each end Traits not undergoing natural selection have a normal distribution.

Normal Distribution in Humans Most individuals lie in the middle and fewer individuals can be found at the ends.

Directional selection favors phenotypes (physical traits) at one extreme.

Directional Selection in Humans More darker skinned individuals closer to equator.

Stabilizing selection favors the intermediate phenotype. A birthweight of 7-8 lbs. is favored. Heavier and lighter birthweights are selected against.

Male – more masculine favored. Female – more feminine favored. Disruptive selection favors both extreme phenotypes. Male – more masculine favored. Female – more feminine favored.

Genetic drift Random change in allele frequencies that occur in small populations. Founder effect – migration of individuals from a population

Speciation Formation of new species Reproductive isolation – two populations cannot interbreed and produce fertile offspring

Mechanisms Leading to Reproductive Isolation Behavioral Isolation – differences in courtship rituals Geographic Isolation – natural barrier lake or mountain Temporal Isolation – time differences in breeding

Behavioral, geographic or temporal?

Behavioral, geographic or temporal?

Behavioral, geographic or temporal?