Evolution in Populations Single Gene Traits Can lead to a shift in allele frequencies. This genetic selection can then lead to the evolution or a change in the species. Polygenic Traits Can lead to a shift in allele frequencies. Changes that can alter the phenotypic make-up of a species. Directional Selection Stabilizing Selection Disruptive Selection

Evolution in Populations Single Traits

Evolution in Populations Polygenic Traits

Evolution in Populations Polygenic Traits

Evolution Caused by a Genetic Change Genetic Drift A shift in allele frequencies in a population due to chance. Bottlenecks An abrupt and severe reduction in the number of individuals during the history of a species, resulting in the loss of diversity from the gene pool. The generations following the bottleneck are more genetically homogenous than would otherwise be expected. Bottlenecks often occur in consequence of a catastrophic event. The American Heritage® Science Dictionary

Genetic Drift

Founders Effect The frequency of the “a” allele is low in the initial population, but a small subset, in which one individual is Aa, is removed from the large population and founds a new population. The frequency of “a” is markedly higher in this new population, due to its relatively high frequency in the founders. www.blackwellpublishing.com/korfgenetics/figu...

Temporal Isolation Breed at differing times of the year. Western Spotted Skunk (Spilogale gracilis) Breeds in late summer Eastern Spotted Skunk (Spilogale putorius) Breeds in late winter

Behavioral Isolation Two species that are capable of interbreeding, but don’t because of courtship rituals or other similar behaviors.

Mechanical Isolation When structural differences prevent successful mating. For example, in many closely related species of plants, there are certain differences, such as differences in color, that help attract different kinds of pollinators.

Gamete Isolation (Lock and Key Isolation) Fertilization cannot take place if the gametes are not compatible.

Geographical Isolation Individuals of a species that are physically separated by a barrier may form subspecies such as these spotted owls.