Evolution of Populations Marie Černá

Microevolution Genetic drift in small population Gene flow by migration Mutation Nonrandom mating Natural selection

Genetic Drift

Genetic Drift - small populations

Genetic Drift - large populations

Genetic Drift - Bottle neck effect

northern elephant seals of the coast of Mexico During the 1890’s, their population was reduced due to hunting to about 20 individuals. (It is also likely that one male would have fathered the offspring of the entire group.)

Genetic Drift - Founder principle Drosophila subobscura (fruit fly) European → American

Genetic Drift - Founder principle

Gene flow - migration

Mutation

Nonrandom mating inbreeding assortative mating

Natural selection the differential ability to survive and produce viable, fertile offspring is likely to adapt a population to its environment (accumulates and maintains favorable genotypes)

Genetic variation: the substrate for natural selection within populations (polymorphism) between populations (geographical variation)

Polymorphism

Geographic variation

Geographic variation - Cline

Sources of Genetic variation Mutation (affection of function) Sexual recombination

Crossing over in meiosis

Independent assortment

Preservation of Genetic variation Diploidy (heterozygote protection) Balanced polymorphism

Heterozygote advantage

Frequency-dependent selection

Modes of natural selection

Directional selection is most common - during periods of environmental change - when members of a population migrate to some new habitat with different environmental conditions

Directional selection peccaries Desert cactus population

Diversifying selection

a species of finch lives in Cameroon, West Africa small-billed birds (soft seeds) large-billed birds (hard seeds)

Stabilizing selection This mode of selection reduces variation and maintains the status quo for a particular phenotypic character. It keeps the majority of human birth weights In the 3 – 4 kg range. For babies much smaller or larger than this, infant mortality is greater.

Stabilizing selection Desert cactus population

Literature Biology, eighth edition, Campbell, Reece Unit four: Mechanisms of Evolution Chapter 23: The Evolution of Populations Pages 468 – 486