Mechanisms for Evolution Natural selection traits that improve survival or reproduction accumulate in the population ADAPTIVE change Genetic drift frequency of traits changes in a population due to chance events RANDOM change Mutation, Gene Flow, Nonrandom Mating

Natural Selection Selection acts on any trait that affects survival or reproduction predation selection (speed, camouflage, defenses) physiological selection (disease resistance, protection from injury) sexual selection (attractiveness, fertility)

Modes of Selection Directional: Darker mice are favored because they live in dark rocks. Disruptive: Intermediate color at a disadvantage. Mice live in both light and dark rocks. Stabilizing: Intermediate color is advantaged, extremes are not favored. Examples: Directional – beaks of finches selected to accommodate food source; Stabilizing – birth weight -> either extreme can be harmful to baby or mother; Disruptive – black bellied seed crackers of South Africa -> seeds come in large and small variety so intermediate beak size is selected against.

Genetic Drift Chance events change frequency of traits in a population not adaptation to environmental conditions Founder effect Bottleneck

Genetic drift (can greatly affect small populations) CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

Founder effect (Genetic drift) A new population is started by a small group of individuals or are isolated from other population just by chance some rare traits may be at high frequency; others may be missing skews the gene pool of new population less genetic diversity Small founder group, less genetic diversity than Africans All white people around the world are descended from a small group of ancestors 100,000 years ago (Chinese are white people!) albino deer Seneca Army Depot

Example: Distribution of blood types Distribution of the O type blood allele in native populations of the world reflects original settlement South & Central American Indians were nearly 100% type O for the ABO blood system. Since nothing in nature seems to strongly select for or against this trait, it is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type

Bottleneck effect (genetic drift) A large population drastically reduced by a disaster famine, natural disaster, loss of habitat… loss of variation by chance event narrows the gene pool

Example: Cheetahs All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat

Gene Flow Migration changes allelic frequencies in the gene pool

Nonrandom Mating Similar genotypes mate with one another Inbreeding Assortive mating – increasing frequency of homozygous individuals (Ex: plant selfing)

Mutation Ultimate source of variaton Random – only passed if in gametes Mutagens

Question??? What do you think is the definition of a species? Pair up and write down your definition of a species and how do we get new species?