Evolutionary mechanisms

Evolutionary mechanisms1. Mutation: the only source of new genetic information.

Mutation: any heritable change in the structure or amount of genetic material.

Different levels of mutation DNA: point and frame shift mutations (mistakes made

during DNA replication)Arrangements of DNA+/- of single chromosomes+ complete sets of chromosomes.

Spontaneous Only mutations occurring during meiosis can affect the next

generation.

2. Evolutionary forces: in small populations

• Natural populations are finite in size.• Geographically structured so that mating, within a species,

is not random.• A. Genetic Drift. Process occurring in small, isolated

populations (demes).• The frequency of different alleles between generations can

fluctuate BY CHANCE.• Therefore, by definition, genetic drift is an evolutionary

force.• But, mutations cannot be anticipated; therefore, cannot

reliably produce adaptations.

Dots = demes

Geographic distribution of a species

Computer modeling ofgenetic drift

Start with 4 heterozygousindividuals (A1/A2)

Example 1 of genetic drift

Desert: 8,000 to 4,000 ybpContiguous with SW desertThen retreat of deserts to SW

Present: oak-hickoryforestRelictual populationseach with. c.12 lizards.

• Example 2 of genetic drift• Tristan da Cunha; island; in southern Atlantic; one of most

isolated places on earth; administered by Great Britain. • 1816: William Glass, wife, two daughters• Joined later by a few additional settlers from England• 1961: volcano eruption forced removal of all residents (294)• In England, tested for various genetic traits.• All residents homozygous (alleles fixed) for nine genetic

markers.• Clinodactyly (dominant) present in the Glass family.• Alternative glucose-6-phosphate dehydrogenase allele arrived

in 1827…..gene flow.

B. Founder Effect Occurs when a small segment of a population becomes

geographically isolated from the larger population The smaller population has only a fraction of the genetic

variation of the larger population.

C. Population bottleneck: What does it do?

3. Gene Flow: Individuals carry alleles from one population to another population.

Gene flow refers to migrationInfluenced by culture and social structure.

Gene flow (exchange of genes between populations) has increased over time.

4. Natural selection: directional selection

Natural selection: e.g.: Stabilizing selection

19.2 lbs. = c. 8.5 kg