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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