Transcript Evolutionary forces: in small populations

Evolutionary mechanisms
Evolutionary mechanisms
1. 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 of
genetic drift
Start with 4 heterozygous
individuals (A1/A2)
Example 1 of genetic drift
Desert: 8,000 to 4,000 ybp
Contiguous with SW desert
Then retreat of deserts to SW
Present: oak-hickory
forest
Relictual populations
each 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 migration
Influenced 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