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Chapter 7

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BIOL 359
Jonathan Witt

BIOL359 - Evolution Winter 2013 Chapter 7: Mendelian Genetics in Populations II: Migration, Genetic Drift & Nonrandom Mating 7.1. Migration - Migration: The movement of alleles between populations “gene flow – transfer of allele from gene pools” - Migration as a mechanism of evolution o Migration changes the frequency of alleles, most important in preventing populations from diverging o E.g. Water snake of Lake Erie lives on mainland & islands. The color pattern is determined by a single locus with 2 alleles, banded allele is dominant over the unbanded allele  Mainland: Most snakes are banded & Island: Most snakes are unbanded  Natural selection by predators  Island: Unbanded individuals are more cryptic on island rocks  higher survival rate  Migration from mainland to island brings the allele for banded coloration, preventing the unbanded allele from fixing on the islands (Homogenization opposes NS) o Migration tends to homogenize allele frequencies across populations if allowed to proceed unopposed by any other mechanisms of evolution o E.g. The homogenization of the red bladder campion (an insect-pollinated perennial wildflower)  Geological uplift, new islands are constantly rising out of the water  Red bladder campion seeds transports through wind & water, the plant is among the first to colonize new islands  Young populations: Vary in allele frequencies “genetic drift”  Intermediate age populations: Homogeneous in allele frequencies “migration”  Old populations: Vary in allele frequencies “selection”  Calculate FSTalues (0 to 1), higher = more variable 7.2. Genetic Drift - Genetic Drift: The purely luck mechanism of evolution other than natural selection, results from violation of the assumption of infinite population size o In populations of finite size, chance events in the form of sampling error in drawing gametes from the gene pool can cause evolution. Genetic drift is most powerful in small populations, it’s power diminishes as population size increases o Sampling error: The random discrepancy between the theoretical expectations & actual results (theoretical outcome of each random selection follows bell curve), If more samples are chosen, population gradually settles down towards the theoretical value - Founder effect: The allele frequencies in the new population is likely to be different by chance from the allele frequencies of the source population o E.g. Zosterops lateralis “silvereyes”, documented migration 5 times. New population contains a randomly chosen subset of allele that were present in the source population that the founders came from, and the allele diversity in their population decreased along their route of travel o Given sufficient time, genetic drift can produce substantial changes in allele frequencies even in populations that are fairly large, if no other evolutionary mechanisms are at work  Allele frequencies wonder between 0 and 1  Eventually driven to fixation or loss, frequency of heterozygotes declines in finite population
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