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BIO120 Lecture 7: Geographic Variation (Genetic drift increase…
BIO120 Lecture 7: Geographic Variation
Population structure
Phenotypic differentiation
Plasticity - ability of genotype to modify phenotype in response to (unpredictable) environmental change
Ex. evolution of human skin pigmentation reflects trade-off (high UV is harmful b/c it interferes with folate but selects for increased pigmentation; low UV reduces vitamin D synthesis so selects for decreased pigmentation
Genetic differentiation among populations across geographic range
Reciprocal transplant studies (i.e. cloned genotypes in different environments compared) and genomics provide evidence for local adaptation -- enables measurement of selection against alien genotypes
Genetic drift increase differentiation between populations
Stochastic (unpredictable, ex. mutation, recombination)
Deterministic (predictable, ex. natural selection)
Loss of alleles
Population bottleneck - single sharp reduction (especially in small populations) causes loss in diversity
Founder events - colonization by few individuals that start new population w/ limited diversity when compared to source population (explains loss of genetic diversity when humans migrated away from Africa)
Populations isolated from each other for longer have had more opportunity for genetic drift
Evidence for interbreeding between humans and Neanderthals
Migration of gene flow homogenizes; causes populations to stay similar
Measuring
Problems
Difficult to measure
Difference between potential and actual
Distinguish between gamete and individual
Techniques
Neutral genetic markers used
Polymorphic neutral genetic variation used to study population processes affecting genetic diversity
Ex. fast vs. slow allele in two populations --> frequency of heterozygotes gives estimate of gene flow