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Lecture 18

Lecture 18 Notes

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University of Toronto St. George
Spencer C H Barrett

Lecture 18: Population structure, gene flow and genetic drift  Multiple populations across the landscape; the genetic variation that gives rise to speciation- population differentiation. o ie: patterns of variability in the Rat Snake. Hypothesis: soil color o Geographic variability  Fisher and Wright had very different viewpoints: how important genetic drift is as an evolutionary process o Wright saw an important role for population structure and genetic drift in evolution.  If you looked at real populations (not hypothetical), would see a lot of population structure across the landscape- some big, some small- small populations could play an important role in evolutionary change. STRUCTURE across the LANDSCAPE in causing differentiation- suggested that genetic drift was a player, along with natural selection. o Fisher disagreed and argued that most evolution occurred in large populations by natural selection.  Most important evolutionary change that gives rise to adaptation occurred in large populations through selection  Structure and genetic differentiation: o o Two habitats are close together; trees occur on both sides. South side warms up much earlier in the spring; has a head start. Genetic differentiation between insects, plants that occur between the two. Scale of genetic differentiation can be quite small.  Gene Flow is important because acts as cohesive force that stops differentiation from occurring.  Stochastic means random forces.  Some definitions: o Population: A group of individuals of a single species occupying a given area at the same time  Mark Recapture Studies dedicated to estimating populations o Migration: The movement of individuals from one population to another  Important for animals, prerequisite for gene flow.  Ecological phenomenon o Gene flow: The movement of genes from one population to another  Genetic phenomenon  Recap: Key questions when considering genetic variability with populations o How much of the observed variation among individuals is genetic in origin?  Interested in heritable genetic variation o Does the variation contribute to fitness differences among individuals?  Key questions when looking at populations across the landscape o Looking at whole collection of populations; what proportion of the total amount of variability in the sample is distributed within versus between populations?  Statistical analysis. Analysis of Variants  ie) were individuals who believed that human species should be broken up into series of species because of recognizable differences between different ethnic groups. Have come to finding (through electrophoresis) that the vast proportion of the variability in the human species is within populations. o Are some traits more differentiated than others? (Are some loci or traits more differentiated than the genome-wide average?)  Is this between population differentiations due to local adaptation?  Balancing act between natural selection and gene flow: o o Two primary process that cause populations A and B to diverge; arrows indicate divergence o ie) ancestral population that get into two different geographical areas; snow on mountains o Over time, natural selection will drive them apart as they become adapted. (This is the process of genetic differentiation) o Another process is genetic drift- random process by which genes become different in populations- driven by small population size. Loss of rare alleles. o Drift and selection drive these populations apart; gene flow act as cohesive force that will stop populations from pulling apart.  How do we measure gene flow? (movement of genes from one population to another) o Difficult to observe and measure (Can’t actually see genes move) o Distinguish between potential (migration) and actual gene flow o Distinguish between gamete and individual: what is actually moving? Animals walking from one population to the next; organisms that dump gametes out (marine organisms, wind pollinating plants). Genes move as gametes or as individuals- different genetic impacts o Use experimental approaches: Use neutral genetic markers (Polymorphic neutral genetic variation used to study population processes affecting genetic diversity). ie) look at pattern of bands on a gel to infer what kind of reproductive system- clonal? Selfing? Out crossing? Using genes as markers.  2 experimental populations – screen populations for allele that is fixed in one population but not in the other; other population is fixed for alternate allele. o Both populations are homozygous, but are homozygous for alternate alleles o Diagnostic markers: markers that are only in that populati
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