Lecture 18: Population structure, gene flow and genetic drift
Multiple populations across the landscape; the genetic variation that gives rise to speciation-
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
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
o Fisher disagreed and argued that most evolution occurred in large populations by natural
Most important evolutionary change that gives rise to adaptation occurred in large
populations through selection
Structure and genetic differentiation:
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.
o Population: A group of individuals of a single species occupying a given area at the same
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.
o Gene flow: The movement of genes from one population to another
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 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