biologists use the term population to refer to individuals of a particular species that might encounter each other in a lifetime (thus are potential mates or rivals)

we have seen that populations of the same species in different places often differ in size, coloration, or features of morphology (averages and ranges of measurements differ among populations) -- analysis of amino acid sequences in proteins or of base-pair sequences in DNA usually reveals even more differences among populations of the same species

what produces these genetic differences among populations?

four processes change frequencies of alleles in populations . . .

gene flow, mutation, genetic drift, natural selection

natural selection is the topic for a later section of this course -- for the moment, notice that selection (either geographically uniform or local) can influence the frequencies of alleles at particular loci -- it seldom if ever affects all loci equally -- in contrast, the other three processes affect all genetic loci about equally

mutation is discussed in many other courses in this department -- for our purpooses, notice that mutations occur randomly at all genetic loci (even if not exactly equally at every locus)

genetic drift also affects alleles at all genetic loci randomly -- drift becomes important when populations are very small -- any few individuals are unlikely to contain a representative set of the alleles from a large population -- a small population thus usually has allele frequencies that . . .

• differ from the large population from which the small one comes
• differ from any other small population even from the same source

notice that genetic drift tends to decrease genetic variation within any one small population -- but to increase genetic variation between different small populations

small populations can occur for several reasons -- a few individuals might get lost and colonize a new area (founder effect) -- some catastrophe might reduce a population to only a few survivors (bottleneck effect) -- only a few individuals in a population might reproduce successfully (reproductive skew)

these are all special cases of a general rule -- whenever mortality or reproduction has a random (accidental) component, genetic drift occurs (allele frequencies change randomly) -- especially in small populations

gene flow is crucial in keeping populations genetically similar -- it is the only homogenizing process that affects all loci equally -- gene flow results from movement of genes (sometimes called gene migration) -- which in turn results from movements of individuals between birth and reproduction (called natal dispersal)

when geographical barriers reduce natal dispersal, local populations exchange genes less frequently -- we might expect that such populations would be especially likely to evolve genetic and phenotypic differences

geographic variation often reveals the importance of geographical barriers for the genetic differentiation of populations

for example, consider the subspecies of birds in North America (subspecies are differentiated populations within a widespread species) . . .

• birds isolated on islands -- Newfoundland has 8 endemic subspecies among 82 species of breeding land birds (in biology endemic means occuring nowhere else) -- remember the Hairy Woodpeckers on Newfoundland
• birds isolated on mountains -- Song Sparrow has 3 subspecies in eastern North America, 28 out west -- where mountain forests are islands in a sea of desert and semidesert

even in absence of geographical barriers, natal dispersal sometimes occurs only over short distances -- so populations much farther apart than the average distance of natal dispersal exchange few genes -- biologists often call this situation isolation by distance -- populations isolated by distance can differ progressively in a cline (gradual change) -- for example the cline in sizes of Hairy Woodpeckers from Gulf coast to northern Canada

of the four ways that gene frequencies change, two tend to make populations genetically similar --

• gene flow (gene migration resulting from natal dispersal)
• geographically uniform natural selection

three tend to make populations genetically different --

• random mutations
• genetic drift (variation among small populations)
• local differences in natural selection

genetic difference between two populations thus depends on a balance between the processes that tend to make local populations similar and those that tend to make them different

as a result we cannot explain the evolution of geographic variation unless we can measure these four processes -- it comes down to the numbers