MAJOR ISSUES AND CONCEPTS
Population Genetics and Evolution
The crucial insight of population genetics is that changes in the frequency of traits in a population can be
related to changes in the frequency of genes for those traits. From a population genetic perspective,
evolution is a change across generations in the frequencies of genes and combinations of genes.
Five primary forces affect the genetic composition of populations and cause evolution.
1. Natural Selection—Natural selection occurs when individuals in a population either do not
survive equally well, do not breed equally well, or do not survive and breed equally well due to
inherited differences.
2. Mutation—Mutation can produce new variations of genes (alleles), new genes, new
combinations of genes, new chromosomes, and new physical types in a population.
3. Gene Flow—New genes flow into a population through gametes and through “genetically
different” individuals migrating into a population and breeding.
4. Genetic Drift—Small populations are more likely to vary in distinct ways due to chance effects
changing gene and genotype frequencies. Because small populations are more likely to lose
genetic and phenotypic variation by chance alone, population specific differences in gene and
genotype frequencies increase. These changes in gene and genotype frequencies may be due to
" random fluctuation in gene and genotype frequency, " the genetic difference produced in a
“newly established” population due to the atypical genetic make-up of the “founders,” or " the
genetic difference produced in an existing population due to few individuals surviving an
extreme level of mortality and reproducing.
5. Mate Choice—Non-random mate choice (mate choice is affected by phenotype/genotype)
only changes genotype-frequency across generations; the allele frequencies in the population
stay the same. These five evolutionary forces can occur simultaneously and interact. The overall
outcome of their interaction determines the specific ways allele and genotype frequencies
change across generations in a population. The overall outcome of their interaction determines
evolution.
Consider three of the many different ways to characterize species: (1) Biological Species Concept, (2)
Phylogenetic Species Concept, and (3) Morphological Species Concept.
1. The biological species concept is used by many zoologists and is the legal definition of a
species found in the Endangered Species Act. The primary requirement of the biological species
is reproductive isolation. If populations do not naturally interbreed, or if they fail to produce
fertile offspring, then they are reproductively isolated and represent “good species.” The effect
of reproductive isolation is “no gene flow.”
2. According to the phylogenetic species concept, the evolutionary history of a group is its
phylogeny. A phylogenetic tree is a branching figure summarizing this evolutionary history. The
“tree” represents the pattern of evolution. If the biological units being compared are species,
phylogeny identifies the “phylogenetic species.” Each “species” uniquely occupies the tip of a
“tree branch.” The assumption is that the species that are most closely related are most
genetically or physically similar.
3. Under the morphological species concept, unique species are identified based on consistent,
distinguishing physical differences. This species concept is widely applied to dead, extinct, or
fossilized specimens, for which the evolutionary history might not be known, or organisms with
unknown or untested reproductive compatibility.