The Evolution of Populations

 Emphasizes the extensive genetic variation within populations and recognizes the importance of quantitative characteristics.  Modern Synthesis – comprehensive theory of evolution that integrates discoveries and ideas from many different fields, including paleontology, taxonomy, biogeography, and, of course, population genetics.  Modern synthesis emphasizes the importance of populations as the units of evolution, the central role of natural selection as the most important mechanism of evolution, and the idea of gradualism to explain how large changes can evolve as an accumulation of small changes occurring over long periods of time.

 Population – localized group of individuals belonging to the same species  Species – a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature  Gene Pool – total aggregate of genes in a population at any one time consisting of all alleles at all gene loci in all individuals of the population

 Hardy-Weinberg theorem – the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than Mendelian segregation and recombination of alleles  Hardy-Weinberg equilibrium – Mendelian system has no tendency to alter allele frequencies between generations  Hardy-Weinberg equation: p2 + 2pq + q2

 1. Very large population size.  2. No migration.  3. No net mutations.  4. Random mating.  5. No natural selection.

 Microevolution – Change in the gene pool on the smallest scale  Genetic Drift – Change in the population’s allele frequencies due to chance.  Bottleneck Effect – Genetic drift due to a drastic reduction in population size.  Founder Effect – Genetic drift in a new colony.

 Natural Selection – Differential success in reproduction.  Gene Flow – When a population gains or loses alleles.  Mutation – Change in an organism’s DNA.  Polymorphic – If two or more distinct morphs are each represented in high enough frequencies to be readily noticeable.  Gene Diversity – Average percent of genetic loci that are heterozygous.  Nucleotide Diversity – Measured by comparing the nucleotide sequences of DNA samples from two individuals and then pooling the data from many such comparisons of two individuals.  Geographic Variation – Differences in gene pools between populations or subgroups of populations.

 Balanced Polymorphism – Ability of natural selection to maintain stable frequencies of two or more phenotypic forms in a population. Two mechanisms:  Heterozygote Advantage – If individuals who are heterozygous at a particular locus have greater survivorship and reproductive success than any type of homozygote, then two or more alleles will be maintained at that locus by natural selection.  Frequency-Dependent Selection – The survival and reproduction of any one morph declines if that phenotypic form becomes too common in the population.  Neutral Variation – Confers no selective advantage for some individuals over others, i.e. fingerprints.

 Darwinian Fitness – contribution of an individual to the gene pool of the next generation relative to the contributions of other individuals.  Relative Fitness – contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus.

 Directional Selection – most common during periods of environmental change or when members of a population migrate to some new habitat with different environmental conditions.  Diversifying Selection – occurs when environmental conditions are varied in a way that favors individuals on both extremes of a phenotypic range over intermediate phenotypes.  Stabilizing Selection – acts against extreme phenotypes and favors the more common intermediate variants.

 Sexual Dimorphism – marked differences, called secondary sexual characteristics, not effected by reproduction  Intrasexual Selection – direct competition among individuals of one sex for mates of the opposite sex

 1. Evolution is limited by historical constraints.  2. Adaptations are often compromises.  3. Not all evolution is adaptive.  4. Selection can only edit existing variations.