Chapter 23 The Evolution of Populations
Darwin and Mendel
Population genetics - genetics which emphasizes the extensive genetic variation within
populations and recognizes the importance of quantitative characters
Modern synthesis - a comprehensive theory of evolution that integrated ideas from
paleontology, taxonomy, biogeography, and population genetics
Population - a 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
Alleles in Populations
Gene Pool - the total aggregate of genes in a population at any one time
Allele frequency - the relative proportion of alleles in the gene pool
Genetic structure - a population’s frequencies of alleles and genotypes
Hardy-Weinberg
Hardy-Weinberg Theorem - 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 (Fig 23.3)
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equation - p2 + 2pq + q2 =1
Hardy-Weinberg Assumptions
Very large population size
No migration
No net mutations
Random mating
No natural selection
Microevolution
Evolution is a generation-to-generation change in a population’s frequencies of alleles or
genotypes - a change in a population’s genetic structure
Causes of Microevolution
Genetic drift - Changes in the gene pool of a small population due to chance (Fig
23.4)
The Bottleneck Effect (Fig 23.5)
Founder Effect
Gene flow
Mutation
Nonrandom Mating
Inbreeding - mating between closely related partners
Assortative mating - individuals select partners that are like themselves in
certain phenotypic characters
Natural Selection
Genetic Variation
Polymorphism - two or more contrasting forms are each represented in a population
Geographical variation - differences in genetic structure between populations
Cline - a graded change in some trait along a geographic axis (Fig 23.8)
What Causes Genetic Variation?
Mutation
Sexual recombination
What Preserves Genetic Variation?
Diploidy
Balanced polymorphism - the ability of natural selection to maintain diversity in a
population
Heterozygous advantage
Hybrid vigor
Frequency-dependent selection - the reproductive success of any one
morph declines if that phenotypic form becomes too common in
the population
Neutral variation
Adaptive Evolution
Fitness
Darwinian fitness - the measure that is critical to selection, is the relative
contribution an individual makes to the gene pool of the next generation
Relative fitness - the contribution of a genotype to the next generation compared
to the contributions of alternative genotypes for the same locus
Selection (Fig 23.12)
Stabilizing
Directional
Diversifying
Sexual selection - selection based on variation in secondary sex characteristics
Sexual dimorphism - the distinction between the secondary sex characteristics of
males and females (Fig 23.16)
Why Not Perfection?
Organisms are locked into historical constraints
Adaptations are often compromises
Not all evolution is adaptive
Selection can only edit variations that exist