Chapter 20: Genetic Variation and Evolution — Study Notes

Genetic Variation, Genotype, and Phenotype

• Genetic variation is the presence of different alleles of genes within individuals of a population.

• The genotype is the combination of alleles that an individual has for a gene or group of genes.

• The phenotype is the observable appearance or behavior produced by the genotype.

• Relationship: phenotype results from the expression of alleles; genotype is the blueprint, phenotype is the visible outcome.

• Example: cat fur color demonstrates how two alleles for a gene can yield different genotypes and a shared phenotype through dominance relationships.

• In sexually reproducing organisms, an individual typically has two alleles for a given gene (unless structural duplication on a chromosome occurs, which is a complication not covered here).

• There can be more than two alleles in a population for a given gene (e.g., ABO blood group with alleles A, B, and O), but any given individual still has only two alleles for that gene.

• Dominant vs recessive: a dominant allele is expressed if present; a recessive allele is expressed only when two copies are present (homozygous recessive).

• Different genotypes can produce the same phenotype when dominance relationships cause the same visible trait (e.g., homozygous dominant and heterozygous both producing black fur in the example).

• Alleles arise from genetic mutations (changes in the gene sequence).

Evolutionary change and how it starts from variation

• Evolution is a genetic change in a population over generations; individuals do not evolve. Change is observed across generations in populations.

• Natural selection is a key mechanism that can drive evolutionary change: individuals with advantageous inherited traits tend to leave more surviving offspring.

• Example: Arctic mammals with mutations leading to thicker fur may have a selective advantage in cold environments; the advantage is contextual and changes with the environment (e.g., climate warming can reduce or reverse the advantage).

• Peppered moths illustrate natural selection: allele frequency shifts in response to environmental changes (pre-industrial light-colored moths vs. post-industrial dark moths due to soot-darkened tree bark).

• Before industrialization: light-colored (peppered) moths had higher fitness on light bark; after soot deposition, dark moths were better camouflaged and more likely to survive and reproduce.

• Natural selection is a major driver of evolving populations, but not the only process that changes allele frequencies.

• Other processes can change allele frequencies and drive evolution (e.g., mutations, migration, drift, nonrandom mating).

• The environment determines whether a trait is advantageous; what is advantageous is relative to a specific environment and time.

Allele frequencies and Hardy–Weinberg expectations

• Allele frequency: the proportion of a particular allele among all alleles for a given gene in a population.

• For a gene with two alleles (dominant P and recessive q): the sum of allele frequencies is 1: $p + q = 1.$

• Under ideal conditions, genotype frequencies follow Hardy–Weinberg proportions: $p^2 + 2pq + q^2 = 1,$ where:

  • $p^2$ is the frequency of homozygous dominant genotype,

  • $2pq$ is the frequency of the heterozygous genotype,

  • $q^2$ is the frequency of the homozygous recessive genotype.

• These equations apply only if five assumptions are met (Hardy–Weinberg equilibrium):

  • No mutations occur,

  • No gene flow (no movement of alleles in/out of the population),

  • Mating is random,

  • The population is very large (no genetic drift),

  • No natural selection occurs.

• Hardy–Weinberg equilibrium is a baseline model; most natural populations do not meet all five assumptions, but the model helps detect whether evolution is occurring by comparing observed genotype frequencies to expected frequencies.

• Example: peppered moths genotype frequencies are given as: 4% homozygous dominant (p^2 = 0.04), 32% heterozygous (2pq = 0.32), 64% homozygous recessive (q^2 = 0.64).

• From the genotype frequencies, compute allele frequencies:

  • $p^2 = 0.04 \Rightarrow p = \sqrt{0.04} = 0.20$

  • $q^2 = 0.64 \Rightarrow q = \sqrt{0.64} = 0.80$

  • Alternatively, using the heterozygote term: $2pq = 0.32,$ and since $p + q = 1,$ one can also derive $p = p^2 + \tfrac{1}{2}(2pq) = 0.04 + 0.16 = 0.20.$ and $q = q^2 + \tfrac{1}{2}(2pq) = 0.64 + 0.16 = 0.80.$

• Thus, the dominant allele frequency is $p = 0.20$ and the recessive allele frequency is $q = 0.80.$

• Important note: a dominant allele does not automatically mean it is more prevalent; in this example the dominant allele is only 20% of the alleles, while the recessive allele is 80%.

• When a population is in Hardy–Weinberg equilibrium, allele frequencies are expected to remain constant from generation to generation, regardless of which trait is dominant. In reality, populations rarely meet all five assumptions, so allele frequencies often change over time, indicating evolution.

Five agents of evolutionary change (the five assumptions and their processes)

• Mutation

  • Mutations are spontaneous, often random, permanent genetic changes that create genetic variation.

  • Mutation rate is generally low, roughly on the order of once per 100,000 cell divisions.

  • Mutation types (based on downstream effects):

  • Deleterious mutations: disadvantageous but not necessarily lethal.

  • Lethal mutations: cause death before or during reproduction.

  • Neutral mutations: have no effect on fitness, often occurring in noncoding regions.

  • Advantageous (adaptive) mutations: increase fitness in a given environment and can spread through a population.

  • The adaptive value of a mutation is environment-dependent and can change with environmental shifts (e.g., peppered moths before vs. after industrial soot deposition).

  • Mutations provide the raw material for evolution by introducing new genetic variation.

• Gene flow (migration)

  • Movement of alleles between populations due to individuals moving or gametes (e.g., pollen) moving.

  • Gene flow changes allele frequencies and can increase genetic variation within a population by introducing new alleles from other populations.

  • Over time, regular gene flow tends to make gene pools more similar between populations, potentially reducing differences in phenotype frequencies.

  • Gene flow can both increase or decrease variation depending on the context and amount of migration.

  • Examples include human migrations reducing genetic differences among populations and historical barriers (e.g., geographical separation) increasing differences.

• Nonrandom mating

  • Individuals choose mates based on phenotype or genotype, not at random.

  • Two main types:

  • Assortative mating (positive assortative mating): phenotypically similar individuals mate, increasing homozygosity; often leads to an excess of homozygotes in the next generation and deviations from Hardy–Weinberg expectations.

  • Disassortative mating (negative assortative mating): phenotypically different individuals mate, increasing heterozygosity and producing more heterozygotes than expected.

  • Nonrandom mating can influence genotype frequencies without necessarily changing allele frequencies directly.

• Genetic drift (in small populations)

  • Random fluctuations in allele frequencies due to sampling error, especially pronounced in small populations.

  • Can lead to the loss of alleles or fixation of others independent of their adaptive value.

  • Effects are more pronounced in bottlenecks and founder events.

• Natural selection

  • Differential survival and reproduction based on inherited traits.

  • Alleles that confer an advantage tend to increase in frequency; those that are disadvantageous tend to decrease.

  • Relative to the environment and time; an advantageous trait in one environment may be neutral or deleterious in another.

Population genetics, molecular evidence, and practical examples

• Population genetics studies genetic variation within and between populations to understand evolutionary processes.

• Phenotypic variation can be obvious (e.g., differences in skin color, eye color, height, etc.) or subtle; when phenotypic variation is evident, it often reflects underlying genetic variation.

• In some cases, phenotypic variation is not obvious, so genetic differences are assessed directly by sequencing genes or whole genomes.

• Example of molecular variation: a segment of the mitochondrial cytochrome c oxidase subunit I gene (COI) is used as a barcoding gene because part of it is highly conserved across animals and can be aligned to compare nucleotide differences.

  • Alignment shows columns of nucleotides; identical nucleotides are the same color, differences are different colors.

  • Mutations can be single-nucleotide changes (e.g., a third guanine replaced by adenine) or larger changes such as insertions or deletions.

  • A large pink region in an alignment could represent a deletion of nearly 50 nucleotides; other colored regions may indicate insertions.

  • Such differences across individuals reflect genetic variation that can lead to diversification, even if the phenotypic effect is not immediately visible in a small sample.

• In 1988, Hardy and Weinberg formulated a principle establishing genotype frequencies from allele frequencies under the five assumptions; the principle enables predictions and tests for evolutionary change.

• The same framework helps explain that evolution can be inferred when observed genotype frequencies deviate from Hardy–Weinberg expectations, indicating that one or more of the five agents is at work.

• Practical relevance: Hardy–Weinberg provides a baseline to detect and interpret evolutionary forces in natural populations, which can inform conservation and management strategies (e.g., assessing extinction risk and genetic health).

• Real-world note: the northern white rhinoceros is functionally extinct, illustrating how populations can lose genetic diversity and be unable to adapt to changing environments.

Important takeaways and connections

• Genetic variation is the substrate for evolution; both genotype and phenotype are central concepts for understanding variation.

• Evolution operates at the population level; allele frequencies can be tracked generation to generation to assess whether evolution is occurring.

• The Hardy–Weinberg framework gives a baseline expectation under ideal conditions; deviations from this baseline point to evolutionary processes in action.

• The five agents of evolutionary change—mutation, gene flow, nonrandom mating, genetic drift, and natural selection—can act alone or in combination to alter allele frequencies and drive evolution.

• Real-world examples (peppered moths, COI barcoding, ABO blood group) illustrate how genetic variation manifests, how populations respond to environmental change, and how scientists study and interpret these changes.

• Dominance is a relationship describing phenotype expression, not a universal indicator of allele frequency or fitness; a dominant allele can be rarer than a recessive allele in a given population.

• The environment shapes the adaptive value of alleles; what is advantageous now may become neutral or disadvantageous if conditions change.

• Population genetics provides a bridge between molecular variation and macroevolution, linking DNA-level changes to population-level outcomes and species diversification.