Lecture 3 fall 2025.elms

Key Terms and Core Concepts

• Natural selection: differential reproduction of individuals based on heritable variation in traits.
• Evolution: genetic change in a population over time.
• Population: a group of individuals of a single species that live and interbreed in a particular geographic area at the same time.
• Adaptation: a trait (structural, physiological, behavioral) that increases an organism’s chance of survival and reproduction; also, the evolutionary process by which traits change to improve survival/reproduction.
• Fitness ( = $R0$ ): the total number of offspring produced by an individual during its lifetime. $R</em>0$

Components of Fitness and Reproductive Strategies

• Components contributing to fitness:
  • Survival to reproductive age
  • Mating success
  • Fecundity (number of offspring)
  • Age at first reproduction (e.g., cost of migration due to delayed reproduction)
  • Duration of reproductive life (survival after reproduction is not directly relevant)
  • Degree of parental care; paternal care often depends on offspring’s ability to survive with/without care
• Reproductive strategies (varying provisioning and care):
  • Large number of gametes with little or no provisioning beyond yolk
  • Retention or care of eggs
  • Bi-parental care (typical of birds) — generally associated with monogamy
  • Maternal care (typical of mammals) — generally associated with polygamy
• Fitness is the capacity of an individual to pass on genes to reproducing offspring.

Outcomes of Natural Selection

• Natural selection can increase fitness and produce:
  • Stabilizing selection
  • Directional (differential) selection
  • Disruptive selection
• These outcomes depend on how fitness varies with phenotype.

How Natural Selection Can Operate: Interpreting Graphs (Conceptual)

• Fitness graphs show fitness of individuals with different phenotypes for a trait.
• Population phenotype distribution changes from before (dashed line) to after (solid line) selection.

Stabilizing Selection

• Favors intermediate variants of a trait; reduces variation and maintains the status quo.
• Acts against extreme phenotypes; leads to a more homogeneous population over time.
• Reduces variation but does not change the mean trait value.
• Often called purifying selection: selection against deleterious mutations in the usual gene sequence.
• Example: Stabilizing selection on human birth weight.
• Example: Stabilizing selection in Siberian Husky size optimum for working in snow (35–60 lbs).

Directional (Differential) Selection

• Individuals at one extreme of a trait distribution have higher survival and reproduction, contributing more offspring.
• For a single gene locus, directional selection may favor a particular variant (positive selection).
• Change per generation depends on:
  • Strength of selection
  • Proportion of phenotypic variation that is genetic (heritability, $h^2$)
• Heritability: the proportion of the phenotypic variance of a trait in a population that is due to additive genetic variance; an indicator of the genetic component of the trait.
• Selection differential ($S$): the difference between the mean of the trait in the original population and the mean of the trait in the selected reproductive population.
  • $S = \bar{X}<em>{Selected} - \bar{X}</em>{Original}$
• The Breeder’s Equation (predicting response to selection):
  • $R = h^2 S$
• The Breeder’s Equation can also be rearranged to estimate heritability: $h^2 = \frac{R}{S}$
• Environmental factors can drive directional selection (e.g., climate change, resource variation, predation pressure).
• Examples:
  • Climate change selecting for heat-tolerant traits.
  • Coloration changes in snowshoe hares depending on snow cover.
  • Disease resistance increasing in populations facing new pathogens.
  • Texas Longhorn cattle as a case of directional selection for certain hardy traits.

Disruptive Selection and Polymorphisms

• Disruptive selection favors extreme phenotypes at both ends of the distribution; average/intermediate phenotypes are disfavored.
• Results in increased variation; can produce a bimodal distribution.
• Example: Bill sizes in the Black-bellied Seedcracker (Pyrenestes ostrinus) – larger and smaller bills favored; intermediate sizes disfavored.
• Consequences: can maintain or increase polymorphisms in a population across environments.

Polymorphisms and Balancing Selection

• Polymorphism: existence of two or more distinct forms/phenotypes of a trait within a population.
• Mechanisms maintaining polymorphisms (Balancing Selection):
  • Heterozygote advantage (overdominance)
  • Frequency-dependent selection
• Frequency-dependent selection: fitness of a phenotype depends on its frequency in the population; can maintain polymorphism when rare morphs gain advantage (predator/prey dynamics, predator switching).
• Heterozygote advantage: heterozygotes have higher fitness than either homozygote in certain environments; maintains both alleles in the population.

Clines and Geographic Variation

• A cline is a gradient in a trait across a geographic range.
• Clines can be smooth or abrupt; a single population can have multiple independent clines for different traits.
• Examples:
  • Australian birds: smaller size toward the north; plumage color intensity correlates with humidity and is reduced toward arid centers.
  • White clover: cyanide production deters herbivores; frost increases mortality for cyanide-producing plants, causing geographic variation in cyanide production.
  • Europe: gradual clover phenotype changes with winter temperature gradients (cline).
• Geographic variation reflects different selective pressures across environments.

Cyanide Production in White Clover: Geographic Gradient

• Proportion of cyanide-producing individuals in European populations depends on winter temperatures.
• Frost events reduce cyanide producers in colder areas, shaping geographic distribution.

Co-evolution: Sickle Cell Anemia and Malaria

• Sickle-cell anemia is caused by a point mutation in the beta-globin gene, producing hemoglobin S (HbS).
• HbS homozygotes (HbSS) have severe disease; HbS heterozygotes (HbS HbA) have sickle-cell trait with milder symptoms.
• Heterozygotes gain malaria resistance, increasing survival in malaria-endemic regions.
• This is a classic case of balancing selection via heterozygote advantage.
• Visual: geographic distribution of HbS and Plasmodium falciparum malaria in Africa illustrates co-evolution.

Frequency-Dependent Selection: Predator–Prey and Morph Cycles

• Predation can favor rare morphs; when a morph becomes common, it becomes targeted, reducing its fitness, allowing the rare morph to increase in frequency.
• Classic examples include prey switching and frequency-dependent predation cycles.
• Three-throat-m morphs in male side-blotched lizards (orange, blue, yellow) provide different mating strategies and interact in a rock–paper–scissors cycle; no single morph dominates across seasons, producing ongoing cycles.
• Scale-eating fish Perissodus microlepis exhibit left- and right-mouth morphs maintained by frequency dependence.

Hardy-Weinberg Principle: A Null Model

• The Hardy-Weinberg (HW) principle is a null model describing allele and genotype frequencies in the absence of evolutionary forces and with random mating.
• Assumptions (no evolutionary agents acting):
  • No mutations
  • No migration (gene flow)
  • No natural selection
  • No genetic drift
  • Random mating
• HW provides a baseline to predict genetic makeup if none of these forces are acting and mating is random.
• History: Developed by Godfrey Hardy and Wilhelm Weinberg in the early 20th century.

HW Algebra and Genotype Frequencies

• Let p be the frequency of the dominant allele, q the frequency of the recessive allele (
  p + q = 1
  ).
• Genotype frequencies expected under HW:
  • Homozygous dominant: $f(AA) = p^2$
  • Heterozygous: $f(Aa) = 2pq$
  • Homozygous recessive: $f(aa) = q^2$
• The sum of allele frequencies: $p + q = 1$
• The sum of genotype frequencies: $p^2 + 2pq + q^2 = 1$
• In HW equilibrium, observed frequencies should match these predictions if no evolutionary forces are acting.

Testing HW: PKU Example (Heredity and Genotype Frequencies)

• PKU (phenylketonuria) is an autosomal recessive disease.
• Phenotype: affected individuals are aa; disease frequency observed: 1 in 10,000.
• Question: If HW applies, what is the frequency of the a allele? How many carriers (Aa) are expected in 10,000 individuals?
• Setup: Let A be the wild-type allele, a be the recessive disease allele.
• Observed: $f(aa) = q^2 = frac{1}{10{,}000} = 0.0001$
• Solve: $q = \sqrt{0.0001} = 0.01$
• Then $p = 1 - q = 0.99$
• Predicted genotype frequencies:
  • $f(AA) = p^2 = 0.9801$
  • $f(Aa) = 2pq = 2(0.99)(0.01) = 0.0198$
  • $f(aa) = q^2 = 0.0001$
• In 10,000 individuals:
  • Affected (aa): 1 person