The Evolution of Populations Vocabulary

Overview of Population Evolution

• Evolution is defined as the process by which allele frequencies within a population change over time.

• The central question explored is: What mechanisms cause the evolution of populations?

• Genetic variation is the foundation of evolution, resulting in variations such as different colors in a hypothetical beetle population.

• The three main mechanisms of allele frequency change are:

  • Natural selection

  • Genetic drift

  • Gene flow

• Natural selection is the only mechanism that consistently leads to adaptive evolution.

• Mutation is a critical factor that must not be forgotten as a source of variation.

Microevolution and the Unit of Evolution

• Microevolution is defined as the change in allele frequencies in a population over generations.

• A common misconception is that individual organisms evolve; however, only populations can evolve.

• While natural selection acts on individuals (affecting their survival and reproductive success), the evolutionary impact is measured by changes in the population over time.

• An example provided is a finch population that evolved, rather than its individual members changing their own genetic makeup.

Genetic Variation: The Prerequisite for Evolution

• Evolution by natural selection requires variation in heritable traits.

• Phenotypic variation often reflects underlying genetic variation, which is defined as differences in the composition of genes or other DNA sequences among individuals.

• Phenotype is the product of the interaction between the genotype and the environment: $Phenotype = Genotype + Environment$.

• Types of phenotypic variation:

  • Traits on an "either-or" basis: Usually determined by a single gene.

  • Gradations along a continuum: Determined by the influence of many genes (e.g., horse coat color).

• Natural selection can only act on phenotypic variation that has a genetic component. Nonheritable variation, such as caterpillars changing appearance (resembling oak flowers or twigs) based on chemicals in their diet (oak flowers vs. oak leaves), cannot be evolved.

• Sources of genetic variation include:

  • New alleles/genes: Produced via mutation (change in nucleotide sequence) or gene duplication.

  • Sexual reproduction: Rearranges existing genes through meiosis (crossing over and independent assortment) and random fertilization.

• Measuring genetic variation:

  • Gene variability: Average percent of loci that are heterozygous.

  • Nucleotide variability: Measured at the molecular level. Most variations occur in noncoding introns (resulting in little phenotypic change), while variations in coding exons may alter amino acid sequences.

Gene Pools and Allele Frequencies

• A population is a group of individuals of the same species living in the same area that interbreed to produce fertile offspring.

• A gene pool consists of all alleles for all loci in a population.

• In diploid organisms, each individual has two copies of each allele.

• For a locus with two alleles, the frequencies are represented by $p$ and $q$.

• The sum of all allele frequencies in a population must equal 1:

  • $p + q = 1$

• Example calculation for a wildflower population with incomplete dominance:

  • 320 red flowers ($C^R C^R$)

  • 160 pink flowers ($C^R C^W$)

  • 20 white flowers ($C^W C^W$)

  • Total individuals = 500; total alleles = 1000.

  • Number of $C^R$ alleles: $(320 \times 2) + 160 = 800$.

  • Frequency of $C^R$ ($p$): $\frac{800}{1000} = 0.8$.

  • Number of $C^W$ alleles: $(20 \times 2) + 160 = 200$.

  • Frequency of $C^W$ ($q$): $\frac{200}{1000} = 0.2$.

The Hardy-Weinberg Equation

• The Hardy-Weinberg (HW) equation describes the expected genetic makeup of a population that is not evolving at a specific locus.

• If observed data deviates from the HW expected values, the population may be evolving at that locus.

• Hardy-Weinberg Equilibrium: A state where allele and genotype frequencies remain constant from generation to generation because mating is random and evolutionary mechanisms are absent.

• The HW equation for two alleles:

  • $p^2 + 2pq + q^2 = 1$

  • $p^2$: Frequency of homozygous dominant genotype.

  • $q^2$: Frequency of homozygous recessive genotype.

  • $2pq$: Frequency of heterozygous genotype.

Conditions for Hardy-Weinberg Equilibrium

• For a population to be in HW equilibrium, five conditions must be met:

  1. No Mutations: The gene pool is modified if mutations occur or if genes are deleted/duplicated.

  2. Random Mating: Inbreeding or subset mating changes genotype frequencies.

  3. No Natural Selection: Success differences in survival/reproduction change allele frequencies.

  4. Extremely Large Population Size: In small populations, genetic drift causes chance fluctuations in allele frequencies.

  5. No Gene Flow: Movement of alleles into or out of the population alters frequency.

Solving Genetic Frequency Problems

• Key steps for solving problems:

  1. Identify given variables.

  2. Identify requested variables.

  3. Use $p + q = 1$ and $p^2 + 2pq + q^2 = 1$.

• Example 1: 1% of a population is $aa$ homozygotes.

  • $q^2 = 0.01$

  • $q = 0.1$

  • $p = 1 - 0.1 = 0.9$

• Example 2: 64% display the dominant trait.

  • $p^2 + 2pq = 0.64$

  • $q^2 = 1 - 0.64 = 0.36$

  • $q = 0.6$

  • $p = 1 - 0.6 = 0.4$

  • Heterozygous percentage ($2pq$): $2 \times (0.4 \times 0.6) = 0.48$ or 48%.

• Example 3 (Red Pandas): Jorge and Claire examine 160 homozygous $F$, 137 heterozygotes, and 29 homozygous $f$. They test if this population is in HW equilibrium.

Mechanisms of Evolutionary Change

• Natural Selection: Differential reproductive success. Results in adaptive evolution where favorable traits increase over time (e.g., DDT resistance in D. melanogaster fruit flies).

• Genetic Drift: Random fluctuations in allele frequencies. Significant in small populations; can lead to loss of genetic variation or fixation of harmful alleles.

  • Founder Effect: A few individuals start a new population.

  • Bottleneck Effect: A sudden environmental change drastically reduces population size, resulting in a gene pool that no longer reflects the original.

• Gene Flow: Movement of alleles among populations (migration of individuals or gametes like pollen). Tends to reduce genetic differences between populations and can affect local adaptation.

• Relative Fitness: The contribution an individual makes to the gene pool of the next generation relative to others.

Modes of Natural Selection

• Selection acts directly on phenotypes and indirectly on genotypes.

• Three modes of selection:

  • Directional Selection: Favors individuals at one end of the phenotypic range (e.g., birds with larger beaks surviving a drought).

  • Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes, maintaining the average trait.

  • Disruptive Selection: Favors individuals at both extremes of the phenotypic range; the intermediate is selected against. This can lead to speciation.

• Selection types on alleles:

  • Positive: Increases frequency of a favorable allele.

  • Negative: Decreases frequency of a deleterious allele.

  • Balancing Selection: Maintains two or more phenotypic forms in a population.

  • Heterozygote Advantage: Heterozygotes have higher fitness than both homozygotes (e.g., $SA$ genotype for sickle-cell provides malaria protection without severe disease).

  • Frequency-Dependent Selection: Fitness of a phenotype depends on its commonness; balancing selection keeps phenotypes near 50% frequency.

Sexual Selection

• Sexual selection is natural selection for mating success, which can lead to sexual dimorphism (marked differences between sexes in secondary characteristics).

• Intrasexual Selection: Competition within the same sex (usually males) for access to the other sex. Involves "weapons" like horns or antlers and aggressive displays. Costs include being energetically taxing and attracting predators.

• Intersexual Selection: Often called mate choice; individuals of one sex (usually females) are choosy in selecting mates.

• Anisogamy: The fusion of dissimilar gametes (many small sperm vs. fewer large eggs). Females invest more energy per gamete and offspring, making them the choosier sex.

• Benefits of choice:

  • Direct: Territory, food, parental care, health.

  • Indirect: "Good genes" that enhance offspring survival or foraging skills.

Constraints on Natural Selection

• Natural selection cannot fashion perfect organisms because:

  1. Selection can only act on existing variations.

  2. Evolution is limited by historical constraints.

  3. Adaptations are often compromises.

  4. Chance, natural selection, and the environment interact.

Administrative Reminders

• Unit 4 Mastering assignments are due April 22nd at 11:59 pm.

• Exam 4: Thursday, April 23rd (50 questions, Chapters 13-15, 18, and 21).

• Final Exam: Thursday, April 30th at 10:45 am (100 questions, Chapters 1-15, 18, and 21).