Chapter 23: Evolution of Populations – Study Notes

Evolution of Populations

Big Idea: Populations Evolve, Individuals Do Not

Evolution is a process that occurs at the population level, not within individual organisms. While natural selection acts on individuals, only populations evolve over generations as allele frequencies change.

• Natural selection acts on individuals, but only populations can evolve.

• Example: On Daphne Major Island, finches with larger beaks survived a drought by eating larger seeds, leading to an increase in alleles for large beaks in the population.

Microevolution

Microevolution refers to changes in allele frequencies within a population over generations. It is driven by several mechanisms:

• Natural selection

• Genetic drift

• Gene flow

Only natural selection consistently leads to adaptive evolution (increased fitness).

Genetic Variation

Genetic variation is essential for evolution. It arises from differences in DNA sequences, but phenotype is influenced by both genotype and environment.

• Phenotype = genotype + environmental influence

• Example: Caterpillars of the same species may look different if they eat different foods due to environmental effects.

• Natural selection can only act on heritable (genetic) variation.

Types of Traits

• Discrete characters: Traits with distinct categories (e.g., red or white flower color).

• Quantitative characters: Traits that vary along a continuum (e.g., height, beak size).

Measuring Genetic Variation

• Gene variability: Percentage of gene loci that are heterozygous (average heterozygosity).

• Nucleotide variability: Differences in DNA sequences among individuals.

Geographic Variation

Populations in different locations may have different gene pools.

• Cline: A gradual change in a trait across a geographic area.

• Example: Mummichog fish have different cold-adaptation alleles along a temperature gradient from Maine to Georgia.

Sources of Genetic Variation

• Mutations: Changes in nucleotide sequence. Only mutations in gamete-producing cells are heritable.

• Point mutations: Can be neutral, harmful, or beneficial.

• Chromosomal mutations: Large-scale changes (deletions, rearrangements) usually harmful.

• Gene duplication: Duplicated genes can evolve new functions; less harmful since the original gene remains.

• Example: Odor-detecting genes have undergone multiple duplications.

Sexual Reproduction and Variation

Sexual reproduction increases genetic variation through:

• Crossing over

• Independent assortment

• Random fertilization

Sexual recombination is usually a greater source of genetic variation than mutation.

Hardy-Weinberg Equilibrium

Population and Gene Pool

• Population: A group of individuals of the same species that can interbreed and produce fertile offspring.

• Gene pool: All alleles at all loci in a population.

• If a locus is fixed, all individuals are homozygous for the same allele (no variation at that locus).

Allele Frequencies

For a gene with two alleles (A and a):

• p = frequency of dominant allele (A)

• q = frequency of recessive allele (a)

They must sum to 1:

$p + q = 1$

Example: If there are 800 dominant and 200 recessive alleles in a population of 1000 alleles:

• $p = \frac{800}{1000} = 0.8$

• $q = \frac{200}{1000} = 0.2$

Hardy-Weinberg Equation

The Hardy-Weinberg equation predicts genotype frequencies under certain conditions:

$p^2 + 2pq + q^2 = 1$

• $p^2$ = frequency of homozygous dominant genotype (AA)

• $2pq$ = frequency of heterozygous genotype (Aa)

• $q^2$ = frequency of homozygous recessive genotype (aa)

Example: If $p = 0.8$ and $q = 0.2$:

• $p^2 = 0.64$ (64% homozygous dominant)

• $2pq = 0.32$ (32% heterozygous)

• $q^2 = 0.04$ (4% homozygous recessive)

Conditions for Hardy-Weinberg Equilibrium

For a population to remain in Hardy-Weinberg equilibrium (no evolution at a locus), these conditions must be met:

1. No mutations

2. Random mating

3. No natural selection

4. Very large population size

5. No gene flow

These conditions are rarely met in nature. A population may be in equilibrium for some genes but not others.

PKU Example

Phenylketonuria (PKU) is a recessive genetic disorder occurring in about 1 in 10,000 births.

• $q^2 = 0.0001$

• $q = \sqrt{0.0001} = 0.01$

• $p = 1 - q = 0.99$

• Carrier frequency: $2pq = 2(0.99)(0.01) = 0.0198$ (about 2% are carriers)

Mechanisms of Evolution

Natural Selection

Natural selection changes allele frequencies because individuals with advantageous traits survive and reproduce more successfully.

• Example: Insects with DDT resistance alleles survived pesticide use and passed on resistance.

• Only natural selection consistently leads to adaptive evolution.

Genetic Drift

Genetic drift is random change in allele frequencies, especially significant in small populations.

• Occurs by chance

• Can reduce genetic variation

• Can cause harmful alleles to become fixed

• Does not produce adaptation

• Example: If only a few flowers reproduce, some alleles may be lost by chance.

Founder Effect

The founder effect occurs when a small group starts a new population, leading to different allele frequencies than the original population.

Bottleneck Effect

The bottleneck effect is a drastic reduction in population size due to events like natural disasters or habitat loss.

• Survivors may not represent the original gene pool.

• Example: Greater Prairie Chickens in Illinois lost genetic variation after habitat loss; introducing birds from other states increased variation and reproductive success.

Gene Flow

Gene flow is the movement of alleles between populations via migration, immigration, emigration, or gamete movement (e.g., pollen).

• Can increase or decrease fitness depending on the context.

• Example (decreased fitness): Great tit birds on an island received alleles from mainland birds, some of which reduced fitness.

• Example (increased fitness): Mosquitoes gained insecticide-resistance alleles through gene flow, aiding survival.

Natural Selection in More Detail

Fitness

Fitness is the genetic contribution an individual makes to the next generation. It is measured by reproductive success, not just physical strength.

Modes of Natural Selection

• Directional selection: Favors one extreme phenotype (e.g., larger beaks during drought).

• Disruptive selection: Favors both extreme phenotypes; intermediate forms are selected against.

• Stabilizing selection: Favors intermediate phenotypes; extremes are selected against.

Adaptations

Adaptations are traits that improve survival or reproduction. Adaptive evolution increases the match between organisms and their environment, but adaptation is ongoing as environments change.

• Examples: Cuttlefish camouflage, snake jaws that stretch to swallow large prey.

Sexual Selection

Sexual Selection

Sexual selection is a form of natural selection related to mating success. It often leads to differences between males and females (sexual dimorphism).

Intrasexual Selection

Competition among members of the same sex (usually males) for mates.

• Example: Males fighting for access to females.

Intersexual Selection

Mate choice, usually by females, based on certain traits in males.

• Example: Peacocks with elaborate feathers attract mates, though such traits may reduce survival.

Maintaining Genetic Variation

Neutral Variation

Genetic variation that does not affect fitness (no selective advantage or disadvantage).

Diploidy

Diploidy preserves genetic variation by hiding recessive alleles in heterozygotes, preventing their removal by selection.

Balancing Selection

• Heterozygote advantage: Heterozygotes have higher fitness than either homozygote (e.g., sickle-cell allele confers malaria resistance).

• Frequency-dependent selection: The fitness of a phenotype decreases as it becomes more common (e.g., scale-eating fish with left- and right-mouthed forms).

Why Natural Selection Does Not Make Perfect Organisms

• Natural selection can only act on existing variation.

• Evolution is limited by historical constraints.

• Adaptations often involve compromises.

• Chance events can affect evolution.

• Environments are constantly changing.