Population Genetics and Speciation: Study Notes

Population Genetics and Speciation

Overview

This topic explores how genetic variation within populations enables evolution, the mechanisms that alter allele frequencies, and the processes leading to speciation. Key concepts include gene pools, Hardy-Weinberg equilibrium, microevolutionary forces, and reproductive barriers.

Population Genetics

Genetic Variation and Evolution

Genetic variation is the foundation of evolutionary change. It refers to differences in DNA sequences among individuals in a population.

• Population: A group of individuals of the same species living in a specific area.

• Genetic variation: Differences in alleles and genes among individuals.

• Microevolution: Change in allele frequencies in a population over time.

• Natural selection: Acts on individuals, but evolution occurs at the population level as allele frequencies shift.

• Directional selection: Selection that favors one extreme phenotype, shifting allele frequencies in one direction.

Example: If beneficial alleles increase survival and reproduction, their frequency rises in the population over generations.

Sources of Genetic Variation

Variation arises from both genetic and environmental factors.

• Heritable variation: Differences due to genes, passed from parents to offspring.

• Nonheritable variation: Differences due to environmental influences (e.g., diet affecting caterpillar color).

• Genetic variation mechanisms:

  • Mutations (changes in DNA sequence)

  • Errors in meiosis (crossing over, independent assortment)

  • Spontaneous mutations during DNA replication

  • Induced mutations (mutagens, chemicals, radiation)

• Neutral variation: Mutations in noncoding regions or silent mutations that do not affect phenotype.

Example: Human globin genes evolved by duplication and divergence, resulting in different globin genes on separate chromosomes.

Gene Pools and Allele Frequencies

Gene Pool

The gene pool is the total collection of alleles at every locus in all individuals of a population.

• Fixed allele: Only one allele present at a locus in the population (all individuals homozygous).

• Allele frequency: Proportion of a specific allele among all alleles for a gene in the population.

Example Calculation: In a population of 500 diploid plants (1000 alleles for flower color): - 320 red (CRCR): 640 CR alleles - 20 white (CWCW): 40 CW alleles - 160 pink (CRCW): 160 CR, 160 CW alleles Total CR = 800, Total CW = 200 Frequency CR = 0.8, Frequency CW = 0.2

Hardy-Weinberg Equilibrium

Describes a population that is not evolving, where allele and genotype frequencies remain constant from generation to generation.

• Allele frequencies: $p$ and $q$ (e.g., $p = 0.8$, $q = 0.2$)

• Genotype frequencies: $p^2$ (homozygous dominant), $2pq$ (heterozygous), $q^2$ (homozygous recessive)

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

Example: For CR and CW alleles: - CRCR: $0.8^2 = 0.64$ (64%) - CRCW: $2 imes 0.8 imes 0.2 = 0.32$ (32%) - CWCW: $0.2^2 = 0.04$ (4%)

Assumptions of Hardy-Weinberg Equilibrium

Populations must meet these conditions to remain at equilibrium:

Condition	Consequence if Not Met
No mutations	Allele frequencies change due to new alleles
Random mating	Non-random mating alters genotype frequencies
No natural selection	Allele frequencies change if some alleles confer advantage
Extremely large population size	Genetic drift affects allele frequencies in small populations
No gene flow	Migration introduces new alleles

Hardy-Weinberg Example

Phenylketonuria (PKU): A recessive inherited disease. If 1 in 12,000 births has PKU, what percent of the population are heterozygote carriers?

• Let $q^2 = 1/12,000$

• Calculate $q = \sqrt{1/12,000}$

• Calculate $2pq$ for carrier frequency

Microevolution: Causes and Mechanisms

Genetic Drift

Random changes in allele frequencies, especially significant in small populations.

• Genetic drift: Chance events cause alleles to be over- or underrepresented in the next generation.

• Bottleneck effect: Sudden reduction in population size due to environmental events (e.g., floods, fires), drastically altering allele frequencies.

• Founder effect: When a small group establishes a new population, allele frequencies may differ from the original population.

• Genetic drift can lead to loss of genetic variation and fixation of harmful alleles.

Example: British colonists on Tristan da Cunha had a higher rate of progressive blindness due to the founder effect.

Gene Flow

Movement of alleles between populations due to migration of individuals or gametes.

• Gene flow: Transfer of alleles into or out of a population.

• Reduces genetic differences between populations.

• Can introduce new alleles or remove existing ones.

Example: Plant pollen carried by wind can introduce new alleles to a population.

Natural Selection

Process by which beneficial alleles increase in frequency and harmful alleles decrease over generations.

• Selection for beneficial alleles: Increases their frequency.

• Selection against harmful alleles: Decreases their frequency.

• Can lead to adaptation and directional selection.

Example: Insects preferring red flowers select against the CR allele, causing its frequency to decrease.

Speciation (Preview)

Barriers to Reproduction

Speciation occurs when populations become reproductively isolated, leading to the formation of new species.

• Prezygotic barriers: Prevent mating or fertilization between species.

• Postzygotic barriers: Prevent hybrid offspring from surviving or reproducing.

Example: Geographic isolation can lead to allopatric speciation.

Types of Speciation

• Allopatric speciation: Occurs when populations are geographically separated.

• Sympatric speciation: Occurs without geographic separation, often due to genetic changes.

• Autopolyploidy and allopolyploidy: Speciation due to changes in chromosome number.

Example: Polyploidy in plants can result in new species.

Additional info: Academic context and examples have been expanded for clarity and completeness. Table reconstructed from slide content.