population genetics

Population Genetics, Microevolution and Natural Selection

Understanding Evolution

• Evolution occurs at the population level. To understand how evolution works, it's essential to look at populations.

Population Genetics

• Definition: Population genetics is concerned with the nature of the gene pool of a population and how it changes over time.

  • Gene Pool: The sum total of all the genes in a population at any given time.

  • Characterization: The gene pool is characterized by allele frequencies, which are the relative frequency or proportion of a given allele in a gene pool.

Alleles and Genotype Frequencies

• Notation:

  • A: dominant allele (green wings)

  • a: recessive allele (blue wings)

  • Genotype Frequencies:

  • Total population (N) = 200

  • Genotype frequencies:

    • AA: 100 individuals → Frequency = $\frac{100}{200} = 0.5$

    • Aa: 50 individuals → Frequency = $\frac{50}{200} = 0.25$

    • aa: 50 individuals → Frequency = $\frac{50}{200} = 0.25$

  • Allele Frequencies:

  • p (frequency of dominant allele, A) = 0.625

  • q (frequency of recessive allele, a) = 0.375

Hardy-Weinberg Theorem and Equation

• To understand how populations change, one must first understand how they stay the same. This is where the Hardy-Weinberg Theorem and Equation are crucial.

Hardy-Weinberg Questions

• Common Misconception: Shouldn’t recessive traits slowly disappear from the population?

• Question: Why, if having six fingers is a dominant gene, don’t ¾ of the people in class have six fingers?

Hardy-Weinberg Theorem

• States that allele and genotype frequencies in a population’s gene pool remain constant over time unless acted upon by agents.

Hardy-Weinberg Equation

• Key Equations:

  • $p + q = 1$ (represents allele frequencies)

  • $p^2 + 2pq + q^2 = 1$ (represents genotype frequencies)

• Describes the allele frequencies of a population that is NOT evolving.

Application Example

• Population Generation:

  • Next generation of butterflies: N = 200 with genotype distributions:

  • AA: 90 → Frequency = $\frac{90}{200} = 0.45$

  • Aa: 15 → Frequency = $\frac{15}{200} = 0.075$

  • aa: 95 → Frequency = $\frac{95}{200} = 0.475$

  • New allele frequencies:

  • p = 0.4875 (frequency of dominant allele)

  • q = 0.5125 (frequency of recessive allele)

  • Conclusion: Some factor other than random sexual recombination is acting upon this population.

Conditions for Hardy-Weinberg Equilibrium

• 1. Very large population size

• 2. Isolation from other populations

• 3. No net mutations

• 4. Random mating

• 5. No natural selection

Hardy-Weinberg Equilibrium in Human Populations

• Useful for tracking recessive diseases/conditions in human populations.

Estimating the Frequency of Carriers in a Population

• Example: Albinism

  • A recessive condition (aa) occurs at a frequency of 0.00005 in European Caucasians.

  • Calculate:

    • $q^2 = 0.00005$

    • $q = \sqrt{0.00005} = 0.0071$

    • $p = 1 - 0.0071 = 0.9929$

    • Carrier frequency: $Aa = 2pq = 2 * 0.0071 * 0.9929 = 0.014$

Availability of Hardy-Weinberg Theorem

• Most populations do not achieve equilibrium, but the theorem can identify factors that change the population.

Evolution Overview

• Evolution Definition: Descent with modification.

• Microevolution: Change in the gene pool of a species/population over time.

• Macroevolution: The origin/rise of new taxonomic groups such as species or genera; refers to large-scale changes.

Causes of Microevolution

1. Genetic Drift: Random shifts in allele frequencies, more significant effects in small populations.

   • Bottleneck Effect: A sharp reduction in population size due to environmental events, leading to loss of genetic diversity.

   • Founder Effect: Occurs when a small group from a population starts a new population with limited genetic variation.

2. Gene Flow: The loss or gain of alleles due to immigration or emigration.

3. Mutations: Chance alterations of genetic material; they may substitute alleles and introduce new alleles.

4. Nonrandom Mating: Certain individuals have a greater chance of being chosen as mates.

5. Natural Selection: Differential reproductive success based on certain phenotypes leading to increased prevalence in future generations.

Genetic Drift Explained

• Process: Allele frequencies fluctuate randomly, leading to one allele becoming fixed while the other becomes lost.

• Neutrality: The particular alleles involved in genetic drift are typically neutral, meaning they do not confer a survival advantage.

• Time Frame: The average time to fixation of a newly arisen neutral allele is approximately $4N$ generations, where $N$ is the population size.

Case Studies

• Example 1: Kruger National Park indicated recovery after supplemental immigration; Addo Elephant National Park’s isolation led to slow recovery.

• Example 2: Illustrated the extreme effects of genetic drift through generations of limited survival and reproduction.

Evolution by Gene Flow

• Vital for maintaining genetic diversity in isolated populations.

• Can counteract allele fixation due to genetic drift, thus preserving variation among populations.

Mutations

• Mutations serve as the original source of genetic variation and are crucial for natural selection.

Nonrandom Mating

• Definition of Hardy-Weinberg Assumption: The population is panmictic, meaning members mate randomly.

• Inbreeding: Mating between closely related individuals; it increases the proportion of homozygotes and can lead to inbreeding depression due to the expression of deleterious alleles.

Inbreeding Depression Data

• Depicts offspring mortality rates relative to relatedness of parents, highlighting increased risks in closer relatives.

Nonrandom Mating Types

• Assortative Mating: “Like mates with like” (ex: weight in human populations).

Natural Selection Fundamentals

• Definition: It involves differential reproductive success and survival, where certain phenotypes/genotypes are better suited for survival and reproduction. This mechanism relies on genetic variation as raw material.

Example of Natural Selection

• Case Study: Geospiza fortis (ground finches):

  • Normal conditions present a variety of beak sizes.

  • In drought conditions, larger beaks are favored as they can handle tough seeds better, leading to an evolutionary increase in beak size in subsequent generations.

Fitness in Natural Selection

• Definition of Fitness: Measured by the proportionate contribution an individual makes to future generations, which includes both survival and reproduction.

• Adaptation: Any heritable trait that increases fitness in a given environment, representing the end product of natural selection.

Common Misconceptions about Evolution

• Misconception: “Struggle for existence” denotes survival is purely about fighting strength.

• Misconception: Individuals adapt, when in fact, evolution can only occur at the population level.

• Not all traits are susceptible to natural selection; it primarily acts on heritable traits.

Selection Patterns

• Directional Selection: Simplest form; favors phenotypes at one extreme.

• Stabilizing Selection: Favors phenotypes near the population mean, maintaining the population's mean traits.

• Disruptive/Diversifying Selection: Rare in nature; favors extreme phenotypes over intermediates, usually leading to reproductive isolation of extremes.

Implications of Natural Selection

• Genetic variation is necessary for evolutionary changes, with polymorphism contributing to either phenotypic or genotypic diversity.

  • Polymorphism: High enough frequency of two or more distinct morphs or types in a population.

Measuring Genetic Variation in Populations

• Heterozygosity: The average percentage of loci that are heterozygous in individuals.

• Nucleotide Variability: Differences in nucleotide sequences among individuals within a population.

Implications for Endangered Species

• Very small populations often lose genetic variability, which can inform strategies for species management, e.g., in the Adder (Vipera berus) population leading to high homozygosity and poor offspring viability.

Geographical Variation

• Results from natural selection acting on diverse environments, genetic drift affected by population size, and gene flow depending on location.

Sources of Genetic Variation

• Mutations: Serve as the primary source, affecting populations differently based on reproduction rates, especially among bacteria and insects.

Lastly, Seeking Clarification on Natural Selection Dynamics

• Question raised: If natural selection enhances certain phenotypes, why do unfavorable alleles not disappear?

• Heterozygote Protection: Recessive alleles are shielded within heterozygote carriers, providing a mechanism for maintaining genetic variation despite selection pressures.

• Balancing Selection: Maintains stable frequencies of two or more forms in a population through mechanisms like heterozygote advantage and frequency-dependent selection.

Specific Example: Warfarin Resistance

• Historical use of Warfarin as a pesticide led to rapid changes in resistance allele frequencies, explained by the roles of dominant and recessive genotypes in fitness relative to environmental conditions.

• Heterozygote Advantage: In a scenario with warfarin, SS (susceptible) die, RR (resistant) suffer inefficiencies while RS (heterozygotes) demonstrate resistance and maintain health, thus becoming more common across generations.