Population Genetics and Evolution Notes

Population Genetics

• Definition: Population genetics studies the distribution and change in frequency of alleles (gene variants) within populations, integrating Mendel's work on single-gene traits and Darwin's focus on polygenic variation. It explores how evolutionary processes such as selection, drift, mutation, and gene flow affect genetic diversity and structure in populations, allowing scientists to understand the genetic basis of inherited traits and evolution.

• Key terms:

• Population: A group of individuals of the same species located in a specific area capable of interbreeding to produce fertile offspring. Populations share a common gene pool, and their genetic composition can change over generations.

• Gene Pool: The complete set of genetic information within a population, including all the alleles for every gene present. A diverse gene pool is essential for the adaptability and survival of a population under changing environmental conditions.

Patterns of Evolution

• Evolution can manifest through various patterns:

• Speciation: The formation of new species as populations diverge and adapt to different environments. This can occur through mechanisms such as allopatric isolation, where geographic barriers lead to reproductive isolation, or sympatric speciation, where new species arise without physical barriers.

• Divergent Evolution: An evolutionary pattern where species evolve and differentiate from a common ancestor due to adaptation to varied environments, leading to homologous structures that indicate a shared evolutionary origin.

• Convergent Evolution: This occurs when unrelated species develop similar traits or adaptations because they occupy similar ecological niches or face similar environmental challenges, resulting in analogous structures that perform similar functions but do not share a recent common ancestor.

• Adaptive Radiation: A rapid diversification from a single ancestor species into multiple forms that adapt to different ecological niches. This phenomenon is often observed in island biogeography, such as Darwin’s finches in the Galapagos Islands, where each species evolved distinctive beak shapes based on available food sources.

Speciation

• Definition of Species: A species is defined as a group of organisms that can reproduce and create fertile offspring under natural conditions, maintaining genetic continuity over generations.

• Hybrids: Offspring produced from the mating of two different species, which can often be sterile (e.g., mules from horses and donkeys, ligers from lions and tigers), showcasing the genetic barriers that separate distinct species.

• Speciation: The process through which new species arise from existing species due to genetic changes or environmental shifts that result in reproductive isolation, ultimately leading to the inability of populations to interbreed.

Methods of Speciation: Reproductive Isolation

• Reproductive Isolation: Mechanisms that prevent species from mating and producing viable offspring, helping maintain species boundaries. They are categorized into:

• Prezygotic Barriers: Mechanisms that prevent mating or fertilization between species.

• Postzygotic Barriers: Mechanisms that prevent hybrid offspring from developing successfully after fertilization.

Prezygotic Barriers

1. Temporal Isolation: Distinct mating seasons or times of day can prevent species from interbreeding.

2. Behavioral Isolation: Unique mating rituals or behaviors can inhibit pairing between different species.

3. Mechanical Isolation: Structural differences in reproductive organs can prevent successful mating.

4. Gametic Isolation: Incompatibility between egg and sperm of different species can hinder fertilization.

Postzygotic Barriers

• These barriers occur after fertilization leading to hybrid offspring issues:

• Chromosomal mismatches can cause lethality or developmental problems.

• Fertile hybrids may still be sterile (e.g., mules), preventing gene flow between species.

Types of Speciation

• Allopatric Speciation: Occurs due to geographic isolation that prevents interbreeding among populations within the same species (e.g., Kaibab and Abert squirrels separated by the Grand Canyon).

• Sympatric Speciation: Arises without physical barriers, often through mechanisms like polyploidy in plants or behavioral shifts in populations leading to reproductive isolation within the same geographic region (e.g., North American Apple maggot flies that preferentially mate on different host plants).

Divergent vs. Convergent Evolution

• Divergent Evolution: Species become increasingly different as they adapt to diverse environments, resulting in homologous structures that reflect their common ancestry.

• Convergent Evolution: Unrelated species evolve similar traits due to adaptation to similar environmental pressures, leading to analogous structures that serve similar functions.

Adaptive Radiation

• Definition: A specific form of divergent evolution where species rapidly evolve to occupy various ecological niches, exemplifying the role of natural selection in diversification.

• Example: Darwin’s finches in the Galapagos Islands showcase adaptive radiation, where various species evolved unique beak shapes and sizes tailored to the distinct food sources available on specific islands.

Tempo of Speciation

• Evolution can happen at different rates, with two main theories describing the pace of species change:

1. Gradualism: Proposes slow, steady evolution driven by small, continuous changes across generations, leading to transitional forms.

2. Punctuated Equilibrium: Suggests that long periods of stability in species are interrupted by brief, dramatic changes leading to rapid speciation events.

Evolutionary Forces Causing Speciation

1. Genetic Drift: Random changes in allele frequencies, particularly pronounced in small populations, can lead to significant evolutionary changes over time.

• Bottleneck Effect: A reduction in population size due to random events (e.g., a natural disaster) that significantly decreases genetic diversity.

• Founder Effect: Occurs when a new population is established by a small number of individuals leading to reduced genetic variation compared to the original population.

1. Gene Flow: The movement of genes between populations through migration and interbreeding, which can enhance genetic diversity and reduce differences between populations.

2. Mutations: Random alterations in DNA sequences that create new alleles, serving as the original source of genetic variation in populations.

3. Non-random Mating: Preferences for certain phenotypes can affect allele frequencies and ultimately influence genetic diversity within populations.

4. Natural Selection: Environmental pressures favor the survival and reproduction of individuals with advantageous traits, shaping the evolution of populations over time.

Patterns of Natural Selection

• Stabilizing Selection: Favors average traits within a population and eliminates extreme variations (e.g., maintaining optimal human birth weight).

• Directional Selection: Favors one extremity of the trait distribution, leading to shifts towards one side (e.g., giraffe neck size increasing over generations).

• Disruptive Selection: Favors extreme traits and removes average traits, promoting diversity (e.g., Chinook Salmon exhibiting varied body sizes).

Heterozygous Advantage

• Heterozygotes often showcase enhanced survival and reproductive success compared to homozygotes, a phenomenon illustrated by sickle cell anemia, where heterozygous individuals have a survival advantage against malaria.

Hardy-Weinberg Equilibrium

• Definition: A theoretical model representing a non-evolving population where allele frequencies remain constant across generations, providing a baseline for understanding evolutionary dynamics.

• Conditions for Hardy-Weinberg:

1. No natural selection or environmental pressures acting on the population.

2. No mutations introducing new alleles into the population.

3. No migration that adds or removes individuals from the population.

4. A sufficiently large population size to minimize the effects of genetic drift.

5. Random mating occurs, ensuring that each individual has an equal chance of contributing to the next generation of offspring.

Hardy-Weinberg Equations

• For a gene with two alleles (p and q):

• Allele Frequencies: p + q = 1

• Genotype Frequencies: p² + 2pq + q² = 1, where p² represents the frequency of homozygous dominant individuals, 2pq expresses heterozygous individuals, and q² indicates the frequency of homozygous recessive individuals.

Practice Problems

1. Calculate genotype frequencies given allele frequencies using Hardy-Weinberg equations.

2. Determine allele frequencies from a population sample with known numbers of individuals exhibiting specific phenotypes.

3. Apply Hardy-Weinberg principles to solve for unknown allele frequencies based on observed phenotype distributions within a population.