Evolution of Populations: Mechanisms, Adaptation, and Genetic Equilibrium

Evolution of Populations (Chapter 23)

Learning Objectives

This section introduces the foundational concepts of population evolution, including mechanisms such as natural selection, adaptation, fitness, and genetic equilibrium. Students will learn to explain the relationship between genetic variation, mutation, and natural selection, and identify genotype frequencies using the Hardy-Weinberg principle.

• Define evolution, natural selection, adaptation, and fitness.

• Explain the connection between genetic variation, mutation, and natural selection.

• Describe the concept and rules of genetic equilibrium.

• Identify genotype frequencies in populations.

• List the five conditions required for Hardy-Weinberg equilibrium and describe how changes affect populations.

Descent with Modification and Diversity of Life

Origins of Species Differences

Species accumulate differences from their ancestors as they adapt to different environments over many generations. This process, known as descent with modification, explains both the similarities and differences among Earth's species.

• Ancient common ancestor: All species share a common origin, leading to shared characteristics.

• Adaptation: Species change over time to better fit their environments.

• Diversity of life: Accumulation of differences has resulted in the vast variety of species observed today.

• Example: Orchid, mantid, and stick insect all evolved from a common ancestor but adapted to different ecological niches.

Mechanisms of Evolution

Natural Selection

Natural selection is the process by which traits that enhance survival and reproduction increase in frequency in a population over time. It is the primary mechanism for evolution.

• Inherited traits: Characteristics are passed from parents to offspring.

• Variation: Offspring show genetic differences due to mutation and recombination.

• Differential survival: Individuals with advantageous traits are more likely to survive and reproduce.

• Example: Ladybird beetles show variation in color and pattern, which may affect survival.

Adaptation

Adaptation refers to features that allow organisms to survive and reproduce in specific environments. Over time, natural selection leads to evolutionary adaptation.

• Darwin's finches: Beak shapes adapted for different diets (cactus-eater, insect-eater, seed-eater).

• Example: Finches on the Galápagos Islands evolved distinct beak shapes to exploit different food sources.

Evolutionary Fitness

Definition and Measurement

Fitness is a measure of an organism's ability to survive and reproduce in its environment. Evolutionary fitness is determined by the number of offspring an individual contributes to the next generation.

• Survival of the fittest: Individuals with higher fitness are more likely to pass on their genes.

• Evolution acts on populations: Populations, not individuals, evolve over time.

Genetic Variation and Evolution

Gene Pool and Genetic Diversity

The gene pool is the sum of all alleles present in a population. Genetic diversity is essential for evolution, as it provides the raw material for natural selection.

• Allele frequency: The proportion of a specific allele among all alleles at a genetic locus in the population.

• Genotype frequency: The proportion of a specific genotype among all individuals in the population.

• Example: Flower color in a plant population may be determined by allele frequencies.

Hardy-Weinberg Equilibrium

Concept and Conditions

Hardy-Weinberg equilibrium describes a population in which allele and genotype frequencies remain constant from generation to generation, provided certain conditions are met.

• No mutations

• Random mating

• No natural selection

• Extremely large population size

• No gene flow (no migration)

If any of these conditions are not met, evolution may occur.

Hardy-Weinberg Equation

The Hardy-Weinberg equation predicts genotype frequencies in a non-evolving population:

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

Where:

• $p$ = frequency of one allele (e.g., dominant)

• $q$ = frequency of the other allele (e.g., recessive)

• $p^2$ = frequency of homozygous dominant genotype

• $2pq$ = frequency of heterozygous genotype

• $q^2$ = frequency of homozygous recessive genotype

Example Calculation

Given a gene pool with 80% CR (p = 0.8) and 20% CW (q = 0.2):

• $p^2 = 0.64$ (CRCR genotype)

• $2pq = 0.32$ (CRCW genotype)

• $q^2 = 0.04$ (CWCW genotype)

These frequencies will remain constant in subsequent generations if Hardy-Weinberg conditions are met.

Mechanisms of Evolutionary Change

Genetic Drift

Genetic drift refers to random changes in allele frequencies, especially significant in small populations. It can lead to loss of genetic variation and fixation of harmful alleles.

• Founder effect: A small group establishes a new population, leading to different allele frequencies than the original population.

• Bottleneck effect: A sudden reduction in population size due to disaster or other events, resulting in a loss of genetic diversity.

• Example: Hereditary blindness in isolated populations, reduced genetic diversity in prairie chickens after population bottlenecks.

Mechanism	Description	Example
Founder Effect	Small group starts new population; allele frequencies differ from original	Colonization of isolated islands
Bottleneck Effect	Population size drastically reduced; genetic diversity lost	Prairie chicken population decline

Summary of Genetic Drift

• Significant in small populations

• Can cause allele frequencies to change at random

• May lead to loss of genetic variation

• Can cause harmful alleles to become fixed

Additional info:

• Gene flow (migration) and mutation are also important mechanisms of evolution, though not covered in detail in the provided notes.

• Selection, drift, and gene flow interact to shape genetic diversity in populations.