Evolution: Population Genetics and Change in Species (Hardy-Weinberg, Evidence for Evolution)

Unit 5: Evolution – Population Genetics and Change in Species

Overview

This unit focuses on the mechanisms of evolution at the population level, emphasizing population genetics, the Hardy-Weinberg equilibrium, and the evidence supporting evolutionary theory. Key concepts include gene pools, allele frequencies, and the processes that drive genetic change in populations over time.

Topic A: Genetics, Populations, and Gene Pools

Population Genetics and Evolution

• Evolution is defined as a change in the genetic composition of a population over generations.

• A gene pool consists of all the alleles present in a population.

• Population genetics studies the distribution and change of allele frequencies under the influence of evolutionary processes.

The Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium is a mathematical model that describes a non-evolving population.

• It provides a baseline to measure if and how populations are evolving.

• The model is based on five assumptions:

  1. Large population size (no genetic drift)

  2. No migration (no gene flow)

  3. No mutations

  4. Random mating

  5. No natural selection

• The Hardy-Weinberg equation for two alleles is:

  • $p + q = 1$ (where p and q are the frequencies of the two alleles)

  • $p^2 + 2pq + q^2 = 1$ (where $p^2$ = frequency of homozygous dominant, $2pq$ = frequency of heterozygotes, $q^2$ = frequency of homozygous recessive)

• Deviations from Hardy-Weinberg equilibrium indicate that evolution is occurring.

Mechanisms of Microevolution

• Genetic drift: Random changes in allele frequencies, especially in small populations (e.g., bottleneck effect, founder effect).

• Gene flow: Movement of alleles between populations through migration.

• Mutation: Random changes in DNA that introduce new alleles.

• Non-random mating: Mating that is not random can change genotype frequencies.

• Natural selection: Differential survival and reproduction of individuals due to differences in phenotype.

Application Example

• Calculating allele frequencies in a population using Hardy-Weinberg equations to determine if evolution is occurring.

Topic B: Multiple Lines of Evidence for Evolution

Evidence Supporting Evolution

• Fossil record: Shows changes in organisms over time and the appearance of new species.

• Comparative anatomy: Homologous structures indicate common ancestry; vestigial structures are remnants of features that served important functions in ancestors.

• Embryology: Similar embryonic development among different species suggests evolutionary relationships.

• Molecular biology: DNA and protein similarities among species reflect shared ancestry.

• Biogeography: Geographic distribution of species supports patterns of descent with modification.

Scientific Theories and Evidence

• A scientific theory is a well-supported explanation of natural phenomena, based on evidence and subject to testing and revision.

• Evolution is supported by multiple, independent lines of evidence from various scientific disciplines.

Application Example

• Using molecular evidence (such as DNA sequences) to construct evolutionary trees and infer relationships among species.

Key Vocabulary

• Microevolution

• Population genetics

• Gene pool

• Hardy-Weinberg Equilibrium Model

• Gene flow

• Genetic drift

• Population

• Natural selection

• Adaptation

• Theory

• Variation

• Diversity

• Homologous structures

• Vestigial structures

• Bottleneck effect

• Fitness

• Speciation

• Mutation

• Non-random mating

• Evolution

Hardy-Weinberg Equilibrium Table

Symbol	Meaning
p	Frequency of dominant allele
q	Frequency of recessive allele
p + q	Sum of allele frequencies (should equal 1)
p^2	Frequency of homozygous dominant genotype
2pq	Frequency of heterozygous genotype
q^2	Frequency of homozygous recessive genotype

Summary

• Evolution at the population level is driven by changes in allele frequencies due to mechanisms such as natural selection, genetic drift, gene flow, mutation, and non-random mating.

• The Hardy-Weinberg equilibrium provides a model for understanding genetic stability and detecting evolutionary change.

• Multiple lines of evidence, including fossils, anatomy, embryology, and molecular data, support the theory of evolution.