Chapter 21: The Evolution of Populations

Introduction to Population Evolution

Evolution in biology refers to changes in the genetic composition of populations over time. It is important to note that individuals do not evolve; rather, populations evolve as allele frequencies shift across generations.

• Microevolution: Defined as a change in allele frequencies in a population over generations.

• Population: All members of a single species living in the same area, capable of interbreeding and producing fertile offspring.

• Gene pool: The complete set of all possible genes and alleles present within a population.

Genetic Variation and Its Sources

Genetic variation is essential for evolution, as it provides the raw material for natural selection and other evolutionary processes. Variation arises from several sources:

• Mutation: The formation of new alleles due to changes in DNA sequence.

• Gene duplication: Altering gene number or position, which can create new genetic material.

• Rapid reproduction: Especially in organisms like viruses (e.g., HIV), rapid reproduction increases mutation rates.

• Sexual reproduction: Produces new allele combinations through processes such as crossing over and independent assortment.

Causes of Microevolution

Microevolution is driven by three main mechanisms that alter allele frequencies within populations:

• Natural Selection: Favorable traits increase in frequency as they enhance survival and reproduction. Occurs by chance, influenced by biotic and abiotic environmental factors. Not a predetermined process; often summarized as "survival of the fittest."

• Genetic Drift: Random changes in allele frequencies due to chance events. Includes:

  • Bottleneck effect: Drastic reduction in population size due to natural disasters, limiting genetic diversity among survivors.

  • Founder effect: When a small group becomes isolated from the larger population, rare alleles may become more common.

• Gene Flow: Movement of alleles between populations due to migration and interbreeding, which can introduce new genetic material.

Natural Selection and Sexual Selection

Natural selection acts on heritable traits, increasing the frequency of advantageous adaptations. A subset of natural selection is sexual selection, which involves nonrandom mating based on traits that improve mating success.

• Sexual selection: Individuals with certain traits are more likely to obtain mates, leading to changes in allele frequencies.

• Example: Brightly colored plumage in birds may increase mating success but also predation risk.

The Hardy-Weinberg Principle

The Hardy-Weinberg Principle provides a mathematical model to study genetic variation in populations. It states that allele frequencies in a gene pool remain constant from one generation to the next in the absence of evolutionary forces.

• Conditions for Hardy-Weinberg Equilibrium:

  • No mutations (no changes in alleles)

  • Random mating (no selective breeding or preference)

  • No natural selection (all genotypes equally favored)

  • Extremely large population size (no genetic drift)

  • No gene flow (no migration in or out)

• Application: Used to determine if a population is evolving by comparing observed and expected allele frequencies.

Hardy-Weinberg Equations

These equations allow calculation of allele and genotype frequencies in a population:

• Where p is the frequency of the dominant allele, and q is the frequency of the recessive allele.

• Where p^2 is the frequency of homozygous dominant genotype, 2pq is the frequency of heterozygotes, and q^2 is the frequency of homozygous recessive genotype.

Worked Examples

These examples illustrate how to use Hardy-Weinberg equations to analyze population genetics:

• Example 1: In a population of 100 pea plants, 9 are homozygous recessive (short).

  • Frequency of homozygous recessive ():

  • Frequency of recessive allele ():

  • Frequency of dominant allele ():

  • Frequency of homozygous dominant ():

  • Frequency of heterozygotes ():

• Example 2: In a population of 700 individuals: 85 AA, 320 Aa, 295 aa.

  • Frequency of AA ():

  • Frequency of Aa ():

  • Frequency of aa ():

  • Frequency of A ():

  • Frequency of a ():

• Example 3: For a genetic disorder with prevalence 1 in 6,600 (homozygous recessive), the carrier frequency can be estimated using Hardy-Weinberg equations.

  • Frequency of affected individuals ():

  • Frequency of recessive allele ():

  • Frequency of carrier (heterozygote, ):

Summary Table: Hardy-Weinberg Equilibrium

The following table summarizes the key variables and their meanings in Hardy-Weinberg calculations:

Key Takeaways

• Evolution occurs at the population level, not the individual level.

• Genetic variation is necessary for evolution and arises from mutation, gene duplication, rapid reproduction, and sexual reproduction.

• Microevolution is driven by natural selection, genetic drift, and gene flow.

• The Hardy-Weinberg Principle provides a baseline to detect evolutionary change in populations.

• Hardy-Weinberg equations allow calculation of allele and genotype frequencies, useful for studying population genetics and disease prevalence.