Evolution of Populations

Introduction to Microevolution

Microevolution refers to changes in the genetic composition of populations over time. By examining the distribution of phenotypes (observable traits) and the underlying genetic variation, biologists can determine whether a population is evolving. The primary mechanisms that drive microevolution are natural selection, genetic drift, and gene flow.

• Phenotype: The observable physical or physiological traits of an organism, determined by its genotype and environment.

• Microevolution: Evolutionary change within a population, especially changes in allele frequencies over generations.

• Key mechanisms: Natural selection, genetic drift, and gene flow.

• Example: Changes in fur color in a population of animals due to predation pressure.

Population Genetics

Allele and Genotype Frequencies

Population genetics studies the distribution and change of allele frequencies under the influence of evolutionary processes. Understanding the difference between allele frequency and genotype frequency is fundamental.

• Allele frequency: The proportion of a specific allele among all alleles for a gene in a population.

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

• Relationship: Genotype frequencies can be used to calculate allele frequencies and vice versa, especially under Hardy-Weinberg equilibrium.

• Example: In a population with 100 individuals: 25 AA, 50 Aa, 25 aa. Frequency of A = (2*25 + 50)/(2*100) = 0.5; frequency of a = 0.5.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in a population at equilibrium (not evolving). It predicts genotype frequencies from allele frequencies under certain conditions (no selection, mutation, migration, genetic drift, or non-random mating).

• Equations:

  • Allele frequencies:

  • Genotype frequencies:

  • Where = frequency of dominant allele, = frequency of recessive allele.

• Expected genotype frequencies:

  • Homozygous dominant (AA):

  • Heterozygous (Aa):

  • Homozygous recessive (aa):

• Example: If and , then expected frequencies are: AA = 0.36, Aa = 0.48, aa = 0.16.

Population Genetics in Action

Changes in allele frequencies over generations indicate that evolution is occurring. Selection pressures, such as predation, can shift these frequencies, as shown in the example of fur color in mice.

• Example Table: Change in Genotype Frequencies Over Generations

• Interpretation: As the frequency of the A allele increases, the frequency of the AA genotype increases, while aa decreases.

Mechanisms of Evolution

Natural Selection

Natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution. It acts on phenotypic variation, favoring individuals with higher fitness in a given environment.

• Directional selection: Favors one extreme phenotype, shifting the population mean.

• Disruptive selection: Favors both extremes over intermediate phenotypes.

• Stabilizing selection: Favors intermediate phenotypes, reducing variation.

• Balancing selection: Maintains genetic diversity, e.g., heterozygote advantage.

• Example: Sickle-cell allele in humans is maintained in populations where malaria is prevalent due to heterozygote advantage.

Genetic Drift

Genetic drift is the random fluctuation of allele frequencies in a population, especially significant in small populations. It can lead to the loss of genetic variation and fixation of alleles.

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

• Founder effect: When a few individuals start a new population, their allele frequencies may differ from the original population.

• Example: A population of plants reduced from 2000 to 50 individuals after a storm may have different allele frequencies than the original group.

Gene Flow

Gene flow is the movement of alleles between populations due to migration of individuals or gametes. It can introduce new alleles and increase genetic variation, or homogenize populations.

• Example: Migration of rabbits across a mountain range introduces new alleles to a previously isolated population.

Mini-Quiz Examples

• Calculating allele frequencies: In a cattle population with 36% red (RR), 16% white (WW), and 48% roan (RW), the frequency of the white allele (W) is calculated as follows:

  • Frequency of W = (2*number of WW + number of RW) / (2*total number of individuals)

  • Frequency of W = (2*0.16 + 0.48) / 2 = 0.4

• Hardy-Weinberg and disease carriers: If the frequency of cystic fibrosis (recessive, q2) is 0.04, then q = 0.2, p = 0.8. Carrier frequency (2pq) = 2*0.8*0.2 = 0.32 (32%).

• Identifying mechanisms: If two rabbit populations are reconnected and interbreed, gene flow is the primary mechanism of evolution.

• Type of selection: If intermediate-colored mice are predated more than light or dark, disruptive selection is occurring.

Additional info: Some quiz questions and examples were inferred and expanded for clarity and completeness.