Microevolution: Evolution of Populations

Microevolution: Evolution of Populations

Introduction to Microevolution

Microevolution refers to changes in allele frequencies within a population over time. These small-scale evolutionary changes are the foundation for understanding how populations adapt and evolve.

• Microevolution is the change in genetic makeup of a population from one generation to the next.

• It is measured by changes in allele frequencies within a population.

• Microevolution is distinct from macroevolution, which involves larger evolutionary changes such as speciation.

Goals of Studying Microevolution

• Calculate allele frequencies in a population.

• Determine whether a population is evolving by comparing allele frequencies over time.

• Compare actual population genetics to expectations based on the Hardy-Weinberg equilibrium.

• Differentiate among mechanisms of evolution (e.g., natural selection, genetic drift, gene flow).

• Identify how natural selection can lead to adaptive evolution.

Key Concepts in Evolution

• Evolution is defined as descent with modification.

• It is specifically measured as a change in allele frequencies over time.

• To determine if a population is evolving, scientists assess whether allele frequencies change from one generation to the next.

Genetic Variation and Evolution

Genetic variation within a population is essential for evolution to occur. Without variation, populations cannot adapt to changing environments.

• Genetic variation arises from mutations, gene shuffling during sexual reproduction, and other sources.

• Variation provides the raw material for natural selection and other evolutionary mechanisms.

• Example: The image of horses with different coat colors illustrates genetic variation within a population.

Phenotypes and Genotypes: Flower Color Example

Phenotypes are observable traits, while genotypes are the genetic makeup underlying those traits. The example of flower color demonstrates how different genotypes produce different phenotypes.

• Phenotype: The physical appearance (e.g., red, white, or pink flowers).

• Genotype: The genetic constitution (e.g., CRCR, CWCW, CRCW).

• In the example, three genotypes correspond to three phenotypes:

  • CRCR: Red flowers

  • CWCW: White flowers

  • CRCW: Pink flowers

Calculating Allele and Genotype Frequencies

Allele and genotype frequencies are fundamental for understanding population genetics. They allow scientists to quantify genetic variation and track evolutionary changes.

• Given a population of N = 90 individuals:

  • 30 individuals with genotype CRCR

  • 30 individuals with genotype CWCW

  • 30 individuals with genotype CRCW

• To calculate allele frequencies:

  • Count the total number of alleles: Each individual has 2 alleles, so total alleles = $2 \times 90 = 180$.

  • Count the number of CR alleles:

    • Each CRCR individual contributes 2 CR alleles: $30 \times 2 = 60$

    • Each CRCW individual contributes 1 CR allele: $30 \times 1 = 30$

    • Total CR alleles: $60 + 30 = 90$

  • Frequency of CR allele: $\frac{90}{180} = 0.5$

  • Frequency of CW allele: $\frac{90}{180} = 0.5$

Hardy-Weinberg Equilibrium (Additional info)

The Hardy-Weinberg equilibrium provides a mathematical model to predict genotype frequencies in a non-evolving population. Deviations from this equilibrium indicate that evolution is occurring.

• Genotype frequencies can be predicted using the equation:

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

• Where p and q are the frequencies of two alleles.

• p^2: Frequency of homozygous dominant genotype

• 2pq: Frequency of heterozygous genotype

• q^2: Frequency of homozygous recessive genotype

Additional info: The Hardy-Weinberg principle assumes no mutation, migration, selection, or genetic drift, and random mating.

Summary Table: Genotypes and Phenotypes

Phenotype	Genotype	Number of Individuals
Red	CRCR	30
White	CWCW	30
Pink	CRCW	30