GENERAL INFORMATION:
Chapter 23: The Evolution of Populations
I. Introduction to Microevolution
● Definition: Microevolution refers to changes in allele frequencies within a population
over successive generations.
● Key Point: While natural selection acts on individuals, it is populations that evolve over
time.
II. Sources of Genetic Variation
● A. Mutations
○ Point Mutations: Changes in a single nucleotide base in DNA.
■ Effects:
■ Often neutral due to redundancy in the genetic code.
■ Can be harmful or, occasionally, beneficial.
○ Chromosomal Mutations: Alterations that delete, duplicate, or rearrange
chromosome segments.
■ Impact:
■ Typically harmful, but gene duplications can lead to new functions
over time.
● B. Sexual Reproduction
○ Mechanisms Promoting Variation:
■ Crossing Over: Exchange of genetic material between homologous
chromosomes during meiosis.
■ Independent Assortment: Random distribution of maternal and paternal
chromosomes into gametes.
■ Fertilization: Combining of genetic material from two parents, increasing
variability.
III. The Hardy-Weinberg Principle
● A. Concept
○ Describes a non-evolving population where allele and genotype frequencies
remain constant.
, ● B. Conditions for Equilibrium
○ No mutations.
○ Random mating.
○ No natural selection.
○ Extremely large population size.
○ No gene flow.
● C. Mathematical Representation
○ Allele Frequencies: p + q = 1
■ p = frequency of dominant allele
■ q = frequency of recessive allele
○ Genotype Frequencies: p² + 2pq + q² = 1
■ p² = frequency of homozygous dominant
■ 2pq = frequency of heterozygous
■ q² = frequency of homozygous recessive
IV. Mechanisms of Microevolution
● A. Natural Selection
○ Process: Differential reproductive success leads to certain alleles being passed
to the next generation in greater proportions.
○ Types:
■ Directional Selection: Favors individuals at one end of the phenotypic
range.
■ Disruptive Selection: Favors individuals at both extremes of the
phenotypic range.
■ Stabilizing Selection: Favors intermediate variants and acts against
extreme phenotypes.
● B. Genetic Drift
○ Definition: Random fluctuations in allele frequencies, especially in small
populations.
○ Examples:
■ Bottleneck Effect: A sudden reduction in population size due to
environmental events, leading to a loss of genetic diversity.
■ Founder Effect: A few individuals become isolated from a larger
population, establishing a new population with a different allele frequency.
● C. Gene Flow
○ Definition: Movement of alleles between populations through migration of
individuals or gametes.
○ Impact:
■ Can introduce new alleles into a population.
■ May reduce genetic differences between populations over time.
Chapter 23: The Evolution of Populations
I. Introduction to Microevolution
● Definition: Microevolution refers to changes in allele frequencies within a population
over successive generations.
● Key Point: While natural selection acts on individuals, it is populations that evolve over
time.
II. Sources of Genetic Variation
● A. Mutations
○ Point Mutations: Changes in a single nucleotide base in DNA.
■ Effects:
■ Often neutral due to redundancy in the genetic code.
■ Can be harmful or, occasionally, beneficial.
○ Chromosomal Mutations: Alterations that delete, duplicate, or rearrange
chromosome segments.
■ Impact:
■ Typically harmful, but gene duplications can lead to new functions
over time.
● B. Sexual Reproduction
○ Mechanisms Promoting Variation:
■ Crossing Over: Exchange of genetic material between homologous
chromosomes during meiosis.
■ Independent Assortment: Random distribution of maternal and paternal
chromosomes into gametes.
■ Fertilization: Combining of genetic material from two parents, increasing
variability.
III. The Hardy-Weinberg Principle
● A. Concept
○ Describes a non-evolving population where allele and genotype frequencies
remain constant.
, ● B. Conditions for Equilibrium
○ No mutations.
○ Random mating.
○ No natural selection.
○ Extremely large population size.
○ No gene flow.
● C. Mathematical Representation
○ Allele Frequencies: p + q = 1
■ p = frequency of dominant allele
■ q = frequency of recessive allele
○ Genotype Frequencies: p² + 2pq + q² = 1
■ p² = frequency of homozygous dominant
■ 2pq = frequency of heterozygous
■ q² = frequency of homozygous recessive
IV. Mechanisms of Microevolution
● A. Natural Selection
○ Process: Differential reproductive success leads to certain alleles being passed
to the next generation in greater proportions.
○ Types:
■ Directional Selection: Favors individuals at one end of the phenotypic
range.
■ Disruptive Selection: Favors individuals at both extremes of the
phenotypic range.
■ Stabilizing Selection: Favors intermediate variants and acts against
extreme phenotypes.
● B. Genetic Drift
○ Definition: Random fluctuations in allele frequencies, especially in small
populations.
○ Examples:
■ Bottleneck Effect: A sudden reduction in population size due to
environmental events, leading to a loss of genetic diversity.
■ Founder Effect: A few individuals become isolated from a larger
population, establishing a new population with a different allele frequency.
● C. Gene Flow
○ Definition: Movement of alleles between populations through migration of
individuals or gametes.
○ Impact:
■ Can introduce new alleles into a population.
■ May reduce genetic differences between populations over time.
