The Evolution of Populations (Chapter 23, Campbell Biology)

Chapter 23: The Evolution of Populations

Introduction to Population Evolution

Evolution at the population level involves changes in the genetic makeup of populations over time. This chapter explores the mechanisms that drive these changes, focusing on how allele frequencies shift and the consequences for populations.

• Population: A group of individuals of the same species living in the same area and interbreeding.

• Evolution (in this context): A change in allele frequencies in a population over generations.

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

• Example: Medium ground finches on Daphne Major evolved larger beaks in response to a drought, as birds with larger beaks survived better and passed on their traits.

Mechanisms of Evolution

Genetic Variation

Genetic variation is the foundation of evolution, providing the raw material for natural selection and other evolutionary processes.

• Genetic Variation: Differences in genes or other DNA sequences among individuals.

• Phenotype: The observable traits of an organism, resulting from the interaction of its genotype and the environment.

• Sources of Genetic Variation:

  • Mutation (changes in DNA sequence)

  • Gene duplication

  • Sexual reproduction (recombination of alleles)

• Quantifying Genetic Variation:

  • Percentage of heterozygous loci in a population

  • Nucleotide variability (comparing DNA sequences between individuals)

• Neutral Variation: Genetic variation that does not provide a selective advantage or disadvantage, often due to redundancy in the genetic code.

• Example: Point mutations in noncoding regions usually result in neutral variation.

Formation of New Alleles

New alleles arise primarily through mutation, which is a change in the nucleotide sequence of DNA. Most mutations are neutral or harmful, but occasionally they can be beneficial.

• Mutation: A change in the DNA sequence, which can be caused by errors in DNA replication or exposure to radiation/chemicals.

• Point Mutation: A change in a single nucleotide; can have significant effects if it alters protein function.

• Heritability: Only mutations in cells that produce gametes are passed to offspring in multicellular organisms.

Sexual Reproduction and Genetic Variation

Sexual reproduction increases genetic variation by recombining existing alleles through several mechanisms:

• Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.

• Independent Assortment: Random distribution of chromosomes into gametes during meiosis.

• Fertilization: Random combination of gametes from two parents.

The Hardy-Weinberg Principle

Gene Pools and Allele Frequencies

The gene pool of a population consists of all copies of every type of allele at every locus in all members of the population. Allele frequencies can be calculated to determine the genetic structure of a population.

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

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

• Example Calculation: In a population of 500 wildflowers:

  • 320 red (CRCR), 160 pink (CRCW), 20 white (CWCW)

  • Genotype frequencies: CRCR = 0.64, CRCW = 0.32, CWCW = 0.04

  • Allele frequencies: CR = 0.8, CW = 0.2

The Hardy-Weinberg Equation

The Hardy-Weinberg equation predicts the expected genotype frequencies in a population that is not evolving.

• Equation: $p^2 + 2pq + q^2 = 1$ Where:

  • $p$ = frequency of one allele (e.g., CR)

  • $q$ = frequency of the other allele (e.g., CW)

  • $p^2$ = frequency of homozygous genotype (CRCR)

  • $2pq$ = frequency of heterozygous genotype (CRCW)

  • $q^2$ = frequency of homozygous genotype (CWCW)

• Allele Frequency Relationship: $p + q = 1$

• Use: To test whether a population is evolving at a particular locus.

Conditions for Hardy-Weinberg Equilibrium

For a population to remain in Hardy-Weinberg equilibrium (no evolution), five conditions must be met:

Condition	Consequence if Not Met
No mutations	Gene pool is modified if mutations occur or if entire genes are deleted/duplicated.
Random mating	Non-random mating (e.g., inbreeding) changes genotype frequencies.
No natural selection	Allele frequencies change if individuals with different genotypes have different survival or reproductive success.
Extremely large population size	Small populations experience genetic drift, causing allele frequencies to fluctuate by chance.
No gene flow	Movement of alleles into or out of populations alters allele frequencies.

Mechanisms That Alter Allele Frequencies

Natural Selection

Natural selection is the only mechanism that consistently causes adaptive evolution. It is based on differential survival and reproduction of individuals with certain heritable traits.

• Key Point: Individuals with traits better suited to the environment tend to leave more offspring.

• Example: Beak size in finches changes in response to food availability.

Genetic Drift

Genetic drift is the random fluctuation of allele frequencies in small populations due to chance events.

• Founder Effect: When a few individuals become isolated from a larger population, the new population's gene pool may differ from the original.

• Bottleneck Effect: A sudden reduction in population size (e.g., due to a natural disaster) can drastically alter allele frequencies.

• Consequences:

  • Significant in small populations

  • Can lead to loss of genetic variation

  • Can cause harmful alleles to become fixed

Gene Flow

Gene flow is the movement of alleles between populations, often through migration of individuals or gametes (e.g., pollen).

• Effects:

  • Reduces genetic differences between populations

  • Can introduce new alleles, increasing genetic variation

  • Can either increase or decrease a population's fitness

• Example: Migration of insecticide-resistant mosquitoes spreads resistance alleles to new populations.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Effect on Genetic Variation
Natural Selection	Favors alleles that increase fitness	Can increase or decrease variation, depending on selection type
Genetic Drift	Random changes in allele frequencies	Reduces variation, especially in small populations
Gene Flow	Movement of alleles between populations	Can increase or decrease variation

Additional info:

• Sexual selection, a form of natural selection, can lead to sexual dimorphism (differences in secondary sexual characteristics between sexes).

• Examples of sexual dimorphism include differences in size, color, ornamentation, and behavior between males and females.