Ch. 21 Evolution of Populations: Mechanisms and Patterns

Chapter 21: The Evolution of Populations

Overview: The Smallest Unit of Evolution

Evolution is a fundamental concept in biology, describing how populations—not individual organisms—change genetically over time. A common misconception is that individuals evolve during their lifetimes, but in reality, it is populations that evolve as allele frequencies shift from one generation to the next.

• Natural selection acts on individuals, but only populations evolve.

• Genetic variation within a population is essential for evolution to occur.

Genetic Variation: The Foundation of Evolution

Genetic variation refers to differences in DNA sequences among individuals, which lead to variation in traits. This variation is the raw material for evolution.

• Gene: A segment of DNA that influences an organism’s traits (e.g., flower color gene).

• Allele: Different versions of a gene (e.g., white flower allele, red flower allele).

• Homozygous: Both alleles are the same (e.g., two red alleles).

• Heterozygous: Alleles are different (e.g., one red, one white allele).

• Genotype: The genetic makeup of an individual (the alleles they possess).

• Phenotype: The physical manifestation of the genotype (e.g., flower color).

Genetic variation can affect not only physical traits but also biochemical and behavioral traits.

Sources of Genetic Variation

• Mutations: Changes in DNA sequence during replication can create new alleles.

• Gene rearrangement: Errors during DNA replication can lead to new gene combinations.

• Sexual reproduction: Shuffles existing genes into new combinations, increasing variation.

Allele Frequencies and Population Evolution

Changes in allele frequency within a population are used to test whether evolution is occurring. The gene pool is the collection of all alleles in a population.

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

• If a population is not evolving, allele and genotype frequencies remain constant (Hardy-Weinberg equilibrium).

Mechanisms That Alter Allele Frequencies

There are several mechanisms by which allele frequencies can change in a population, leading to evolution.

• Natural selection: Alleles that confer beneficial traits increase in frequency.

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Gene flow: Movement of alleles between populations through migration.

• Non-random mating: Mating preferences can alter genotype frequencies.

Table: Mechanisms Affecting Allele Frequencies

Mechanism	Description	Effect on Population
Natural Selection	Alleles for beneficial traits increase	Adaptive evolution
Genetic Drift	Random changes, especially in small populations	Loss of genetic variation, fixation of alleles
Gene Flow	Exchange of alleles between populations	Increases genetic similarity between populations
Non-random Mating	Preference for certain mates	Changes genotype frequencies

Genetic Drift: Founder Effect and Bottleneck Effect

Genetic drift is a mechanism of evolution that has a stronger effect in small populations. It can lead to significant changes in allele frequencies due to random events.

• Founder effect: A few individuals start a new population, leading to reduced genetic variation.

• Bottleneck effect: A sudden reduction in population size (e.g., due to disaster) drastically reduces genetic diversity.

Table: Bottleneck Effect Examples

Location	Population Before	Population After	Genetic Diversity
Illinois Prairie Chickens	Thousands	~50	Reduced
Cheetahs	Large	Very small	Very low
Additional info: Other species may also experience bottlenecks due to human activity or natural disasters.

Natural Selection and Adaptive Evolution

Natural selection is the only mechanism that consistently leads to adaptive evolution, where traits that enhance survival and reproduction become more common.

• Natural selection can cause adaptive evolution in three ways:

  • Directional selection: Favors one extreme phenotype.

  • Disruptive selection: Favors both extreme phenotypes over intermediate forms.

  • Stabilizing selection: Favors intermediate phenotypes, reducing variation.

Sexual Selection and Sexual Dimorphism

Sexual selection is a form of natural selection where certain traits increase an individual's chances of mating.

• Intrasexual selection: Competition among individuals of the same sex (usually males).

• Intersexual selection: Mate choice, often by females.

• Sexual dimorphism: Differences in appearance between males and females of a species.

Preservation of Genetic Variation

Genetic variation is maintained in populations through several mechanisms, allowing populations to adapt to changing environments.

• Heterozygote advantage: Heterozygous individuals have higher fitness than either homozygote (e.g., sickle cell trait and malaria resistance).

• Frequency-dependent selection: The fitness of a phenotype depends on how common it is in the population.

• Recessive alleles: Can persist in heterozygotes, even if harmful in homozygotes.

Limits of Natural Selection

Natural selection cannot produce perfect organisms due to several constraints:

• Can only act on existing variation.

• Evolution is limited by historical constraints (e.g., ancestral body plans).

• Adaptations are often compromises between competing demands.

• Chance, natural selection, and the environment interact in complex ways.