How Populations Evolve: Mechanisms and Evidence of Evolution

Chapter 13: How Populations Evolve

Introduction to Evolution

The theory of evolution explains the diversity of life by proposing that living species are descendants of ancestral species that were different from present-day organisms. Evolution is an ongoing process, with the environment playing a significant role in shaping species over time.

• Evolution: The process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.

• Descent with modification: Darwin's phrase for evolution, indicating that all organisms are related through descent from an ancestor and have accumulated adaptations over time.

Darwin’s Theory of Evolution

Darwin’s Voyage and the Origin of Species

Charles Darwin’s observations during his voyage on the HMS Beagle led him to develop the theory of evolution by natural selection. His work, On the Origin of Species, challenged the prevailing view of a young, unchanging Earth.

• Common ancestry: All life is connected through common descent.

• Adaptation: Descendants accumulate modifications suited to their environments over time.

• Theory: In science, a theory is a broad explanatory idea supported by a large body of evidence.

Evidence for Evolution: The Fossil Record

Fossils provide strong evidence for evolution by documenting differences between past and present organisms and revealing that many species have become extinct. The fossil record shows the historical sequence in which organisms have evolved.

• Fossil: Imprints or remains of organisms from the past.

• Transitional fossils: Fossils that show intermediate states between ancestral forms and descendants.

Transitional Forms and Whale Evolution

Transitional fossils link extinct species with living species, illuminating evolutionary origins. For example, fossils of Pakicetus and Rodhocetus show that whales evolved from land-dwelling, cloven-hoofed ancestors, as supported by both fossil and molecular evidence.

Homology: Evidence from Structure and Development

Homology refers to similarity resulting from common ancestry. Related species may have similar structures that serve different functions. Homologies can be anatomical, molecular, or developmental. Vestigial structures are remnants of features that served important functions in ancestors.

• Homology: Similarity due to shared ancestry.

• Vestigial structures: Features that are reduced or functionless but were functional in ancestors.

Evolutionary Trees

Biologists use evolutionary trees to represent patterns of descent. Homologous structures and molecular data help determine the branching sequence of these trees, illustrating relationships among species.

Mechanisms of Evolution

Natural Selection

Darwin proposed natural selection as the mechanism of evolution. Artificial selection, such as selective breeding in agriculture, demonstrates how selection can bring about significant change. Natural selection acts on heritable traits within populations, not individuals, and is not goal-directed.

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

• Natural selection: The process by which individuals with advantageous traits survive and reproduce more successfully.

Observing Natural Selection

Natural selection can be observed in real time, such as the evolution of pesticide resistance in insects. Natural selection edits existing variation and is contingent on current environmental conditions.

Sources of Genetic Variation

Genetic variation arises from mutations and sexual reproduction. In sexually reproducing organisms, most variation results from the unique combination of alleles through crossing over, independent assortment, and random fertilization.

• Mutation: The ultimate source of genetic variation.

• Sexual reproduction: Shuffles existing alleles to produce new combinations.

Population Genetics and Microevolution

Microevolution is a change in allele frequencies within a population’s gene pool. The Hardy-Weinberg equilibrium describes a non-evolving population, where allele and genotype frequencies remain constant if certain conditions are met.

• Gene pool: All copies of every allele in a population.

• Hardy-Weinberg equilibrium: Describes a population that is not evolving.

Hardy-Weinberg Equation:

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

where $p$ and $q$ are the frequencies of two alleles in the population.

Mechanisms of Microevolution

Three main causes of evolutionary change are:

1. Natural selection

2. Genetic drift (including the bottleneck and founder effects)

3. Gene flow

• Bottleneck effect: A sharp reduction in population size leads to loss of genetic diversity.

• Founder effect: A few individuals colonize a new habitat, leading to a new gene pool.

Adaptive Evolution and Fitness

Only natural selection consistently leads to adaptive evolution, increasing the frequency of traits that enhance survival and reproduction. Relative fitness measures an individual’s contribution to the next generation compared to others.

Modes of Natural Selection

Natural selection can alter phenotypic variation in three ways:

• Stabilizing selection: Favors intermediate phenotypes.

• Directional selection: Favors one extreme phenotype.

• Disruptive selection: Favors both extreme phenotypes over intermediates.

Sexual Selection

Sexual selection is a form of natural selection where individuals with certain traits are more likely to obtain mates. It can lead to pronounced differences between males and females (sexual dimorphism). Intrasexual selection involves competition within one sex, while intersexual selection (mate choice) involves preferences by one sex for certain traits in mates.

Evolution of Drug-Resistant Microorganisms

Antibiotic resistance evolves through natural selection. Overuse and misuse of antibiotics accelerate the spread of resistant strains, posing a significant public health threat.

Preserving Genetic Variation

Diploidy and balancing selection help maintain genetic diversity. Heterozygote advantage occurs when heterozygous individuals have higher fitness than either homozygote, maintaining multiple alleles in the population.

Limits of Natural Selection

Natural selection cannot produce perfect organisms due to several constraints:

• Selection acts only on existing variation.

• Evolution is limited by historical constraints.

• Adaptations are often compromises.

• Chance, natural selection, and the environment interact unpredictably.

Summary Table: Modes of Selection

Mode of Selection	Effect on Phenotype	Example
Stabilizing	Favors intermediate variants	Human birth weight
Directional	Favors one extreme	Antibiotic resistance in bacteria
Disruptive	Favors both extremes	Beak size in finches

Key Terms and Concepts

• Evolution

• Natural selection

• Mutation

• Gene pool

• Hardy-Weinberg equilibrium

• Genetic drift

• Gene flow

• Relative fitness

• Sexual selection

• Balancing selection