How Populations Evolve: Key Concepts in Evolutionary Biology

CH-13: How Populations Evolve

Introduction to Evolutionary Biology

Evolution is a central concept in biology, explaining how populations change over time and adapt to their environments. The study of evolution encompasses the mechanisms that drive genetic change and the evidence supporting these processes.

• Evolution is the process by which populations of organisms change genetically over generations.

• Real-world example: In the 1950s, the World Health Organization (WHO) attempted to eradicate malaria, but resistance evolved in mosquito populations, demonstrating evolution in action.

• Drug resistance in malaria and other diseases is a direct result of evolutionary processes.

Main Topics for Discussion

• Darwin's Theory of Evolution

• The Evolution of Populations

• Mechanisms of Microevolution

Environmental Effects on Evolution

Environmental factors can influence evolutionary outcomes, such as determining the sex of certain species. For example, temperature changes are affecting the sex ratios of Costa Rican crocodiles.

• Environmental sex determination is a phenomenon where environmental conditions, such as temperature, influence the sex of offspring in some reptiles.

• Link: Changing sex of Costa Rican crocodiles

Darwin's Theory of Evolution

Historical Context and Darwin's Voyage

Charles Darwin's observations during his voyage on the HMS Beagle were foundational to the development of evolutionary theory. His studies of diverse species and their adaptations led to the formulation of natural selection.

• Descent with modification: Darwin proposed that species change over time and share common ancestors.

• His book, On the Origin of Species, introduced the concept of evolution by natural selection.

Key Concepts of Darwin's Theory

• Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

• Evolution is a theory supported by extensive evidence, not just a hypothesis.

• Populations, not individuals, evolve over time.

• Natural selection acts only on heritable traits.

• Evolution is not goal-directed; it does not produce perfectly adapted organisms.

Examples and Applications

• Drug resistance in mosquitoes and bacteria is a modern example of natural selection.

• Environmental changes, such as climate or habitat alteration, can drive evolutionary change.

Summary Table: Key Differences Between Natural Selection and Artificial Selection

Natural Selection	Artificial Selection
Occurs naturally in populations	Directed by humans (e.g., breeding)
Driven by environmental pressures	Driven by desired traits
Results in adaptation to environment	Results in traits beneficial to humans

The Evolution of Populations

Genetic Variation and Its Sources

Genetic variation is essential for evolution. It arises from mutations, sexual reproduction, and recombination.

• Mutation: Random changes in DNA that introduce new genetic variants.

• Sexual reproduction: Shuffles alleles through meiosis and fertilization, increasing genetic diversity.

• Gene pool: The total collection of genes in a population.

Microevolution

Microevolution refers to changes in allele frequencies within a population over time.

• Occurs through mechanisms such as natural selection, genetic drift, gene flow, and mutation.

• Example: Evolution of pesticide resistance in insects.

Mechanisms of Microevolution

Natural Selection

Natural selection is the only mechanism that consistently leads to adaptive evolution, improving the fit between organisms and their environment.

• Relative fitness: The contribution an individual makes to the gene pool of the next generation compared to others.

• Favorable traits increase in frequency over time.

Sexual Selection

Sexual selection is a form of natural selection where individuals with certain traits are more likely to obtain mates.

• Intrasexual selection: Competition among individuals of the same sex for mates.

• Intersexual selection: Individuals of one sex (usually females) choose mates based on specific traits.

Balancing Selection

Balancing selection maintains genetic diversity in a population by preserving multiple alleles.

• Heterozygote advantage: Heterozygous individuals have higher fitness than homozygotes, maintaining genetic variation.

• Example: Sickle cell trait in humans provides resistance to malaria.

Limitations of Natural Selection

• Selection can act only on existing variations; advantageous alleles do not arise on demand.

• Evolution is limited by historical constraints; existing structures are adapted for new functions.

• Adaptations are often compromises; traits may serve multiple functions.

• Chance, natural selection, and environmental changes interact unpredictably.

Summary: Key Learning Objectives

• Explain how Darwin's voyage influenced his thinking about evolution.

• Describe why evolution is regarded as a scientific theory.

• Explain how fossils form and why the fossil record is incomplete.

• Discuss how homologies, fossils, and molecular biology support evolution.

• Describe how evolutionary trees represent ancestral relationships.

• Summarize Darwin's observations and inferences about natural selection.

• Explain why individuals cannot evolve and why evolution does not produce perfectly adapted organisms.

• Describe how mutation and sexual reproduction produce genetic variation.