Lec: 7 Natural Selection and Evidence for Evolution

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Natural Selection and Evidence for Evolution

Introduction

This study guide covers the theory of natural selection, its subtleties, and the evidence supporting evolution. It includes Darwin's observations, examples of natural selection in action, and the role of biogeography in understanding evolutionary processes.

Theory of Natural Selection

Subtleties of Natural Selection

Heritable Variation: Evolution by natural selection requires variation in traits that are heritable, meaning they can be passed from parents to offspring.
Progressiveness: Natural selection is not inherently progressive; it does not always lead to greater complexity or improvement, but rather to better adaptation to the current environment.

Darwin’s Observations and Inferences (Ernst Mayr’s Summary)

Observation 1: All species have the capacity to increase in number geometrically if all offspring survive to reproduce.
Observation 2: Populations tend to remain stable in size.
Observation 3: Environmental resources are limited.
Inference 1: There is a struggle for existence; only a fraction of individuals survive to reproduce.
Observation 4: No two individuals in a population are exactly alike.
Observation 5: Some variation is heritable.
Inference 2: Survival depends partly on genetic makeup; individuals with traits best suited to the environment are more likely to survive and reproduce (natural selection).
Inference 3: Unequal survival and reproduction lead to gradual changes in the population (evolution).

Darwin’s Observations in the Galapagos

Island Species and Adaptation

Darwin observed that small, isolated populations on the Galapagos Islands developed distinct traits compared to mainland populations, providing evidence for evolution by natural selection.

Iguanas: Marine and land iguanas on the Galapagos differ from mainland iguanas, showing adaptation to different environments.

Marine iguanas, land iguanas, and mainland iguanas

Mockingbirds: Five different species, each unique to specific islands, differing from mainland species.

Different species of mockingbirds from the Galapagos and mainland

Finches: Thirteen species with different beak shapes adapted to different food sources, all similar to a mainland ancestor.

Galapagos finches with different beak shapes and a mainland grassquit

Tortoises: Shell shape varies from island to island, adapted to different food sources.

Galapagos tortoises with different shell shapes and island map

Influences on Darwin’s Theory

Artificial Selection

Humans breed plants and animals for desirable traits, demonstrating that selection can cause significant changes in populations over generations.

Artificial selection in dogs, all descended from the gray wolf

The Writings of T.R. Malthus

Malthus’s essay on population growth influenced Darwin’s understanding of the struggle for existence and the role of limited resources in natural selection.

Portrait of T.R. Malthus

Key Concepts in Natural Selection

Variation: Natural selection acts on existing variation; it does not create new traits. New variations arise from mutation, genetic recombination, and sexual reproduction.
Heritability: Only heritable (genetic) traits can be acted upon by natural selection. Acquired traits are generally not inherited, with some exceptions due to epigenetic changes.
Fitness: Defined as the ability to survive and reproduce relative to others in the population. Fitness is environment-dependent.
Population as the Unit of Evolution: Evolution occurs at the population level, not in individuals.
Non-progressiveness: Evolution does not have a predetermined direction or goal; it is shaped by environmental pressures.

Examples of Natural Selection in Action

Industrial Melanism: The increase in dark-colored (melanic) moths in polluted environments due to better camouflage from predators.
Drug Resistance in HIV: Mutant viruses resistant to drugs like 3TC survive and proliferate when the drug is present, illustrating selection for pre-existing variation.
Pesticide Resistance: Insect populations develop resistance to pesticides as individuals with resistance genes survive and reproduce.
Colouration in Guppies: Guppy coloration varies with predation pressure; brighter males are favored by females but are more visible to predators, leading to a balance shaped by natural selection.

Biogeography as Evidence for Evolution

Biogeography studies the distribution of species across geographic areas. Closely related species are often found in the same region, supporting the idea of common ancestry and adaptive radiation.

Example: The genetic similarity between the silversword and mountain dubautia in Hawaii, despite different habitats, is greater than between the silversword and the physically similar queen of the Andes in South America. This demonstrates that geographic proximity, not just physical similarity, reflects evolutionary relationships.

Summary Table: Key Evidence for Evolution

Type of Evidence	Example	Key Concept
Direct Observation	Industrial melanism, drug resistance	Natural selection acts on existing variation
Comparative Anatomy	Galapagos finches, tortoises	Adaptation to local environments
Biogeography	Silversword and dubautia in Hawaii	Geographic clustering of related species