Evolution—Descent with Modification

Introduction

Evolution is a central concept in biology, describing how populations and species change over time. This section explores the mechanisms, evidence, and historical context of evolutionary theory, focusing on the contributions of Charles Darwin and Alfred Russel Wallace.

Historical Context and Contributors

Darwin and Wallace

• Charles Darwin (1809–1882): British naturalist who developed the theory of natural selection as a mechanism for evolution. His ideas were influenced by geology, economics, and his voyage on the HMS Beagle.

• Alfred Russel Wallace: Independently conceived the mechanism of natural selection while working in the Amazon and Southeast Asia. He corresponded with Darwin, leading to the joint presentation of their ideas in 1858.

• Both scientists recognized that species change over time and that natural selection drives this process.

Additional info: Darwin's book "On the Origin of Species" (1859) laid the foundation for modern evolutionary biology.

Earlier Ideas

• Jean-Baptiste Lamarck: Proposed the inheritance of acquired characteristics, suggesting that organisms could change in response to their environment and pass these changes to offspring.

• Thomas Malthus: His essay on population growth influenced Darwin's thinking about competition and survival.

Key Terms and Definitions

• Descent with modification: The process by which species change over time, giving rise to new species while sharing a common ancestor.

• Homology: Similarity in characteristics resulting from shared ancestry.

• Homologous structure: Anatomical features in different species that originated from a common ancestor (e.g., vertebrate forelimbs).

• Analogous structure: Features in different species that have similar functions but evolved independently (e.g., wings of bats and insects).

• Convergent evolution: The independent evolution of similar traits in species of different lineages due to similar environmental pressures.

• Vestigial structure: Remnants of structures that served important functions in ancestors but are reduced or unused in descendants (e.g., whale hind limb bones).

• Evolutionary tree: A diagram showing evolutionary relationships among species.

• Biogeography: The study of the geographic distribution of species.

• Artificial selection: The process by which humans breed plants and animals for desired traits.

• Adaptation: A heritable trait that increases an organism's fitness in a particular environment.

Mechanisms of Evolution

Biological Evolution

• Definition: Heritable change in one or more characteristics of a population or species from one generation to the next.

• Microevolution: Small-scale changes within a population, such as changes in allele frequency.

• Macroevolution: Large-scale changes that result in the formation of new species or groups of species.

Natural Selection

Natural selection is the primary mechanism of evolution, as described by Darwin and Wallace.

• Variation: Individuals in a population vary in their traits.

• Heredity: Some of these traits are heritable and can be passed to offspring.

• Differential Reproductive Success: Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

Example: During a drought in the Galapagos, finches with deeper beaks survived better because they could eat larger seeds, leading to an increase in average beak depth in the population.

Adaptive Radiation

• The rapid evolution of multiple species from a common ancestor, often when new ecological niches become available.

• Example: Darwin's finches on the Galapagos Islands evolved different beak shapes to exploit different food sources.

Evidence for Evolution

Five Lines of Evidence

1. Fossil Record: Fossils provide chronological evidence of past life and evolutionary transitions. Transitional forms show intermediate characteristics between ancestral and descendant groups.

2. Biogeography: The geographic distribution of species supports evolution. Isolated continents and islands often have unique (endemic) species, and similar species on distant continents suggest past connections.

3. Convergent Evolution: Unrelated species evolve similar traits due to similar environmental pressures (e.g., dolphins and sharks both have streamlined bodies).

4. Direct Observations: Evolution can be observed directly, such as changes in beak size in finches or antibiotic resistance in bacteria. Artificial selection by humans is another form of direct observation.

5. Homologies: Similarities among organisms due to shared ancestry, including anatomical, developmental, and molecular homologies.

Types of Homology

• Anatomical Homology: Similar body structures among different species (e.g., vertebrate limb bones).

• Developmental Homology: Similarities in embryonic development (e.g., presence of gill slits in vertebrate embryos).

• Molecular Homology: Similarities in DNA, RNA, or protein sequences among different species.

Sources of Genetic Variation

• Mutations: Random changes in DNA sequence, such as point mutations, can introduce new genetic variation.

• Exon Shuffling: Rearrangement of exons can create new genes or gene variants.

• Sexual Reproduction: Increases genetic variation through recombination and independent assortment of alleles.

• Horizontal Gene Transfer: Movement of genetic material between different species, especially common in bacteria.

Table: Comparison of Homologous and Analogous Structures

Summary

• Evolution explains the diversity of life through descent with modification.

• Natural selection is a key mechanism, requiring variation, heredity, and differential reproductive success.

• Multiple lines of evidence support evolutionary theory, including fossils, biogeography, homologies, and direct observations.

• Genetic variation arises from mutations, recombination, and gene transfer, providing the raw material for evolution.