Evolution—Descent with Modification (BSC 2011: Biological Diversity)

Evolution—Descent with Modification

Introduction

Evolution is a central concept in biology, describing how populations and species change over time. This topic 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 evolution by natural selection, 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.

• Both scientists presented the idea of natural selection in 1858.

Additional info: Darwin's work was influenced by Thomas Malthus's essay on population growth, which highlighted the struggle for resources.

Earlier Ideas

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

• Georges Cuvier: Studied vertebrate fossils and contributed to paleontology.

• Charles Lyell: Advocated for gradual geological change, influencing Darwin's thinking.

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.

• Analogous structure: Features in different species that have similar functions but evolved independently (not from a common ancestor).

• 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.

• 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 passed to offspring.

• Differential Reproductive Success: Individuals with advantageous traits are more likely to survive and reproduce.

Example: During a drought in the Galapagos, finches with deeper beaks survived better due to the availability of larger seeds, leading to a shift in beak depth in the population.

Sources of Genetic Variation

• Mutations: Random changes in DNA sequence (e.g., point mutations).

• Exon shuffling: Rearrangement of exons can create new genes.

• Sexual reproduction: Increases variation through recombination and independent assortment.

• Horizontal gene transfer: Movement of genetic material between organisms other than by descent.

Evidence for Evolution

1. Fossil Record

• Fossils are preserved remains or traces of past organisms, usually found in sedimentary rock layers (strata).

• The depth of a fossil can indicate its relative age.

• The fossil record shows gradual evolutionary change and transitional forms (intermediate between ancestral and descendant species).

• Example: Evolution of horses shows changes in size, foot anatomy, and tooth morphology over time.

2. Biogeography

• Study of the distribution of species and ecosystems in geographic space and through geological time.

• Isolated continents and islands often have many endemic species (found nowhere else).

• Similar species on distant continents suggest past physical connections (e.g., continental drift).

3. Direct Observations and Artificial Selection

• Natural selection can be observed directly in nature (e.g., changes in finch beak size).

• Artificial selection demonstrates how selective breeding can produce significant changes in organisms over relatively short periods.

4. Homologies

• Anatomical homology: Similar structures in different species due to common ancestry (e.g., forelimbs of humans, cats, whales, and bats).

• Developmental homology: Similarities in embryonic development among different species (e.g., presence of gill slits and tails in human embryos).

• Molecular homology: Similarities in DNA and protein sequences among different species; closely related species have more similar sequences.

5. Convergent Evolution

• Unrelated species evolve similar traits independently due to similar environmental pressures.

• Example: Wings of bats and birds, or streamlined bodies of dolphins and sharks.

Summary Table: Types of Evidence for Evolution

Type of Evidence	Description	Example
Fossil Record	Traces of past organisms showing gradual change	Horse evolution, transitional fossils
Biogeography	Geographic distribution of species	Endemic species on islands
Direct Observation	Observed evolutionary changes in real time	Finch beak size changes
Artificial Selection	Human-directed breeding for specific traits	Dog breeds, crop varieties
Homologies	Similarities due to shared ancestry	Vertebrate forelimbs, embryonic development
Convergent Evolution	Similar traits in unrelated lineages	Wings in bats and birds

Key Concepts and Theories

Major Theories of Darwinism (Ernst Mayr)

• Perpetual change: The world is constantly changing, and organisms change over time.

• Common descent: All organisms share a common ancestor.

• Multiplication of species: Species multiply by splitting and forming new species.

• Gradualism: Evolutionary change occurs gradually over long periods.

• Natural selection: The mechanism by which better-adapted individuals survive and reproduce.

Formulas and Equations

• Hardy-Weinberg Equation (for genetic equilibrium):

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

Where:

• $p$ = frequency of dominant allele

• $q$ = frequency of recessive allele

Conclusion

Evolution is supported by multiple lines of evidence and is driven by mechanisms such as natural selection, genetic variation, and adaptation. Understanding these concepts is fundamental to the study of biological diversity.