Evolutionary Theory: Historical Context and Evidence (Chapter 22 Study Notes)

Evolutionary Theory: Historical Context and Evidence

Intellectual Context of Darwin

The development of evolutionary theory was influenced by several key thinkers and discoveries prior to Charles Darwin. Understanding their contributions provides context for Darwin's work.

• Jean-Baptiste Lamarck (1809): Proposed that organisms change over time through the inheritance of acquired characteristics. He suggested that traits developed during an organism's lifetime could be passed to offspring.

• Thomas Malthus (1798): Published "Essay on the Principle of Population," arguing that populations grow faster than resources, leading to competition and survival pressures.

• James Hutton (1795): Introduced the principle of gradualism, stating that geological changes occur slowly over long periods.

• Georges Cuvier (1812): Documented extensive studies of vertebrate fossils, establishing extinction as a scientific fact.

• Charles Lyell (1830): Published "Principles of Geology," advocating for uniformitarianism—the idea that the same geological processes operate today as in the past.

• Alfred Russel Wallace (1858): Independently conceived the hypothesis of natural selection while studying species in the Malay Archipelago.

• Charles Darwin (1809–1882): Traveled on HMS Beagle (1831–1836), observed diverse species, and developed the theory of evolution by natural selection. Published "On the Origin of Species" in 1859.

Example: Darwin's observations of finches and iguanas in the Galápagos Islands were crucial in formulating his ideas about adaptation and speciation.

Pre-Darwinian Theories of Evolution

Before Darwin, several theories attempted to explain how organisms change over time.

• Lamarckian Theory: Suggested that use or disuse of traits leads to modifications that are inherited by offspring.

• Cuvier's Catastrophism: Proposed that sudden events (catastrophes) caused extinctions and shaped life on Earth.

• Gradualism (Hutton and Lyell): Emphasized slow, continuous processes shaping Earth's geology and life.

Additional info: Modern genetics has shown that acquired traits are generally not inherited, refuting Lamarck's mechanism.

Darwin's Theory of Evolution by Natural Selection

Darwin and Wallace identified natural selection as the primary mechanism driving evolutionary change and diversification.

• Descent with Modification: All species are related through common ancestry and change over time.

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

Key Observations:

• Individuals in a population vary in their heritable characteristics.

• Organisms produce more offspring than the environment can support.

Key Inferences:

• Individuals well-suited to their environment tend to leave more offspring.

• Favorable traits accumulate in the population over time.

Equation:

$\text{Evolution by natural selection requires: Variability + Heritability + Differential Reproductive Success}$

Types of Selection

Selection can occur in various forms:

• Natural Selection: Driven by environmental pressures.

• Artificial Selection: Humans select for desired traits (e.g., breeding plants or animals).

• Other Forms: Sexual selection, stabilizing selection, directional selection, etc.

Example: Artificial selection in wild mustard has produced diverse vegetables such as cabbage, Brussels sprouts, and kale by selecting for different plant parts.

Evidence Supporting Evolution

Multiple lines of evidence support the theory of evolution:

• Direct Observation: Case studies and artificial selection demonstrate evolutionary change in real time.

• Homology: Similarities in anatomy, embryology, and biochemistry among related species.

• Fossil Record: Provides chronological evidence of species change and transitional forms.

• Biogeography: Study of geographic distribution of species, revealing patterns consistent with evolutionary history.

Direct Observation

• Case Study: Soapberry bugs in Florida have evolved beak lengths in response to changes in available food sources (balloon vine vs. goldenrain tree).

• Artificial Selection: Selective breeding in plants and animals demonstrates rapid evolutionary change.

Homology

• Comparative Anatomy: Homologous structures (e.g., mammalian forelimbs) indicate common ancestry.

• Comparative Embryology: Similar developmental stages (e.g., pharyngeal arches, post-anal tail) among vertebrates.

• Comparative Biochemistry: Molecular similarities (e.g., DNA, proteins) among species.

• Vestigial Structures: Remnants of features that served functions in ancestors but are no longer functional (e.g., human appendix).

Fossil Record

• Transitional fossils (e.g., ankle bones in cetaceans and ungulates) show evolutionary links between major groups.

• Fossils document the transition from land to sea in mammals (e.g., whales).

Biogeography

• Species distribution patterns reflect historical events (e.g., continental drift, Pangaea).

• Closely related species found on widely separated continents (e.g., Galaxiidae fish in South America and Australia).

Summary Table: Lines of Evidence for Evolution

Type of Evidence	Description	Example
Direct Observation	Observed evolutionary changes in populations	Soapberry bug beak length adaptation
Homology	Similar structures due to common ancestry	Mammalian forelimbs, embryonic development
Fossil Record	Chronological evidence of species change	Transitional fossils (whale evolution)
Biogeography	Geographic distribution of species	Galaxiidae fish on separate continents

Tree Thinking

Modern evolutionary biology uses phylogenetic trees to represent relationships among species, based on genetic, anatomical, and fossil evidence.

• Phylogenetic Tree: Diagram showing evolutionary relationships.

• Common Ancestor: Point where lineages diverge.

Additional info: Molecular data (e.g., DNA sequences) are increasingly used to resolve evolutionary relationships.