Foundations of Evolutionary Thought
Early Ideas on Species Change
Pierre-Louis Moreau de Maupertuis (1698-1759) suggested that the multiplication of species resulted from accidental recombination of particles from both parents, leading to offspring with varied traits from both parents.
Georges-Louis Leclerc, Comte de Buffon (1707-1788) proposed that animals had not been created as they appeared; rather, modern forms modified from common ancestors. Species change over time.
Jean-Baptiste Lamarck (1744-1829)
Lamarck believed animals possessed an internal force or desire to change. He proposed two main ideas:
- Law of Use and Disuse: Useful body parts, like muscles, became larger and stronger, while unused parts would get smaller and eventually disappear.
- Law of Inheritance of Acquired Characteristics: Organisms could pass on traits acquired during their lifetime to their offspring.
Lamarck saw this as a mechanism for evolution, asserting that species change over time and the environment is a major factor for change. For example, a giraffe develops a longer neck as a result of continually reaching for higher leaves.
Darwin, Wallace, and Their Influences
Alfred Russel Wallace (1823-1913)
Co-developed the theory of evolution by natural selection independently of Darwin.
Charles Lyell (1797-1875)
A geologist who believed the Earth was very old and had changed gradually over time. This gave Darwin the idea that populations of organisms could also change over long periods.
Thomas Malthus (1766-1834)
An economist who observed that populations tend to grow faster than resources (more babies being born than people dying). His Malthusian Doctrine proposed that only factors like war, famine, and disease could balance the population. This influenced Darwin’s concept of competition.
Charles Darwin (1809-1882)
During his five-year voyage on the HMS Beagle, visiting South America and the South Seas (including the Galapagos Islands), Darwin gathered evidence to formulate his Theory of Natural Selection.
Natural Selection: Mechanism for Evolution
Natural selection, often summarized as Survival of the Fittest, explains how evolution occurs:
- Individuals within a population possess inherited traits that help them survive and reproduce in their specific environment.
- These beneficial traits are more likely to be passed down successfully to the next generation.
- Over time, organisms become better adapted to their environment.
Adaptations
Adaptations are inherited traits that enhance an organism’s ability to survive and reproduce. Examples include:
- Structural: Different beak types in Galapagos finches; a woodpecker’s hammering beak.
- Physiological: Llamas’ specialized hemoglobin for high altitudes; an octopus changing color for camouflage.
- Behavioral: Migration; hibernation.
- Biochemical: Chemicals preventing ice formation in the bodies of Arctic fish.
Patterns of Natural Selection
- Stabilizing Selection: Favors the average form of a trait, providing the greatest fitness (e.g., intermediate lizard tail length might be optimal – too short affects balance, too long attracts predators).
- Directional Selection: Favors an extreme form of a trait, providing the greatest fitness (e.g., longer tongues in anteaters allow reaching termites deeper in mounds).
- Disruptive Selection: Favors both extreme forms of a trait over the intermediate form, providing fitness advantages in different niches (e.g., limpets in an environment with black and white rocks might favor black and white shells over grey).
Evidence Supporting Evolutionary Theory
Divergent Evolution and Common Descent
Divergent Evolution occurs when one species gives rise to multiple new species. The evolutionary history (phylogeny) of organisms can often be traced back to a single ancestor, supporting the concept of common descent.
Embryological Evidence
Similarities observed during the earliest stages of development (ontogeny) across different species suggest shared ancestry.
Anatomical Evidence
- Homologous Structures: Similar underlying body structures in different organisms, potentially used for different functions, indicating a common ancestor (e.g., the forelimbs of humans, cats, dogs, and whales).
- Analogous Structures: Structures with similar functions but different underlying anatomy, evolving independently in different species due to similar environmental pressures (e.g., the wings of a butterfly and a bird).
Physiological Evidence
Shared physiological features, such as birds and reptiles both having an amniotic egg with a hard shell for protection.
Biochemical Evidence
Similarities in proteins, chemicals, and DNA sequences across species. DNA hybridization involves extracting and comparing DNA fragments to assess genetic similarities.
Other Mechanisms Driving Evolution
Genetic Drift
Another mechanism for evolution where allele frequencies in a population change due to random chance, especially significant in small populations. Events like natural catastrophes (bottlenecks), migration (founder effect), or random mating patterns can cause the frequency of a particular allele to increase or decrease unpredictably.
Gene Flow
The movement of genes (alleles) from one population to another, typically through migration, which can introduce new genetic variation or alter existing allele frequencies.
The Pace of Evolutionary Change
Gradualism
The view that evolutionary change occurs slowly and steadily over long periods. For example, tiger stripes might have evolved from lighter-colored markings, with individuals having slightly better camouflage surviving and reproducing more successfully each generation, making the trait more common over time.
Punctuated Equilibrium
The view that species often remain relatively unchanged for long periods (stasis), interrupted by short bursts of rapid evolutionary change. These changes might be driven by mutations affecting key traits or significant environmental shifts. For example, tiger stripes could potentially arise more rapidly under strong selective pressure.