Principles of Evolution

Introduction: The Strategic Context of Evolutionary Theory

Evolutionary theory is the central, unifying principle of modern biology. It explains both the diversity of life and the presence of vestigial structures—features that persist in organisms even after their original function has diminished. Examples include human wisdom teeth and the wings of flightless ostriches. These features are evidence of descent with modification, the process by which populations change over time, retaining ancestral traits. Understanding evolution allows biologists to move from simple description to a mechanism-based explanation of adaptation and diversity.

Historical Timeline: From Fixed Forms to Fluid Change

Pre-Darwinian Narrative

• Ancient Greek Philosophy: Plato proposed that every object on Earth was a temporary reflection of a perfect, divinely inspired "ideal form." Aristotle, his student, expanded on this by arranging all organisms into a linear hierarchy called the Scala Naturae (Great Chain of Being).

• 18th Century: Increased exploration revealed greater diversity of life. Georges-Louis Leclerc, Comte de Buffon suggested that a small number of founding species had evolved through natural processes.

• Fossil Discoveries: The discovery of fossils—preserved remains of organisms—provided evidence that life had changed over time. William Smith recognized that certain fossils were always found in specific, consistent rock layers, indicating a progression of forms.

• Catastrophism: To explain fossil patterns without evolution, Georges Cuvier proposed catastrophism: successive natural disasters destroyed species, which were then fossilized in rock layers.

• Uniformitarianism: In contrast, geologists like James Hutton and Charles Lyell developed the principle of uniformitarianism, suggesting that the same gradual processes shaping the Earth today have operated over vast timescales, indicating the Earth is exceedingly old.

• Early Evolutionary Mechanisms: Jean-Baptiste Lamarck proposed the (now refuted) inheritance of acquired characteristics, suggesting that traits changed by use or disuse during an organism's lifetime could be passed to offspring.

Key Scientists and Their Contributions

The Mechanics of Natural Selection: Darwin’s Four Postulates

Darwin and Wallace shifted the focus of biology from individual organisms to populations. They recognized that while natural selection acts on individuals, it is populations that evolve over time. This mechanism explains how adaptation occurs through non-random differential reproductive success.

Darwin and Wallace’s Four Postulates

1. Individuals in a Population Vary: No two individuals are exactly alike, largely due to random mutations.

   • Example: In a population of crickets, some may have a mutation that affects wing structure.

2. Traits Are Passed from Parent to Offspring: Characteristics are heritable.

   • Example: Offspring inherit wing structure from their parents.

3. Some Individuals Fail to Survive and Reproduce: More organisms are born than can survive in their environment.

   • Example: Crickets that are easily found by parasites are less likely to survive and reproduce.

4. Survival and Reproduction Are Not Determined by Chance: Reproductive success depends on an individual’s specific characteristics.

   • Example: Crickets with silent wings avoid parasites and are more likely to survive and reproduce.

Critical Comparison: Blended Inheritance vs. Mendelian Genetics

• Blended Inheritance: The outdated idea that offspring are a simple mixture of parental traits (e.g., tall AA and short aa parents produce only medium Aa offspring). This would eliminate variation over generations, halting evolution.

• Mendelian Genetics: Demonstrates that alleles remain discrete. In a cross of two heterozygotes (Aa × Aa), both extreme phenotypes (AA and aa) reappear in the next generation, preserving variation necessary for natural selection.

  • Genotypic Ratio Example:

Empirical Evidence: How We Know Evolution Occurred

Multiple scientific disciplines provide independent lines of evidence for evolution, all supporting the concept of descent with modification.

Evidence from Anatomy and Development

• Homologous Structures: Anatomical features that differ in function but share the same evolutionary origin. Example: The bone structure of a human arm and a chicken wing.

• Vestigial Structures: "Useless" structures inherited from ancestors that serve no apparent purpose in the modern organism. Example: Human wisdom teeth, wings on flightless ostriches.

• Analogous Structures: Structures similar in function but different in internal anatomy and origin, resulting from convergent evolution. Example: Wings of insects vs. wings of birds.

• Embryological Similarities: Early developmental stages reveal shared ancestry. Example: All vertebrate embryos develop a tail and gill slits.

The Molecular Record

• All living organisms use DNA as genetic material, RNA and ribosomes for translation, the same 20 amino acids to build proteins, and ATP as an energy carrier.

• The degree of DNA similarity between organisms can be used as a "molecular clock" to estimate evolutionary relatedness.

Evolution in Action: Current Topics and Real-World Events

Evolution is ongoing and observable, especially under strong selective pressures, including those imposed by humans.

Current Event Case Studies

1. The Silent Crickets of Kauai: A mutation silenced male crickets, making them less detectable by a parasitic fly. These silent males survived and reproduced more, causing the silent trait to become common.

2. Chemical Resistance: Herbicide and pesticide use selects for resistant individuals. Mutations that confer resistance allow survival and reproduction, leading to rapid evolution of resistant populations.

3. Artificial Selection: Humans breed plants and animals for desired traits (e.g., dog breeds, high-yield crops), demonstrating how selection for heritable traits can rapidly change populations.

Application Questions

1. Silent Crickets of Kauai: The arrival of the parasitic fly changed the criteria for non-random success (Postulate 4) by making survival and reproduction dependent on the ability to avoid detection, favoring the silent mutation.

2. Herbicide Resistance: It is inaccurate to say plants "developed resistance because they needed it" (Postulate 1). Variation already exists in the population; selection acts on individuals with pre-existing resistance mutations.

3. Artificial Selection and Inheritance: Artificial selection in dog breeding validates Postulate 2, as desired traits are inherited from parent to offspring, demonstrating heritability.

Additional info: The study guide emphasizes the transition from philosophical to scientific explanations of biological change, the importance of genetic variation, and the role of empirical evidence in supporting evolutionary theory.