Evolution: Mechanisms and Evidence

Introduction

Evolution is a central concept in biology that explains the diversity of life on Earth. This topic covers the historical development of evolutionary theory, the mechanisms by which evolution occurs, and the evidence supporting evolutionary change.

Pre-Darwinian Ideas and Lamarck's Hypothesis

Early Theories of Evolution

• Pre-Darwinian Ideas: Before Charles Darwin, several naturalists proposed that species could change over time, but their explanations varied.

• Lamarck's Hypothesis: Jean-Baptiste Lamarck suggested that organisms could acquire traits during their lifetime and pass these traits to their offspring. This is known as the inheritance of acquired characteristics.

• Example: Lamarck believed that giraffes developed long necks because their ancestors stretched to reach higher leaves, and this trait was inherited by future generations.

Additional info: Lamarck's ideas were later shown to be incorrect, as acquired traits are not inherited genetically.

Darwin's Voyage and the Theory of Natural Selection

Darwin's Observations and Conclusions

• Darwin's Voyage: Charles Darwin traveled on the Beagle, observing diverse species and fossils, especially in the Galápagos Islands.

• Theory of Natural Selection: Darwin proposed that individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

• Key Points of Natural Selection:

  • Variation exists within populations.

  • Some variations are heritable.

  • More offspring are produced than can survive.

  • Individuals with favorable traits have higher fitness.

• Example: Finches on the Galápagos Islands have different beak shapes adapted to their food sources.

Mechanisms of Evolution: Natural Selection

How Natural Selection Drives Evolution

• Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

• Population Changes: Over generations, advantageous traits become more common in the population.

• Artificial Selection: Humans select for desirable traits in plants and animals, demonstrating the power of selection.

• Example: Breeding dogs for specific traits is an example of artificial selection.

Evidence for Evolutionary Change

Types of Evidence

• Direct Observation: Evolution can be observed in real time, such as bacteria developing antibiotic resistance.

• Homology: Similarities in structure or genetics due to shared ancestry.

  • Anatomical Homology: Structures like the forelimbs of mammals have similar bone arrangements.

  • Molecular Homology: Similar DNA or protein sequences among different species.

• Fossil Record: Fossils show changes in species over time and document extinct organisms.

• Biogeography: The geographic distribution of species supports evolutionary relationships. For example, unique species on islands resemble those on the nearest mainland.

Key Terms and Definitions

Summary Table: Types of Evolutionary Evidence

Formulas and Diagrams

• Fitness (w): The relative reproductive success of a genotype.

• Hardy-Weinberg Equation: Describes genetic equilibrium in a population. Where and are the frequencies of two alleles in a population.