Evolution by Natural Selection: Foundations and Mechanisms

Evolution by Natural Selection

Introduction

Evolution by natural selection is a foundational concept in biology, explaining how populations change over time and how new species arise. This process is driven by genetic variation, differential survival, and reproduction among individuals in a population.

The Rise of Evolutionary Thought

Historical Perspectives

• Plato & Typological Thinking: Plato believed in unchanging, ideal forms ("types"). Variation was considered unimportant.

• Aristotle's "Scale of Nature": Aristotle arranged organisms on a linear scale from simple to complex, with humans at the top. He viewed species as fixed and unchanging.

Lamarck and the Idea of Evolutionary Change Through Time

• Lamarckian Evolution: Proposed that species change over time through the inheritance of acquired characteristics (e.g., giraffes stretching their necks).

• Key Point: Although incorrect, Lamarck's ideas introduced the concept that species are not fixed.

Darwin and Wallace: Evolution by Natural Selection

• Darwin & Wallace: Independently proposed that evolution occurs by natural selection, where individuals with advantageous traits survive and reproduce more successfully.

• Key Mechanism: Natural selection acts on heritable variation within populations.

Evidence for Evolution

• Fossil Record: Shows transitional forms and changes in species over time.

• Vestigial Traits: Structures with no current function but present due to ancestry (e.g., human tailbone).

• Current Evolution: Observable changes in populations (e.g., antibiotic resistance in bacteria).

• Homology: Similarities due to shared ancestry, including genetic, developmental, and structural homologies.

Misconceptions about Evolution

• Evolution does not occur in individuals, but in populations over generations.

• Evolution is not goal-directed or progressive.

• Not all traits are adaptive; some are vestigial or neutral.

Natural Selection

Darwin's Postulates

• Organisms vary in their traits.

• Some of this variation is heritable.

• More offspring are produced than can survive; there is competition for resources.

• Individuals with advantageous traits survive and reproduce more, passing those traits to the next generation.

Fitness

• Definition: Fitness is the ability of an individual to survive and reproduce in its environment.

• Relative Fitness: The contribution of an individual to the gene pool of the next generation compared to others.

Important Points about Natural Selection

• Selection acts on individuals, but evolution occurs in populations.

• Only heritable traits are affected by natural selection.

• Traits that increase reproductive success become more common over generations.

• Natural selection does not create new traits; it acts on existing variation.

Types of Selection

• Directional Selection: Favors one extreme phenotype, shifting the population mean.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

• Disruptive Selection: Favors both extremes, increasing variation.

Constraints on Evolution

• Genetic constraints: Limited by available genetic variation.

• Historical constraints: Evolution builds on existing structures.

• Trade-offs: Adaptations may have costs as well as benefits.

• Non-adaptive traits: Some traits persist due to genetic drift or linkage to adaptive traits.

Key Terms Table

Term	Definition
Natural Selection	Process by which individuals with advantageous traits survive and reproduce more successfully.
Fitness	Ability to survive and reproduce in a given environment.
Adaptation	Heritable trait that increases an organism's fitness.
Homology	Similarity due to shared ancestry.
Vestigial Trait	Structure with reduced or no function, inherited from ancestors.

Example: Antibiotic Resistance

• Bacteria with mutations that confer resistance to antibiotics survive and reproduce, leading to an increase in resistant populations.

Additional info: Modern evolutionary biology integrates genetics (the Modern Synthesis), emphasizing the role of mutation, gene flow, genetic drift, and natural selection in shaping populations.