Evolution by Natural Selection: Historical Context and Mechanisms

Evolution by Natural Selection

Learning Objectives

• Explain the concept of typological thinking and the significance of Aristotle's scale of nature.

• Describe Lamarck’s hypotheses of use and disuse and inheritance of acquired traits.

• Explain how Darwin and Wallace revolutionized evolutionary thought.

• Summarize evidence supporting species change over time and relatedness.

• Describe the four conditions necessary for evolution by natural selection.

• Differentiate between three major types of natural selection.

• Identify and correct common misconceptions about natural selection.

Causes of Evolution (Evolutionary Mechanisms)

Hardy-Weinberg Assumptions and Evolutionary Mechanisms

Evolution occurs when the assumptions of Hardy-Weinberg equilibrium are violated, resulting in changes to the gene pool over time.

Assumption of H-W Equilibrium	Possible Outcomes if Broken
Large population size	Genetic drift: Especially relevant in small populations, where random chance events can lead to loss of alleles.
Random mating	Non-random mating: Due to inbreeding or sexual selection via mate choice.
No mutations	Mutations: The only mechanism that generates new alleles in a population by chance.
No gene flow	Gene flow: Tends to equalize allele frequencies in populations and can bring in new alleles via migration.
No natural selection	Natural selection: Favors certain alleles, leading to adaptations.

Evolution by natural selection is the foundational mechanism of change in populations over time.

History on the Rise of Evolutionary Thought

Typological Thinking

Historically, humans have marveled at the diversity of life and questioned its origins and relationships. Typological thinking began with the Greek philosopher Plato (2,400 years ago), who claimed that species were examples of a perfect essence created by God, remaining unchanged through time. Variation was seen as imperfection.

Aristotle and the Scale of Nature

Aristotle, Plato’s student, organized species into a linear scheme called the great chain of being or scale of nature. This hierarchy began with minerals, then plants, invertebrates, vertebrates, humans, angels, and God. This view, combined with religious beliefs, dominated for over 2,000 years.

Persistence of Typological Thinking

Typological thinking remained popular into the 1700s, with ideas of “higher” and “lower” organisms and human separation from other species persisting as cultural habits.

Aristotle's Scale of Nature:

• Higher: Humans, mammals

• Lower: Plants, minerals

Breakdown of Typological Thinking

In the 1800s, advances in geology challenged long-held beliefs:

• Earth is much older than 6,000 years.

• Earth predates humans.

• Fossil findings proved extinction of some species.

• Fossils illustrated transitions to modern species.

James Hutton: Founder of modern geology, predicted Earth's age based on rock cycles. Georges Cuvier: Studied fossils, highlighted catastrophes as drivers of extinction. Charles Lyell: Popularized Hutton’s work, contextualized fossils in rock layers.

Key Terms and Concepts

• Typological thinking: The belief that species are fixed and unchanging.

• Scale of nature: Hierarchical organization of life from simple to complex.

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Gene flow: Movement of alleles between populations.

• Mutation: Random changes in DNA that introduce new alleles.

• Natural selection: Differential survival and reproduction of individuals due to differences in phenotype.

Formula: Hardy-Weinberg Principle

The Hardy-Weinberg equation describes allele and genotype frequencies in a non-evolving population:

$p^2 + 2pq + q^2 = 1$

Where $p$ and $q$ are the frequencies of two alleles.

Example: Genetic Drift

In a small population, a rare allele may be lost simply due to random chance, not because it is disadvantageous.

Example: Natural Selection

Natural selection can lead to adaptations, such as the development of antibiotic resistance in bacteria.

Comparison Table: Mechanisms of Evolution

Mechanism	Effect on Population	Source of Variation
Genetic Drift	Random changes, loss of alleles	Chance events
Gene Flow	Homogenizes populations	Migration
Mutation	Introduces new alleles	DNA changes
Natural Selection	Favors adaptive traits	Heritable variation

Additional info: These notes expand on the provided slides by including definitions, examples, and a summary table for mechanisms of evolution, ensuring a self-contained study guide for exam preparation.