Evolution by Natural Selection

Introduction to Evolution by Natural Selection

Evolution by natural selection is a foundational theory in modern biology, independently formulated by Charles Darwin and Alfred Russel Wallace. This theory explains how populations adapt to diverse environments and reveals key attributes of life, including the fact that populations of organisms evolve over time.

• Definition: Evolution by natural selection is the process by which heritable traits that increase an organism's fitness become more common in a population over generations.

• Key Point: Populations, not individuals, evolve.

• Example: Arctic foxes have thick fur for cold climates, while desert foxes have large ears for heat dissipation.

Scientific Theories: Pattern and Process

Scientific theories are composed of two main components:

• Pattern: Observations about the natural world (e.g., species change over time).

• Process: The mechanism that produces the observed pattern (e.g., natural selection).

The Rise of Evolutionary Thought

Historical Context: Special Creation and Typological Thinking

For over 2000 years, the idea of special creation dominated Western thought, positing that all species were created independently and are unchanging. This view is known as typological thinking, where each organism is seen as a perfect, immutable example of its type.

• Plato: Proposed that every organism is a perfect, unchanging essence or type.

• Typological Thinking: The belief that species are fixed and do not change over time.

Aristotle and the Scale of Nature

Aristotle expanded on typological thinking by organizing species into a linear hierarchy known as the great chain of being (scala naturae).

• Species were fixed types.

• Hierarchy: Species were arranged by increasing size and complexity, starting with minerals and plants, and culminating with humans, angels, and God.

• Central Claim: Some species are 'higher' or more complex than others.

Lamarck and Evolution as Change Through Time

In the early 1800s, Jean-Baptiste de Lamarck proposed the first formal theory of evolution, suggesting that simple organisms arise by spontaneous generation and evolve by moving up the chain of being.

• Lamarckian Evolution: Organisms evolve by acquiring traits during their lifetime and passing them to offspring (inheritance of acquired characteristics).

• Example: Giraffes stretch their necks to reach food, and their offspring inherit longer necks.

• Progressive Evolution: Evolution produces larger, more complex, or 'better' species over time.

Darwin, Wallace, and Population Thinking

Darwin and Wallace proposed that species change over time through variation among individuals in populations. Their theory is based on population thinking, which emphasizes the importance of variation within populations.

• Key Point: Individuals with certain traits produce more offspring than others, leading to changes in trait frequencies over generations.

• Population: A group of individuals of the same species living in the same area at the same time.

The Pattern of Evolution: Species Change and Are Related

Descent with Modification

Darwin described evolution as descent with modification: modern species are modified descendants of ancestral species.

• Prediction: Species change through time and are related by common ancestry.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution by natural selection:

1. The Vastness of Geologic Time

• Fossil Record: Fossils are traces of organisms from the past, organized by relative ages in sedimentary rock layers.

• Geologic Time Scale: Earth is about 4.6 billion years old; earliest life appears in rocks 3.4–3.8 billion years old.

2. Extinction Changes Species Over Time

• Extinct Species: Fossils show that many species have gone extinct, indicating that species are dynamic and change over time.

• Fact: Over 99% of all species that have ever lived are now extinct.

3. Transitional Features

• Transitional Features: Traits in fossil species that are intermediate between ancestral and derived species, supporting gradual change.

• Law of Succession: Fossil species in a region resemble living species in the same area.

4. Vestigial Traits

• Vestigial Trait: A reduced or incompletely developed structure with little or no function, similar to functioning structures in related species.

• Examples: Whales with tiny hip bones, ostriches with reduced wings, human coccyx (tailbone).

5. Observed Evolutionary Change

• Contemporary Evolution: Evolution can be observed in real time, such as antibiotic resistance in bacteria and changes in finch beak shape.

6. Species in the Same Geographic Area

• Common Ancestry: Closely related species are often found in the same geographic region, as seen in Galapagos finches.

• Phylogenetic Tree: Diagram showing ancestor-descendant relationships among taxa.

7. Homologies

• Homology: Similarity due to common ancestry, observed at three levels:

  • Genetic Homology: Similarity in DNA, RNA, or amino acid sequences.

  • Developmental Homology: Similarity in embryonic development (e.g., pharyngeal pouches in vertebrate embryos).

  • Structural Homology: Similarity in adult morphology (e.g., limb bones in vertebrates).

8. Formation of New Species (Speciation)

• Speciation: The process by which one species splits into two or more descendant species, observed in contemporary populations (e.g., killer whales).

The Process of Evolution by Natural Selection

Darwin’s Four Postulates

Darwin identified four criteria for evolution by natural selection:

1. Variation exists among individuals in a population.

2. Some trait differences are heritable.

3. Survival and reproductive success are variable.

4. Individuals with certain traits are more likely to survive and reproduce (not a random sample).

Modern Synthesis: Evolution by natural selection occurs when heritable variation leads to differential reproductive success.

Key Definitions

• Fitness: The ability of an individual to produce surviving, fertile offspring relative to others in the population.

• Adaptation: A heritable trait that increases an individual's fitness in a particular environment.

• Selection: Differential reproduction as a result of heritable variation.

Case Study: Antibiotic Resistance in Mycobacterium tuberculosis

• Background: M. tuberculosis causes tuberculosis (TB). Antibiotic resistance has led to a resurgence of TB.

• Mechanism: Resistance is often due to a point mutation in the rpoB gene, altering RNA polymerase and preventing antibiotic binding.

• Natural Selection: Drug-resistant bacteria survive and reproduce, increasing the frequency of resistance alleles.

Case Study: Beak Shape in Galapagos Finches

• Observation: Beak form and body size are heritable in finches.

• Natural Experiment: Drought led to selection for deeper beaks, as these were better for cracking large seeds.

• Result: Alleles for deep beaks increased in frequency after the drought.

Limits and Misconceptions of Natural Selection

Acclimatization vs. Adaptation

• Acclimatization: Phenotypic changes in an individual in response to the environment; not heritable.

• Adaptation: Genetic changes in a population due to natural selection; heritable.

Natural Selection Is Not Goal-Directed

• Evolution does not occur to solve problems or because organisms 'want' to adapt.

• Evolution is not progressive; complex traits can be lost or simplified.

Fitness Trade-Offs

• Definition: Compromises between traits, as selection acts on many traits simultaneously.

• Examples: Trade-off between egg size and number, rapid growth and lifespan, bright coloration and predation risk.

Historical and Environmental Constraints

• Adaptations are limited by genetic and historical factors.

• Natural selection occurs in changing environments, which can constrain adaptation.

Other Evolutionary Processes

• Natural selection is not the only process that drives evolution. Other processes include:

  • Genetic Drift: Random changes in allele frequencies.

  • Gene Flow: Movement of alleles between populations.

  • Mutation: Introduction of new genetic variation.

Summary Table: Evidence for Evolution