Ch 2: Evolutionary Theory

Chapter 2: Evolutionary Theory

Natural Selection

Evolution by Natural Selection

• Foundational contributions by Charles Darwin and Alfred Russel Wallace: Both scientists independently arrived at the theory of natural selection during the mid-19th century.

• Influences and Intellectual Context:

  • Thomas Malthus: His essay on the Principle of Population argued that populations grow exponentially while resources grow arithmetically, leading to a "struggle for existence."

  • Charles Lyell: A geologist who proposed Uniformitarianism, suggesting that the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in the past. This provided the vast geological time scale ($millions of years$) necessary for slow evolutionary change.

  • International Travel and Exploration:

  • Darwin's voyage on the HMS Beagle (specifically observations in the Gal pagos Islands) allowed him to see biogeographical patterns.

  • Wallace's work in the Malay Archipelago helped him observe the limits of species distribution (The Wallace Line).

Key Historical Moment

• Publication of Darwin's (1859) Origin of Species: This work synthesized decades of research. Its full title, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, fundamentally changed the biological sciences by providing a mechanism for descent with modification.

Definitions and Principles of Natural Selection

Necessary and Sufficient Conditions

For natural selection to occur, three specific conditions must be met:

1. Variation: Individuals within a population must differ in their traits (phenotypes).

2. Heritability: These variations must have a genetic basis so they can be passed from parents to offspring.

3. Differential Fitness: Some variations must provide a survival or reproductive advantage, leading to higher Relative Fitness ($w$) for those individuals compared to others.

Mechanisms and Adaptations

• Adaptation: A trait that has been shaped by natural selection to enhance the survival or reproduction of an organism in a specific environment.

• Case Studies:

  • Leafy Sea Dragon: Uses extreme camouflage to mimic floating seaweed, avoiding predation in Australian waters.

  • Rafflesia Flower: A parasitic plant that lacks roots or leaves; it evolved a "corpse-like" smell to attract carrion flies for pollination.

  • Darwin’s Hawk Moth: Possesses a proboscis specifically evolved to reach the deep nectar spurs of the Angraecum sesquipedale orchid, an example of co-evolution.

Selection and Genetic Framework

Selection Impact on Phenotypes

• Selection acts directly on phenotypes, but the result is a change in the allele frequencies within the gene pool over time.

• The Fundamental Formula:

  $G \times E = P$

  • $G$ (Genotype): The genetic makeup.

  • $E$ (Environment): External factors affecting development.

  • $P$ (Phenotype): The physical manifestation or trait.

• Phenotypic Plasticity: The ability of a single genotype to produce different phenotypes in response to different environments.

Understanding Environment

• Environment is not just the physical climate; it is a complex web of stressors and supports:

  • Abiotic Factors: Solar radiation, temperature, and altitude.

  • Biotic Factors: Nutrition, predators, parasites, and pathogens.

  • Social Factors: Family dynamics, social hierarchies, and sociopolitical structures that influence access to resources.

Evolutionary Developmental Biology (Evo-Devo)

• Sean B. Carroll (2005): Emphasizes that much of human evolution stems from changes in Gene Regulation (switches) rather than changes in the protein-coding sequences themselves. This explains how humans and chimpanzees can be so genetically similar yet morphologically different.

Levels of Selection

• Selection can operate at various hierarchies:

  • Gene Level: "Selfish genes" that ensure their own replication.

  • Cellular Level: Competitive lineages within a body (relevant to cancer biology).

  • Individual Level: The standard Darwinian focus on organismal survival.

  • Group/Kin Selection: Selection favoring traits that benefit relatives or social groups, potentially explaining altruism.

Sexual Selection

• Definition: A form of natural selection where individuals with certain inherited characteristics are more likely than other individuals to obtain mates.

• Modes of Sexual Selection:

  • Intrasexual: Competition within one sex (usually males) for access to the other (e.g., antlers in deer).

  • Intersexual: Individuals of one sex (usually females) are choosy in selecting their mates (e.g., peacock feathers).

• Trade-offs: Elaborate traits used for mating (like bright colors) may attract predators, creating a balance between reproductive success and longevity.

Constraints on Adaptationism

Evolutionary Mismatches

• Vitamin C (Ascorbic Acid): Most mammals synthesize their own Vitamin C. Humans lost this ability due to a mutation in the GULO gene because our ancestors had a fruit-rich diet where Vitamin C was abundant. In modern environments with processed diets, this leads to scurvy.

Physical and Structural Constraints

• Scaling Laws: As insects get larger, their tracheal system (for oxygen diffusion) becomes inefficient. Historically, insects were larger only when atmospheric oxygen levels were much higher.

• Buoyancy: Marine mammals like gray whales can grow to immense sizes because water supports their body weight, a constraint that limits land mammals.

Trade-offs and History

• The Obstetrical Dilemma: The human female pelvis is a compromise between the narrowness required for efficient Bipedalism and the width required for the birth of large-brained infants.

• Historical Contingency: Evolution can only work with existing structures.

  • The Blind Spot: In the vertebrate eye, nerve fibers pass in front of the retina to reach the optic nerve, creating a hole in the visual field. This is an "unintelligent" design resulting from the specific path evolution took in early chordates.