Natural Selection and Evolution: Mechanisms and Evidence

Natural Selection and Evolution

Introduction

Natural selection is a fundamental mechanism of evolution, explaining how populations change over time. This process, first described by Charles Darwin and Alfred Russel Wallace, provides a scientific explanation for the diversity of life and adaptation to environments. This study guide summarizes the key concepts, evidence, and case studies related to natural selection and evolution.

The Process of Evolution: How Does Natural Selection Work?

Overview

Evolution by natural selection describes how heritable traits that enhance survival and reproduction become more common in populations over generations. Darwin and Wallace's theory of natural selection explains the pattern of descent with modification observed in nature.

• Evolution: Change in the genetic composition of a population over time.

• Natural selection: The process by which individuals with advantageous traits survive and reproduce more successfully, passing those traits to the next generation.

• Descent with modification: The passing on of traits from parent organisms to their offspring, with changes accumulating over generations.

• Example: Variation in color and pattern among beetles of the species Cyrtorrhinus chilensis illustrates natural variation within a population.

Darwin’s Four Postulates

Introduction

Darwin identified four key postulates that must be met for natural selection to occur. These postulates form the foundation for understanding how evolution operates in natural populations.

Postulate 1: Variation in a Trait in a Population

• Variation exists among individuals in a population for any given trait (e.g., beak size in finches).

• Such variation is observable and measurable.

• Example: In Galápagos finches, beak depth varies among individuals, as shown in histograms of beak measurements.

Postulate 2: Some Trait Differences Are Heritable

• Heritability refers to the proportion of trait variation that can be attributed to genetic differences among individuals.

• Traits must be passed from parents to offspring for natural selection to act on them.

• Example: A positive correlation between parent and offspring beak depth in finches indicates heritability.

• Equation: Heritability ($h^2$) can be estimated as: $h^2 = \frac{V_G}{V_P}$ where $V_G$ is genetic variance and $V_P$ is phenotypic variance.

Postulate 3: More Offspring Are Produced Than Can Survive

• Populations produce more offspring than the environment can support, leading to competition for resources.

• Environmental factors (e.g., drought, food availability) limit survival and reproduction.

• Example: During drought years on Daphne Major, food scarcity led to a decline in finch population size.

Postulate 4: Survival and Reproduction Are Not Random

• Individuals with traits better suited to the environment are more likely to survive and reproduce.

• This differential survival and reproduction leads to an increase in advantageous traits in the population.

• Example: After a drought, finches with deeper beaks (better for cracking hard seeds) survived at higher rates, shifting the population's average beak depth.

Case Study: Beak Size and Shape Variation in Galápagos Finches

Introduction

Peter and Rosemary Grant conducted a long-term study of medium ground finches (Geospiza fortis) on Daphne Major in the Galápagos Islands, providing direct evidence for natural selection in action.

• Beak size and shape are variable and heritable traits in finch populations.

• Environmental changes, such as drought, alter food availability and select for certain beak characteristics.

• Data show that all four of Darwin’s postulates are met, resulting in observable evolutionary change.

Which Genes Are Under Selection?

Genetic Basis of Traits

• Many traits, such as beak size, are polygenic (influenced by multiple genes).

• Each gene may have a small effect, but together they shape the phenotype.

• Key genes:

  • ALX1: Regulates beak shape.

  • HMGA2: Regulates beak size.

• Natural selection acts on phenotypes, but evolutionary change occurs at the genetic (genotype) level.

• Example: Medium ground finches may have a mixture of blunt (B) and pointed (b) alleles at the ALX1 gene, affecting beak shape.

Summary Table: Darwin’s Four Postulates

Postulate	Description	Example (Finches)
1. Variation	Individuals in a population vary in traits	Beak depth varies among finches
2. Heritability	Some trait differences are heritable	Beak depth is correlated between parents and offspring
3. Overproduction	More offspring are produced than can survive	Finch population declines during drought due to limited food
4. Non-random survival	Survival and reproduction depend on traits	Finches with deeper beaks survive better during drought

Additional Information

• Polygenic traits often show continuous variation (e.g., beak depth) rather than discrete categories.

• Natural selection is not the only process of evolution; other mechanisms include genetic drift, gene flow, and mutation.

• Misconceptions: Natural selection does not create variation; it acts on existing variation. Evolution does not have a predetermined direction or goal.