Natural Selection and Evidence for Evolution: Study Notes

Natural Selection: Causes and Mechanisms

Causes of Natural Selection

Natural selection is a fundamental mechanism of evolution, first described by Charles Darwin. It explains how certain traits become more common in a population over time due to differential survival and reproduction.

• Variation: Individuals in a population vary in their traits (e.g., coloration, size, speed).

• Heritability: Some of these traits are heritable and can be passed from parents to offspring.

• Differential Survival and Reproduction: Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

Darwin's Theory of Natural Selection: Key Aspects

• Overproduction: More offspring are produced than can survive.

• Variation: There is variation among individuals in a population.

• Competition: Individuals compete for limited resources.

• Adaptation: Traits that improve survival and reproduction become more common over generations.

Natural Selection as a Mechanism

Natural selection is a mechanism of evolution, driving changes in allele frequencies within populations over time.

Effects of Natural Selection on Populations

Fitness in Biological Terms

Fitness refers to an organism's ability to survive and reproduce in its environment. It is often measured by the number of offspring an individual contributes to the next generation.

Influence of Biotic and Abiotic Factors

• Biotic Factors: Living components such as predators, prey, competition, and disease can affect selection pressures.

• Abiotic Factors: Non-living components like temperature, water availability, and climate also influence which traits are advantageous.

Examples of Natural Selection

• Peppered Moth: During the Industrial Revolution, darker moths became more common due to pollution darkening tree bark, making them less visible to predators.

• Antibiotic Resistance: Bacteria with mutations that confer resistance to antibiotics survive and reproduce, leading to populations dominated by resistant strains.

Phenotypic Variation and Its Importance

Role of Variation in Fitness

Phenotypic variation within a population is crucial for natural selection. Variations can affect an organism's fitness depending on environmental conditions.

• Environmental Influence: A trait that is beneficial in one environment may be disadvantageous in another.

• Example: Fur thickness in animals may be advantageous in cold climates but not in warm climates.

Human Impact on Population Diversity

Human Influence on Diversity

Humans can affect genetic diversity within populations through activities such as selective breeding, habitat alteration, and introduction of invasive species.

• Selective Breeding: Humans breed plants and animals for specific traits, reducing genetic diversity.

• Habitat Destruction: Reduces population sizes and genetic variation.

Artificial vs. Natural Selection

Comparison Table

Aspect	Natural Selection	Artificial Selection
Agent	Environment	Humans
Purpose	Survival and reproduction	Desired traits
Speed	Gradual	Often rapid
Genetic Diversity	Maintained or increased	Often reduced

Evidence for Evolution

Types of Scientific Evidence

• Geographic: Distribution of species supports common ancestry (e.g., finches on Galápagos Islands).

• Geological: Fossil records show changes in species over time.

• Physical: Comparative anatomy reveals homologous structures (e.g., limb bones in vertebrates).

• Biochemical: DNA and protein similarities indicate evolutionary relationships.

• Mathematical: Population genetics models (e.g., Hardy-Weinberg equilibrium) predict allele frequency changes.

Example: Hardy-Weinberg Equation

The Hardy-Weinberg equation describes genetic equilibrium in a population:

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

where p and q are the frequencies of two alleles.

Additional info: These notes expand on the brief questions by providing definitions, examples, and a comparison table for artificial vs. natural selection, as well as a key population genetics equation.