Foundations of Evolution and Population Genetics: Study Notes for General Biology

The Scientific Method

Overview of the Scientific Method

The scientific method is a systematic approach used by scientists to investigate natural phenomena, develop hypotheses, and test predictions. It ensures that scientific inquiry is logical, repeatable, and objective.

• Observation: Gathering information about the natural world through senses or instruments.

• Question: Formulating a question based on observations.

• Hypothesis: Proposing a tentative explanation or answer to the question.

• Prediction: Making a logical statement about what will happen if the hypothesis is correct.

• Test: Conducting experiments or further observations to test the prediction.

• Analysis: Comparing results to predictions.

• Revision: If results do not support the hypothesis, revise or propose a new hypothesis; if results support the hypothesis, develop additional predictions and test them.

Example: Testing whether sunlight affects plant growth by growing plants in sunlight and shade, then measuring their growth.

The Evolution of the Theory of Evolution

Darwin's Observations and Influences

Charles Darwin developed the theory of evolution by natural selection based on his observations during his voyage on the HMS Beagle and the influence of other scientists.

• Biogeography: Darwin noted that animals and plants on distant islands were more similar to those on nearby continents, suggesting common ancestry.

• Population Observations:

  • More individuals are born than survive to reproduce.

  • Individuals in a population show variation.

  • Some variations are heritable.

  • Individuals compete for limited resources.

• Influence of Other Scientists:

  • Charles Lyell (Geologist): Proposed that natural processes occur slowly and continually, both in geology and biology.

  • Thomas Malthus (Economist): Suggested that human suffering is a consequence of populations growing faster than resources, leading to competition.

Example: The similarity between horses and zebras is due to their descent from a common ancestor.

Darwin's Hypothesis and Natural Selection

Darwin hypothesized that individuals with traits better adapted to their environment are more likely to survive and reproduce, passing those traits to offspring. This process is known as natural selection.

• Descent with Modification: Species change over time, and new species arise from ancestral forms.

• Natural Selection: The mechanism by which evolution occurs, favoring individuals with advantageous heritable traits.

• Differential Reproductive Success: Fitter individuals leave more offspring than less fit individuals.

Example: Insecticide resistance in insects is an example of natural selection in action.

Natural Selection

Mechanism of Natural Selection

Natural selection operates on heritable variation within populations, leading to changes in allele frequencies over generations.

• Heritable Variation: Genetic differences must exist in the population.

• Selection Pressure: Environmental factors favor certain traits.

• Survival and Reproduction: Individuals with favorable traits survive and reproduce, passing those traits to offspring.

• Population Change: Over time, the population becomes better adapted to its environment.

Key Points:

• Natural selection is an editing mechanism, not a creative process.

• Populations evolve, not individuals.

• Evolution is tracked by changes in allele frequencies in populations.

Example: The evolution of insecticide resistance in beetles.

Population Genetics

Introduction to Population Genetics

Population genetics combines Darwinian evolution and Mendelian genetics to study changes in allele frequencies within populations over time (microevolution).

• Gene: Segment of DNA that carries genetic information for a heritable trait.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: The observable characteristics of an organism.

• Allele: Different versions of a gene at a specific locus.

• Homozygous: Two identical alleles at a locus (e.g., AA or aa).

• Heterozygous: Two different alleles at a locus (e.g., Aa).

• Dominant allele: Masks the effect of the recessive allele in the phenotype.

Example: Mendel’s Law of Segregation explains how alleles are separated during gamete formation.

Gene Pool and Microevolution

The gene pool is the total collection of genes and alleles in a population. Microevolution refers to changes in allele frequencies within a population over time.

• Environmental influences cause changes in the gene pool.

• Evolution occurs when allele frequencies change from one generation to the next.

Hardy-Weinberg Equilibrium

Principles of Hardy-Weinberg Equilibrium

The Hardy-Weinberg Equilibrium describes a theoretical state in which allele frequencies in a population remain constant from generation to generation, provided certain conditions are met.

• Conditions Required:

  • No genetic drift (large population size)

  • No gene flow (no migration)

  • No mutation

  • Random mating

  • No natural selection

• Equation: The Hardy-Weinberg equation is: $p^2 + 2pq + q^2 = 1$ where $p$ and $q$ are the frequencies of two alleles, and $p + q = 1$.

Example: In a population of 100 ants, if 80% of alleles are B and 20% are b, the genotype frequencies can be calculated using the equation above.

Genotype	Frequency
BB	$p^2$
Bb	$2pq$
bb	$q^2$

Mechanisms of Microevolution

Factors Causing Changes in Allele Frequencies

Several mechanisms can cause microevolution, leading to changes in the genetic makeup of populations.

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

• Bottleneck Effect: Drastic reduction in population size due to disaster, altering the gene pool.

• Founder Effect: Small group colonizes a new area, leading to a different gene pool.

• Gene Flow: Movement of alleles between populations through migration or gamete transfer.

• Mutation: Changes in DNA that create new alleles; ultimate source of genetic variation.

• Nonrandom Mating: Selection of mates based on specific traits, affecting allele frequencies.

• Natural Selection: Differential reproductive success leads to adaptive changes.

Types of Natural Selection

Patterns of Selection

Natural selection can act on populations in different ways, shaping the distribution of traits.

• Stabilizing Selection: Favors intermediate phenotypes, reducing extremes. Example: Optimal clutch size in birds.

• Disruptive Selection: Favors both extremes over intermediate phenotypes, leading to polymorphism or genetic divergence.

• Directional Selection: Favors one extreme phenotype, shifting the population average. Example: Evolution of pesticide resistance in insects or changes in salmon size due to fishing.

Neutral Variations

Some genetic variations do not affect reproductive success and are not acted on by natural selection. These may be influenced by genetic drift and can become important if environmental conditions change.

• Example: Human fingerprints are highly variable but do not confer a selective advantage or disadvantage.

*Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables and equations have been formatted for study purposes.*