Study Guide: Evolution, Natural Selection, and Phylogeny

Evolution by Natural Selection

Scientific Advances and Darwin's Theory

Understanding the process of evolution by natural selection is fundamental to modern biology. Charles Darwin's theory was shaped by scientific advances and observations of natural variation.

• Scientific Advances: Developments in geology, paleontology, and biology helped Darwin construct his theory of evolution.

• Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

• Darwin's Four Postulates: (see also "pocket mice box" and other examples)

  1. Individuals in a population vary in their traits.

  2. Some of these variations are heritable.

  3. More offspring are produced than can survive.

  4. Individuals with advantageous traits are more likely to survive and reproduce.

• Examples: The evolution of coat color in pocket mice is a classic example of natural selection in action.

Genes, Pools, Hardy-Weinberg, Evolutionary Processes

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations. It predicts how gene frequencies will be inherited from generation to generation in the absence of evolutionary forces.

• Gene Pool Model: All the alleles in a population make up its gene pool.

• Hardy-Weinberg Equation: Used to calculate allele and genotype frequencies.

Equation: $p^2 + 2pq + q^2 = 1$ where $p$ = frequency of one allele, $q$ = frequency of the other allele.

• Assumptions: No mutation, random mating, no gene flow, infinite population size, and no selection.

• Application: Used as a null hypothesis to detect if evolution is occurring in a population.

Evolutionary Processes

Several mechanisms can cause changes in allele frequencies, leading to evolution.

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

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

• Gene Flow: Movement of alleles between populations.

• Mutation: Introduction of new genetic variation.

Types of Natural Selection:

• Stabilizing Selection: Favors intermediate variants and reduces variation.

• Directional Selection: Favors one extreme phenotype.

• Disruptive Selection: Favors both extreme phenotypes over intermediates.

Example: The evolution of antibiotic resistance in bacteria is an example of directional selection.

Species & Speciation

Species Concepts

Defining what constitutes a species is a central question in biology. Several concepts are used to classify species.

• Biological Species Concept: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

• Other Species Concepts: Morphological, phylogenetic, and ecological species concepts are also used, especially when reproductive data is unavailable.

Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It often involves the development of reproductive isolation.

• Reproductive Isolation: Prevents gene flow between populations. Can be pre-zygotic (before fertilization) or post-zygotic (after fertilization).

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often through polyploidy or behavioral changes.

Table: Types of Reproductive Isolation (from Table 24.1)

Type	Pre-zygotic	Post-zygotic
Geographic	Yes	No
Behavioral	Yes	No
Hybrid Sterility	No	Yes
Other Mechanisms	Yes/No	Yes/No

Additional info: Table entries inferred for clarity; refer to textbook Table 24.1 for full details.

Phylogeny

Phylogenetic Trees and Systematics

Phylogeny is the study of evolutionary relationships among organisms. Systematics is the broader field that includes phylogeny and taxonomy.

• Phylogenetic Tree: A diagram that represents evolutionary relationships among species.

• Monophyletic Group: Includes an ancestor and all its descendants.

• How to Build a Phylogenetic Tree:

  1. Identify homologous traits (shared due to common ancestry).

  2. Use molecular or morphological data to infer relationships.

  3. Apply principles such as parsimony (the simplest explanation is preferred).

Example: The classification of mammals based on shared derived characteristics (such as hair and mammary glands).