Evolution, Population Genetics, and Phylogeny: Study Guide

Evolution by Natural Selection

Introduction to Evolution

Evolution is the process by which populations of organisms change over generations through variations in heritable traits. Natural selection is a key mechanism driving evolutionary change.

• Definition: Evolution is the change in the genetic composition of a population over time.

• Key Concept: Natural selection acts on heritable variation within a population, leading to adaptation.

• Example: Darwin's finches on the Galápagos Islands evolved different beak shapes to exploit different food sources.

Darwin's Four Postulates

Darwin identified four key components necessary for natural selection to occur:

• Variation: Individuals in a population vary in their traits.

• Heritability: Some of these variations are heritable and can be passed to offspring.

• Overproduction: More offspring are produced than can survive.

• Differential Survival and Reproduction: Individuals with advantageous traits are more likely to survive and reproduce.

Scientific Advances Supporting Evolution

Modern scientific advances, such as genetics and molecular biology, have provided evidence supporting Darwin's theory of evolution by natural selection.

• Examples: DNA sequencing, fossil records, and observed evolutionary changes in laboratory and natural populations.

Population Genetics

Gene Pools and Hardy-Weinberg Equilibrium

Population genetics studies the genetic composition of populations and how it changes over time. The gene pool is the total collection of genes in a population.

• Hardy-Weinberg Principle: Describes a non-evolving population where allele and genotype frequencies remain constant from generation to generation.

• Equation:

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

• p: Frequency of the dominant allele

• q: Frequency of the recessive allele

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

Quantitative Genetics

Quantitative traits are influenced by multiple genes and can be measured on a continuous scale (e.g., height, weight).

• Example: The beak depth of finches is a quantitative trait affected by several genes.

Non-Random Mating and Assortative Mating

Non-random mating occurs when individuals select mates based on certain traits, leading to changes in genotype frequencies.

• Assortative Mating: Individuals mate with others that are similar (positive assortative) or dissimilar (negative assortative) in certain traits.

Evolutionary Processes Affecting Populations

Several mechanisms can cause changes in allele frequencies in populations:

• 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 through migration.

• Mutation: Random changes in DNA that introduce new genetic variation.

• Non-random Mating: Alters genotype frequencies but not allele frequencies directly.

Random vs. Non-Random Processes

It is important to distinguish between random processes (e.g., genetic drift, mutation) and non-random processes (e.g., natural selection).

Speciation

Definition and Types of Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species.

• Allopatric Speciation: Occurs when populations are geographically separated.

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

• Hybrid Speciation: New species arise from hybridization between two different species.

• Polyploidy: Speciation due to changes in chromosome number, common in plants.

• Dispersal and Vicariance: Mechanisms that lead to geographic isolation and speciation.

Pre-zygotic and Post-zygotic Barriers

Reproductive isolation is essential for speciation and can be classified as pre-zygotic or post-zygotic.

• Pre-zygotic Barriers: Prevent mating or fertilization (e.g., temporal, behavioral, mechanical isolation).

• Post-zygotic Barriers: Reduce viability or fertility of hybrids (e.g., hybrid sterility).

Table: Types of Reproductive Isolation

Type	Pre-zygotic	Post-zygotic
Temporal	Yes	No
Behavioral	Yes	No
Mechanical	Yes	No
Hybrid Sterility	No	Yes
Hybrid Inviability	No	Yes

Phylogeny and Systematics

Phylogeny as Evolutionary History

Phylogeny is the study of the evolutionary relationships among species. Phylogenetic trees are diagrams that represent these relationships.

• Monophyletic Group: Includes a common ancestor and all its descendants.

• Non-monophyletic Group: Does not include all descendants of a common ancestor.

• Characters and Species: Used to construct phylogenetic trees based on shared derived traits.

Building and Evaluating Phylogenetic Trees

Phylogenetic trees are constructed using morphological, molecular, or genetic data. The best tree is the one that requires the fewest evolutionary changes (principle of parsimony).

• Steps to Build a Tree:

  1. Identify characters and their states for each species.

  2. Group species by shared derived characters.

  3. Construct the tree using the principle of parsimony.

• Evaluating Trees: Compare alternative trees and select the one with the least number of evolutionary steps.

Applications of Phylogenetics

• Understanding evolutionary relationships

• Classifying organisms

• Tracing the origin and spread of diseases

Additional info: Some content was inferred and expanded for academic completeness, such as definitions, examples, and the table on reproductive isolation.