Chapter 13: How Populations Evolve – Study Notes

How Populations Evolve

Introduction

This chapter explores the mechanisms by which populations change over time, focusing on the theory of evolution, the evidence supporting it, and the processes that drive microevolution within populations. Key concepts include Darwin's theory, natural selection, genetic variation, and the structure of populations.

Darwin’s Theory of Evolution

Historical Context

• Charles Darwin developed the theory of evolution by natural selection after his voyage on the HMS Beagle.

• His observations of species such as iguanas and tortoises in the Galápagos Islands contributed to his ideas.

• Published The Origin of Species in 1859, introducing the concept of natural selection.

Definition of Theory

• A scientific theory is a widely accepted explanatory idea that is broader than a hypothesis, generates new hypotheses, and is supported by a large body of evidence.

• Example: Evolution by natural selection.

Evidence for Evolution

Fossil Record

• The fossil record shows the sequence in which fossils appear in sedimentary rock, providing evidence for evolutionary change over time.

• Examples include ammonite casts, dinosaur tracks, fossilized leaves, insects in amber, and preserved remains such as Otzi, the Ice Man.

Homology

• Homologous structures are anatomical features in different species that are similar due to shared ancestry, even if they serve different functions (e.g., human arm, cat leg, whale flipper, bat wing).

• Embryological similarities, such as pharyngeal pouches and post-anal tails in chick and human embryos, further support common ancestry.

Vestigial Structures

• Vestigial structures are remnants of features that served important functions in ancestors but are no longer functional in the current organism.

Evolutionary and Phylogenetic Trees

• Evolutionary trees illustrate patterns of descent and common ancestry among groups of organisms.

• Phylogenetic trees show evolutionary relationships based on shared characteristics, such as vertebrae, limbs, and eggs with shells.

Mechanisms of Evolution

Natural Selection

• Natural selection is the process by which individuals with certain traits are more likely to survive and reproduce than those without those traits.

• Three important points:

  1. Individuals do not evolve; populations evolve.

  2. Natural selection can amplify or diminish only heritable traits.

  3. Evolution is not goal-directed.

• Example: Pesticide resistance in insects – alleles conferring resistance become more common after repeated pesticide application.

Artificial Selection

• Artificial selection is the intentional breeding of organisms for desired traits, as seen in Darwin’s pigeons and the cultivation of vegetables from wild mustard.

Genetic Variation

• Genetic variation is essential for evolution and arises from:

  1. Mutations – changes in DNA sequence.

  2. Recombination through reproduction – random pairing of gametes during sexual reproduction.

Population Genetics

Population

• A population is a group of individuals of the same species that live in the same area and interbreed.

Gene Pool and Allele Frequency

• The gene pool consists of all the alleles of all individuals in a population.

• Allele frequency is calculated as: $\text{Allele Frequency} = \frac{\text{number of specific alleles}}{\text{total number of alleles}}$

• Example: If there are 20 green alleles out of 73 total, the frequency of green is $\frac{20}{73}$.

Microevolution

• Microevolution refers to changes in the relative frequencies of alleles in a population over generations.

Causes of Microevolution

• Natural Selection – differential survival and reproduction.

• Genetic Drift – random changes in allele frequencies due to chance events.

  • Bottleneck Effect: Catastrophes reduce population size, altering genetic makeup.

  • Founder Effect: A small group establishes a new population with a different allele frequency.

• Gene Flow – movement of alleles between populations due to migration.

Summary Table: Mechanisms of Microevolution

Mechanism	Description	Example
Natural Selection	Traits that improve survival/reproduction become more common	Pesticide resistance in insects
Genetic Drift	Random changes in allele frequency	Bottleneck effect in prairie chickens
Gene Flow	Movement of alleles between populations	Migration of individuals between populations

Additional info:

• Images of aquatic iguanas, tortoises, and other animals illustrate evolutionary adaptations and diversity.

• Examples of artificial selection in plants (wild mustard derivatives) show how human intervention shapes genetic traits.