Chapter 13: How Populations Evolve

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How Populations Evolve

Introduction: The Cheetah and Genetic Diversity

Endangered species, such as the cheetah, face extinction risks due to a lack of genetic diversity. Genetic diversity is crucial for populations to adapt to changing environments and survive threats.

Genetic Diversity: The variety of genes within a population. Low diversity can make species vulnerable to disease and environmental changes.
Example: Cheetahs have experienced genetic bottlenecks, reducing their ability to adapt.

Biology and Society: Mosquitoes and Evolution

Efforts to eradicate malaria by killing mosquitoes with DDT led to the evolution of DDT-resistant mosquito populations, demonstrating evolution in action.

Evolution: The change in genetic composition of populations over time.
Application: Monitoring drug resistance in malaria parasites is essential for public health.

The Diversity of Life

Taxonomy and Classification

Taxonomy is the science of identifying, naming, and classifying species. The Linnaean system organizes species into a hierarchy, aiding our understanding of biological diversity.

Taxonomy: Branch of biology for classification.
Linnaean System: Uses binomial nomenclature (genus and species) and hierarchical categories.
Example: Panthera pardus (leopard) is classified as follows:

Rank	Example
Species	Panthera pardus
Genus	Panthera
Family	Felidae
Order	Carnivora
Class	Mammalia
Phylum	Chordata
Kingdom	Animalia
Domain	Eukarya

Naming and Classifying Species

Each species is given a two-part Latinized name (binomial): the genus and the specific epithet. This system helps distinguish closely related species.

Genus: Group of closely related species.
Specific epithet: Distinguishes species within a genus.
Example: Ursus americanus (American black bear) and Ursus maritimus (polar bear) are more closely related than Ursus americanus and Bufo americanus (American toad).

Explaining the Diversity of Life

Evolutionary Theory

Charles Darwin's theory of evolution, published in 1859, explains the origin of species and the diversity of life through natural selection.

Evolution: Descent with modification; species change over time.
Natural Selection: Mechanism by which individuals with advantageous traits survive and reproduce.

The Idea of Fixed Species

Historically, species were thought to be unchanging. Fossil discoveries challenged this view, suggesting that species could change over time.

Fixed Species: The belief that species are permanent and do not evolve.
Fossils: Imprints or remains of past organisms, providing evidence for evolution.

Lamarck and Evolutionary Adaptations

Jean-Baptiste de Lamarck proposed that traits acquired during an organism's lifetime could be passed to offspring, a concept later replaced by Darwin's theory.

Adaptation: Traits that improve an organism's ability to survive and reproduce.
Lamarckism: Use and disuse of body parts leads to inheritance of acquired traits (now discredited).

Charles Darwin and The Origin of Species

Darwin's observations during his voyage on the HMS Beagle led to the development of the theory of evolution by natural selection.

Descent with Modification: Species evolve over generations, accumulating changes.
Natural Selection: Individuals with beneficial traits are more likely to survive and reproduce.

Evidence for Evolution

Multiple lines of evidence support evolution, including fossils, homologies, and molecular biology.

Fossil Record: Ordered sequence of fossils in rock layers, showing changes over time.
Homology: Similarity due to common ancestry, seen in anatomical structures and DNA sequences.
Vestigial Structures: Remnants of features that served functions in ancestors (e.g., whale pelvis, human goose bumps).
Comparative Embryology: Similar developmental stages in vertebrate embryos indicate common ancestry.

Structure	Species
Forelimb	Human, Cat, Whale, Bat

Evolutionary Trees

Evolutionary trees illustrate patterns of descent and relationships among species, based on anatomical and molecular homologies.

Evolutionary Tree: Diagram showing evolutionary relationships.
Branching Sequence: Determined by shared homologous traits.

Key Points About Natural Selection

Population-Level Evolution

Natural selection acts on individuals, but populations evolve over generations as adaptive traits become more common.

Population: Group of interbreeding individuals of the same species.
Evolution: Change in allele frequencies in a population over time.

Artificial Selection

Selective breeding in domesticated species demonstrates how selection can change traits over generations.

Artificial Selection: Human-directed breeding for desirable traits.
Variation and Heritability: Essential for selection to occur.

Mechanism of Natural Selection

Natural selection results from limited resources, competition, and differential survival and reproduction.

Struggle for Existence: Only some offspring survive to reproduce.
Adaptive Traits: Traits that improve survival and reproduction are passed on.

Natural Selection in Action

Examples include the evolution of pesticide resistance in insects and antibiotic resistance in bacteria.

Pesticide Resistance: Not caused by pesticides, but by selection for pre-existing resistant individuals.
Antibiotic Resistance: Overuse of antibiotics selects for resistant bacteria (e.g., MRSA).

Sources of Genetic Variation

Mutation

Mutation is the ultimate source of genetic variation, creating new alleles.

Mutation: Change in DNA sequence; can be beneficial, neutral, or harmful.
Chromosomal Mutations: Large-scale changes can be harmful, but gene duplications may lead to new functions.

Sexual Reproduction

Sexual reproduction shuffles alleles, increasing genetic diversity through independent assortment, crossing over, and random fertilization.

Independent Assortment: Random distribution of chromosomes during meiosis.
Crossing Over: Exchange of genetic material between homologous chromosomes.
Random Fertilization: Combination of gametes from two parents.

Population Genetics

Gene Pool and Hardy-Weinberg Equilibrium

The gene pool consists of all alleles in a population. Hardy-Weinberg equilibrium describes a non-evolving population.

Gene Pool: All alleles at all loci in a population.
Hardy-Weinberg Equation:

Application: Used to calculate carrier frequencies for genetic diseases (e.g., PKU).

Microevolution

Microevolution is the change in allele frequencies within a population over time.

Genetic Equilibrium: No change in allele frequencies; population is not evolving.
Microevolution: Small-scale evolutionary changes.

Mechanisms of Evolution

Natural Selection, Genetic Drift, and Gene Flow

Three main mechanisms drive evolution: natural selection, genetic drift, and gene flow.

Natural Selection: Promotes adaptation.
Genetic Drift: Random changes in allele frequencies, especially in small populations.
Bottleneck Effect: Drastic reduction in population size reduces genetic diversity.
Founder Effect: New population started by a few individuals has different allele frequencies.
Gene Flow: Movement of alleles between populations reduces differences.

Natural Selection: Outcomes and Fitness

Evolutionary Fitness

Fitness is measured by an individual's reproductive success relative to others in the population.

Relative Fitness: Contribution to the next generation's gene pool.

Types of Natural Selection

Natural selection can result in different patterns of trait distribution:

Directional Selection: Favors one extreme phenotype.
Disruptive Selection: Favors both extremes over intermediate phenotypes.
Stabilizing Selection: Favors intermediate phenotypes; most common in stable environments.

Sexual Selection and Dimorphism

Sexual selection favors traits that improve mating success, often leading to differences between males and females (sexual dimorphism).

Sexual Selection: Traits that increase mating success are favored.
Sexual Dimorphism: Distinct differences in appearance between sexes.

Evolution Connection: Antibiotic Resistance

Antibiotic resistance in bacteria is a modern example of evolution by natural selection, driven by the widespread use of antibiotics.

MRSA: Methicillin-resistant Staphylococcus aureus, a dangerous superbug.
Application: Highlights the importance of responsible antibiotic use.