Darwinian Evolution: Mechanisms, Evidence, and Speciation

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Darwinian Evolution

Introduction

Darwinian evolution explains how species change over time through the process of natural selection. This chapter explores the historical context, mechanisms, evidence, and consequences of evolutionary change, as well as how new species arise and are classified.

Darwin's Background

Historical Context and Influences

Charles Darwin published On the Origin of Species in 1859, introducing the concept of "descent with modification" (evolution).
Before Darwin, most believed species were unchanging and Earth was only about 5,000 years old.
Key influences:
- Fossils: Indicated Earth's great age and changing life forms.
- Charles Lyell: Proposed that geological processes occur slowly over long periods.
- Lamarck: Suggested species could change, though his mechanism was incorrect.
- Alfred Russel Wallace: Independently developed the idea of natural selection, prompting Darwin to publish.
The HMS Beagle voyage (1831–1836) exposed Darwin to diverse species, especially on the Galápagos Islands, shaping his ideas about adaptation and speciation.

Natural Selection (Core Concept)

Darwin's Logic and Mechanism

Overproduction: Populations can grow rapidly, producing more offspring than the environment can support.
Limited Resources: Environmental resources are finite.
Competition: Individuals compete for resources; not all survive.
Variation: Individuals differ in inherited traits.
Heritability: Traits are passed from parents to offspring.
Conclusion: Natural selection leads to unequal reproductive success; individuals with advantageous traits survive and reproduce more.

Important clarifications:
- Populations, not individuals, evolve over time.
- Natural selection acts only on heritable traits (not acquired traits like muscle from exercise).
- Evolution is not goal-directed; it responds to current environmental conditions.
Example: Antibiotic resistance in bacteria: resistant individuals survive and reproduce, leading to a population dominated by resistant bacteria.

Evolution in Daily Life

Artificial Selection and Adaptation

Artificial selection: Humans select which organisms reproduce, shaping traits in crops, livestock, and pets.
Adaptation: The accumulation of favorable traits in a population over generations.
Evolution explains both the unity (shared DNA and cellular structures) and diversity (variety of forms) of life.

The Fossil Record

Evidence from Fossils

Fossil record: Ordered sequence of fossils in rock layers; older fossils are found deeper.
Radiometric dating: Uses radioactive isotopes (e.g., carbon-14) to determine fossil ages.
Carbon-14 half-life: years; after each half-life, half the C-14 decays to nitrogen.
By measuring the ratio of C-14 to C-12, scientists estimate the age of fossils.
Types of fossils: Sedimentary, amber, frozen organisms, trace fossils (e.g., footprints).
Transitional forms: Fossils showing intermediate features (e.g., Basilosaurus, an ancient whale with hind legs) provide evidence for evolution.

Other Evidence for Evolution

Biogeography, Comparative Anatomy, and Bioinformatics

Type	What it is	Example
Biogeography	Geographic distribution of species	Marsupials dominate Australia (isolated 50 million years ago)
Comparative Anatomy	Comparing body structures	Bat wing, porpoise flipper, and human arm share the same bones
Bioinformatics	Comparing DNA/protein sequences	Human and chimpanzee DNA are ~96% similar

Homologous structures: Similar bone structures with different functions, indicating shared ancestry.

Populations and Gene Pools

Genetic Variation and Microevolution

Population: Group of the same species in the same place and time; the smallest unit that can evolve.
Gene pool: All versions of all genes in a population.
Genetic variation: Arises from mutations (new genes) and sexual recombination (new combinations of genes).
Microevolution: Generation-to-generation change in the gene pool.

Mechanisms of Evolution

Drivers of Evolutionary Change

Mechanism	What it is
Natural Selection	Favorable traits increase in the gene pool
Genetic Drift	Random changes in the gene pool, especially in small populations
Bottleneck Effect	Disaster drastically reduces population size and genetic diversity (e.g., cheetahs)
Founder Effect	Small group colonizes a new area, resulting in a limited gene pool
Gene Flow	Migration of individuals in or out, making populations more similar
Sexual Selection	Traits that attract mates become more common (e.g., peacock feathers)

Darwinian fitness: Measured by the number of healthy, reproducing offspring an individual leaves behind, not by strength or speed.

Geologic Record

Earth's History and Evolution

Earth's history is divided into four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
Tectonic plates move, reshaping continents and influencing evolution (explains biogeography).
Pangea: A single supercontinent ~200 million years ago, later broke apart.
Key events:
- 3.5 billion years ago: Oldest prokaryote fossils
- 530 million years ago: Cambrian explosion (rapid diversification of life)
- 251 million years ago: Largest mass extinction (90% of species lost)
- 65 million years ago: Dinosaur extinction (meteor impact), followed by mammal diversification

Macroevolution

Large-Scale Evolutionary Changes

Macroevolution: Large-scale changes such as the origin of new species, mass extinctions, and novel features.
Speciation: Formation of new species.
Nonbranching evolution: One species gradually transforms into another (no increase in species number).
Branching evolution: One species splits into two or more, increasing diversity.
Mass extinctions: Five major events, each followed by rapid diversification of survivors.

What is a Species?

Species Concepts and Reproductive Barriers

Species: A population capable of naturally interbreeding to produce healthy, fertile offspring.
This definition does not apply to asexual organisms (e.g., bacteria) or extinct species known only from fossils.

Barrier	How it works	Example
Behavioral isolation	Different mating rituals or signals	Bird songs or dances
Habitat isolation	Live in different areas, never meet	Garter snakes in water vs. land
Mating time differences	Breed at different times of year	Skunks breeding in different seasons
Mechanical incompatibility	Anatomy does not fit	Insect genitalia differences
Gametic incompatibility	Sperm cannot fertilize the egg	Sea urchin species
Hybrid weakness	Hybrid offspring are sterile or unfit	Mules (horse × donkey)

How Speciation Happens

Models and Mechanisms of Speciation

Graduated model: Slow, gradual change over millions of years (e.g., horse evolution).
Punctuated equilibrium: Long periods of little change interrupted by rapid bursts of evolution.
Allopatric speciation: Physical barrier separates populations, leading to divergence (e.g., Grand Canyon squirrels).
Sympatric speciation: New species arise within the parent population without geographic separation, often via chromosome duplication in plants (e.g., modern bread wheat).

Taxonomy

Naming and Classifying Life

Taxonomy: The science of naming and classifying organisms.
Three domains: Bacteria, Archaea, Eukarya.
Hierarchical classification: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.
Binomial nomenclature: Each species is given a two-part Latin name (Genus species), e.g., Homo sapiens.

Quick Concept Check

Key Distinctions

Microevolution vs. Macroevolution: Small gene pool changes vs. large-scale species changes.
Natural selection vs. Genetic drift: Selection is non-random (based on fitness); drift is random.
Allopatric vs. Sympatric speciation: Geographic barrier vs. no geographic barrier.
Artificial vs. Natural selection: Humans choose vs. environment chooses.
Graduated vs. Punctuated equilibrium: Slow, steady change vs. bursts of rapid change.

Sample High-Probability Test Questions

What are Darwin's 5 key observations/conclusions leading to natural selection?
What is the half-life of carbon-14 and how is it used in dating?
What is the difference between the bottleneck effect and the founder effect?
Name 3 lines of evidence for evolution outside the fossil record.
What makes mules an example of hybrid weakness?
What type of speciation occurred with the Grand Canyon squirrels?
What is Darwinian fitness actually measured by?
Why can't individuals evolve—only populations?