Chapter 14: How Biological Diversity Evolves

Chapter 14: How Biological Diversity Evolves

Biology and Society: Humanity’s Footprint

Human activities have profoundly impacted Earth's ecology and geology, leaving a lasting mark known as the Anthropocene epoch.

• Human Footprint: Includes the global transport of organisms, widespread agriculture, domestication of animals, production of plastics and concrete, nuclear radioactivity, and climate-altering emissions.

• Anthropocene: Proposed new epoch characterized by irreversible changes, high extinction rates, and rapid environmental transformation.

• Evolutionary Impact: Human-driven changes foster evolutionary adaptations, such as pesticide-resistant insects and antibiotic-resistant bacteria.

The Origin of Species

Speciation is the evolutionary process by which new biological species arise, contributing to the diversity of life.

• Speciation: The splitting of one species into two or more distinct species.

• Darwin’s Legacy: Modern discoveries have expanded our understanding of speciation since Darwin’s On the Origin of Species.

What Is a Species?

Species are defined by their ability to interbreed and produce fertile offspring, though alternative definitions exist.

• Biological Species Concept: A species is a group of populations whose members can interbreed and produce fertile offspring.

• Limitations: Does not apply to asexual organisms or fossils.

• Other Definitions: Based on physical traits, molecular data, or shared ancestry (phylogenetic species concept).

Reproductive Barriers Between Species

Reproductive barriers prevent closely related species from interbreeding, maintaining species boundaries.

• Prezygotic Barriers: Prevent mating or fertilization (e.g., temporal, habitat, behavioral, mechanical, gametic isolation).

• Postzygotic Barriers: Occur after fertilization, resulting in hybrid inviability, sterility, or breakdown.

Barrier Type	Example
Behavioral Isolation	Different courtship rituals
Habitat Isolation	Species occupy different habitats
Temporal Isolation	Species breed at different times
Reduced Hybrid Fertility	Horse × Donkey = Sterile mule

Mechanisms of Speciation

Speciation can occur through geographic isolation (allopatric) or within the same area (sympatric).

• Allopatric Speciation: Geographic barriers isolate populations, leading to reproductive isolation and divergence.

• Sympatric Speciation: New species arise within the same geographic area, often due to polyploidy, habitat complexity, or sexual selection.

Allopatric Speciation

• Geologic processes (mountains, rivers) can isolate populations.

• Reproductive barriers evolve, maintaining species separation even if contact resumes.

Sympatric Speciation

• Polyploidy: Chromosome doubling creates new species, common in plants.

• Habitat Complexity: Different niches within the same area reduce gene flow.

• Sexual Selection: Preferences for specific traits can drive divergence.

Polyploid Speciation Type	Description
Autopolyploidy	Chromosome doubling in a single species (2n → 4n)
Allopolyploidy	Hybridization between two species, followed by chromosome doubling

Example: Many crop plants (oats, potatoes, bananas) are polyploids.

Human Activities and Speciation

Human-mediated transport of organisms can facilitate hybridization and speciation.

• Case Study: Introduction of goatsbeard plants in the U.S. led to polyploid speciation and the formation of new species (Tragopogon mirus).

Island Showcases of Speciation

Islands provide unique environments for speciation due to isolation and diverse habitats.

• Galápagos Islands: Home to 14 species of finches with specialized beaks adapted to different diets.

• Structure-Function Correlation: Beak shape reflects feeding habits and ecological niche.

Finch Type	Diet
Cactus-seed-eater	Seeds
Insect-eater	Insects
Tool-using insect-eater	Insects (uses tools)

Observing Speciation in Progress

Speciation is often slow, but ongoing divergence can be observed in nature.

• Populations may diverge by exploiting different food resources or breeding in different habitats.

• Fossil record documents speciation events over thousands to millions of years.

Earth History and Macroevolution

Macroevolution encompasses evolutionary changes above the species level, including mass extinctions and the origin of major adaptations.

• Geologic Time Scale: Divides Earth's history into periods based on fossil evidence.

• Fossil Record: Sequence of fossils in rock strata provides evidence for macroevolution.

The Fossil Record

• Radiometric Dating: Determines ages of rocks and fossils using radioactive isotope decay.

• Relative Dating: Dates fossils by their position between volcanic layers.

Plate Tectonics and Biogeography

Continental drift has shaped the distribution and evolution of life on Earth.

• Plate Tectonics: Earth's crust is divided into plates that move atop the mantle.

• Continental Drift: Movement of plates causes geographic isolation and environmental change.

• Pangaea: Supercontinent formed ~250 million years ago, later broke apart, leading to isolated evolutionary arenas.

• Biogeography: Study of the distribution of organisms; explains unique faunas such as Australian marsupials.

Earth Layer	Description
Crust	Thin, solid outer layer
Mantle	Hot, viscous material beneath crust
Outer Core	Liquid metal
Inner Core	Solid metal

Additional info: These notes expand on the provided slides with definitions, examples, and tables for clarity and completeness, suitable for exam preparation in General Biology.