Chapter 14: Speciation and Extinction – Study Notes

Speciation and Extinction

Introduction to Speciation and Extinction

Speciation is the evolutionary process by which populations evolve to become distinct species. Extinction marks the end of a species. This chapter explores the mechanisms of speciation, the barriers that maintain species boundaries, and the patterns and causes of extinction.

Mutations and Evolutionary Change

Mutations as Raw Material for Evolution

• Mutation: A change in the DNA nucleotide sequence. Mutations can alter protein function or gene expression, leading to new phenotypes.

• Mutations provide the genetic variation necessary for evolution.

• Over time, accumulated mutations can lead to divergence between populations, eventually resulting in new species.

• Example: Mutations in a common ancestor's DNA can lead to the evolution of two modern species with distinct traits.

Microevolution and Macroevolution

• Microevolution: Small evolutionary changes within a population, often observable over a few generations.

• Macroevolution: Large-scale evolutionary changes that result in the formation of new species or groups, occurring over longer timescales.

• Microevolutionary changes accumulate to drive macroevolution.

Defining Species

What is a Species?

• Species are distinct groups of organisms that can interbreed and produce fertile offspring.

• The definition of "species" has evolved over time, reflecting advances in biology and genetics.

Linnaeus and Darwin: Appearance-Based Definitions

• Carolus Linnaeus (1700s) developed a binomial naming system, using genus and species names (e.g., Gavialis gangeticus for the gharial).

• Early definitions relied on physical appearance to distinguish species.

Modern Biological Species Concept

• The biological species concept defines species by their ability to interbreed and produce fertile offspring.

• Reproductive isolation is key: new species form when populations can no longer interbreed.

Limitations of the Biological Species Concept

• Does not apply to asexually reproducing organisms or fossil species.

• Some organisms rarely interbreed in nature, even if capable.

DNA Analysis in Species Identification

• Researchers compare gene sequences to determine relatedness.

• In bacteria and archaea, >97% DNA sequence identity often defines the same species.

Reproductive Barriers and Speciation

Reproductive Isolation

Reproductive isolation prevents gene flow between populations, leading to speciation. Barriers can be prezygotic or postzygotic.

Prezygotic Barriers (Prevent Fertilization)

Barrier	Description	Example
Habitat isolation	Different environments	Ladybugs feed on different plants
Temporal isolation	Active or fertile at different times	Field crickets mature at different rates
Behavioral isolation	Different courtship activities	Frog mating calls differ
Mechanical isolation	Physical incompatibility	Sage species use different pollinators
Gametic isolation	Gametes cannot unite	Sea urchin gametes are incompatible

Postzygotic Barriers (After Fertilization)

Barrier	Description	Example
Hybrid inviability	Hybrid offspring fail to mature	Hybrid eucalyptus seeds are not viable
Hybrid infertility (sterility)	Hybrid offspring are sterile	Liger (lion-tiger cross) is infertile
Hybrid breakdown	Second-generation hybrids have reduced fitness	Hybrid mosquitoes have abnormal genitalia

Types of Speciation

Spatial Patterns of Speciation

• Allopatric speciation: Physical barrier separates populations (e.g., Galápagos tortoises on different islands).

• Parapatric speciation: Populations share a border area; most mating occurs within each population (e.g., little greenbuls in different habitats).

• Sympatric speciation: Populations diverge genetically while living together, often due to microenvironment differences or polyploidy (e.g., cichlid fish, cotton plants).

Polyploidy and Instant Speciation

• Polyploidy: The condition of having more than two sets of chromosomes, leading to reproductive isolation from diploid ancestors.

• Common in plants, polyploidy can result in the rapid formation of new species.

Rates and Patterns of Speciation

Gradualism vs. Punctuated Equilibrium

• Gradualism: Evolution proceeds in small, incremental changes over long periods.

• Punctuated equilibrium: Evolution occurs in rapid bursts, separated by periods of little change.

• Both patterns are supported by fossil evidence.

Adaptive Radiation

• Rapid speciation occurs when a population enters a new or heterogeneous environment, leading to multiple specialized forms.

• Often follows mass extinction events, as surviving species exploit new ecological niches.

Extinction

Background and Mass Extinctions

• Background extinction rate: The normal pace of species loss due to environmental change.

• Mass extinction: A rapid loss of many species due to catastrophic events (e.g., meteorite impacts, climate change).

• Human activities are accelerating extinction rates, especially on islands and in vulnerable habitats.

Summary Table: Types of Speciation

Type	Definition	Example
Allopatric	Physical barrier separates populations	Galápagos tortoises
Parapatric	Populations share a border area	Little greenbuls
Sympatric	Populations diverge in the same area	Cichlid fish, cotton plants (polyploidy)

Key Terms

• Mutation: Change in DNA sequence

• Microevolution: Small-scale evolutionary change

• Macroevolution: Large-scale evolutionary change

• Species: Group of organisms capable of interbreeding

• Reproductive isolation: Barriers preventing gene flow

• Polyploidy: Extra sets of chromosomes

• Gradualism: Slow, steady evolutionary change

• Punctuated equilibrium: Rapid bursts of change

• Adaptive radiation: Rapid diversification of a lineage

• Extinction: End of a species