Chapter 22: The Origin of Species

Introduction to Species Concepts

The concept of a species is fundamental to understanding biological diversity and evolution. Several species concepts exist, but the biological species concept is widely used in biology due to its emphasis on reproductive isolation.

• Species: A group of individuals capable of interbreeding in nature and producing viable, fertile offspring.

• Reproductive isolation is key to maintaining species boundaries.

• Speciation can occur with or without geographic separation.

Biological Species Concept

Definition and Key Features

The biological species concept defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.

• Members of a species share a gene pool, which is kept together by gene flow.

• Reduction of gene flow between populations can lead to the formation of new species.

Reproductive Isolation

Reproductive isolation refers to the existence of biological barriers that prevent members of different species from interbreeding and producing fertile offspring.

• Prezygotic barriers: Prevent fertilization from occurring.

• Postzygotic barriers: Occur after fertilization, resulting in inviable or sterile offspring.

• Barriers can be prezygotic (before zygote formation) or postzygotic (after zygote formation).

Prezygotic Barriers

Types of Prezygotic Barriers

Prezygotic barriers prevent mating or fertilization between species. The main types include:

• Habitat isolation: Species live in the same area but occupy different habitats and rarely encounter each other.

• Temporal isolation: Species breed at different times (day, season, or year).

• Behavioral isolation: Courtship rituals and other behaviors unique to a species prevent mating.

• Mechanical isolation: Morphological differences prevent successful mating.

• Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species.

Examples:

• Habitat isolation: Aquatic and terrestrial garter snakes (Thamnophis spp.)

• Temporal isolation: Spotted skunks (Spilogale spp.) breed in different seasons.

• Behavioral isolation: Blue-footed boobies perform unique courtship dances.

• Mechanical isolation: Shells of snails (Bradybaena spp.) spiral in opposite directions, preventing mating.

• Gametic isolation: Sea urchin sperm cannot fertilize eggs of a different species; pollen-stigma incompatibility in plants.

Postzygotic Barriers

Types of Postzygotic Barriers

Postzygotic barriers reduce the viability or reproductive capacity of hybrid offspring.

• Reduced hybrid viability: Hybrids fail to develop properly or are frail.

• Reduced hybrid fertility: Hybrids are sterile (e.g., mule from horse and donkey).

• Hybrid breakdown: First-generation hybrids are viable and fertile, but their offspring are feeble or sterile.

Examples:

• Reduced hybrid viability: Ensatina salamander hybrids are frail and have low survival.

• Reduced hybrid fertility: Mules (horse × donkey) are sterile.

• Hybrid breakdown: Rice hybrids show reduced fitness in the second generation.

Limitations of the Biological Species Concept

The biological species concept has several limitations:

• Cannot be applied to asexual organisms (e.g., bacteria).

• Not useful for fossils or extinct organisms.

• Difficult to apply when little is known about reproduction.

Other Species Concepts

• Morphological species concept: Characterizes species by body shape and structural features.

• Ecological species concept: Defines species by their ecological niche.

Speciation: With or Without Geographic Separation

Speciation is the process by which one species splits into two or more species. It can occur with or without geographic separation.

• Allopatric speciation: Occurs when populations are geographically separated, leading to interruption of gene flow and divergence.

• Sympatric speciation: Occurs without geographic separation, often through genetic changes or ecological factors.

Allopatric Speciation

Gene flow is interrupted when a population is divided into geographically isolated subpopulations. Over time, these populations may evolve independently and become distinct species.

Sympatric Speciation

Speciation occurs in populations that live in the same geographic area. Mechanisms include polyploidy, habitat differentiation, and sexual selection.

Summary Table: Types of Reproductive Barriers

Key Equations and Terms

• Gene flow: The transfer of alleles or genes from one population to another.

• Speciation: The process by which one species splits into two or more species.

• Reproductive isolation: The existence of biological factors that impede members of two species from producing viable, fertile offspring.

Equation for gene flow (migration rate):

where is the migration rate, is the number of migrants, and is the population size.

Equation for speciation rate (conceptual):

Additional info: Polyploidy is a common mechanism of sympatric speciation in plants, where an error in cell division results in extra sets of chromosomes.