Speciation and the Biological Species Concept

Speciation

Introduction to Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. Understanding how species are defined and how new species arise is fundamental to the study of biology and biodiversity.

• Key Question: How do scientists define a species?

• Key Question: Are organisms truly divided into discrete units called species?

The Nature of Species

Defining Species

The concept of species must account for two main phenomena:

• Distinctiveness: Species that occur together at a single locality are often clearly different from one another.

• Connection: There is a connection among different populations belonging to the same species, even if they are geographically separated.

Ranges of Species

Sympatric and Allopatric Species

Species can be classified based on the overlap of their geographic ranges:

• Sympatric species: Occur together in the same area, are phenotypically different, utilize different parts of the habitat, and behave separately.

• Allopatric species: Do not have overlapping ranges; they are geographically separated.

Within a single species, populations in different areas may appear distinct but are usually connected by gene flow through intermediate populations.

The Biological Species Concept

Definition and Limitations

The Biological Species Concept (BSC), proposed by Ernst Mayr, defines a species as:

• "Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."

• Species are composed of populations whose members mate with each other and produce fertile offspring.

Limitations: The BSC does not apply well to asexual organisms, fossils, or organisms where reproductive isolation is difficult to observe.

Reproductive Isolation

Reproductive isolation is the existence of biological barriers that prevent two species from producing viable, fertile offspring. These barriers:

• Block gene flow between different species

• Reduce the formation of hybrids

Reproductive Isolating Mechanisms

Types of Barriers

Reproductive isolation can be achieved through various mechanisms, which are classified as prezygotic or postzygotic barriers.

Prezygotic Isolating Mechanisms

• Habitat (Ecological) Isolation: Species occupy different habitats and thus do not encounter each other to mate.

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

• Behavioral Isolation: Unique courtship rituals or behaviors prevent mating between species.

• Mechanical Isolation: Morphological differences prevent successful mating.

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

Postzygotic Isolating Mechanisms

• Reduced Hybrid Viability: Hybrid offspring have impaired development or survival.

• Reduced Hybrid Fertility: Hybrid offspring are sterile or have reduced fertility.

• Hybrid Breakdown: First-generation hybrids are viable and fertile, but subsequent generations are weak or sterile.

Geography of Speciation

Allopatric vs. Sympatric Speciation

Speciation can occur in two main geographic contexts:

• Allopatric Speciation: Occurs when populations are geographically isolated, leading to divergence and reproductive isolation.

• Sympatric Speciation: Occurs without geographic separation, often through mechanisms such as polyploidy (especially in plants) or disruptive selection.

Process of Speciation

• Initially identical populations must diverge.

• Reproductive isolation must evolve to maintain these differences.

• Gene flow can homogenize populations unless they are isolated.

Character Displacement

Role in Sympatric Speciation

During sympatric speciation, natural selection favors individuals in each species that use resources not used by the other species, reducing competition. This leads to:

• Greater fitness for individuals with unique traits

• Increased frequency of trait differences over time

• Divergence of species

Example: Beak size differences in sympatric populations of finches reduce competition for food resources.

Summary Table: Types of Reproductive Isolating Mechanisms

Type	Mechanism	Example
Prezygotic	Habitat Isolation	Two species occupy different habitats and do not meet
Prezygotic	Temporal Isolation	Species breed at different times
Prezygotic	Behavioral Isolation	Different courtship rituals
Prezygotic	Mechanical Isolation	Incompatible reproductive structures
Prezygotic	Gametic Isolation	Sperm cannot fertilize egg
Postzygotic	Reduced Hybrid Viability	Hybrid offspring do not survive
Postzygotic	Reduced Hybrid Fertility	Hybrid offspring are sterile
Postzygotic	Hybrid Breakdown	Later generations of hybrids are weak or sterile

Key Terms

• Species: A group of organisms capable of interbreeding and producing fertile offspring.

• Gene flow: The transfer of genetic material between populations.

• Polyploidy: The condition of having more than two complete sets of chromosomes, common in plant speciation.

• Disruptive selection: Natural selection that favors individuals at both extremes of a trait, potentially leading to speciation.

Additional info: The images of fish species (Etheostoma spp.) illustrate phenotypic differences among closely related species, which can be a result of speciation processes discussed above.