Speciation: Mechanisms and Concepts (Ch. 24)

Speciation

Introduction to Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It is a fundamental concept in biology, explaining the diversity of life on Earth.

• Species: Groups of organisms that can interbreed and produce fertile offspring.

• Speciation occurs when populations become genetically isolated and diverge.

• Genetic isolation can result from physical separation (allopatry) or other mechanisms.

• Divergence occurs due to mutation, natural selection, and genetic drift.

How Are Species Defined and Identified?

Species Concepts

Biologists use several concepts to define and identify species, each with strengths and limitations.

• Biological Species Concept: Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

• Morphospecies Concept: Species are identified by differences in morphological (shape, structure) features.

• Phylogenetic Species Concept: Species are the smallest monophyletic groups on a phylogenetic tree, sharing a common ancestor.

Biological Species Concept

• Relies on reproductive isolation (no gene flow between populations).

• Prezygotic isolation: Prevents individuals from mating (e.g., temporal, habitat, behavioral, mechanical, gametic barriers).

• Postzygotic isolation: Offspring do not survive or are sterile.

• Limitations: Not applicable to asexual organisms or fossils; difficult to assess in populations that do not overlap geographically.

Morphospecies Concept

• Uses observable morphological traits to distinguish species.

• Useful for fossils and asexual species.

• Limitations: May miss cryptic species (those that look similar but are genetically distinct); subjective in trait selection.

Phylogenetic Species Concept

• Defines species based on evolutionary history and monophyly (single common ancestor).

• Uses DNA sequence data and phylogenetic trees.

• Limitations: Requires extensive genetic data; may result in many more species being recognized.

Comparison Table: Species Concepts

Concept	Criteria for Defining Species	Advantages	Disadvantages
Biological	Reproductive isolation	Directly relates to gene flow	Not applicable to fossils/asexuals; hard to test in allopatric populations
Morphospecies	Distinct morphological features	Works for fossils and asexuals	Subjective; may miss cryptic species
Phylogenetic	Smallest monophyletic group	Applicable to all organisms; based on evolutionary history	Requires genetic data; may split species too finely

Isolation and Divergence in Allopatry

Allopatric Speciation

Allopatric speciation occurs when populations are geographically separated, preventing gene flow and leading to divergence.

• Dispersal: A population moves to a new habitat, colonizes it, and forms a new population.

• Vicariance: A physical barrier splits a population into two or more isolated groups.

• Genetic drift, mutation, and selection cause populations to diverge.

• Examples: Formation of new species on islands, river formation splitting populations.

Isolation and Divergence in Sympatry

Sympatric Speciation

Sympatric speciation occurs without geographic isolation, often through genetic changes or ecological niche differentiation.

• Disruptive selection: Favors individuals at both extremes of a trait, leading to reproductive isolation.

• Polyploidy: Chromosome duplication (autopolyploidy or allopolyploidy) can instantly create reproductive isolation.

• Ecological niche partitioning: Populations exploit different resources or habitats within the same area.

• Example: Apple maggot flies specializing on different host plants.

What Happens When Isolated Populations Come into Contact?

Secondary Contact and Its Outcomes

When previously isolated populations meet, several outcomes are possible depending on the degree of reproductive isolation.

• Fusion: Populations interbreed and merge back into a single species.

• Reinforcement: Selection favors traits that prevent interbreeding, strengthening reproductive barriers.

• Hybrid zones: Regions where interbreeding occurs and hybrid offspring are common.

• Speciation by hybridization: Hybrids form a new species with unique characteristics.

Summary Table: Possible Outcomes of Secondary Contact

Outcome	Process	Example
Fusion of populations	Populations interbreed and merge	Hybrid zone formation with gene flow
Reinforcement	Selection for traits preventing hybridization	Prezygotic isolation increases
Hybrid zone formation	Stable region where hybrids are common	Hybrid zones in European toads
Extinction of one population	One population outcompetes the other	Competitive exclusion
New species formation	Hybrids become reproductively isolated	Hybrid speciation in sunflowers

Key Terms and Concepts

• Allopatry: Populations are geographically separated.

• Sympatry: Populations live in the same geographic area.

• Prezygotic isolation: Barriers to mating or fertilization.

• Postzygotic isolation: Barriers after fertilization, such as hybrid sterility.

• Monophyletic group: A group consisting of an ancestor and all its descendants.

• Polyploidy: Having more than two sets of chromosomes.

Example: Speciation in Darwin's Finches

• Finches on the Galápagos Islands evolved into multiple species through allopatric speciation.

• Different beak shapes and sizes adapted to specific food sources.

Additional info: Polyploidy is especially common in plants and can lead to rapid speciation. Hybrid zones may be stable or transient, depending on fitness of hybrids and environmental conditions.