Speciation (Ch. 24): Mechanisms and Concepts

Speciation: The Formation of New Species

Introduction to Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It is a central concept in evolutionary 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 due to mutation, natural selection, and genetic drift.

• Genetic Isolation: Required for speciation; prevents gene flow between populations.

• Divergence: Accumulation of genetic differences, often due to selection or 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 based on morphological (shape, size, structure) differences.

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

Biological Species Concept

• Species are reproductively isolated if they do not interbreed or fail to produce fertile offspring.

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

• Postzygotic isolation: Mating occurs, but offspring are inviable or sterile.

Morphospecies Concept

• Species are distinguished by morphological features.

• Useful for fossils and asexual organisms, but can be subjective due to variation within species.

Phylogenetic Species Concept

• Species are identified as the smallest group sharing a common ancestor (monophyletic group or clade).

• Relies on genetic, developmental, and structural traits.

• Useful for identifying cryptic species and understanding evolutionary relationships.

Comparison of Species Concepts

Species Concept	How Species Are Identified	Advantages	Disadvantages
Biological	Reproductive isolation	Directly relates to gene flow and evolution	Not applicable to fossils or asexual organisms; difficult to test in practice
Morphospecies	Differences in morphological traits	Widely applicable; useful for fossils	Subjective; may miss cryptic species
Phylogenetic	Smallest monophyletic group on phylogenetic tree	Applicable to all organisms; identifies cryptic species	Requires extensive genetic data

Isolation and Divergence in Allopatry

Allopatric Speciation

Allopatric speciation occurs when populations are geographically separated, leading to genetic isolation and divergence.

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

• Vicariance: A physical barrier splits a population into isolated groups (e.g., mountain formation, river change).

• Genetic drift, mutation, and selection cause divergence in isolated populations.

Isolation and Divergence in Sympatry

Sympatric Speciation

Sympatric speciation occurs without geographic isolation, often due to genetic, ecological, or behavioral differences within the same area.

• Disruptive selection: Favors extreme phenotypes, leading to reproductive isolation.

• Polyploidy: Chromosome duplication (especially in plants) can result in instant reproductive isolation.

• Ecological niche differentiation: Use of different resources or habitats within the same area can reduce gene flow.

What Happens When Isolated Populations Come into Contact?

Secondary Contact and Its Outcomes

When diverged populations come back into contact, several outcomes are possible:

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

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

• Fusion: Populations interbreed extensively, merging into a single species.

• Stability: Hybridization continues, but parent species remain distinct.

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

Summary Table: Possible Outcomes of Secondary Contact

Pattern of the Populations	Process	Examples
Reinforcement	Selection for traits that prevent interbreeding	Hybrid zones with low fitness hybrids; selection for prezygotic isolation
Fusion	Gene flow erases differences between populations	Hybrid zones with high gene flow; populations merge
Stability	Hybridization continues, but parent species remain distinct	Stable hybrid zones; hybrids persist but do not replace parent species
Hybrid speciation	Hybrid offspring form a new species	Hybrid sunflowers with unique traits

Key Terms and Concepts

• Gene flow: Movement of alleles between populations.

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Monophyletic group (clade): An ancestor and all its descendants.

• Polyploidy: Having more than two sets of chromosomes; common in plant speciation.

• Hybrid zone: Geographic area where interbreeding occurs and hybrids are common.

Example: Apple Maggot Flies

• Originally laid eggs only in hawthorn fruit.

• Some shifted to apples after their introduction in North America.

• Temporal isolation (different fruiting times) and habitat isolation (different host plants) led to reproductive isolation and divergence.

Additional info: Polyploidy is especially important in plant speciation, where new species can arise in a single generation due to chromosome duplication.