Speciation: Concepts, Mechanisms, and Examples

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 and the formation of new species over time.

• Species: An evolutionarily independent population or group of populations.

• Speciation is ongoing and can be observed in various organisms, such as the diverse cichlid fish shown in the provided image.

Defining and Identifying Species

Criteria for Identifying Species

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

• Biological Species Concept

• Morphological Species (Morphospecies) Concept

• Phylogenetic Species Concept

Biological Species Concept

Reproductive Isolation as the Main Criterion

The biological species concept defines species based on reproductive isolation, which prevents gene flow between populations.

• Reproductive Isolation: Members of different populations do not interbreed, or if they do, they fail to produce viable, fertile offspring.

• Results in lack of gene flow between populations.

Mechanisms of Reproductive Isolation

• Prezygotic Isolation: Individuals of different species are prevented from mating successfully.

• Postzygotic Isolation: Hybrid offspring do not survive or reproduce.

Types of Reproductive Isolation

Type	Prezygotic Isolation	Postzygotic Isolation
Habitat	Populations are isolated because they breed in different habitats.	Zygotes are produced but do not survive or are sterile.
Behavioral	Populations do not interbreed because they have different courtship displays.	Hybrid offspring do not develop normally and die at some point during early development.
Mechanical	Mating fails because male and female reproductive structures are incompatible.	Hybrid offspring are sterile.
Gametic Barrier	Mating fails because eggs and sperm are incompatible.	Hybrid offspring are sterile.

Limitations of the Biological Species Concept

• Cannot be evaluated in fossils or species that reproduce asexually.

• Cannot be applied to populations that do not overlap geographically.

Morphological Species (Morphospecies) Concept

Identification Based on Physical Traits

The morphological species concept identifies species based on differences in size, shape, or other morphological features.

• Distinguishing features arise if populations are independent and isolated from gene flow.

• Widely applicable, especially when there is no data on gene flow.

• Equally applicable to sexual, asexual, and fossil species.

Limitations of the Morphospecies Concept

• Polymorphic species may be classified as more than one species.

• Cannot identify cryptic species that differ in non-morphological traits.

• Features used to distinguish species under this concept are subjective.

Phylogenetic Species Concept

Identification Based on Evolutionary History

The phylogenetic species concept identifies species based on their evolutionary history and relationships.

• All species are related by common ancestry.

• A monophyletic group consists of an ancestral population plus all of its descendants.

• Species are identified by synapomorphies (unique forms), which are homologous traits found in a common ancestor and its descendants but missing in more distant ancestors.

Advantages and Limitations

• Can be applied to any type of population.

• Logical: different species have different synapomorphies due to lack of gene flow and independent evolution.

• Phylogenies are currently available for only a subset of populations.

• May lead to recognition of many more species than other concepts.

Mechanisms of Speciation

Allopatric vs. Sympatric Speciation

Speciation can occur in populations that are geographically separated (allopatric) or in populations that share the same geographic area (sympatric).

• Allopatric Speciation: A population forms a new species while geographically isolated from its parent population.

• Sympatric Speciation: A subset of a population forms a new species without geographic separation.

Allopatric Speciation

• Genetic isolation results from geographic separation.

• Populations that live in different areas are in allopatry.

• Mechanisms include dispersal (movement to a new habitat) and vicariance (physical splitting of a habitat).

Sympatric Speciation

• Populations live in the same geographic area and are close enough to interbreed.

• Speciation occurs due to disruptive selection or chromosomal mutations.

• Disruptive Selection: Populations adapt to different ecological niches, leading to reproductive isolation.

• Niche: The range of ecological resources that a species can use and the conditions it can tolerate.

Sympatric Speciation by Polyploidization

• Polyploidy: An error in meiosis or mitosis results in more than two sets of chromosomes.

• Autopolyploid: Chromosomes are all from the same species.

• Allopolyploid: Chromosomes are from different species due to hybridization.

Reinforcement and Hybridization

Reinforcement

When postzygotic isolation occurs, there is strong natural selection against interbreeding. Natural selection for traits that prevent interbreeding among populations is called reinforcement.

• Fitness of hybrid offspring is typically lower than that of parents.

• Reinforcement strengthens reproductive barriers.

Hybridization and Secondary Contact

Secondary contact of two populations can produce a range of outcomes:

• Fusion of the populations

• Reinforcement of divergence

• Formation of hybrid zones

• Extinction of one population

• Formation of new species

Summary Table: Species Concepts Comparison

Species Concept	Main Criterion	Advantages	Limitations
Biological	Reproductive isolation	Clear mechanism; relates to gene flow	Not applicable to fossils, asexual species, or geographically isolated populations
Morphological	Physical traits	Widely applicable; useful for fossils	Subjective; cannot identify cryptic species
Phylogenetic	Evolutionary history	Logical; applies to all populations	Requires phylogenetic data; may split species excessively

Example: The cichlid fish in African lakes are a classic example of rapid speciation, with many closely related but distinct species evolving due to ecological and behavioral differences.

Additional info: Polyploidy is especially common in plants, leading to instant speciation. Hybrid zones are regions where different species meet and interbreed, producing hybrids with varying fitness.