Speciation: Mechanisms and Concepts

Speciation

Introduction to Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It typically involves two main steps: genetic isolation and genetic divergence. - Genetic isolation: Occurs when a barrier to gene flow isolates populations, preventing interbreeding. - Genetic divergence: Mutation, natural selection, and genetic drift cause populations to diverge genetically over time. - Speciation: The splitting event that creates two or more distinct species from an ancestral species.

How Are Species Defined and Identified?

Species Concepts

Biologists use three main criteria to identify species: 1. Biological species concept 2. Morphospecies concept 3. Phylogenetic species concept

Biological Species Concept

The biological species concept defines species based on reproductive isolation. - Reproductive isolation: Lack of gene flow between populations; members do not interbreed or fail to produce viable, fertile offspring. - Prezygotic isolation: Individuals are prevented from mating successfully. - Postzygotic isolation: Hybrid offspring do not survive or reproduce. Disadvantages: Cannot be applied to fossils, asexual species, or populations that do not overlap geographically.

Isolation Type	Mechanism	Example
Temporal	Populations breed at different times	Bishop pines and Monterey pines release pollen at different times
Habitat	Populations breed in different habitats	Mainland mice and beach mice live in different habitats
Behavioral	Populations have different courtship displays	Birds have different songs
Mechanical	Genitalia incompatible	Genitalia of insects are incompatible
Gametic barrier	Eggs and sperm incompatible	Differences in proteins prevent sperm from penetrating eggs
Hybrid inviability	Hybrid offspring do not develop	When ring-necked doves mate with rock doves, hybrids do not hatch
Hybrid sterility	Hybrid offspring mature but are sterile	Horses and donkeys produce sterile mules

Morphospecies Concept

The morphospecies concept identifies species based on morphological differences. - Advantages: Widely applicable, including to sexual, asexual, and fossil species. - Disadvantages: May misidentify polymorphic species, miss cryptic species, and is subjective.

Phylogenetic Species Concept

The phylogenetic species concept identifies species based on evolutionary history and monophyletic groups (clades). - Monophyletic group: An ancestral population plus all its descendants. - Synapomorphies: Unique traits found in a group but missing in more distant ancestors. - Advantages: Widely applicable, based on testable criteria. - Disadvantages: Few well-estimated phylogenies available; may recognize more species than other concepts.

Criterion for Identifying Populations as Species	Advantages	Disadvantages
Biological Species	Reproductive isolation = evolutionary independence	Not applicable to asexual or fossil species; difficult to assess if populations do not overlap geographically
Morphospecies	Widely applicable	Misidentifies polymorphic species; misses cryptic species; subjective
Phylogenetic Species	Widely applicable; based on testable criteria	Relatively few well-estimated phylogenies are currently available

Species Definitions in Action: Elephant Species

Systematics and taxonomy are used to classify relationships among organisms. - Example: Morphological, genetic, and phylogenetic analyses support the existence of three distinct elephant species: African savanna, African forest, and Asian elephants.

Isolation and Divergence in Allopatry

Allopatric Speciation

Allopatric speciation occurs when populations become geographically separated (allopatry). - Dispersal: Movement of individuals to a new habitat. - Vicariance: Physical splitting of a habitat by a barrier.

Evidence for Vicariance

- Example: DNA analysis of trumpeters in the Amazon basin shows speciation by vicariance due to river formation and habitat subdivision.

Isolation and Divergence in Sympatry

Sympatric Speciation

Sympatric speciation occurs among populations within the same geographic area. - External events: Disruptive selection based on ecological niches or mate preferences. - Internal events: Chromosomal mutations, such as polyploidy.

Disruptive Selection

- Example: Killer whales in the Northeast Pacific occupy different ecological niches and exhibit distinct behaviors, leading to potential speciation.

Ecotype	Group Size	Feeding Culture
Resident	Larger groups	Fish-eating
Transient	Smaller groups	Mammal-eating

Disruptive Selection for Mate Choice

- Example: The mFAS gene in fruit flies affects both ecological divergence and mate choice, leading to prezygotic isolation.

Polyploidization

Polyploidy is a mutation resulting in more than two sets of chromosomes, important in plant speciation. - Autopolyploid: Chromosome doubling within a single species. - Allopolyploid: Chromosome doubling from hybridization between species.

Outcomes of Secondary Contact

Fusion or Extinction

- Fusion: Populations interbreed, erasing distinctions. - Extinction: One population outcompetes and drives the other to extinction.

Reinforcement of Divergence

- Reinforcement: Natural selection favors traits preventing interbreeding when hybrid fitness is low.

Hybrid Zone Formation

- Hybrid zone: Geographic area where interbreeding and hybrid offspring are common. - Example: Townsend’s and hermit warblers form a moving hybrid zone, potentially leading to extinction of one species.

New Species through Hybridization

- Hybridization: Interbreeding between species can produce new, ecologically distinct species. - Example: Hybrid sunflowers occupy new habitats and have unique genetic combinations.

Key Insights

Summary of Speciation

- Speciation is a two-step process involving genetic isolation and divergence. - It is ongoing, dynamic, and sometimes complex.

Process	Example
Fusion of populations	Open-water whitefish population fused with benthic population after introduction of competitor
Extinction	Townsend’s warblers may drive hermit warblers to extinction
Reinforcement	Fruit fly species in overlapping ranges
Hybrid zone formation	Hybrid zone between hermit and Townsend’s warblers
Formation of new species	Hybridization between sunflowers gave rise to a new species