Speciation: Mechanisms, Concepts, and Outcomes

Speciation: Mechanisms, Concepts, and Outcomes

Mechanisms of Reproductive Isolation

Reproductive isolation is a key process in the formation of new species, preventing gene flow between populations. It can be classified as prezygotic (before fertilization) or postzygotic (after fertilization).

Type	Mechanism	Explanation	Example
Prezygotic Isolation	Temporal	Populations breed at different times.	Barking frogs and leopard frogs release their pollen at different times of year.
	Habitat	Populations breed in different habitats.	Mainland mice and beach mice mate in different habitats.
	Behavioral	Populations have different courtship behaviors.	Male songbirds sing species-specific songs.
	Mechanical	Reproductive structures are incompatible.	The genitalia of some insects are not compatible for mating.
	Gametic Barrier	Eggs and sperm are incompatible.	Differences in proteins on egg and sperm surfaces prevent fusion.
Postzygotic Isolation	Hybrid Inviability	Hybrid offspring do not develop normally and die as embryos.	Some frog hybrids fail to develop past early embryonic stages.
	Hybrid Sterility	Hybrid offspring mature but are sterile as adults.	Horse × donkey = mule (sterile).

Example of Hybrid Sterility

• Horse × Donkey = Mule: Mules are sterile hybrids, illustrating postzygotic isolation.

The Morphological Concept of a Species

The morphological (or morphospecies) concept defines species based on observable physical traits such as size, shape, or other morphological features.

• Can be widely applied, including to sexual, asexual, or fossil species.

• Does not require knowledge of gene flow.

• Some species are difficult to distinguish morphologically (cryptic species).

• Example: Oak titmouse and juniper titmouse were once considered a single species until DNA sequencing revealed differences.

The Phylogenetic Concept of a Species

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

• A monophyletic group (clade) consists of an ancestral population and all its descendants.

• Monophyletic groups are identified by synapomorphies (shared, derived traits).

• Example: Fur or milk-producing glands in mammals are synapomorphies.

• A species is the smallest monophyletic group on the tree of life.

How Do New Species Arise?

Speciation can occur through geographic or genetic mechanisms that reduce gene flow between populations.

• Allopatric Speciation: Genetic isolation due to geographic separation.

• Sympatric Speciation: Occurs among populations within the same geographic area.

Allopatric Speciation

By Dispersal

• Genetic drift via the founder effect: Small populations colonize new areas, leading to rapid divergence.

• Natural selection: Divergence occurs if the new environment differs from the original.

• Example: Darwin’s finches on the Galápagos Islands.

By Vicariance

• Physical splitting of habitat divides populations (e.g., river formation, mountain uplift).

• Example: Snapping shrimp populations separated by the closing of the Isthmus of Panama.

Sympatric Speciation

Sympatric speciation occurs without geographic separation, often through disruptive selection or chromosome mutations.

• External events: Disruptive selection based on ecological niches or mate preferences.

• Internal events: Chromosome mutations (e.g., polyploidy).

Example: Killer Whale Ecotypes

• Different feeding cultures and hunting behaviors lead to disruptive selection and reproductive isolation.

Example: Apple Maggot Flies

• Apple maggot flies feed and mate on apples; hawthorn flies on hawthorns.

• Despite living in the same area, they do not interbreed due to habitat preference (prezygotic isolation).

Sympatric Speciation by Polyploidization

• Polyploidy: Chromosome-level mutations increase chromosome number.

• Common in plants; can result in instantaneous speciation.

Types of Polyploidy

• Autopolyploid: Chromosome doubling within a single species due to mutation.

• Allopolyploid: Chromosome doubling following hybridization between different species.

Type	Process	Result
Autopolyploid	Nondisjunction during meiosis leads to diploid gametes; self-fertilization produces tetraploid offspring.	Offspring have doubled chromosome number (e.g., 4n).
Allopolyploid	Hybridization between two species followed by chromosome doubling.	Offspring have chromosomes from both parent species and are often fertile.

• Example: Tragopogon species in North America formed new polyploid species.

What Happens When Isolated Populations Come into Contact?

• If divergence and prezygotic isolation exist, gene flow is minimal and populations continue to diverge.

• If prezygotic isolation does not exist, populations may interbreed, leading to fusion.

Possible Outcomes of Secondary Contact

Process	Explanation	Example
Fusion	Populations freely interbreed.	Occurs when populations of the same species come into contact.
Reinforcement	Natural selection increases reproductive isolation if hybrids have low fitness.	Common in fly species that occupy the same area.
Hybrid Zone Formation	Hybridization occurs in a well-defined geographic area; hybrids may persist.	Townsend’s and hermit warblers hybridize where ranges overlap.
Extinction	If one population is a better competitor, the other may go extinct.	Townsend’s warblers may drive hermit warblers to extinction.
New Species Formation	Hybrids with unique characteristics may form a new species.	Hybridization between sunflower species gave rise to a new species.

Hybrid Zones

• Regions where two species interbreed and produce hybrid offspring with intermediate characteristics.

• Hybrid zones may shift or lead to extinction of one parent species.

• Example: Townsend’s and hermit warblers in the Pacific Northwest.

Practice Questions and Applications

• Dog breeders: Maintaining breed purity by separating fertile dogs is an example of prezygotic isolation (habitat or behavioral isolation).

• Fruit fly courtship rituals: Different courtship behaviors represent behavioral isolation.

• Fungal hybrid viability: Reduced viability of hybrid ascospores is an example of postzygotic isolation (hybrid inviability).

• Moth scent preference: Genetic divergence based on host plant preference suggests sympatric speciation by disruptive selection.

• Ants on a flooded island: Geographic isolation due to habitat fragmentation leads to allopatric speciation by vicariance.

• Fleas on different hosts: Divergence based on host preference is an example of sympatric speciation by disruptive selection.

• Dwarf elephants on Mediterranean islands: Allopatric speciation by vicariance (isolation after glacial retreat) led to the evolution of a new, miniaturized population.