Speciation Notes

Prezygotic and Postzygotic Barriers

• Prezygotic: Before fertilization.
• Postzygotic: After fertilization.
• The zygote is the single diploid cell that results from fertilization.

Postzygotic Barriers

• Prevent the hybrid zygote from developing into a viable, fertile adult.
• Hybrids: Offspring resulting from mating between different species.
• Zygote often unviable or has reduced hybrid viability, meaning it's not vigorous enough to reproduce.
• Reduced hybrid fertility: The hybrid is not fertile.
• Hybrid breakdown: Progressively less viable and/or fertile over generations.

Reproductive Isolation

• Prezygotic Barriers:
  • Habitat Isolation
  • Temporal Isolation
  • Behavioral Isolation
  • Mechanical Isolation
  • Gametic Isolation
• Postzygotic Barriers:
  • Reduced hybrid viability
  • Reduced hybrid fertility
  • Hybrid breakdown

Limitations of the Biological Species Concept

• Cannot be applied to fossils or asexual organisms (including all prokaryotes).
• Emphasizes absence of gene flow, but gene flow can occur between morphologically and ecologically distinct species.
• This limitation is addressed by the evolutionary species concept.

Other Definitions of Species (Species Concepts)

• Morphological species concept: Defines a species by structural features.
• Ecological species concept: Defines a species in terms of its ecological niche.
• Most species studies rely on DNA and use the Evolutionary Species Concept

Speciation and Geographic Separation

• Speciation can occur with or without geographic separation.
• Allopatric speciation: Gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations.
  • Observed in nature, e.g., sister species of snapping shrimp (Alpheus) diverged 3 to 9 million years ago due to the formation of the Isthmus of Panama.

Sympatric Speciation

• Occurs in populations that live in the same geographic area.
• Can occur if gene flow is reduced by factors, including:
  • Polyploidy
  • Sexual selection
  • Habitat differentiation

Polyploidy

• The presence of extra sets of chromosomes due to accidents during cell division.
• Much more common in plants than in animals.
• Polyploid individuals may be able to reproduce with other polyploids but not with members of the original population (non-polyploids).
  • Example: Cope’s gray treefrogs (2N) and Gray treefrogs (4N) are sympatric across much of southeastern US and look the same, but their calls are different.

Sexual Selection

• Can drive sympatric speciation.
  • Example: Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria.

Habitat Differentiation

• Sympatric speciation can also result from the appearance of new ecological niches within a habitat (”microhabitats”).
  • Populations of North American maggot fly feed on both apples and hawthorns and appear to be diverging into different species.
  • Flies prefer to mate with flies reared on the same fruit.
  • Flies that use different host species experience both habitat and temporal isolation.

Allopatric and Sympatric Speciation: A Review

• Allopatric speciation
  • Geographic isolation restricts gene flow between populations.
  • If contact is restored between populations, interbreeding is prevented.
• Sympatric speciation
  • Reproductive barrier isolates a subset of a population without geographic separation from the parent species.
  • Can result from polyploidy, sexual selection, habitat differentiation, and temporal isolation.