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.