7.10 Speciation
- Syllabus
- 2025
- Topic
- 7.10
- Level
- —
Learning objectives
7.10A—Describe the conditions under which new species may ariseDescribe the conditions under which new species may arise.• Speciation occurs when two populations become reproductively isolated from each other.• The biological species concept provides a commonly used definition of a species for sexually reproducing organisms. It states that species can be defined as a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring.7.10B—Describe the rate of evolution and speciation under different ecological conditionsDescribe the rate of evolution and speciation under different ecological conditions.• Punctuated equilibrium is when evolution occurs rapidly after a long period of stasis. Gradualism is when evolution occurs slowly over hundreds of thousands or millions of years.• Divergent evolution occurs when adaptation to new habitats results in phenotypic diversification. Speciation rates can be especially rapid during times of adaptive radiation as new habitats become available.• Convergent evolution occurs when similar selective pressures result in similar phenotypic adaptations in different populations or species.7.10C—Explain the processes and mechanisms that drive speciationExplain the processes and mechanisms that drive speciation.• Sympatric speciation occurs in populations with geographic overlap. Allopatric speciation occurs in populations that are geographically isolated.• Various pre-zygotic and post-zygotic mechanisms can maintain reproductive isolation and prevent gene flow between populations.
When Populations Become Separate Species
Speciation occurs when two populations become reproductively isolated: they no longer exchange genetic information through successful interbreeding. With gene flow interrupted, the populations can continue to diverge independently.
For sexually reproducing organisms, the biological species concept asks whether individuals can interbreed and produce offspring that are both viable and fertile. If they can, they belong to the same species under this concept; if reproductive isolation prevents that exchange, they may form separate species.
Two populations may look different yet remain one species if they still exchange genes through viable, fertile offspring. Conversely, mating that produces no surviving offspring or only sterile offspring does not maintain gene flow between the populations.
Physical separation alone is not the definition of a new species. The decisive condition is reproductive isolation, and the biological species concept is specifically framed for sexually reproducing organisms.
Rates and Patterns of Evolutionary Change
Evolution and speciation do not proceed at one constant rate. Ecological conditions can be associated with long periods of little change, steady accumulation of change, rapid diversification, or similar adaptations in separate lineages.
| Rate model | Pattern through time |
|---|---|
| Punctuated equilibrium | Long periods of stasis are interrupted by relatively rapid evolutionary change |
| Gradualism | Evolutionary change accumulates slowly over hundreds of thousands or millions of years |
| Pattern | Ecological condition and outcome |
|---|---|
| Divergent evolution | Adaptation to different or newly available habitats produces increasing phenotypic differences |
| Adaptive radiation | Many newly available habitats can support especially rapid diversification and speciation |
| Convergent evolution | Similar selective pressures lead different populations or species to evolve similar phenotypic adaptations |
For example, access to several distinct habitats can expose populations to different selective pressures, promoting divergent adaptation and rapid adaptive radiation. By contrast, similar environments can favor similar solutions in separate lineages, producing convergence.
Similar traits produced by convergent evolution do not by themselves show that the species are closely related. Convergence describes a similar response to similar selective pressures, not shared recent ancestry.
How Reproductive Isolation Drives Speciation
Speciation is driven when barriers prevent gene flow and maintain reproductive isolation. The populations then evolve independently because alleles are no longer regularly exchanged between them.
| Speciation context | Population relationship | How isolation can persist |
|---|---|---|
| Allopatric | Populations are geographically isolated | A physical separation prevents regular interbreeding and gene flow |
| Sympatric | Populations overlap geographically | Reproductive barriers separate groups even though they occupy the same general area |
| Barrier type | When it acts | Examples of the outcome |
|---|---|---|
| Prezygotic | Before a zygote forms | Habitat, temporal, behavioral, mechanical, or gametic isolation prevents mating or fertilization |
| Postzygotic | After fertilization | Hybrid offspring have reduced viability or are sterile, so they do not sustain gene flow |
Barrier reduces successful interbreeding → gene flow falls → populations remain reproductively isolated → independent evolutionary change can accumulate → separate species may arise.
Sympatric does not mean that no isolation exists; it means the isolation is not geographic. Allopatric describes geographic separation, but speciation requires reproductive isolation to be established or maintained.