Speciation
Speciation is the formation of new species. One biology textbook describes it as "the formation of two species from one original species".[1] Darwin pictured this process as a branching tree.[1]
What is a species?
Under the biological species concept, a species is a group of individuals that actually or potentially interbreed.[1] Species are therefore defined by whether they can reproduce with one another.[2] Even where two species do sometimes interbreed, they stay separate.[3]
Reproductive isolation
The crucial event in the origin of a new species is reproductive isolation, when two groups stop interbreeding.[4] Barriers to interbreeding are of two main kinds:[5][4]
- Prezygotic barriers stop fertilisation from happening. For example, two groups may have different courtship behaviour, choose mates differently, or breed at different times of year.[4][5] The reproductive organs of some damselflies fit only those of their own species. The shape of a flower can also decide which pollinators visit it: foxgloves are visited by bees and trumpet creepers by hummingbirds.[6]
- Postzygotic barriers act after fertilisation. The two groups can mate, but their hybrid offspring die young or are sterile.[4][6] The mule, a cross between a horse and a donkey, is sterile. This is probably because horses have 64 chromosomes and donkeys 62, so the chromosomes cannot pair properly when sex cells are made.[7][8]
Ways that species form
Allopatric speciation
If a new species evolves when a population is geographically separated from its ancestors, the process is called allopatric speciation.[9][2] Almost all biologists accept that this happens.[9] Laboratory experiments support it. When populations of fruit flies were kept apart and fed on different foods, they evolved different digestive enzymes. They also came to prefer mates from their own line.[10] In most of a series of 14 such experiments, some reproductive isolation appeared.[10]
Sympatric and parapatric speciation
If a new species evolves within the same area as its ancestor, the process is called sympatric speciation.[9] One way this can happen is if groups within a population flower at different times.[2] Parapatric speciation happens in a population whose range is next to that of its ancestor. Both sympatric and parapatric speciation are more disputed than allopatric speciation.[9] A possible example is the fly Rhagoletis pomonella. From 1864 some of these flies moved from hawthorn trees to apple trees, and the two groups now differ in their enzymes and in how long they take to develop.[11]
Speciation by polyploidy
In plants, a new species can arise when a hybrid doubles its number of chromosomes. This is called polyploidy, and it allows the chromosomes to pair properly again, so the hybrid becomes fertile.[12] Wheat, cotton and tobacco are examples of plants that arose when sex cells from two species combined. Polyploidy is common in plants but rare in animals.[13] Hybrid speciation is probably commoner in plants than in animals.[14]
Hybrid zones
Where two closely related groups meet and interbreed, they form a hybrid zone. If the hybrids are less fit than their parents, natural selection may strengthen the barriers between the groups, a process called reinforcement. If the hybrids are just as fit, the two groups may merge into one species.[15][16] The meeting zone of hooded crows and carrion crows is an example of speciation that is not yet complete.[17]
How fast does speciation happen?
Some biologists think new species form gradually. Others support the idea of punctuated equilibrium, in which species change quickly and then stay the same for long periods.[18]
See also
References
- ↑ 1.0 1.1 1.2 Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 735. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ 2.0 2.1 2.2 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 319–320. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 375. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ 4.0 4.1 4.2 4.3 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. pp. 381–382. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ 5.0 5.1 Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 741. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ 6.0 6.1 Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 742. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 320–321. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 373. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ 9.0 9.1 9.2 9.3 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 382. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ 10.0 10.1 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 384. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 412. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 405. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 740. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 408. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 744. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 399. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 410. ISBN 1-4051-0345-0.
{{cite book}}:|access-date=requires|url=(help) - ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 745. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.