Speciation

Speciation is about how species form. It is a major part of evolutionary biology.

Darwin thought most species came directly from pre-existing species. This is called anagenesis: species by changing, or 'phyletic evolution'. For much of the 20th century, scientists thought most species came when earlier species split. This is called cladogenesis.^[1][2] The general view was that most species splitting is caused or helped on its way by isolating mechanisms.^[3][4]

No doubt the physical separation of species which once lived together is a main factor. It is illustrated by so many examples, some of which are discussed below.

However, work in the last 20 years has shown some other causes. Analysing the DNA sequence of living things has shown that there is often some hybridisation between related species. That means genes have been transferred by these crosses. In turn, that means reproductive isolation is not the only definition of a species, and speciation does not always need allopatry (species to be reproductively separated).^[5][6][7] The sections below illustrate the idea that physical separation was of prime importance in the formation of new species.

1. ↑ Cook O.F. (1906). "Factors of species-formation". Science. 23 (587): 506–507. Bibcode:1906Sci....23..506C. doi:10.1126/science.23.587.506. PMID 17789700.
2. ↑ Cook O.F. (1908). "Evolution without isolation". American Naturalist. 42 (503): 727–731. doi:10.1086/279001. S2CID 84565616.
3. ↑ King, David (2003). "The concept of species". Zoology 304, Evolution. Archived from the original on 2007-05-29. Retrieved 2007-07-18.
4. ↑ Mallet, J.L.B. (1998). "Isolating mechanisms". Retrieved 2007-07-18.
5. ↑ Mallet J. 2001. The speciation revolution. J. Evolutionary Biology 14, 887-888.
6. ↑ Wang R.L; Wakeley J. & Hey J. 1997. Gene flow and natural selection in the origin of Drosophila pseudoobscura and close relatives. Genetics' 147 1091-1106.
7. ↑ Shapiro B.J; Le Ducq J.B. & Mallet J. 2016. What is speciation? PLoS Genetics 12 (3):e1005860. [1]