Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the terms speciation for the ‘splitting’ of lineages or cladogenesis as opposed to ‘anagenesis’ or ‘phyletic evolution’ occurring within lineages. Whether genetic drift is a minor or major contributor to speciation is the subject matter [of] much on going discussion.
There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: Allopatric, Peripatric, Parapatric and Sympatric. Speciation may also be induced artificially through animal husbandry, agriculture or laboratory experiments.
Observed examples of each kind of speciation are provided throughout:
- Phyletic gradualism, above, consist of relatively slow change over geologic time.
- Punctuated equilibrium, bottom, consist of morphological stability and rare, relatively rapid burst of evolutionary change.
All forms of natural speciation have taken place over the course of evolution; however it still remains a subject of debate as to the relatively importance of each mechanism in driving biodiversity. One example of natural speciation is the diversity of the three-spined stickleback, a marine fish that, after the last ice age, has undergone speciation into new freshwater colonies in isolated lakes and streams. Over an estimated 10,000 generations, the sticklebacks show structural differences that are greater than those seen between different genera of fish, including variations of fins, changes in the number or size of their bony plates, variable jaw structures, and color differences.
Speciation rates: There is debate as to the rate at which speciation occurs over geologic time. While some evolutionary biologists claim that speciation has remained relatively constant over time, some paleontologists like Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals; a view known as punctuated equilibrium.
- Allopatric: During allopatric (from the ancient Greek allos, other + Greek patrā, fatherland) geographically isolated population (for e.g. by habitat fragmentation due to geographical changes such as mountain building). The isolated populations then undergo genotypic and phenotypic divergence as:
- They become subjected to dissimilar selective pressures.
- They independently undergo genetic drift.
- Different mutations arise in the two populations. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.
- Peripatric: In peripatric speciation, new species are formed in isolated peripheral populations, prevented from exchanging genes with the main population.
- Parapatric: In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography.
- Sympatric: It refers to the formation of two or more species from a single ancestral species occupying the same geographical location.
