Also called the natural state model of species, this was based on the ideas of Greek philosopher Aristotle (384-322 BC), and applied by Swedish botanist Carolus Linnaeus (1707-1778) and others in their search for the perfect ‘type specimen’ for each species.
It is the concept that all members of a species share a common natural state that serves to define and separate them from other species, with observed variations in individuals of a species being caused by forces that interfere with the organism’s attainment of its natural state.
This pre-evolution perspective holds that species are fixed entities, a view no longer accepted by biologists.
Also see: theory of speciation, theory of species
Compare with: POPULATION THINKING
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.
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. Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion.
Rapid sympatric speciation can take place through polyploidy, such as by doubling of chromosome number; the result is progeny which are immediately reproductively isolated from the parent population. New species can also be created through hybridisation followed, if the hybrid is favoured by natural selection, by reproductive isolation.