People have thought about the origins of closely related species for decades. In fact, scientific research on speciation can be traced all the way back into the early 19th century. By looking at older literature, we can understand old arguments, as well as, provide us with ideas toward current problems.
The evolution of biology is a like a history of unanswered problems that have been worked on by some of the most original thinkers of our time. This is very different from a field like molecular biology, where much of the older literature may be irrelevant due to advancement of the field. One of the first things that you should know is that researchers cannot come up with one universal definition of a species.
Speciation is a process that can be looked at from many points of view, whether it be by Behaviorists, Phylogeneticists, Systematists or Evolutionary biologists. So, when trying to decide on a single definition, many conflicting ideas come into play depending on the angle you are examining it from. Most often, the definition for a species, depends on the specific criteria you have set for boundaries.
As you can see, this is not as easy to define as it initially seemed. So, how do we begin to understand speciation?
First and foremost, it is best to understand the current definitions that are accepted for a species.
Then, tackle the problem of understanding the difference between concepts to finally be able to choose a definition that works best for you.
Species Concepts for Speciation:
1. Biological Species Concept (Isolation Concept) (BSC)
"Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups" (Mayr 1963)
"Systems of populations, the gene exchange between these systems is limited or prevented in nature by a reproduction isolating mechanism or by a combination of such mechanisms" (Dobzhansky 1970)
2. Recognition Species Concept
3. Cohesion Species Concept
4. Phylogenetic Species Concept (Character-based)
5. Genealogical Species Concept
6. Evolutionary Species Concept
7. Genotypic Species Cluster Definition
"...clusters are recognized by a deficit of intermediates, both at a single loci (heterozygote deficits) and at multiple loci (strong correlations or disequilibria between loci that are divergent between clusters)" (Mallet 1995)
From this list, 1-3 are process based, while 4-7 are pattern based. Many researchers are adamant that pattern not process should form the basis of any species definition while the others are just as adamant for the opposite.