18. Speciation: The division of a population into separate, distinct, and re productively isolated groupings involves significant alterations in genes, although the amino acid changes may be quite small. 
The two methods by which speciation occurs are:
1) Allopatric speciation [two populations are geographically isolated and over time evolve into different species].
2) Sympatic speciation [one species splits in two when a small proportion of individuals within a population reproduce exclusively with each other]. Sympatic speciation–more common in plants than animals–is rarer than allopathic speciation and may be the result of non-random mating or a mutation that leads to reproductive isolation. Geographic isolation is not necessary for this division within a population. Both may demonstrate either or both postzygotic hybrid sterility or inviability (mules are the sterile offspring of horses and donkeys). There are also prezygotic isolating mechanisms: habitat isolation, temporal isolation–organisms breed at different times–behavioral isolation–incompatible courtship rituals and other mating cues, mechanical isolation–incompatible genitalia–gametic isolation–gametes cannot fuse at the time of fertilization.
There are two forms of sympatic speciation that are worthy of mention, but will not be described in depth. They are allopolyploid speciation and autopolyploidy. Allopolyploidy does not begin with physical separation of populations, but rather it starts with hybridization of two different species which occupy the same territory. The process requires that each species’ chromosome types and/or numbers differ from the other species. Proper pairing cannot occur and any offspring will be sterile. However, if both species’ chromosomes double–which, strangely enough, is not rare–they then have pairable chromosomes and can produce new, viable, and fertile offspring which is by all definitions a new species, one that is incapable of successful breeding with either of the parental species. Autopolyploidy occurs when a single species doubles its chromosomes, again not a rare phenomenon, in fact a common one. When such an organism pairs with another organism from the same species which has doubled its chromosomes, viable offspring result. However, neither the organism with the doubled chromosomes nor the offspring of such a mating will survive and be fertile. For either type of event to occur a rare event has to occur in two generations. The remarkable thing is that such events are not rare in organisms that have a rapid generational turnover in a large population. Single plants produce millions of eggs and pollen grains making such an improbable event become probable. Autopolyploidy is so common, in fact, that it is estimated that 70% of all extant plants had a polyploidy event occur sometime/somewhere in their ancestry and is a common way that plants speciate (except for trees). Polyploidy is much rarer in animals, but does occur occasionally in fish, insects, worms, and reptiles. The red viscacha rat of Argentina has 112 chromosomes.
Large chromosomal rearrangements such as the fusion of two chromosomes in the chimpanzee/human ancestor that produced the human chromosome 2 do not necessarily change gene function, but do generally result in reproductive isolation. In a study of the lungless salamander, Ensatina eschscholtzi, the classical Darwinian process of natural selection leading to evolution and finally to speciation that is even currently in progress is clearly elucidated. The study, Patterns and Processes of Species Formation in Salamanders, by David B. Wake, published in the Ann. Missouri, Bot. Gard. 93:8-23, 2006 revealed that the deciding factor in that animal group was geographic isolation. If individuals become geographically isolated from members of their populations, any mutations occurring in these isolated individuals will further remove them genetically from the original population. With sufficient divergent evolution, the groups, if brought into physical contact, may not be able to produce offspring; and if they are able reproduce, the offspring may be sterile. Some other changes that might accomplish this are:
- ~seasonal incompatibility (different breeding seasons preclude cross-copulation).
- ~Environmental isolation, e.g. pollination by bees versus hummingbirds due to flowers living in different areas with attendant evolutionary changes to accommodate the different selection pressures plus mutations.
- ~behavioral incompatibility (nonmeshing courtship rituals).
- ~gamete incompatibility (sperm and eggs from different groups cannot form a zygote).and embryo mortality (the zygote cannot develop past a premature state).
Dr. Wake’s research involved studies of the salamanders’ enzymes, and nuclear and mitochondrial DNA which demonstrated the underlying genomic evidence of a species complex breaking up—evolutionary development. continued…