Accumulated microevolution, or changes in allele frequencies in a population, leads to macroevolution, which is speciation. In this sense, variation that exists within a population eventually increases to become variation between populations. Evolution at the species level is the result of cumulative microevolution. Fishes did not and do not evolve into humans, instead a fish-like ancestor gave rise to all amphibians, reptiles, birds, and mammals, including humans. Step-wise changes accumulated through time in the fins of fish to become weight-bearing legs for walking, and the rest is (pre)history.
Allopatric speciation occurs when a geographic barrier like a river, a highway, or islands separated by rising sea levels isolates a subset of the population and the forces of evolution go to work independently on the two populations. The accumulated changes lead to reproductive incompatibility.
Parapatric speciation, a muddy model of speciation, occurs when a subset of a population that covers a large geographic distribution, over a variety of habitats, becomes isolated by distance and selection acts differently on the gene pools according to their different habitats. These populations can be held together as one highly variable species by hybrid zones where interbreeding occurs. Baboons, which are monkeys that range from North to South Africa, are a good example of this phenomenon as some scientists lump them into one diverse species, but others see the subpopulations as their own individual species.
Sympatric speciation is considered a rare mode of speciation because it occurs without geographic or physical boundaries. Sympatric speci-ation is the formation of two species living in the same place. Behavioral boundaries, like differences in vocalizations or courting rituals, that prevent full gene pool-wide interbreeding or gene flow can permit natural selection and sexual selection to act differentially within the population and result in a subpopulation actually becoming a separate species.
The types of reproductively isolating factors that can drive speciation include failure to recognize mates, behavioral differences, habitat preferences, timing, morphological or mechanical incompatibility during mating, nonfertilization of egg, and nonviable hybrid offspring. None of these things prevent gene flow between human populations despite the diverse and variable nature of the species.
Species are ultimately arbitrary classification categories placed on continuous, overlapping portions of the Tree of Life. There are many different definitions of species, or species concepts, which work for different organisms in space and time. The morphological and the genetic species concepts group organisms together that possess similar anatomy and genetic codes, respectively. The ecological species concept groups those that may have the same morphology but have a different niche. The biological species concept groups organisms together that can successfully reproduce viable, fertile offspring.
The best concept to apply to fossil organisms is debated. As of now there are no fossils of hominins killed in the act of mating. (Although there are dinosaurs from Mongolia that fossilized in the act of fighting and there is a fossil ichthyosaur (an extinct dolphin-like reptile) that got buried while giving birth.) In fossils too old to preserve ancient DNA it is impossible to knowwhich ones could interbreed successfully. Therefore, the biological species concept, which is the concept most commonly used to group living mammals, is of no use for hominin paleontology. Instead, scientists must look to morphological variation within living species of humans, monkeys, and apes to gauge and predict the amount of variation one expects to find within species in the hominin fossil record.
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