In case you have not figured it out with your PHD and all, there are no mechanisms to prevent what does not exist. microevolution is just a poor synonym for adaptation.
And BTW, (borrowing coyoteman style), So sorry to tell you, but dont bet the rent money on appealing to science for authority. When PHD’s can be had based on imaginary ape like creatures that became man, science is already on very shaky ground.
Macroevolution means evolution at or above the species level.
1. New species have arisen in historical times. For example:
* A new species of mosquito, isolated in London’s Underground, has speciated from Culex pipiens (Byrne and Nichols 1999; Nuttall 1998).
* Helacyton gartleri is the HeLa cell culture, which evolved from a human cervical carcinoma in 1951. The culture grows indefinitely and has become widespread (Van Valen and Maiorana 1991).
A similar event appears to have happened with dogs relatively recently. Sticker’s sarcoma, or canine transmissible venereal tumor, is caused by an organism genetically independent from its hosts but derived from a wolf or dog tumor (Zimmer 2006; Murgia et al. 2006).
* Several new species of plants have arisen via polyploidy (when the chromosome count multiplies by two or more) (de Wet 1971). One example is Primula kewensis (Newton and Pellew 1929).
2. Incipient speciation, where two subspecies interbreed rarely or with only little success, is common. Here are just a few examples:
* Rhagoletis pomonella, the apple maggot fly, is undergoing sympatric speciation. Its native host in North America is Hawthorn (Crataegus spp.), but in the mid-1800s, a new population formed on introduced domestic apples (Malus pumila). The two races are kept partially isolated by natural selection (Filchak et al. 2000).
* The mosquito Anopheles gambiae shows incipient speciation between its populations in northwestern and southeastern Africa (Fanello et al. 2003; Lehmann et al. 2003).
* Silverside fish show incipient speciation between marine and estuarine populations (Beheregaray and Sunnucks 2001).
3. Ring species show the process of speciation in action. In ring species, the species is distributed more or less in a line, such as around the base of a mountain range. Each population is able to breed with its neighboring population, but the populations at the two ends are not able to interbreed. (In a true ring species, those two end populations are adjacent to each other, completing the ring.) Examples of ring species are
* the salamander Ensatina, with seven different subspecies on the west coast of the United States. They form a ring around California’s central valley. At the south end, adjacent subspecies klauberi and eschscholtzi do not interbreed (Brown n.d.; Wake 1997).
* greenish warblers (Phylloscopus trochiloides), around the Himalayas. Their behavioral and genetic characteristics change gradually, starting from central Siberia, extending around the Himalayas, and back again, so two forms of the songbird coexist but do not interbreed in that part of their range (Irwin et al. 2001; Whitehouse 2001; Irwin et al. 2005).
* the deer mouse (Peromyces maniculatus), with over fifty subspecies in North America.
* many species of birds, including Parus major and P. minor, Halcyon chloris, Zosterops, Lalage, Pernis, the Larus argentatus group, and Phylloscopus trochiloides (Mayr 1942, 182-183).
* the American bee Hoplitis (Alcidamea) producta (Mayr 1963, 510).
* the subterranean mole rat, Spalax ehrenbergi (Nevo 1999).
4. Evidence of speciation occurs in the form of organisms that exist only in environments that did not exist a few hundreds or thousands of years ago. For example:
* In several Canadian lakes, which originated in the last 10,000 years following the last ice age, stickleback fish have diversified into separate species for shallow and deep water (Schilthuizen 2001, 146-151).
* Cichlids in Lake Malawi and Lake Victoria have diversified into hundreds of species. Parts of Lake Malawi which originated in the nineteenth century have species indigenous to those parts (Schilthuizen 2001, 166-176).
* A Mimulus species adapted for soils high in copper exists only on the tailings of a copper mine that did not exist before 1859 (Macnair 1989).
There is further evidence that speciation can be caused by infection with a symbiont. A Wolbachia bacterium infects and causes postmating reproductive isolation between the wasps Nasonia vitripennis and N. giraulti (Bordenstein and Werren 1997).
5. Some young-earth creationists claim that speciation is essential to explain Noah’s ark. The ark was not roomy enough to carry and care for all species, so speciation is invoked to explain how the much fewer “kinds” aboard the ark became the diversity we see today. Also, some species have special needs that could not have been met during the flood (e.g., fish requiring fresh water). Creationists assume that they evolved from other, more tolerant organisms since the Flood. (Woodmorappe 1996)
Source: http://www.talkorigins.org/indexcc/CB/CB910.html
29+ Evidences for Macroevolution:
http://www.talkorigins.org/faqs/comdesc/