Many biologists believe that adaptation occurs through the accumulation of small changes, such as the slight differences between parents and their children, some of which can result from mutations. However, an alternative that has been suggested for this process is macromutation, essentially when a sudden large-scale mutation produces a characteristic. This theory has generally been disregarded as the major explanation for adaptation, since a mutation on this scale is regarded as more likely to be detrimental than beneficial.
While macromutations appear to be the only explanation for differences such as the number of body segments among arthropods, at the genetic level where the original change occurs, very few changes to genes may actually be necessary to result in the large physical change. Some genes control other genes, where the higher the level of control, then the larger the change those genes can cause (see Homeobox). Biologists make a distinction between changes to the genotype, and the resulting body structure resulting from those genes phenotype.
For example, polydactyl individuals have a large resulting change in their body structure (extra toes), but that change can result from a small change in their genotype. This is not the only possible cause of such changes. They can also result from errors during development, but such non-gene changes are not inherited by future generations.
Macromutation in Chromosomes
Instantaneous biological adaptation must occur when a non-deleterious or rarely useful change occurs in the number of chromosomes in the organism's phenome. This must have occurred when the ancestral ape produced offspring which possessed 46 instead of 48 chromosomes, leading to the eventual evolution of humanity.
This change cannot have taken place slowly as odd numbers and partial chromosomes in diploid creatures are not reproductively viable. Although the possibility of finding a similarly endowed mate to enable reproduction is rather unlikely, the evolution from a single-celled organism to the complex multi-cellular beings found today was equally unlikely, making the possibility not entirely impossible and a rather viable rationale for the differences in chromosome number between the other Great Apes and humans.
This does not invalidate adaptation taking place as a result of slowly developing variation acted on by natural selection, as the genetic sequencing changes wrought by natural selection may lead to chemical instability of the genome concerned, causing fusion, splitting of chromosomes, usually deleterious, but very occasionally useful.
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