(Continued from the previous page)
Other than the speedy gradualism scenario, the peripheral isolates scenario, and the idea of developmental mutations, the only commonly discussed model (within the context of neo-Darwinian theory) offering an explanation of how new stable somasets might suddenly appear is the idea of position effects. In this scenario, rearranging the position of genes on a chromosome is supposed to have an effect on their function, and thus, on the development of the affected organism. But, while position effects could perhaps play a role in the production of new types of organisms, they are certainly inadequate as a comprehensive explanation of saltational change.
This limitation can be inferred from several facts:
Thus, position effects fail to account for all the data and are poorly documented. But stabilization processes are well documented. Moreover, they can in fact rearrange chromosomes without additions or deletions (possibly producing position effects). But these are not the only sorts of mutations they produce. They can also generate deletions and duplications (creating dosage effects). Moreover, the combination, in a single organism, of genes previously found only in two separate types of organisms, can produce novel genetic interactions, heterosis, synergistic effects, and new combinations of traits. Since the genes are packaged in chromosomes, such changes introduce and/or duplicate and/or delete hundreds, or even thousands, of genes at a time. Genes do not function in isolation. They are affected by the function of other genes. For example, some genes turn on (i.e., become “transcriptionally active”) only when certain other genes are active. So even in the absence of position effects (i.e., in situations where no structural rearrangement of any chromosome has occurred), the introduction, deletion, and duplication of large blocks of genetic material (chromosomes and pieces of chromosomes) would affect this complex interaction between genes and therefore alter the development of the organism. Thus, stabilization theory provides a more plausible, better documented, and more comprehensive explanation of the phenomenon of saltational, than does any theory based on position effects alone.
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology