On the Origins of New Forms of Life
7.13: Position Effects
EUGENE M. MCCARTHY, PHD GENETICS
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(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:
- Chromosomal rearrangement is unnecessary for the production of new somatypes. For example, the mule differs markedly from its parents, the horse and the ass. But the position of genes cannot come into question in this case because the individual chromosomes of a mule are identical to those found in its parents and have not been rearranged in any way. All the developmental effects of hybridization in this case (and in the case of all other F1 hybrids produced by interbreeding between distinct chromosets) are due solely to the chromosomes of the parents being reassorted into a new, combined karyotype. Polyploids are another example of new forms of life being produced without chromosomal rearrangement.
- The amount of DNA found in each cell of a given organism (cellular DNA content) varies from one form treated as a species to another, even in the case of closely related ones such as human and chimpanzee (Manfredi Romanini 1985, Plant DNA C-Values Database, Animal Genome Sizes Database). But cellular DNA content usually varies relatively little between different members of a single somaset. Together, these two facts suggest the process creating new somasets changes the quantity of DNA present. Stabilization processes add and delete DNA, but simple rearrangement of the chromosomes doesn't.
- The existence of distinct chromosets within a seemingly uniform somaset shows even extensive rearrangement of the karyotype can fail to bring about any significant alteration in the form of an organism. For example, the previously mentioned Indian muntjac (Muntiacus muntjak) and Reeves’ muntjac (M. reevesi) are somatically identical and yet have markedly different karyotypes. The former has 46 chromosomes, but the latter, only seven (Capanna 1973: 690; King 1993: 150). In the well-studied fruit flies of the genus Drosophila, too, there is often wide variation of chromosome structure within morphologically uniform somasets.¹
- Other than stabilization processes no forces are known that would create a new karyotype (a position effect presupposes a new karyotype).
- Developmental changes resulting from stabilization processes are far better documented than ones resulting from position effects.
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.