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Genetic Interpretation of Regressive Evolutionary Processes: Studies on Hybrid Eyes of Two Astyanax Cave Populations (Characidae, Pisces)

Horst Wilkens
Evolution
Vol. 25, No. 3 (Sep., 1971), pp. 530-544
DOI: 10.2307/2407352
Stable URL: http://www.jstor.org/stable/2407352
Page Count: 15
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Genetic Interpretation of Regressive Evolutionary Processes: Studies on Hybrid Eyes of Two Astyanax Cave Populations (Characidae, Pisces)
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Abstract

1) The eyes of F1-hybrids of two cave populations of Astyanax mexicanus (Anoptichthys hubbsi and A. antrobius) were examined anatomically. Their average sizes are greater than those of the parental forms. 2) As observed in the parental forms, there are also correlations between total size and degree of differentiation of eyes of F1-hybrids. The largest are characterized by features which none of the parental forms possess (remains of outer nuclear and outer reticular layers, horizontal cells, lens rudiments). 3) This proves again that the eye is a developmental unit, whose various structures are interdependent and whose degree of differentiation is correlated with size. Both are less dependent on which particular eye genes are present than on how many eye genes there are. 4) As regards structure and size, the eyes of both parental forms exhibit practically no differences. Both cave populations possess more or less the same number of eye genes. The enlarged eye size of F1-hybrids demonstrates that both cave populations possess a rudimentary eye gene system, which because of separate evolution is partly non-allelic. 5 a) Random mutations are the basis of evolutionary processes, the number of degenerative ones thereby being higher than that of constructive ones. This must be interpreted as caused by the fact that within a balanced genic system, for example, as exhibited by the genic basis of eye development, rise of disorder is far more likely than persistence of order. b) Usually degenerative mutations are eliminated by selection. In the case of an organ which has lost its function, however, it is not subjected to selection. Thus neither constructive nor degenerative mutations get lost. On account of their large number, degenerative mutations cause a preponderantly regressive development of the organs concerned. c) The type of reduction is determined by developmental interdependence within and between all structures of an organ. It is not the kind but the number of degenerative mutations which is decisive in determining the degree of differentiation and correlated size of an organ. d) Organs in which reduction has started are characterized by variability. This is diminished when at the end of a degenerative process there are only genes left which are connected pleiotropically to others. Rapidity of reduction which is probably high on account of the possibility of all mutations-the whole raw material of evolution-becoming expressed, is now diminished. A rudiment which is almost completely homozygous now will persist for a long time.

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