No. 1, May, 1921] • GENETICS 37 



251. East, E. M. The role of reproduction in evolution, Amer. Nat. 52 : 273-289. 1918. 

 — The role of methods of reproduction in evolution may be interpreted by comparing their 

 efifectiveness in offering selective agencies their raw material. Both asexual and sexual 

 methods of reproduction occur in nearly all groups of animals and plants. In neither king- 

 dom was sex developed as a more rapid means of multiplication; rather it fulfilled some other 

 requirement. After origin of sex many changes in reproductive mechanisms occurred among 

 plants, but almost all of them resulted in greater protection of the gametes, in increased assur- 

 ance of fertilization, or in provision for better distribution, which may be interpreted as 

 variations tending to perfect sexuality. Coincident with this, two important retrogressive 

 developments occurred — apogamy and hermaphroditism — followed by evolution of methods 

 of cross-fertilization, which seems to have been of immense advantage. — Essential evolu- 

 tionary changes affecting reproduction in animals are strikingly similar to those in plants. 

 Although asexual reproduction is found in most of the great groups of animals, it evidently 

 did not meet all requirements since sexual reproduction is established in every phylum. 

 Hermaphroditism is a secondary, not a primitive, phenomenon, and, as in plants, it was not 

 found adequate. Further specialization resulted in mechanisms providing for mixtures of 

 different germ-plasms. — Both animals and plants have adopted methods of reproduction which 

 are identical in their essential features, something that can be said of no other life process. 

 The significant feature is reduction of nuclear material in the gametes. This parallel 

 evolution is of itself valid evidence of the importance of the process. For its interpretation 

 compare sexual and asexual reproduction as an actual means for the transmission of char- 

 acters. Extremely narrow variability of pedigreed inbred strains of Nicotiana and wheat 

 indicate no higher heredity coefficient for sexual reproduction. Among animals it appears that 

 the coefficient of heredity is as high for asexual as for sexual reproduction. But is this also 

 true for germinal variation? It is believed the frequency of bud variations in higher plants 

 propagated asexually shows that it is. — Even though there does not seem to be sufficient dif- 

 ference between sexual and asexual reproduction as regards variation frequency to make it a 

 subject of experimental proof, certain theoretical points raise suspicion that there is such a 

 difference. Parthenogenetic individuals having the haploid number of chromosomes should 

 show proportionately more germinal variations than members of the same species having the 

 diploid number of chromosomes, because both recessive and dominant variations should be 

 recognizable in the former. That bud variations occur more frequently in heterozygotes 

 than homozygotes means simply that bud variations are detected more frequently in hetero- 

 zygotes because the majority of bud variations are retrogressive and therefore show only 

 when the organism is heterozygous for the character affected. — The idea of Matjpas, that 

 continued asexual reproduction is impossible through some protoplasmic limitation, is rejected 

 in favor of Weismann's conclusion, that a mixture of germ-plasms offers sufficient advantages 

 to account for everything, which idea finds its main argument in Mendelian heredity. If N 

 variations occur in the germ-plasm of an asexually reproducing organism only A^ types can be 

 formed to offer raw material to selective agencies. But if A^ variations occur in the germ- 

 plasm of a sexually reproducing organism 2" types can be formed. The advantage is almost 

 incalculable. These advantages remain even though it should be shown later that the more 

 fundamental and generalized characters of an organism are not distributed by Mendelian 

 heredity. The majority of variations seem to be comparatively small, changes in detail, the 

 very kind known to be Mendelian in their inheritance. The prime reason for the success of 

 sexual reproduction is the opportunity it gives for mingling germ-plasms of different con- 

 stitution and thereby furnishing many times the quantity of raw material to selective agencies 

 that could possibly be produced through asexual reproduction. — Minor advantages accruing 

 from asexual reproduction are, first, heterosis or hybrid vigor, which is best explained on the 

 basis of linked dominant characters. Second, division of labor is made possible by secondary 

 sexual characters in general, including those which separate the egg and the sperm. Finally, 

 there is a presumable advantage in sex-linked characters, a mechanism contributing to the 

 mixing of germ-plasms. — The essential feature of the role of reproduction in evolution is the 

 persistence of mechanisms in both the plant and animal kingdoms which offer selective agen- 

 cies the greatest amount of raw material. — E. B. Babcock. 



