RETURN SELECTION.
The plus and minus selection series already described make it clear that one can, in a race of hooded rats, either increase or decrease the average pigmentation at will, and at the same time secure more advanced stages either of pigmentation or of depigmentation than those previously occurring in the race. The question now arises, are these changes permanent; will these displaced means retain their new position, if the race is left to itself; or will the newly obtained stages vanish as soon as selection is suspended? A presumption that the changes will prove permanent is afforded by the gradual decrease of regression and its final reversal in the case of offspring of a particular grade, upon repeated selection made in the same direction. (Seepage 12.) But in order to test the matter more directly and thoroughly, the experiment has been repeatedly made of reversing the course of selection, after it had been in progress for several generations, with a view of ascertaining whether the return toward the former condition of the race would be made more speedily and easily than the original departure from it had been.
The first experiment of this sort was a return selection from generation 6 (and 6½) of the minus selection series. The parents of generation 6 (Table 21) averaged -1.86 in grade; the average grade of their offspring was -1.56, a regression of 0.30. The range of the offspring extended from 0 to -2.50. Some low-grade offspring were chosen for a return selection series (Table 31). The mean grade of the selected pairs ranged from -0.37 to -0.87, their mean being -0.60. These parents produced 118 offspring, whose average grade was -1.28, a regression of 0.68in a direction contrary to that of the regression in theminus selection series. The large amount of the regression might seem to imply that it was even more difficult to return toward the former state of the race (in the neighborhood of 0) than it had been to depart from it, but this can not be insisted on, because the number of individuals under observation is not sufficiently large. To test the reality and permanency of the reversed regression, the selection was repeated five additional times, altogether six successive return selections being made with the idea of undoing what had been effected by six original selections in an opposite direction. The result of the second successive return selection is shown inTable 32. The parents here were of grade -0.50 and they produced 19 offspring of the average grade -0.95, a regression of 0.45away from0 as before.
Table 33shows the result of the third return selection. Individuals entered in Table 32 as offspring appear here as parents. Only those pairs which were of mean grade, -0.25 or -0.37, should really be regarded as athird return selection. They gave offspring with mean grades of -0.63 and -0.86 respectively, which show regression of 0.38 and 0.49away from0.
But Table 33 shows also the character of young produced by -1.12 and -1.25 parents in this same third return-selection generation,i. e., byunselectedparents of the generation in question. Their young also regressaway from0—that is, in the direction of the original selection. The -1.12 parents produced -1.61 offspring, a regression of 0.49, while the -1.25 parents produced -1.35 offspring, a regression of 0.10. For Table 33 as a whole the regressionaway from0 averages 0.31.
A fourth generation in the return-selection series is summarized inTable 34. The parents are of mean grade -0.63; their 50 offspring are of mean grade -1.17, a regression amounting to 0.54away from0 and in the direction of the six generations of original selection.
Table 35contains the results of the fifth generation of the series. The parents are here of mean grade -0.65. The number of offspring is very small (13), but they nevertheless show the reversed regression which characterized the four preceding generations. Their mean was -0.75, a regression of 0.10 away from 0.
A sixth and final generation in this return-selection experiment is summarized inTable 36. It includes 36 offspring of mean grade -0.39, the mean of the parents being -0.26, a regression of 0.13 away from 0. It will be seen, therefore, that the effect of the six original selections had not been entirely overcome by an equal number of return selections. The reason for this is obvious. Much smaller numbers are concerned in the return selections than in the original minus selections. The return selections are accordingly less efficient. Nevertheless, after the sixth return selection we find that 1 in 6 of the offspring have plus grades and their average is lower (that is,less minus) than the offspring in the minus series after a single generation of selection. (Cf. Tables16and36.)
The amount and persistency of the reversed regression in this series show clearly that return selection is not easier or more rapid than the original modification of the race by selection, but that selection in either a plus or minus direction has cumulative and permanent effects.
Further support for this conclusion is furnished by return selections (one each) made from the seventh generation, from the eighth generation, and from the eleventh generation of the minus selection series. (See Tables37, 38, and 39.) Generation 7 (Table 22) was produced by parents of average grade -2.01. Their offspring were of average grade -1.73, a regression (toward 0) amounting to 0.28. Certain pairs of these offspring of grade -0.75 and -0.87 (mean -0.78) constitute the return selection from generation 7 (Table 37). They had 33 offspring of average grade -1.15, a regressionaway from0 amounting to 0.37.
Generation 8 of the minus-selection series (Table 23) was produced by parents of mean grade -2.05. Their offspring were of mean grade -1.80, a regression (toward 0) of 0.25. Certain pairs of these offspring of grades -0.50, -0.62, and -1.00 (mean -0.72), when chosen as parents, produced 41 young of mean grade -1.51, a regressionaway from0 amounting to 0.79. (SeeTable 38.)
Generation 11 of the minus series (Table 26) was produced by parents of mean grade -2.30. The offspring were of mean grade -2.15, a regression of 0.15 toward 0. A pair of the offspring of mean grade -1.62 (Table 39) produced 16 young of mean grade -1.95, a regression of 0.32 away from 0. This result shows that the selected race had now passed the point represented by the grade of the parents (-1.62) and the offspring regressed toward a racial mean as advanced as the most extreme individuals obtained previous to selection.
To show that, in the plus selection series, a return selection has a result similar to that just described, two experiments may be cited:
The sixth generation of the plus selection series was produced by parents of mean grade 3.52, and their offspring were of mean grade 3.11, a regressiontoward0 amounting to 0.41. Certain of these offspring of mean grade 2.00, when chosen as parents, produced 17 young of mean grade 2.36, a regressionaway from0 amounting to 0.36. (SeeTable 40.)
The eleventh generation of the plus selection series (Table 11) was produced by parents of mean grade -3.97; their offspring were of mean grade -3.78, a regression of 0.19 toward 0. Certain of these offspring, ranging in grade from -2.62 to -3.25 (Table 41), mean -2.79, produced 53 young of mean grade -3.32, a regression away from 0 amounting to 0.53. The regression in this case, as in all those previously described, wastoward the racial mean of the previous generation, which, however, it has in no case reached.
This can have but one meaning. The genetic character of the hooded rat is in a general way correctly indicated by its somatic character.Selection is therefore immediately effective, whether plus or minus in character, and whether or not preceded by selection in the same direction or in an opposite direction.But regression may be expected from the character of aberrant parents back toward the normal of the previous generation, yet this regression will in general be less than the departure of the aberrant parents from the normal of their generation. If one desires in such a case to obtain continuous and progressive departure from the normal in either a plus or a minus direction, he need only select continuously in the desired direction.