Two
kinds of selection may be distinguished: (1) natural selection, in which
choice is made automatically by the failure to reproduce (through death
or some other cause) of the individuals who are not "fit" to pass the
tests of the environment (vitality, disease resistance, speed, success
in mating, or what not); and (2) artificial selection, in which the
choice is made consciously by man, as a livestock breeder.
SEX-LIMITED, a term applied to traits which differ in the two
sexes, because influenced by the hormones of the reproductive glands.
Example, the beard.
SEX-LINKED, a term applied to traits which are connected with
sex _accidentally_ and not physiologically in development. The current
explanation is that such traits happen to be in the same chromosome as
the determiner of maleness or femaleness, as the case may be.
Color-blindness is the classical example in man.
SEXUAL SELECTION, the conscious or unconscious preference by
individuals of one sex, or by that sex as a whole, for individuals of
the other sex who possess some particular attribute or attributes in a
degree above or below the average of their sex. If the deviation of the
chosen character is in the same direction (plus or minus) as in the
chooser, the mating is called assortative; if in one direction
independent of the characteristic of the chooser, it is called
preferential.
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