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It is said that "necessity is the mother of invention". To be sure,
wheels and pulleys were invented out of necessity by the tenacious
minds of upright citi zens. Looking at the history of mankind,
however, one has to add that "Ieisure is the mother of cultural
improvement". Man's creative genius flourished only when his mind,
freed from the worry of daily toils, was permitted to entertain
apparently useless thoughts. In the same manner, one might say with
regard to evolution that "natural selection mere(y tnodifted, while
redundanry created". Natural selection has been extremely effective
in policing alleHe mutations which arise in already existing gene
loci. Because of natural selection, organisms have been able to
adapt to changing environments, and by adaptive radiation many new
species were created from a common ancestral form. Y et, being an
effective policeman, natural selection is extremely conservative by
nature. Had evolution been entirely dependent upon natural
selection, from a bacterium only numerous forms of bacteria would
have emerged. The creation of metazoans, vertebrates and finally
mammals from unicellular organisms would have been quite impos
sible, for such big leaps in evolution required the creation of new
gene loci with previously nonexistent functions. Only the cistron
which became redun dant was able to escape from the relentless
pressure of natural selection, and by escaping, it accumulated
formerly forbidden mutations to emerge as a new gene locus.
Natural selection operates among individual organisms which differ
in their genetic constitution. The degree of hereditary variability
within a species is greatly enhanced by cross-fertilization.
Indeed, the mechanism of sexual reproduction occurred very early in
evolution, for it is seen today even in bacteria. In Escherichia
coli, fertilization occurs by passage of the single chromosome from
the male into the female bacterium (LEDERBERG, 1959). In
multicellular organisms, the separation of germ from soma, and the
production of haploid gametes became mandatory. The gametes were of
two types. One, extremely mobile, was designed to seek out and
penetrate the other, which loaded with nutrients, received the
mobile gamete and intiated the development of a new individual. The
foundation for true bisexuality was thus laid. In the primitive
state of bisexuality, whether an individual is to be a
sperm-producing male or an egg-producing female appears to be
decided rather haphazardly. In the worm, Banelia viridis, the
minute males are parasites in the female. Larvae that become
attached to the proboscis of an adult female become males, while
unattached larvae sink to the bottom and become females (BALTZER,
1935). The more sophisticated state of bisexuality was initiated by
setting aside a particular pair of chromosomes for specialization
and making either the male or the female a heterogametic sex. Sex
chromosomes as we know them were thus born.
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