Mutations that take place in the ordinary course of nature, without
man’s interference, are _spontaneous mutations_. Most of these arise out
of the very nature of the complicated mechanism of gene replication.
Copies of genes are formed out of a large number of small units that
must be lined up in just the right pattern to form one particular gene
and no other.
Ideally, matters are so arranged within the cell that the necessary
changes giving rise to the desired pattern are just those that have a
maximum probability. Other changes are less likely to happen but are not
absolutely excluded. Sometimes through the accidental jostling of
molecules a wrong turn may be taken, and the result is a spontaneous
mutation.
We might consider a mutation to be either “good” or “bad” in the sense
that any change that helps a creature live more easily and comfortably
is good and that the reverse is bad.
It seems reasonable that random changes in the gene pattern are almost
sure to be bad. Consider that any creature, including man, is the
product of millions of years of evolution. In every generation those
individuals with a gene pattern that fit them better for their
environment won out over those with less effective patterns—won out in
the race for food, for mates, and for safety. The “more fit” had more
offspring and crowded out the “less fit”.
By now, then, the set of genes with which we are normally equipped is
the end product of long ages of such _natural selection_. A random
change cannot be expected to improve it any more than random changes
would improve any very complex, intricate, and delicate structure.
[Illustration: _Evolution of the horse (skull, hindfoot, and forefoot
shown). Note the changes over a 60-million-year period from the Eocene
era to the present._]
Pleistocene and Recent
Pliocene
Miocene
Oligocene
Eocene
Yet over the eons, creatures have indeed changed, largely through the
effects of mutation. If mutations are almost always for the worse, how
can one explain that evolution seems to progress toward the better and
that out of a primitive form as simple as an amoeba, for instance, there
eventually emerged man?
In the first place, environment is not fixed. Climate changes,
conditions change, the food supply may change, the nature of living
enemies may change. A gene pattern that is very useful under one set of
conditions may be less useful under another.
Suppose, for instance, that man had lived in tropical areas for
thousands of years and had developed a heavily pigmented skin as a
protection against sunburn. Any child who, through a mutation, found
himself incapable of forming much pigment, would be at a severe
disadvantage in the outdoor activities engaged in by his tribe. He would
not do well and such a mutated gene would never establish itself for
long.
If a number of these men migrated to northern Europe, however, children
with dark skin would absorb insufficient sunlight during the long winter
when the sun was low in the sky, and visible for brief periods only.
Dark-skinned children would, under such conditions, tend to suffer from
rickets.
Mutant children with pale skin would absorb more of what weak sunlight
there was and would suffer less. There would be little danger of sunburn
so there would be no penalty counteracting this new advantage of pale
skins. It would be the dark-skinned people who would tend to die out. In
the end, you would have dark skins in Africa and pale skins in
Scandinavia, and both would be “fit”.
In the same way, any child born into a primitive hunting society who
found himself with a mutated gene that brought about nearsightedness
would be at a distinct disadvantage. In a modern technological society,
however, nearsighted individuals, doing more poorly at outdoor games,
are often driven into quieter activities that involve reading, thinking,
and studying. This may lead to a career as a scientist, scholar, or
professional man, categories that are valuable in such a society and are
encouraged. Nearsightedness would therefore spread more generally
through civilized societies than through primitive ones.
Then, too, a gene may be advantageous when it occurs in low numbers and
disadvantageous when it occurs in high numbers. Suppose there were a
gene among humans that so affected the personality as to make it
difficult for a human being to endure crowded conditions. Such
individuals would make good explorers, farmers, and herdsmen, but poor
city dwellers. Even in our modern urbanized society, such a gene in
moderate concentration would be good, since we still need our
outdoorsmen. In high concentration, it would be bad, for then the
existence of areas of high population density (on which our society now
seems to depend) might become impossible.
In any species, then, each gene exists in a number of varieties upon
which an absolute “good” or “bad” cannot be unequivocally stamped. These
varieties make up the _gene pool_, and it is this gene pool that makes
evolution possible.
A species with an invariable set of genes could not change to suit
altered conditions. Even a slight shift in the nature of the environment
might suffice to wipe it out.
The possession of a gene pool lends flexibility, however. As conditions
change, one combination of varieties might gain over another and this,
in turn, might produce changes in body characteristics that would then
further alter the relative “goodness” or “badness” of certain gene
patterns.
Thus, over the past million years, for example, the human brain has,
through mutations and appropriate shifts in emphasis within the gene
pool, increased notably in size.