Darwin’s theory was the utterly random variations which had finally produced
Man. And what was more, Darwin had turned Marv into a product of
something as unsentimental as the struggle for existence.”
“Did Darwin have anything to say about how such random variations
arose?”
“You’ve put your finger on the weakest point in his theory. Darwin had
only the vaguest idea of heredity. Something happens in the crossing. A
father and mother never get two identical offspring. There is always some
slight difference. On the other hand it’s difficult to produce anything really new
in that way. Moreover, there are plants and animals which reproduce by
budding or by simple cell division. On the question of how the variations arise,
Darwin’s theory has been supplemented by the so-called neo-Darwinism.”
“What’s that?”
“All life and all reproduction is basically a matter of cell division. When a
cell divides into two, two identical cells are produced with exactly the same
hereditary factors. In cell division, then, we say a cell copies itself.”
“Yes?”
“But occasionally, infinitesimal errors occur in the process, so that the
copied cell is not exactly the same as the mother cell. In modern biological
terms, this is a mutation. Mutations are either totally irrelevant, or they can
lead to marked changes in the behavior of the individual. They can be directly
harmful, and such ‘mutants’ will be continually discarded from the large
broods. Many diseases are in fact due to mutations. But sometimes a
mutation can give an individual just that extra positive characteristic needed to
hold its own in the struggle for existence.”
“Like a longer neck, for instance?”
“Lamarck’s explanation of why the giraffe has such a long neck was that
giraffes have always had to reach upwards. But according to Darwinism, no
such inherited characteristic would be passed on. Darwin believed that the
giraffe’s long neck was the result of a variation. Neo-Darwinism supplemented
this by showing a clear cause of just that particular variation.”
“Mutations?”
“Yes. Absolutely random changes in hereditary factors supplied one of
the giraffe’s ancestors with a slightly longer neck than average. When there
was a limited supply of food, this could be vital enough. The giraffe that could
reach up highest in the trees managed best. We can also imagine how some
such ‘primal giraffes’ evolved the ability to dig in the ground for food. Over a
very long period of time, an animal species, now long extinct, could have
divided itself into two species. We can take some more recent examples of
the way natural selection can work.”
“Yes, please.”
“In Britain there is a certain species of butterfly called the peppered
moth, which lives on the trunks of silver birches. Back in the eighteenth
century, most peppered moths were silvery gray. Can you guess why,
Sophie?”
“So they weren’t so easy for hungry birds to spot.”
“But from time to time, due to quite chance mutations, some darker ones
were born. How do you think these darker variants fared?”
“They were easier to see, so they were more easily snapped up by
hungry birds.”