| Darwin proposed that evolution could be explained by
the differential survival of organisms following their naturally
occurring variation--a process he termed "natural selection." According
to this view, the offspring of organisms differ from one another and
from their parents in ways that are heritable--that is, they can pass on
the differences genetically to their own offspring. Furthermore,
organisms in nature typically produce more offspring than can survive
and reproduce given the constraints of food, space, and other
environmental resources. If a particular off spring has traits that give
it an advantage in a particular environment, that organism will be more
likely to survive and pass on those traits. As differences accumulate
over generations, populations of organisms diverge from their ancestors.
Darwin's original hypothesis has undergone extensive modification and
expansion, but the central concepts stand firm. Studies in genetics and
molecular biology--fields unknown in Darwin's time--have explained the
occurrence of the hereditary variations that are essential to natural
selection. Genetic variations result from changes, or mutations, in the
nucleotide sequence of DNA, the molecule that genes are made from. Such
changes in DNA now can be detected and described with great precision.
Genetic mutations arise by chance. They may or may not equip the
organism with better means for surviving in its environment. But if a
gene variant improves adaptation to the environment (for example, by
allowing an organism to make better use of an available nutrient, or to
escape predators more effectively--such as through stronger legs or
disguising coloration), the organisms carrying that gene are more likely
to survive and reproduce than those without it. Over time, their
descendants will tend to increase, changing the average characteristics
of the population. Although the genetic variation on which natural
selection works is based on random or chance elements, natural selection
itself produces "adaptive" change--the very opposite of chance.
A View
From the National Academy Sciences |
