14 CHAPTER 1following way: “If variations useful to any organic being ever occur, assuredly
individuals thus characterized will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance, these will tend to
produce offspring similarly characterized. This principle of preservation, or the
survival of the fittest, I have called natural selection.” Unlike Lamarck’s transfor-
mational theory, in which individual organisms change, Darwin’s is a variational
theory of change, in which the frequency of a variant form (i.e., the proportion of
individuals with that variant feature) increases within a population from genera-
tion to generation (FIGURE 1.6B). Darwin proposed (as did Wallace) that fitter
individuals differ only slightly from the norm of the population, but that a feature
such as body size gradually evolves to become more and more different because
new, slightly more extreme, advantageous variants continue to arise.
Darwin’s theory of evolution includes five distinct components [18]:- Evolution as such is the simple proposition that the characteristics of
organisms change over time. Darwin was not the first to have this idea, but
he so convincingly marshaled the evidence for evolution that most scientists
soon accepted that it has indeed occurred. - Common descent: Differing radically from Lamarck, Darwin was the first to
argue that species had diverged from common ancestors and that species
could be portrayed as one great family tree representing actual ancestry (see
Figure 1.5B). - Gradualism is Darwin’s proposition that the differences between even
radically different organisms have evolved by small steps through
intermediate forms, not by leaps (“saltations”). - Populational change is Darwin’s hypothesis that evolution occurs by changes
in the proportions (frequencies) of different variant kinds of individuals
within a population (see Figure 1.6B). This profoundly important, completely
original idea contrasts with the sudden origin of new species by saltation and
with Lamarckian transformation of individuals. For Darwin, the average was
a statistical abstraction; there exist only varied individuals, and there are no
fixed limits to the variation that a species may undergo [10, 18]. - Natural selection was Darwin’s brilliant hypothesis, independently conceived
by Wallace, that accounts for adaptations, features that appear “designed”
to fit organisms to their environment. Because it provided an entirely natural,
mechanistic explanation for adaptive design that had previously been attributed
to a divine intelligence, the concept of natural selection revolutionized not only
biology, but Western thought as a whole.
Darwin proposed that the various species that descend from a common ances-
tor evolve different features because those features are adaptive under differ-
ent “conditions of life”—different habitats or habits. Moreover, the pressure of
competition favors the use of different foods or habitats by different species. He
believed that no matter how extensively a species has diverged from its ancestor,
new hereditary variations continue to arise, so that given enough time, there is
no evident limit to the amount of divergence that can occur.
Where, though, do these hereditary variations come from? This was the great
gap in Darwin’s theory, and he never filled it. The problem was serious because,
according to the prevailing belief in blending inheritance, variation should
decrease, not increase. Because offspring are often intermediate between their
parents in features such as color or size, it was widely believed that character-
istics are inherited like fluids, such as paints of different colors. (This notion01_EVOL4E_CH01.indd 14 3/23/17 8:43 AM