Evolution__3rd_Edition

488 PART 4 / Evolution and Diversity
Figure 16.8
The divergent pattern
of evolution. From
Darwin (1859).
According to Darwin’s principle of
divergence ...
. . . competition pushes species
apart
A B C D E F G H I K L
a natural explanation for adaptation and evolution. As environments change, and
competing species change, species will evolve new adaptations. By itself, this theory
does not account for the tree-like, divergent course of evolution. Darwin was well
aware that evolution had steered such a course, indeed the hierarchical structure, of
groups within groups in classification, had been established as fact in the early nineteenth
century a by (among others) Geoffroy St Hilaire’s morphological, and Milne-
Edwards’s embryologic, work. So striking a fact had to fitted into the theory. Darwin
recalled in his autobiography that in the early (1844) version of his theory:
I overlooked one problem of great importance, and it is astonishing to me how I could
have overlooked it and its solution. This problem is the tendency in organic beings
descended from the same stock to diverge in character as they become modified. That they
have diverged greatly is obvious from the manner in which species of all kinds can be
classed under genera, genera under families, families under suborders and so forth. ...
The solution occurred to me long after I had come to Down. The solution, I believe, is that
the modified offspring of all dominant and increasing forms tend to become adapted to
many and diversified places in the economy of nature.
This was Darwin’s principle of divergence (Figure 16.8). Why should it be that species
apparently push one another apart in evolution? Darwin suggested that it mainly
resulted from the relative strengths of competition for resources from more closely
related individuals on the one hand, and from more distantly related individuals on the
other. An individual of a species will compete strongly against other members of its
own species, fairly strongly against members of other species in its own genus, and
then more weakly against members of more distantly related groups. There is little or
no competition at the taxonomic extremes, between an average plant and an average
animal, for example.
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