Lecture 14: The Evidence for Natural Selection
such processes would have generated new species, closely related, yet each
adapted to its own island. The Galapagos ¿ nches offered a paradigm of
the working of natural selection. Natural selection could explain both why
the ¿ nches were well adapted and why they were closely related. Deistic
arguments could offer no reason why
they should be related.What direct evidence did Darwin
offer for his theory? First, he pointed
to the fossil record. It was already
clear that the fossil record contained
many species that no longer existed.
But there were so many gaps in
the fossil record that to refute the
argument that God had repeatedly
created new species, Darwin needed
to show the existence of “transitional”
species, demonstrating the slow evolution of one species into another. While
a deistic theory of life did not need transitional species, the theory of natural
selection did. Fortuitously, such a fossil was found in 1861, just two years
after his book was published. It belonged to a bird-like species now known
as Archaeopteryx, which lived about 150 million years ago. It was about
the size of a magpie and had feathered wings and a wishbone, features one
would expect of a bird. But it also had some distinctly reptilian features,
including teeth and a bony tail. These made it similar to a small dinosaur.
Archaeopteryx was the perfect “transitional” fossil.Second, Darwin pointed to “homologies,” or unexpected similarities between
species. Many striking features can be found across many different species,
such as the ¿ ngers of mammals. A deistic theory of life naturally supposed
that homologies reÀ ected good design. But not all homologies could be
explained in this way. For example, whales have ¿ ngers, though they have no
use for them. There was no reason why a rational creator should have given
whales ¿ ngers. But natural selection, which worked by constantly tweaking
existing species, provided a natural explanation for such “survivals.” It
suggested that such species were similar because they were related. (We now
know that whales are indeed descended from mammal ancestors.)The rapid evolution of
disease vectors such as
AIDS also provides direct
evidence of natural selection,
as does the declining
effectiveness of antibiotics
as new, resistant strains of
bacteria have evolved.