Biology Reference
In-Depth Information
When we call
Tiktaalik
a transitional species, this does not necessarily mean that
this species was on the direct line of descent from fish to tetrapods - it could repre-
sent an extinct branch on the line of descent between these two groups. But
Tiktaalik
does demonstrate the past existence of a species intermediate in form between fish
and tetrapods, as predicted by evolutionary theory. For this reason the term “transi-
tional fossil” is better than the term “missing link”. The value of such transitional
fossils is that they show us the order of the evolutionary steps that connect one type
of organism, such as fish, with a later type of organism, such as tetrapods. In the
same way,
Archaeopteryx
probably is not on the direct line of descent of modern
birds - remember that the tree of life is highly branched and most species of life are
now extinct!
Logical Inference
Figure 4.20 illustrates another line of evidence in support of evolutionary theory -
the fact that DNA undergoes mutation. Mutation is defined as a change in the base
sequence of the genetic material of the organism. Mutation is the ultimate source of
the variation between individuals in a population that was so well documented by
Darwin in his topic. Remember that Darwin knew nothing about the genetic material
or the mechanism of heredity - he had some ideas about how the latter might work,
but these were hopelessly wrong.
Fig. 4.20
