Biology Reference
In-Depth Information
Fig. 4.1
source of confusion with this term because its meaning has changed over time.
“Homology” was coined by a contemporary, and bitter rival, of Charles Darwin
called Richard Owen. Owen was an anatomist, so he defined the term in 1843
to mean “the same organ in different animals under every variety of form and
function”. Unfortunately, some biologists today use the term “homology” to mean
“evolutionarily related”, so to cite homology in the latter sense as evidence for
evolution is a circular argument.
Then we have direct observation, as the second line of evidence. The rate of evo-
lution depends upon the rate of reproduction, and this is much too slow in the case of
animals like ourselves to be seen directly, but in the case of very rapidly reproducing
organisms like viruses, bacteria and some insects, changes in the genetic composi-
tion can be seen between generations. Thirdly, if one type of organism has changed
into another type of organism over long periods of time, there should be transitional
forms sharing characters from both types in the fossil record. I will show you some
examples that bridge the gap between reptiles and birds, and the gap between fish
and amphibians.
Then we have logical inference - the fourth line of evidence. Evolution depends
upon two processes - the mutation of DNA, and natural selection operating on the
effects of these mutations on the properties of the organism. Both these processes are
directly observable today, so it logically follows that evolution must have happened
if these processes occurred in the past. In fact, if we did not have strong evidence
that evolution had happened, and is still happening, we would have a problem to
explain why evolution has not occurred!
