Claim CB805:
Since evolution says organisms came from a common ancestor and since they
lived in a continuity of environments, we should see a continuum of
organisms. There should be a continuous series of animals between cats
and dogs, so that one could not tell where cats left off and dogs began.
Source:
Morris, Henry M. 1985. Scientific Creationism. Green Forest, AR: Master
Books, pp. 70-71.
Response:
- The claim might be true if there were no such thing as extinction. But
since species do become extinct, intermediates that once existed do not
exist today. Since extinction is a one-way street, species can only
become less connected over time. This is clear if we look at the fossil
record, in which early members of separate groups are much harder to
tell apart.
- Environments (and ecological niches) are not really as continuous as the
claim pretends. Dogs bring down their prey through long chases, and
cats ambush their prey; dogs are made for long-distance running, and
cats are made for short sprints with high acceleration from a standing
start. These requirements are quite different, and it is hard to achieve
both in a single body. Compromises between the two have disadvantages
in competition with specialists for either type, and thus natural
selection culls them. Intermediates are competitive only so long as
specialists are absent; so when specialists evolve, the intermediates
are likely to become extinct.
- In part, distinctness is an illusion caused by our choice of which
groups to give names to. Groups with unclear boundaries tend not to get
separate names, or groups in which intermediate forms exist are chopped
in half arbitrarily (especially obvious if fossil forms are considered;
e.g., the line between dinosaurs and birds is
arbitrary,
increasingly so as new fossils are discovered).
- There are indeed several cases of continua in nature. In many groups,
such as some grasses and leafhoppers, different species are very hard
to tell apart. At least ten percent of bird species are similar enough
to another species to produce fertile hybrids (Weiner 1994, 198-199).
The most obvious continua are called ring species, because in the
classic case (the herring gull complex) they form a ring around the
North Pole. If we start in Western Europe and move west, similar
populations, capable of interbreeding, succeed each other
geographically. When we have traveled all the way around the world and
reach Western Europe again, the final population is different enough
that we call it a separate species, and it is incapable of
interbreeding with herring gulls, even though they are connected by a
continuous chain of interbreeding populations. This is a big problem
for creationists. We expect kinds to be easily determined if they were
created separately, but there are no such obvious divisions:
They are mistaken, who repeat that the greater part of our species
are clearly limited, and that the doubtful species are in a feeble
minority. This seemed to be true, so long as a genus was imperfectly
known, and its species were founded upon a few specimens, that is to
say, were provisional. Just as we come to know them better,
intermediate forms flow in, and doubts as to specific limits
augment. (de Condolle, quoted in Darwin, 1872, chap. 2)
References:
Further Reading:
Hazard, Evan B. 1998. Teaching about "intermediate forms." The
American Biology Teacher 60(5): 359-361.
Darwin, C., 1859. The Origin of Species, 1st Edition. Senate,
London.
http://www.talkorigins.org/faqs/origin/chapter4.html
created 2003-4-26, modified 2006-4-23