The meaning of "convergence" in paleo.
Phillip Bigelow (email@example.com)
Sat, 17 Dec 1994 02:56:06 GMT
Recently, there have been posts on this newsgroup that have referred
to the word "convergence", particularly in discussing the possibility that
humans were semi-aquatic in the geologic past. After I saw the term used in
reference to humans being "convergent with aquatic animals", I got
suspicious and checked some basic text references regarding the term, and
how the term should be used.
According to _Vertebrate Paleontology and Evolution_, by Robert Carroll,
CONVERGENCE is defined as characteristics of organisms that are similar
in structure **and function**, but have arisen separately in different
groups of animals. Convergent characteristics are common in groups that
become adapted to a **similar habitat** or way of life.
Now, regarding the statement that "hairlessness is a convergence with
some aquatic mammals":
The term "convergence" _cannot_ be used in the discussion of the AAT,
because humans and hippos, while they both lack appreciable hair,
do not **now** share the same habitat or way of life. Structures on dolphins and the Mesozoic-age ichthyosaurs show convergence, because we already
**know** that both of these animals lived in the sea. Humans are
terrestrial animals (**now**), whereas hippos are semi-aquatic (**now**).
Ostriches and frogs are _NOT_ convergent with each other, even though both have
long legs. Ostriches are fully terrestrial, whereas frogs are sometimes
terrestrial, sometimes arboreal, and sometimes amphibious (depending on
species). The fact that both have long legs has nothing to do with
"convergence" in this case.
The term should only be used when there is knowledge that both types of
animals share the same habitat. In the case of the AAT, they
are trying to **prove** that hominids and hippos shared a similar habitat.
The AAT proponents are doing things in reverse, at least in this case.
The Aquatic Ape people are misusing the term, primarily out of
unfamiliarity with it (I hope), rather than out of an attempt to fit
in more scientifically.
Reference: _Vertebrate Paleontology and Evolution_, by Robert L. Carroll,
1988, Freeman and Company, New York.