20 December 2005

Rensch’s rules

German zoologist Bernhard Rensch (1900-1990) studied birds, land snails and other animals and was one of the contributors to the evolutionary synthesis during the early to mid-20th century. He is best remembered perhaps by one of his books, Neuere Probleme der Abstammungslehre (Newer problems of the evolutionary theory) that came out in 1947, followed by the second edition in 1954. The latter edition was translated into English with some revisions and published in 1959 as Evolution Above the Species Level. A few years ago I inherited a copy of that book from a late friend’s library. For the past week or so, I have had it on my bedside table. Every nite I read a page or 2 before falling asleep.

Evolution Above the Species Level, now more than 50 years old, still offers some relevant, useful insights. Rensch apparently liked to come up with "rules" to explain many evolutionary patterns. In Chapter 4, Section D, titled "Rules of phylogenetic development", he discusses a number of them.

"One of these rules states that large terrestrial vertebrates must develop heavy columniform legs with disproportionately large bones, because with an increase in body size the volume of the body (and thus its weight) increases by the third power, while that of the bones as the supporting structures increases only by the second. Hence, such columniform legs can be seen in the large species of mammals in various orders: in elephants, rhinoceroses…

Moreover, one may state that sessile animals could evolve only in the water, because the eddying of food particles and spermatozoa towards the body cannot occur in the air, but only in a liquid medium."

After mentioning several others, Rensch goes on to list 20 more. For example:

"6. Animals, i.e. heterotrophic organisms, could not originate before the autotrophic organisms (i.e. plants) providing a source of food."

Number 6, like a few others, is quite obvious and derives from common sense. But when you start thinking about it, you realize that it is a very fundamental notion that cannot be violated without stretching definitions.


"11. Locomotion requires receptors which react to stimuli leading to food."

Other rules are more specific:

"15. In terrestrial animals, large body size can emerge only if an internal skeleton is developed, as an external one tends to become prohibitively heavy. Hence, there are not and never have been any really giant types of terrestrial Arthropoda…"

This is how Rensch explained his rationale behind these rules:

"The numerous ecological studies made during recent decades have proved that the structure of animals is definitely correlated with special modes of life and special habitats. These correlations may be summarized as more or less general rules, often applying to many animal groups. There are further rules pertaining to the functional-anatomical consequences of a certain mode of life."

And summarized his purpose as follows:

"These ecological and functional-anatomical rules may in turn be used to show the directedness of the phylogeny, from which certain lines of future evolution can be predicted.

They are intended to show how and why the evolution of organisms had to follow certain main lines."

Ernst Mayr in Populations, Species, and Evolution (another classic, now >35 yrs old, but that is still useful), extracted 5 of Rensch's rules relating to the evolution of terrestrial animals from marine ancestors to explain why there have been so relatively few transitions from the sea to land. That's a topic I have a special interest in and I will return to it in the 2nd post of this series.

Another short biography of Rensch is available here.


afarensis said...

I periodically reread sections of Rensch...I find his stuff fascinating and wish I spoke German so I could read more.


Glad to hear that. I thought I was the only one who was reading 50 or more years old books on evolution.

afarensis said...

I actually have a list of older books on evolution that I'm working my way through. Some of the ones I'm looking for (Morgan, Fisher, Wiesmann, etc.) are kind of pricey so it's going to take awhile...