21 December 2005

Evolution rules

In yesterday's post I introduced some of the evolutionary rules Bernhard Rensch listed in his 1959 book Evolution Above the Species Level. Rensch’s Rule No. 16 relates to the origin of terrestrial animals:

"16. Terrestrial animals could originate only after the development of the following characters: solid tissues and sturdy organs of locomotion (Archimedes' Principle: in water the body weight is decreased by as much as the weight of the volume of water which is displaced by it), organs for direct aerial respiration, and mechanisms preventing desiccation. As sensory epithelia can function only if kept in a moist condition, these organs had to be 'withdrawn' into the interior of the body."

Of the more than 30 known phyla of animals1, only 6 have terrestrial representatives (Platyhelminthes, Nemertina, Mollusca, Annelida, Arthropoda, Chordata). Ernst Mayr2 in Populations, Species, and Evolution (1970), citing Rensch’s book, attributed the relative scarcity of terrestrial phyla to 5 "facts".

"(1) Weight. Water is 800 times as heavy as air. Only animals with a strong skeleton or armor can become terrestrial. A jellyfish needs no support in water, but collapses completely as soon as it is brought on land. Ciliary locomotion, so common in water, is useless in air. Locomotion on land requires strong muscles.
(2) Protection against the environment. A land animal must be protected against the danger of drying out and against strong fluctuations of temperature. There is a high selective premium in favor of a tough skin, armor, or scales.
(3) The excretory system. Permanent life on land, in contrast to amphibious existence, requires the excretion of metabolic end products in such a manner as to reduce water loss to a minimum.
(4) The respiratory system. It must be possible for the land animal to take in oxygen directly from the air.
(5) Sense organs. On land, there is much greater need than in water for sense organs that are effective at long range, particularly among the rapidly moving land animals; hence land animals must develop long-distance vision and hearing."

Mayr appears to have extracted Nos. 1, 2 and 3 from Rensch’s Rule No. 16 quoted above, and based his No. 5 on Rensch's rule No. 11 that I quoted yesterday. I am not sure if and where in his book Rensch talks about the evolution of the excretory system in relation to terrestrial life.

I am assuming what Mayr meant by calling these "facts" is that he presumed that they are requirements that must be fulfilled during the evolution of a lineage of animals that are increasingly more adapted to living outside of water. Although I don't agree with everything he says, in future posts I will return to Mayr's facts Nos. 1-4 as they relate to the evolution of grades of terrestriality in gastropods. For the time being, I will only point out that Mayr's fact No. 5 that "land animals must develop long-distance vision and hearing" is not a requirement at all for life on land. Land snails, annelids (i.e., earthworms) and many arthropods don't have long-distance vision and hearing and I don't see why there can't be a completely blind and deaf land animal. Therefore, I am dropping "fact" No. 5 from further consideration.


1. The exact number of animal phyla depends on who is counting.
2. Curiously, Mayr wrote in Populations, Species, and Evolution that there were only 3 terrestrial phyla, missing the platyhelminths, nemertines and annelids. Some nematodes (Nematoda) that I often observe in land snail cultures and which are active at 100% humidity in the absence of liquid water may also be considered terrestrial, bringing the total number of phyla with terrestrial representatives to 7.

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