© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

20.1  Biological Evolution

From the "viewpoint" of living organisms at the level of the species, biological evolution must be regarded as a predominantly divergent process. The injection of a few members of a given type of lifeforms into a virgin environment normally results in an explosive "adaptive radiation." Species rapidly multiply to fill all possible available niches.

From the "viewpoint" of the whole environment, however, evolution is primarily convergent. Though there are many exceptions, the general rule is that species evolving in roughly similar environments tend to find similar solutions to similar problems of survival.

For instance, the deep-sea niche (where high speed may be required to feed on fast-swimming prey) has given rise to a striking example of parallel evolution. The large, streamlined "fishy" shape has evolved independently at least four times on Earth: The mosasaur (an extinct Cretaceous marine lizard), the ichthyosaur (an extinct Jurassic sharklike reptile), the tuna (and other modern fishes, including sharks), and the cetaceans (modern mammals, including dolphins).

Examples abound. Ants and termites belong to different insect orders, yet they have evolved similar societal and architectural forms. Marsupial "psuedomamnals" evolved independently in Australia and South America, filling niches identical to those occupied elsewhere by their physiological cousins the mammals. In earlier chapters we discussed the convergent evolution of legs, wings, eyes and other bodily organs. It is probably safe to conclude that "anytime you get extensive convergence of evolution along different lines you must be dealing with an almost certain process."²²

Based on their observations of convergent evolution in similar environments, human zoologists and paleontologists have proposed a number of evolutionary "laws". These "laws" appear generally true on Earth, and may be expected to have some measure of applicability to extraterrestrial lifeforms as well. Here are a few of the author’s favorites:

1. The total biomass of the entire system tends to increase and become maximized over time. (Lotka’s Rule)

2. The general process of evolution involves the development of new organ systems, increasing complexity, and greater efficiency.

3. Progressively more modern forms tend to have fewer, more specialized segments and appendages. (Williston’s Law)

4. Species tend to evolve to larger sizes. (Cope’s Law)

5. Major evolutionary steps, once taken, are never reversed. (Dollo’s Law)

6. Allied races of warm-blooded animals tend to be larger in colder climatic regions. (Bergmann’s Rule)

7. Mammals inhabiting tropical regions tend to have shorter and less woolly coats than related lifeforms in colder climatic regions.

8. Herbivores have hooves; carnivores have claws. (Cuvier’s Law)

9. Limbs and tails of related species are shorter in colder climatic regions. (Allen’s Rule)

10. Organisms living in warm, humid areas tend to be more heavily pigmented than related species living in cool, dry regions. (Gloger’s Rule)

Examples of this sort can be multiplied indefinitely. Dr. Bernhard Rensch lists more than 100 such "laws" of evolution, to illustrate his thesis that "evolution is largely a lawful process, and with regard to the effect of continuous mutation and natural selection it is also a determinate process."²⁸⁹⁷

There is one other general rule of evolution, often ignored but of major significance nevertheless: Species tend to become extinct. This simple truth is rarely appreciated fully. It is a fact that more than 99% of all species that have ever trod the Earth are now extinct. While more than 3,000,000 species are alive today, more than a billion animal and plant species have arisen at one time or another in Earth’s past.^624,2440 Most evolutionary experiments prove to be dead ends or are insufficiently adaptive to changing conditions. Extinction, not survival, is the general rule.¹⁶⁶⁸

Last updated on 6 December 2008