This is the first entry in what I hope will be a series of posts on the subject of miniaturization in animals, specifically in land snails.
I am writing these partly to organize my own thinking, so the content may get technical at times. Also, if this were a paper intended for publication (it actually is, but not yet), I would start out by giving examples of the tiniest animals. But, the nice thing about writing for the Internet is that I can start anywhere I want and post what I’ve written in any order I want. I will revise and reorganize as I go along.
The subject of miniaturization brings 2 broad questions to my mind:
1. What are the evolutionary processes that favor and result in miniaturization of animals?
2. What are the consequences of miniaturization?
Tied in with the last question are additional ones. What are the limits to miniaturization? Or, in other words, what is the smallest animal that has evolved and can theoretically evolve? Also, can one species become very small and still retain the morphological and physiological characteristics of larger species?
Rensch1, early in the 1950s reviewed the then available evidence on the genetics of body size and concluded that “body size is brought about by single genes in some cases and by several genes in others”. This is probably true, but then, he continued with the following statement:
Nevertheless, body size must invariably be considered as a single character in processes of selection, and if this character is favored, a large number of morphological, anatomical, histological, physiological, and developmental relations will be changed in the process.I have to do some more thinking to understand what he exactly meant by this and to decide whether I agree with him or not.
Regarding the anatomical changes that accompany miniaturization, Rensch1, also offered the following generalization: “In approaching the lower size limit of phylogeny, the organisms usually reduce special structures and special organs and only the indispensable mechanisms are maintained.”
Hanken & Wake2 further developed this idea. From their review, I have extracted following 3 possible evolutionary consequences of miniaturization in animals:
1. Underdevelopment or loss of organs.
2. Variation in the presence/absence of an organ.
3. Morphological novelties.
There are numerous examples of underdevelopment or loss of organs and related variation in miniature land snails. I will present examples and continue this discussion in a future post.
1. Rensch, B. 1959. Evolution above the species level. Columbia University Press.
2. Hanken, J. & Wake, D.B. 1993. Miniaturization of Body Size: Organismal Consequences and Evolutionary Significance. Annual Review of Ecology and Systematics, 24:501-519.