Phenetics of Natural Populations. II. Asymmetry and Evolution in a Lizard

Abstract

The minor, "random" differences between bilateral structures in 20 natural populations (18 insular populations in the Gulf of California, Mexico, and 2 continental populations) of lizards were examined. It was discovered that, in general, if a population is relatively asymmetrical for one character, it has a high probability of being asymmetrical for other characters as well. This phenomenon is called the population asymmetry parameter (PAP) and differs from the apparently nonexistent organism wide asymmetry property and is not dependent on it. Many of the populations on islands less than 3 sq. km. in area are very asymmetrical (i.e., the average of the individual''s asymmetry values are high) com- pared to continental populations. This is especially true of the most phenetically distinct of the small island populations. Populations on both recently formed continental islands and large islands tend to be relatively symmetrical. The lowest asymmetry values were found in what is probably the oldest and most distinct of the large island popula-tions-that on Angel de la Guarda. Previous writers are followed in interpreting "fluctuating" asymmetry as resulting from random accidents during development. The level of asymmetry is thought of as an indication of the effectiveness of the developmental control systems in buffering against these accidents. A model is proposed to account for the results. The model incorporates 3 relevant variables: immigration, the kind of selection regime, and time. The input of "foreign" chromosomes will dismantle coadapted gene complexes by recombining with them; this should cause a deterioration in developmental homeostasis, and hence, an increase in asymmetry. Directional (and disruptive) selection, by leading to changes in the types and frequencies of chromosomes, should have the same result, whereas a relatively stabilizing selective regime would favor coadapted gene complexes and should enhance developmental homeostasis. The nature of the ecological determinants of selection regimes is incorporated into the model. The level of coadaptation will also depend on the length of time the population has been isolated. The interactions of these variables is discussed. The utility of the model and method for population biology is discussed briefly.