MANY ICHTHYOLOGISTS have tended to assume extreme positions as to whether observed differences between populations of a given species of fish are caused by genetic differences or merely by dissimilarities of environments. Early workers were inclined to regard differences, especially countable ones, as indicative of genetic divergence alone. Many of these forms were formally recognized as subspecies, or species (examples are cited in Jensen, 1944). The weight of thought swung the opposite direction after the discovery that characters could be altered through manipulation of the environment. Now ichthyologists commonly assume differences between populations of a species are environmentally induced unless a genetic basis can be demonstrated experimentally. The bulk of recent literature, however, is concerned primarily with the elucidation of the effect of environment on a given genetic stock. The problem of genetic divergence between populations of a fish species, or within a single population, has not received as much attention and evidence along these lines, therefore, is meager (but see Gordon, 1957). It is essential to the progress of systematic ichthyology, as well as to the planning of experiments on this problem, that the nature of morphological variation in fishes be properly understood. Since the early papers by Hubbs, only Taning (1952) has reviewed this subject. Taning stressed the role of the environment in determining meristic characters, and also fostered the misleading notion of daydegrees. The primary objective of the article at hand is to put forth arguments to support what is meant to be a more balanced point of view. Morphological clines, especially for those forms which have somewhat disjunct populations, are regarded as the consequences of environmental modifications coupled with adaptive genetic changes. In the course of writing the article it also became necessary to reconsider the problem of the production of variation in fishes, particularly, meristic variation.' As a result, a refinement of Hubbs's (1926) hypothesis has been developed to account for the genesis of meristic variation in the hope it may give direction to the design of future experiments.

Some meristic characters in a population of the fish Poecilichthys exilis: their variation and correlation

Causes and Significance of Morphological Variation in Fishes

It has long been known that meristic characters, and especially the number of vertebrae, in many fishes are affected by environmental factors such as temperature, salinity, oxygen concentration, and light during early development. The temperature response curve of vertebral number for offspring from a single mating is often Ushaped, but may be inversely linear or some other shape. Less is known about response to other variables, but there is generally a clear trend within the tolerance range. The genotype of a fish significantly affects the response curve of vetebral number to environmental factors. A few studies have shown that tetrapod meristic characters are also sesitive to temperature and light. Earlier reviews and recent experiments papers are cited d analyzed to show that these phenomena can now be investigated experimentally. Metabolic rate has been correlated with vertebral number and the possible implications of this are discussed. Theories accounting for the formation of somites are examined. T...

Some meristic characters in a population of the fish Poecilichthys exilis: their variation and correlation

Citations

Causes and Significance of Morphological Variation in Fishes

Control of Vertebral Number in Teleosts- an Embryological Problem

Phylogenetic interpretation of ontogenetic change: sorting out the actual and artefactual in an empirical case study of centrarchid fishes

Skeletal anomalies in fish from polluted surface waters

Variation and Systematic Significance of Vertebral Counts in the American Fishes of the Family Percidae

Related Papers (5)

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Meristic differences between two conspecific fish populations in Australian salt lakes