The Cichlid Fishes of Lake Malawi, Africa

Abstract of Publication

Greenwood, P.H. 1984a. What is a species flock? Pp. 13-19 in Echelle, A.A., and I. Kornfield (eds.), Evolution of fish species flocks. University of Maine at Orono Press.  

[No abstract or summary was published. What follows is an extract consisting of the two introductory paragraphs, and the two concluding paragraphs —M.K. Oliver]

Reviewing the relevant but not very extensive literature on this topic soon discloses a varied interpretation of the phrase species flock, dnd its synonym, species swarm. To some authors it is simply a descriptive term used to embrace a number of taxonomically allied and endemic species occurring in a geographically circumscribed area. For others, there are evolutionary implications, especially with regard to the processes involved in a flock's formation. To yet others there are phylogenetic or genealogical implications to be considered with or without reference to the evolutionary processes involved. Almost the only common factors in those different approaches are that members of a flock should have some taxonomic cohesion, should occupy a very restricted area, usually a lake or island, and that the species should be endemic to that area. It is difficult to discover a unified species flock concept. It would seem to be more a descriptive term than a concept, an idea which, like Harriet Beecher Stowe's Topsy, "one 'spects just growed." Also reminiscent of Topsy, it is difficult to get much information on the term's actual birth, although that child's comment that she ".. was raised by a speculator" seems readily applicable to some of the ideas on how a species flock might have evolved.

Despite its origin, discussed [in this paper] as something little more than a descriptive phrase, the idea of a species flock soon became entangled with, and almost absorbed by, various ideas concerned with modes of speciation. That situation persisted until the early 1940's when, for the first time, there appeared to be some attempt to formulate a species flock concept as a particular aspect of evolutionary theory. [Here follows the bulk of the paper. Greenwood then concludes:]

Clearly then, the central issue in any attempt to formulate a species flock concept and identify species flocks must hinge on the monophyly of its members and the level of universality at which that monophyly is determined. In other words, an aggregate of several species should be identified as a flock only if its members are endemic to the geographically circumscribed area under consideration and are each others' closest living relatives. It is this element of immediate shared common ancestry which principally distinguishes a species flock from other species aggregates occupying geographically circumscribed areas. It is also an important indicator of the particular circumstances, both biotic and abiotic, leading to the flock's genesis, circumstances which are likely to differ substantially from those underlying the origin of other kinds of localized species aggregates.

Seen in this light, the importance of species flocks lies not in their being some near mystical evolutionary phenomenon, but in the empirical evidence which can be derived from them and applied in a broader evolutionary context. If periods of intensive speciation are common in phylogeny, as postulated in the punctuated equilibrium theory of evolution, there may be nothing extraordinary about a species flock, even if the flock's size is more in keeping with that of a Scottish crofter's than an Australian sheep farmer's.




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