Major Patterns of Variation | 275 



trend and the extinction of large organisms, leaving only smaller 

 related forms to represent a group. 



Large size seems to have been selected for in some animals be- 

 cause of concomitant benefits derived through allometry. Allometry 

 refers to differential growth of body parts. The horns and antlers of 

 ruminants, for example, often show a strong positive allometry. As 

 body size increases, they grow proportionately much larger. (The 

 head of a postnatal human being grows less, proportionately, than 

 the rest of the organism as a whole; it shows negative allometry.) 

 Horns and antlers are important both in defense and in battles be- 

 tween males for access to females. It does not seem unreasonable, 

 therefore, to assume that the large size of many ruminants is ac- 

 counted for in part by selection for larger weapons. This is an exam- 

 ple where selection for proportionately larger appendages may have 

 produced, through positive allometry, a correlated increase in body 

 size. 



It is also possible that, in some cases, selection for large body size 

 on the basis of increased efficiency of arrangement of internal organs, 

 say, could have produced appendages overlarge for their supposed 

 function. Such might be the case with the evolution of Megaceros, 

 the Pleistocene Irish elk which had the largest antlers of any known 

 deer. As long as the disadvantage in large antlers did not counter- 

 balance the advantage of large overall size, the trend would con- 

 tinue. The discussion is perhaps overly speculative, but it shows 

 that there is no reason to view the great size of these antlers ( or the 

 giant mandibles of stag beetles, or many other such instances) as 

 the result of some sort of "momentum" that carried once-adaptive 

 trends to nonadaptive extremes. As should be obvious from a con- 

 sideration of population genetics (see Chap. 6), this is a meaning- 

 less analogy. Selection against extremes would bring a trend to an 

 abrupt halt as soon as the adverse effects counterbalanced the bene- 

 ficial effects of the trend. As Simpson has pointed out, organisms 

 that have extreme characteristics may seem bizarre to human eyes, 

 but this is no reason to infer that they are inadaptive. Indeed, the 

 thriving possessors of many bizarre features ( male peacocks, bottle- 

 brush weevils, narwhals, plants with intricate flowers or such com- 

 plicated modes of pollination as pseudocopulation) are hardly sup- 

 port for the idea that these characteristics are liabilities. 



Increase in Complexity 



Perhaps the broadest and most general trend that has been postu- 

 lated is that toward increased complexity. Difficult as it is to specify 



