Changes in Populations 147 



mental garden with uniform conditions. It was found that the grazed 

 half yielded a high proportion of genotypes that produced a low 

 prostrate growth. Those from the ungrazed half were erect and 

 showed no tendency to procumbency. This clearly shows that there 

 was a heavy selection in these populations for adaptive growth forms. 



Mimicry of Flax 



An extremely complex system of mimicry has been found in the 

 genus CameUna, plants of the family Cruciferae. Various types of 

 Camelina occur as weeds in fields of cultivated flax ( Linum, family 

 Linaceae). It has been hypothesized that, as the cultivation of flax 

 became more efficient, Camelina was subjected to a series of in- 

 creasingly severe selection pressures. For instance, there was a selec- 

 tive advantage for the CameUna seeds to remain with the flax seeds 

 during the winnowing process so that they would be sown along with 

 the flax. Plants that grew were from seeds with the correct aero- 

 dynamic properties; the other seeds were never planted. The more 

 thorough the winnowing, the stronger the selection pressure. This 

 selection produced Camelina seeds that mimicked flax seeds, not in 

 appearance but in distance blown by a given amount of wind. Sim- 

 ilarly, selection favored the production of tall spindly Camelinu 

 plants that would not be shaded out of existence in the dense stands 

 of cultivated flax. Such selection has produced flax mimics not only 

 in the genus Camelina but also in other plants, including Spergula 

 and Silene (Caryophyllaceae). It should be noted that the evidence 

 for selection here is more inferential than in the preceding examples. 



Disruptive Selection in Mimetic Butterflies 



When selection favors two or more phenotypic modes, it is said to 

 be disruptive. Experimental work with bristle numbers in Drosophila 

 has shown that a pattern of selection in which extremes are favored 

 over intermediates can produce a bimodal population with increased 

 variance. Apparent examples of the operation of disruptive selection 

 in natural populations are found in arrays of mimetic butterflies. 

 For instance, the widespread and much-studied African swallowtail 

 butterfly Papilio dardamts (Fig. 7.10) has a wide variety of mimetic 

 females, although the males never show mimicry. Presumably this 

 is because "normal" color pattern and wing shape of males are im- 

 portant in making them sexually acceptable to the females. Several 

 different forms of the females commonly occur in the same locality, 

 each one accurately mimicking a different distasteful species of 

 butterflies. 



