258 I The Process of Evolution 



boreal distribution. Often the same species may masquerade under 

 different names in Canada, Europe, and Russia until monographic 

 study, bringing together specimens from all areas for the first time, 

 shows the true distribution. It may seem unnecessary to belabor the 

 point in such detail here. However, it is surprising that biogeog- 

 raphers often accept the results of local floristic and faunistic studies 

 without considering this particular source of bias. 



Reticulate Variation 



A rather different situation, and one not well understood, occurs in 

 various groups of organisms. Here a species is well-marked and 

 easily distinguished. There appears to be little or no morphological 

 intergradation so that each cluster is clear-cut. In the milkweed fam- 

 ily ( Asclepiadaceae), for example, once the rather complicated 

 floral morphology is understood, a series of nonoverlapping taxa 

 (species) are recognized easily. It is interesting that in this family 

 of flowering plants it is very difficult to delimit clusters of these 

 primary taxa. The pattern of variation is such that genera are virtu- 

 ally impossible to define. The condition in which characters con- 

 sidered important by the taxonomist appear in different combina- 

 tions in many species is known as reticulate variation. Perhaps this 

 is a result of the complex obligate insect-pollination mechanisms 

 found in these plants, but it seems to be common at the species level 

 in various groups of plants and animals. 



Reticulate variation must be distinguished from reticulate evolu- 

 tion. Reticulate evolution is the joining of phyletic lines (through 

 crossing) and has been demonstrated clearly in many plants where 

 it usually involves doubling of the chromosome number of the 

 hybrid. Instances of this sort in both plants and animals are dis- 

 cussed in Chap. 9. In the Asclepiadaceae, there is no reason to suspect 

 that union of formerly separate phyletic lines has taken place, and 

 there is no indication that polyploidy has played a role in species 

 formation. 



THE FOSSIL RECORD 



In several senses, time is the crux of the problem of "macroevolu- 

 tion" as opposed to "microevolution." The past history of life can be 

 studied only by examination of fossils and comparison with extant 

 forms. Opportunities for checking these observations and the conclu- 

 sions drawn from them are few indeed. The discovery of "living 

 fossils" such as Latimeria and Metaseqiioia provides such a check. 



