284 



small isolated colonies, ehanges in the 

 relative frequencies of different genes 

 might result from the errors of random 

 sampling in the formation of the germ 

 cells and their fertilization, without 

 involving selection. This concept, ad- 

 vanced bv Sewall Wright and known 

 as "random genetic drift," has been 

 invoked as a possible cause of nonselec- 

 tive, nonadaptixe evolution. It has, 

 however, been invalidated by the re- 

 sults of experimental studies in the 

 field such as those of Fisher and Ford 

 on moths, which have shown that se- 

 lective factors are much more impor- 

 tant than casual nonadaptive factors 

 in determining the relative frequency 

 of genes and in bringing about close 

 adaptation to local environmental con- 

 ditions. 



THE NEW SYSTEMATICS AND 

 THE ORIGIN OF SPECIES 



The researches on industrial melan- 

 ism in the peppered moth, banding 

 and color of snails, mimicry in butter- 

 flies, local adaptation in moths, and 

 sickle cell in man are examples of new 

 techniques of experimental study of 

 evolution in the field. Nobody would 

 have welcomed these developments of 

 biological science more than Darwin 

 himself, as a glance at the last few 

 pages of "The Origin of Species" will 

 show. It is therefore appropriate to 

 return to the problem with which this 

 article began. As is now certain, species 

 are not immutable but have under- 

 gone change, and many examples have 

 been given above. Evolution can take 

 place up to a point without the pro- 

 duction of new species, but if this 

 process continues the time must come 

 when new species can be seen originat- 

 ing, and it is legitimate to ask whether 

 modern research has revealed any evi- 

 dence of this. The answer is that new 

 species can be seen originating in na- 

 ture here and now, and new species 



EVOLUTION 



have been artificially produced in the 

 laboratory. 



As E. Mayr has shown, some form 

 of biological isolation between portions 

 of populations is a necessary condition 

 for divergence leading to the formation 

 of new species and higher groups. 



Among the kinds of isolation that 

 are chicflv responsible for the origina- 

 tion of species, geographical isolation 

 is the most important; it involves phys- 

 ical barriers such as oceans, mountain 

 ranges, or deserts which separate whole 

 populations. Geographical races are the 

 chief raw materials from which new 

 species are formed, and it was the dif- 

 ferent finches on the different Galap- 

 agos Islands which first suggested to 

 Darwin that evolution had occurred. 



Geographical isolation is important 

 for the origin of species of plants as 

 well as of animals, but there is another 

 form of isolation which appears to be 

 restricted to plants and involves the 

 sudden erection of sterility barriers be- 

 tween individuals in the same popula- 

 tion as a result of changes in the chro- 

 mosome mechanism. This is known as 

 genetic isolation. When Primula ver- 

 ticillata is crossed with Primula flori- 

 bunda, hybrid offspring are produced, 

 but they are sterile because the chro- 

 mosomes of one parent species are 

 incompatible with those of the other, 

 and the intricate machinery involved 

 in the formation of germ cells is 

 thrown out of gear. Occasionally, how- 

 ever, the hybrid plant undergoes dou- 

 bling of its chromosomes, a condition 

 known as pohploidy, and when that 

 has occurred the hybrid is able to breed 

 with hybrids similar to itself because all 

 the chromosomes have compatible 

 partners, but it is sterile in respect to 

 both parent species. Furthermore, the 

 hybrid is not only true breeding but 

 is different in structure and in habit 

 from each of its parent species. It 

 therefore fulfills all the criteria of a 

 species and has been called Primula 



