THE CONCEPT OF EVOLUTION 709 



adaptive significance at first, the antlers eventually became so big, with a 

 total spread o£ 11 feet, that the deer could not support them and the 

 species became extinct. 



Our increasing knowledge of how genes act in controlling develop- 

 ment has enabled us to explain whatever straight-line trends in evolu- 

 tion may be real in terms of conventional evolution by mutation and 

 selection. Many different types of developmental patterns may arise by 

 random mutation, yet most of them will result in unharmonious proc- 

 esses, ones which will not interdigitate properly and will lead to the 

 death of the organism. Others, with no particular value for survival, 

 will remain or be eliminated by chance. The ones most likely to survive, 

 perhaps, are those which provide for further improvement in some pe- 

 culiar adaptive structure already present. Thus, orthogenetic series can 

 be explained as the result of random mutation and selection occurring 

 along one of the few possible lines of development. An explanation for 

 the overdevelopment of parts is now possible as well: genes do not func- 

 tion independently, but must operate against the background of many 

 other genes also present. Those controlling larger horns, for example, 

 might cause the horns to be proportionately larger than the rest of the 

 body, and if other genes cause an increase in total body size, the horns 

 may become unmanageably large and finally lethal to their possessors. 



304. The Origin of Species by Hybridization 



The crossing of two different varieties or species, called hybridiza- 

 tion, provides another way in which new species may originate. The new 

 species may combine the best characters of each of the parental species, 

 thereby becoming better able to survive than either of its parents. 

 Hybridization is used routinely by animal and plant breeders to estab- 

 lish new combinations of desirable characters. 



When two different species are crossed, and especially ones with 

 different chromosome numbers, the offspring are usually sterile. The 

 unlike chromosomes cannot pair properly, cannot undergo synapsis m 

 the process of meiosis, and the resulting eggs and sperm do not receive 

 the proper assortment of chromosomes. However, if one of these inter- 

 specific hybrids undergoes a chromosome mutation which results in the 

 doubling of the chromosome number, meiosis can then occur normally 

 and fertile eggs and sperm are produced. The hybrid will breed true 

 thereafter ancf will generally not produce fertile offspring when bred 

 with either of the parental species. It is widely believed that this process 

 has been quite important in the evolution of the higher plants; more 

 than half of the higher plants appear to be polyploids. There are species 

 of wheat with 14, 28 and 42 chromosomes, species of roses with 14, 28, 

 42 and 56 chromosomes, and species of violets with every multiple of 

 six from 12 to 54. The fact that similar series of plants with related 

 numbers of chromosomes can be established by experimental breedmg 

 lends credence to the idea that these natural series arose by successive 

 hybridization and chromosome doubling. 



One of the more famous experimental hybrids was the radish x cab- 



