1930 



EVOLUTION 



Page five 



Present Problems of Evolution 



By EDWIN G. CONKLIN 



IN spite of much antagonism and some legal pro- 

 hibitions in some backward states, the fact of or- 

 ganic evolution stands as the only scientific ex- 

 planation of the origin, distribution and relation- 

 sliips of living things. The onl}' problems of evolu- 

 tion at present in dispute among scientific men con- 

 cern the methods and causes at work, and the only 

 way in which such problems can be solved is by 

 studying evolution as it is going on today. 



/. THE MATERIALS OF EVOLUTION 



Darwin recognized the truth of this and devoted 

 much of his attention to the variations of animals 

 and plants, since he regarded these variations as 

 minor steps in the process of evolution. Such vari- 

 ations occur generally among wild and cultivated 

 species and the extent to which they may go is shown 

 by the various races of domestic animals and cul- 

 tivated plants wliich have been produced under hu- 

 man guidance. Although all students of the sub- 

 ject have known that some variations are inherited 

 and others not, that some are large and others small, 

 it was not until the work of Bateson and deVries 

 some thirty or forty years ago that the importance 

 of these different forms of variations was fully recog- 

 nized. DeVries especially demonstrated that in- 

 herited variations might be very great, so that an 

 "elementary species" miglit be born in a day, and 

 these variations be named "mutations," whereas all 

 variations which are not inherited be called "fluctu- 

 ations." Mutations may be great or small, but they 

 are alwa^^s inherited and consequently they are the 

 materials with which evolution works, for any vari- 

 ation to be of evolutionary value must be inherited. 



//. HOW DO MUTATIONS ARISE AND HOW 

 ARE THEY INHERITED? 



Older students of evolution focussed attention 

 largely upon the transformations of mature organ- 

 isms, for example the gross changes necessary to 

 convert a cabbage into a cauliflower, a rock pigeon 

 into a fantail, largely neglecting the microscopic and 

 generally unseen stages which connect one genera- 

 tion with the next. We now know that tlie only 

 living bonds between generations are the germ cells 

 and any changes that are inherited must be repre- 

 sented in these cells ; consequently the study of the 

 methods and causes of evolution has been trans- 

 ferred from the changes occurring in mature organ- 

 isms to the changes taking place in germ cells. These 

 changes may effect the symmetry, pattern and con- 

 stitution of tlie protoplasm of the egg cell, but a 

 still earlier and more fundamental cause of change 

 is found in the chromosomes of the nucleus, which 

 contain the real "inheritance material." Therefore 



the study of the methods and causes of evolution 

 resolves itself into an investigation of the changes 

 taking place in chromosomes. 



1. Cross-breeding or hybridization is an almost 

 universal process, and it leads to many changes in 

 the combination of chromosomes from different par- 

 ents. The study of iiybrids led to the epoch-making 

 discovery of alternative inheritance by Mendel, and 

 although he knew nothing of chromosomes, which 

 were discovered, we now know that all the phenom- 

 ena of Mendelian heredity depend upon the distri- 

 bution of chromosomes to the germ cells and their 

 combination at the time of the fertihzationof the 

 egg. In this way many new combinations of the 

 materials of heredity are formed and many vari- 

 ations in the developed organisms that come from 

 these new combinations of chromosomes. Bateson 

 said that most of our domestic animals and cultiv- 

 ated plants are the results of deliberate crossings, 

 and Lotzy maintains, contrary to the views of many 

 biologists, that h3'bridization is the chief cause of 

 evolution. 



2. A second cause of mutation is found in abnor- 

 mal numbers of chromosomes, also in the breaking 

 in two of individual chromosomes and the reunion 

 of the pieces in new combinations. Tliese "chromo- 

 some mutations" are responsible for most of the 

 mutations discovered by deVries in the evening 

 primrose and by Blakcslee in the jimpson weed. 



3. A more fundamental cause of mutation is 

 found in the changes that take place in the genes 

 or inheritance units, which lie in the chromosomes 

 like beads on a string. Although these genes are so 

 small that they can rarely if ever be seen even with 

 the most powerful microscope, we have as clear evi- 

 dence that they exist and occasionally undergo 

 changes as that there are atoms and molecules which 

 may also suffer changes. Such "gene mutations" 

 are certainly an important factor of evolution and 

 the manner in which they ai-e caused is one of the 

 leading problems of biology today. Recently it has 

 been found that radium, X-rays and probably other 

 forms of radiant energy may cause gene mutations 

 and thus furnish the materials for evolution, and it 

 seems probable, althougli it is still uncertain, that 

 such mutations may be caused in still other ways. 



///. CAUSES OF ADAPTATIONS 



Mutations are certainly the raw materials of evol- 

 ution, but how are these materials used.'' There 

 is good reason to think that mutations occur in 

 every possible direction ; probably ninety-nine out 

 of every hundred are injurious, perhaps not more 

 than one in a thousand is distinctly advantageous. 

 How then can we explain the fact that animals and 



