THEORIES AND MECHANISM OF EVOLUTION 



655 



important. Let us consider each of these 

 briefly. 



Geographic. Darwin and many other nat- 

 urahsts noted very early that animals living 

 on an island were frequently quite different 

 from those on the neighboring mainland. 

 Other barriers, such as a mountain range, 

 a deep canyon, a wide body of water, or a 

 desert, have been known to separate mem- 

 bers of a species long enough for differences 

 to be detected between the two groups. For 

 example, there are two closely related spe- 

 cies of squirrels on the two opposite rims 

 of the Grand Canyon, different only in 

 color markings. Undoubtedly, at one time 

 they migrated freely from one rim to the 

 other but eventually, because of the great 

 depth of the canyon and the tiubulence of 

 the Colorado River, they were unable to 

 mingle as they had in the past. Today they 

 are gradually changing, though slightly, 

 in spite of the fact that the environment is 

 almost identical. 



It is understandable that when a barrier 

 occurs in the range of a given species so 

 that groups of individuals of that species 

 are completely separated, the continual oc- 

 currence of mutations and the slightly differ- 

 ent environments acting selectively upon 

 them will eventually bring about divergence 

 of the two groups. If the barrier continues 

 for a sufficiently long period of time, the 

 differences may become sufficient to con- 

 stitute two species. If the barrier is then 

 removed so that the two groups once again 

 intermingle they will probably maintain 

 their identity as different species. However, 

 if the barrier is removed at a time when 

 they have not changed enough, that is, 

 while they are still varieties, it is possible 

 that the differences may be lost through 

 interbreeding. The latter instance is illus- 

 trated by the human species. Homo sapiens. 

 Although he has occupied nearly all por- 

 tions of the earth's surface and does show 

 some marked physical variations, the var- 

 ious races have not lost their ability to 

 interbreed. The barriers that have been set 



up between races have not been effective 

 sufficiently long to bring about infertility. 

 With the intervention of modern means of 

 transportation those original geographic 

 barriers are now all but erased, so theoret- 

 ically man should lose what changes have 

 been produced by his isolation, providing 

 there is a free mingling again of all races. 

 But such, as we all know, is not the case. 

 Society places barriers that can be as effec- 

 tive as infertility or vast geographical ob- 

 stacles. However, while it is possible it 

 seems highly improbable that such prac- 

 tices will lead to a new species of mankind. 

 Indeed, the social barriers seem likely to 

 continue to grow less as time passes, with 

 the resultant merging of existing races. 



It is interesting to note that under natural 

 conditions hybridization is only rarely ob- 

 served among closely related species of ani- 

 mals. To be sure, it has been possible to 

 effect it under laboratory conditions — the 

 biscow, for example, as a result of the mat- 

 ing of a bison and a cow — but such cases 

 are unusual and do not normally occur in 

 nature. Since the animals occupy the same 

 environment and are therefore subject to 

 the same conditions, why do they not mate 

 freely and thus reverse the differences es- 

 tablished earlier through isolation, inducing 

 reverse evolution in this way? This can best 

 be answered from a study of genetics. 



Genetic. When animals are separated for 

 long periods of time so that they become 

 slightly different physically, the difference 

 extends to their genes. Then, when two 

 such species or even varieties mate the 

 genes are not as perfectly compatible as 

 they were in the common ancestor. This 

 results in low fertility or in most cases com- 

 plete sterility. Thus such crosses under nat- 

 ural conditions would be unfruitful if they 

 did occur and the hybrid would have little 

 chance of establishing itself. Also, the hy- 

 brid itself is often sterile, as is true of the 

 mule previously referred to (p. 600). Fur- 

 thermore, slight differences in the mating 

 season might act as a barrier even though 



