EVOLUTION IX THE DEEP SEAS 231 



essential, and migration may be more frequent tlie greater the 

 selective advantage of the mutation. Also, fixation of neutral 

 characters in the populations of the demes may occur if the demes 

 are small. The estimate usually given for the maximum population 

 in which this can occur is 1000 individuals. Fixation of neutral 

 characters may be important in giving an explanation of the 

 occurrence of the large numbers of apparently neutral, and usually 

 trivial, characters in natural species. It is unlikely that the more 

 important characters on which the progress of evolution mainly 

 depends can ever be neutral. 



These local demes will hardly ever be sufficiently permanent to 

 allow new species, or even recognizable new forms, to be evolved 

 in them while they remain isolated. Almost always, as the result 

 of change in the environment, they will come into contact with 

 other demes before differentiation has gone far enough for the 

 populations to be described as new species or subspecies. But 

 when two demes fuse owing to breakdown of the isolation between 

 them, any mutational differences between them will be preserved 

 in the fused deme if they are advantageous, and neutral characters 

 may persist if the size of the fused deme is not too large. In re- 

 peated fusions o\'er the range of the species, characters will spread 

 through the range, and the genotype of the species will gradually 

 evolve. 



Where some part of a species is permanently isolated from the 

 rest, it will continue to differentiate until it forms a new sub- 

 species or even species. We then get a divergent type of evolution 

 distinct from the successional type in species with undivided range. 

 In a large undivided range slowness of diffusion may result in the 

 characters of the species differing in parts of the range, or the en- 

 vironmental conditions may so differ over the range that adapta- 

 tional differences occur in the different regions. In both these 

 cases we get a dine, a gradual change of the specific characters 

 over the range. All these types of evolution should be distinguished. 



This is the general theory of microevolution in terrestrial en- 

 vironments to which recent work on the ecology of animals has 

 led. It refers only to bisexual animals. If the reproduction is non- 

 sexual, the process of evolution will be very different; lineages 



