An Analysis of Variability Arising through Recombination.

HE existence of variation among the individual members of a population T has been recognized for centuries ; the biological importance of this variation has begun to be understood only in the past few decades. Formerly the type specimen was regarded as representing the species ; now it is realized that adequate representation is obtainable only through a series of specimens. Population geneticists of the 1920's and 1930's were active in studying the genetic component of this variability. CHETVERIKOV ( 1926, 1927), TIMOFEEFF-RESSOVSKY and TIMOFEEFF-RESSOVSKY (1927) and DUBININ (1934) demonstrated the existence of mutant genes in natural populations of Drosophila. BAUR (1924) and TIMOFEEFF-RESSOVSKY (1934) detected spontaneous and induced mutations with extremely minute phenotypic effects. Combined, this information amounted to the discovery of a vast store of partially concealed genetic variability upon which a species may draw for evolutionary changes. A simplified, symbolic illustration of this concept represents the majority of surviving adults in a population as AABB.. ., A a B B ..., A A B b . . . and AaBb. . .. The interbreeding of these types also produces mBB ..., aaBb ..., Aabb ..., AAbb.. . and aabb.. . individuals but the inclusion of these among the breeding portion of the population depends upon the degree to which they are opposed by selection. The repetitive formation of these individuals in each generation, however, guarantees that these genotypes are available should the nature of selective forces change in their favor. This theory, still the prevailing one, explains the existence of gametic variability beneath a uniform phenotypic facade. If, for example, m and bb in our scheme represent lethal genotypes, surviving ind'ividuals carry A and B in at least the heterozygous condition and possess the invariable phenotype AB. Recent data indicate that this is an oversimplification-real differences exist between different individuals of a population. Following the studies on striking and constant differences indicative of balanced polymorphism made by FISHER (1930), FORD (1945) and TIMOFEEFF-RESSOVSKY (1940), WRIGHT anid DOBZHANSKY (1946) demonstrated conclusively that in spite of the absence of morphological differences individuals of D. pseudoobscura heterozygous foi two different gene arrangements of the third chromosome have higher adaptive values than individuals homozygous for either of these third chromosome