112 I The Process of Evolution 



Simplified and overdrawn as it is, this example demonstrates how 

 balanced polymorphism maij be advantageous to a population be- 

 cause it prevents loss of variability due to fixation at the locus. An- 

 other mechanism that tends to slow the loss of variability at a locus 

 is dominance, which permits the "sheltering" of otherwise unde- 

 sirable mutants in heterozygotes. However, viewed in another way, 

 it might be said that both these phenomena tend to keep "unde- 

 sirable" genes in a population. To understand this point of view, 

 the question of "genetic load" must be examined. 



Genetic Load 



If one genotype in a population at a given time is superior to all 

 others, it may be assumed that a population consisting only of 

 individuals with that genotype would have the highest possible 

 fitness. Thus, if the most fit genotype is assigned an adaptive value 

 W = 1.00, the population fitness W will also equal 1.00, since W = 

 2 Wj gj ( where W,- is the adaptive value of the ith genotype and g,- 

 is the frequency of the ith genotype, i being any number from 1 to 

 n and n the number of different genotypes ) . Under this model, any 

 population consisting of a mixture of genotypes will be "less fit" 

 than the ideal monomorphic population. The amount by which a 

 population differs from this ideal is its genetic load L, which may 

 be viewed roughly ^ as the complement of W or L = 1 — W. 



Given this model, two extreme possibilities may be considered. 

 In one case the "ideal" genotype is homozygous at all, or nearly all, 

 of its loci. At the other extreme all loci may be overdominant with 

 respect to fitness, and the "ideal" genotype in this case is a multiple 

 heterozygote. Considerable controversy has surrounded the question 

 of which of these two extreme possibilities is more realistic. Evi- 

 dence from response by populations to inbreeding is at the moment 

 inconclusive. There is considerable evidence for the importance 

 of overdominance with relation to fitness in animals, and it has been 

 suggested that for many organisms extreme deviant phenotypes may 

 be the result of multiple homozygote genotypes segregating in popu- 

 lations where multiple heterozygotes are the "normal" genotypes. 

 Many plants, however, have genetic systems that seem to ensure 



' Actually it is defined as L = —In W. Thus W = e"^, so that L is tlie average 

 number of potential deaths per individual and W is the probability of genetic 

 survival, that is, the probability of an individual not suffering death because of 

 the properties of its genome. (Of course, an individual with the equivalent of 

 five lethal genes in its genome dies only once, although it contributes five 

 "deaths" to the average. ) The reader familiar with statistics will recognize the 

 expression W = e'^ as the first term of a Poisson distribution. 



