THE EVOLUTIONARY CONCEPT 347 



than he could have postulated from what was known in his time. Segrega- 

 tion and recombination, in conjunction with independent assortment and 

 crossing over, form an exceedingly effective device for disseminating 

 mutant genes and chromosomes throughout an interbreeding population. 

 Linkage tends to hold various genes together long enough to "try out" 

 various patterns of gene combinations. The phenomena of recessiveness 

 permit many genes that are deleterious when homozygous in certain gene 

 combinations to be carried and transmitted in a heterozygous condition. 

 Eventually they may find their way into other genotypes where they 

 may have adaptive advantages. 



When we add to these conditions the almost limitless series of permuta- 

 tions and combinations of genes which results from the operation of 

 bisexual reproduction, it becomes evident that biological inheritance not 

 only provides for the transmission of variation but also tremendously 

 increases the genetic diversity of a stock. From this diversity selection, 

 isolation, and the statistical consequences of segregation and recombina- 

 tion can set apart and fix, modify, or eliminate various portions of a once 

 continuous and uniform interbreeding population. 



Selection 



The concept of natural selection was the central and unique factor 

 of Darwin's theory. Its efficacy (or even its reality) has been the most 

 violently and persistently debated topic in the biological controversies 

 about evolution. As mentioned previously, Darwin, in his search for the 

 cause of the tremendous diversity in natural forms of life, was greatly 

 impressed by the analogous diversity in domesticated animals and plants. 

 Here the marked differences between related breeds were maintained by 

 controlled mating and had unquestionably arisen through man's more or 

 less conscious selection of inherited variation. This was a clue to diversity 

 in nature, if some natural selective mechanism could be discovered and 

 demonstrated. Darwin found a second clue in the Malthusian concept of 

 ever-expanding populations faced with static limits to the number of 

 individuals which could survive. Applied to wild populations, he saw 

 that this must result in a struggle to survive and realized that this could 

 be a continuous and powerful agency for selection. 



The concept was clear and plausible but, like variation and inheritance, 

 hard to limit or define. Darwin's own skilled, accurate, and abundant 

 observations of the actuality of high reproductive rates accompanied by 

 static population sizes have been confirmed and extended; modern 

 naturalists have accurately measured the population sizes and reproduc- 

 tive rates of many organisms. There is no question that most zygotes 

 are eliminated before they can in turn reproduce. To demonstrate that 



