MATING DISCRIMINATION 



flies are offered a choice of mate they may exercise that choice in 

 the avoidance of extreme inbreeding and extreme outbreedins. 

 Courtship behaviour in fishes, birds and mammals elaborately 

 testifies to the same exercise of discrimination. In man the 

 prejudice of individuals against incest and bestiality has become 

 hardened and generalized into social tabu and legal prohibition. 



Looking back on all these different systems we see that in a 

 hundred different ways the same end is achieved, the end being 

 the control of the mating system. The means adopted reflect the 

 circumstances and capacities of the species. Plants make use especially 

 of the diploid style as a sieve for sorting the pollen delivered to it 

 by a pollinating agency, over which it can exercise no direct control. 

 Animals have powers of perception which they use in accepting 

 or rejecting mates. And lastly man uses his unique power of social 

 coercion for the same acknowledged purpose. 



The control of the mating system is evidently such as will give 

 a controlled degree of hybridity generally intermediate between 

 homozygosity and that extreme heterozygosity which we see to 

 be disastrous in crosses between species. In other words it is 

 capable of giving the kind of hybridity optimum we were led 

 to expect. Two consequences of this state of things may now 

 be seen. 



First, bars to crossing between species are, in effect, inbreeding 

 devices and should therefore show a dependence on the mechanisms 

 which control mating within the species. This is clearly seen in 

 discriminative mating which is as strong in flies as it is in mammals. 

 When in Drosophila a male of melanogaster is presented with females 

 o£ simulans and of his own species (which are alike to us) he never 

 makes a mistake. Similarly, an experiment with Petunia shows how 

 the plant style makes its choice. Pollen of the Fj between the species 

 axillaris and violacea, put on the style of either parental species, is 

 sorted out so that the grains carrying the chromosomes prepon- 

 derantly of the same parent get through to the egg more often. 

 This effect is even clearer when mixed pollen of two distinct types 

 is used in Streptocarpus. Self-pollen then has an advantage over 

 other species' pollen (an advantage proportional to the remoteness 

 of the other species): but cousin-poUen has an advantage over 

 both. 



253 



