Section 9 — Population Genetics 



concurrent competition of all life history stages. 

 (2) Competition among developing larvae and 

 pupae only, with the adults separated from the 

 flour at regular intervals. This model simulates 

 natural selection in simpler univoltine species. 

 The fate of two alleles {sooty and black) under 

 both systems of selection seems deterministic 

 and analytical experiments are reported which 

 try to isolate the separate components of selec- 

 tion. Among the findings of interest are: Proof of 

 gene frequency dependent changes of adaptive 

 value in the sooty and black locus, proof of 

 gene frequency and zygotic frequency dependent 

 facilitation of growth processes by the sooty 

 allelomorph, genetic differences in response to 

 environmental conditioning and differences in 

 medium conditioning produced by different 

 genotypes (in the case of the black locus). Density 

 dependent differences in adaptive values are 

 apparently responsible for changes in the 

 direction of selection in population system (2). 

 Related experiments have been carried out with 

 houseflies and are contrasted with the Tribolium 

 findings. 



9.10. Mimetic Multilocus Polymorphism in South 

 American Butterflies (Heliconius spp.) (Lepi- 

 doptera, Nymphalidae). John R. G. Turner 

 (Oxford, Great Britain). 



Heliconius melpomene and H. erato have about 

 thirty morphs each — though they are mono- 

 morphic over large areas — and are mutually 

 mimetic. Experiments (Brower, Brower and 

 Collins) showing that birds find the insects dis- 

 tasteful and confuse the two species, and the 

 existence of beak-marked specimens, showing 

 that birds attack the butterflies in the wild, show 

 that this similarity involves synaposematism ; that 

 theory predicts that aposematic species become 

 monomorphic, that some "mimetic" pairs are 

 allopatric, and that some morphs are non-mimetic, 

 shows that other factors affect the polymorphism. 

 The only one whose action has been shown is 

 visual sexual selection. Features advantageous 

 in courtship are usually dominant, and butter- 

 flies lacking red (the chief releaser) are rare; all- 

 black insects are very scarce. There are several 

 autosomal loci, most of them linked; two 

 constitute a supergene. Population studies show 

 strong epistasis and linkage disequilibrium at 

 this superlocus; never are all four superalleles 

 found together, and one of them is very restricted 

 geographically in melpomene and extremely rare 

 in erato. In erato a third locus is included in the 

 supergene. One cannot tell what maintains the 

 polymorphism, although visual predation alone 



can produce strong epistasis, and there is some 

 correlation between gene distribution and cli- 

 mate; note the similarity to the Rhesus blood- 

 groups, also supergenic and also maintaining 

 their polymorphism despite an unstable equili- 

 brium. 



Some results were obtained jointly with 

 Jocelyn Crane and P. M. Sheppard. Papers on 

 Heliconius from the New York Zoological 

 Society's Tropical Research Station appear in 

 Zoologica (N.Y.). 



9.11. The Evolutionary Potential, as Measured by 

 Seed Germination, of Chromosome Races of 

 Mimulus(Scrophulariaceae). Robert K. Vick- 

 ery Jr. (Salt Lake City, U.S.A.). 



The purpose of this investigation was to explore 

 one facet of the norm of reaction of Mimulus, 

 specifically, the range of temperatures under 

 which seed germination could occur and to see 

 whether there were different responses in races of 

 different chromosome numbers. Seeds of over 

 20 races were tested in various combinations of 

 temperatures ranging from minus 7 to 50°C. 

 They were placed in one temperature for 16 hr 

 and in another for 8 hr to simulate a daily 

 periodicity of temperature changes. 



In the Mimulus glabratus complex, the diploid 

 (/? = 15) and tetraploid (n = 30) races germi- 

 nated in combinations of cooler temperatures, 

 from 4° to 21°, and the hexaploids (n = 45) from 

 4° to 25°. Aneuploids of the diploid and tetra- 

 ploid races exhibited ranges that extended 5° to 

 10° higher than their respective euploid forms. 

 The aneuploids of the hexaploid level regularly 

 tolerated combinations involving 35° and one 

 race even grew well in a combination involving 

 -7°. The M. glabratus complex aneuploids ger- 

 minated and grew under a wider range of temper- 

 atures than their euploid relatives and the ones 

 with the highest numbers showed the widest 

 range of tolerance. 



The results for the closely related M. guttatus 

 complex suggest a note of caution to too easy 

 generalizations. In this group the diploids ex- 

 hibited a far wider range of temperature tolerance 

 than did the tetraploids or aneuploids. However, 

 in both groups the range of temperature tolerance 

 of the races was often wide and not necessarily 

 correlated with the temperature ranges of their 

 native environments which suggests that Mimulus 

 has a wide norm of reaction and much unexploi- 

 ted evolutionary potential at each chromosome 

 number level. 



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