Section 9 — Population Genetics 



per cent total alkaloids was effective; however, 

 the yield of cured leaf decreased. The response 

 to selection for total alkaloids and the accompa- 

 nying decrease in leaf yield was in good agree- 

 ment with the predictions. 



9.24. The Evolution of Flower Colours in Populations 

 of Tulipa schrenkii Rgl. according to Statistical 

 Data. A. J. Kupzow (Moscow, U.S.S.R.). 



The area of Tulipa schrenkii Rgl. extends over 

 the steppes of the European part of the U.S.S.R. 

 and of Northern Kazakhstan. Individuals, in the 

 wild populations of this tulip, have four main 

 colours of their flowers: red (anthocyanine + 

 anthoxanthine), yellow (anthoxanthine), rose 

 (anthocyanine), and white (no pigment). There 

 are also certain modifications of these: orange, 

 light rose, violet, and cases of uneven distribution 

 of pigments (striped). Red flowers are charac- 

 teristic of most of the tulipa species, in sect, 

 eiestemenes, and also of the bulk of individuals 

 of T. schrenkii in the virgin steppes. The in- 

 fluence of man leads to a decrease of the amount 

 of red flowers in the wild populations of T. 

 schrenkii and, thus, to an increase of other 

 colours, especially of yellow. The latter ultima- 

 tely, becomes exclusive in the environs of large 

 towns, such as Odessa. 31 populations of T. 

 schrenkii from different parts of its area have 

 been studied. The correlation between the 

 amount of red flowers and that of yellow, the 

 two main types in these populations, was found 

 to be - 0.9 + 5 ± 0.0150. The percentage of 

 red flowers can be taken as an independent va- 

 riable and that of flowers with other colours 

 as its function. Then, while the percentage of 

 red flowers decreases from 100 per cent to 

 51 per cent, all its functions rise, and so the 

 composition of the populations approaches the 

 ratio: 9 red ± 3 yellow ± rose + 1 white 

 (with 10-15 per cent of modified colours among 

 them, mainly orange and violet as variants of 

 red). In the interval from 51 per cent to per 

 cent, of red flowers, the percentage curve of 

 yellow flowers continues to go up whereas the 

 proportions of rose flowers goes down, the 

 curve of white ones ascends up to 30 per cent 

 of the red ones and then also descends. The 

 percentage of the modified colours tends to 

 go down within this interval. The removal of 

 anthocyanine (in yellow) is observed more often 

 than that of anthoxanthine (in rose). Thus, in the 

 former interval many populations have 5-10 

 per cent of yellow flowers and 0-9 per cent of 

 rose ones. The aforesaid modifications are 



observed only in anthocyanine: orange and 

 violet, as mutants of red being more frequent 

 (up to 10.7 and 6.0 per cent) than light rose, as a 

 mutant of rose (up to 1.5 per cent). 



According to the available data, the following 

 concept is suggested: 



1. The initial colour of the flowers of T. 

 schrenkii was red, and it subsequently produced 

 distinctive mutants deprived of anthocyanine or 

 anthoxanthine, and, also modifier genes (muta- 

 tions affecting an anthoxanthine). 



2. Individuals with red flowers have, in 

 virgin steppes, gained success in the struggle 

 for existence. However, under man's influence, 

 various mutants, especially those with yellow 

 flowers, have become the predominant forms. 



3. High percentage of individuals with red 

 flowers ensures a higher rate of mutation, but, 

 if the amount of red flowers diminishes, this 

 rate decreases in the population. 



4. Any increase of individuals with yellow 

 flowers in a population is accompanied by a 

 decrease of the rate of mutation, and so all 

 individuals with differently coloured flowers 

 gradually become supplanted by these yellow 

 flowers. 



9.25. Evolution in the Tropics. F. G. Brieger 

 (Est Sao Paulo, Brazil). 



The basic mechanism of phylogenetic evolu- 

 tion is likely to be the same in tropical and in 

 north-temperate regions, where most studies 

 so far have been carried out. The course of evolu- 

 tion will have been different, since temperate 

 and tropical areas have had a different geological 

 history since the Tertiary and since ecological 

 conditions are also quite different, causing 

 changes in selective trends. The orchids offer 

 excellent material for such studies, since they are 

 not only one of the youngest families of higher 

 plants, but allow a detailed study of phylogene- 

 tic evolution owing to their dispersal, made pos- 

 sible by the smallness and large number of 

 their seeds. Long distance dispersal is most 

 easily studied between continents, and has 

 been followed, almost exclusively by phylo- 

 genetic diversification into new species or groups 

 of species or into new genera. In the case of the 

 formation of new genera, this has followed 

 generally the same lines as evolution in the 

 original area, but with strong predominance of 

 terrestrial orchids over epiphytes and with a 

 strong tendency to form saprophytic species in 

 the new area. In the cases of formation of spe- 

 cies after dispersal between continents, there 



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