SECTION 9 



POPULATION GENETICS 



9.1. The Diversity and Complexity of Ecotypic 

 Differentiation within Plant Species in Response 

 to Soil Factors. R. W. Snaydon (Reading, 

 Great Britain). 



The Park Grass experimental plots at Rot- 

 hamsted provide a unique opportunity for 

 studying ecotypic differentiation in response to 

 controlled differences in soil factors over short 

 distances and relatively short periods of time. 

 The plots, which measure approximately 20 m 

 X 35 m, have continuously received different 

 fertilizer treatments since 1856 and different 

 liming treatments since 1903. The various treat- 

 ments have led to wide differences in the physi- 

 ognomy and botanical composition of the 

 vegetation, yet the grass species Anthoxanthum 

 odoratum is present on most plots. 



Populations samples of A. odoratum from six 

 plots were grown in spaced plant trials and 

 greenhouse experiments. The results indicate 

 that the populations differ significantly in a 

 number of morphological and physiological 

 characters. The response of populations to 

 contrasting soils was closely correlated with 

 native soil pH. Vegetative height, inflorescence 

 height, inflorescence posture and rust suscepti- 

 bility of the populations were closely correlated 

 with vegetation height on the Park Grass plots. 

 Data of flowering, number of panicles and 

 mildew susceptibility were partially correlated 

 with vegetation height but also with fertilizer 

 treatment. Yield, panicle size and leaf size were 

 to some extent correlated with vegetation height, 

 but the relationship was evidently complex. 

 These results, together with results of the 

 physiological analysis of ecotypic differentiation 

 in response to soil factors in Trifolium repens, 

 demonstrate the complexity of ecotypic response 

 to initially simple soil differences, both through 

 the interactions of soil factors and through the 

 secondary and tertiary effects of the changing 

 physiognomy and botanical composition of the 

 vegetation. 



9.2. The Analysis of Evolutionary Processes involved 

 in the Divergence of Plant Populations. 



A. D. Bradshaw (Bangor, Great Britain). 



It is perhaps usual to imagine that evolutionary 

 divergence of distinct populations within a species 

 can only occur overc onsiderable differences. How- 

 ever, in a number of plant species this can be 

 shown not to be true. In two species, Agrostis 

 tenuis and Anthoxanthum odoratum, populations 

 occurring on toxic soils associated with old mine 

 workings show a tolerance to the toxicities 

 which is not present in normal populations. 

 This characteristic shows a remarkable localized 

 distribution. Populations only a few feet away 

 from the toxic areas fail to show the charac- 

 teristic. In the same manner, in Agrostis stoloni- 

 fera, populations occurring in exposed maritime 

 habitats are found on cultivation to possess a 

 dwarf growth form not found in populations 

 occurring in protected habitats a very short 

 distance away. 



In such situations it can be demonstrated 

 that there is considerable gene flow between 

 contrasting populations tending to obliterate 

 the differences between the populations. How- 

 ever, since the differences between the popu- 

 lations are retained, strong selection must be 

 occurring to remove the effects of gene flow. 

 Evidence for this is available. It appears that the 

 manner and timing of the selection may be very 

 different in the adjoining populations, but in 

 general it occurs in the seedling stage. 



Gene flow between populations appears to 

 occur even between populations much farther 

 apart. From this it may be argued that the 

 continuous interplay of strong selection pressures 

 and gene flow are a normal part of the evolution- 

 ary processes determining the occurrence of 

 distinctive -populations within species. 



9.3. Variability of Growth of Ryegrass (Lolium spp.) 

 Progenies under Artificial Environments. 



S. O. Fejer (Ottawa, Canada). 



Responses of inbred and outcrossed New 

 Zealand ryegrass progenies to some factors 

 of artificial environments in the phytotron of 

 the California Institute of Technology, Pasadena, 

 are reported. Growth was relatively little 

 affected by night temperatures, except in some 



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