POPULATION DIFFERENTIATION IN AGROSTIS TENUIS SIBTH

Abstract

In order to investigate the pattern of population differentiation in Agrostis tenuis, sixty tillers were collected from each of thirty-three areas mainly in central Wales. These were grown under garden conditions in Aberystwyth. In a subsequent experiment five of these areas were sampled again by tillers, and at the same time sampled by seed. These samples were grown under garden conditions in Bangor.Numerous differences between these population samples were recorded. The main conclusions are as follows:(i) The environment is the dominating factor in determining population differentiation.(ii) Although the distribution is continuous, distances of about 50 m or less are sufficient to effectively isolate populations from one another.(iii) Thus the species is able to evolve under the influence of natural selection in response to very local variations in environment.(iv) Since the environment is a graded patchwork of different conditions, the pattern of differentiation is similar.(v) It is, therefore, not possible to classify effectively such intraspecific variation.(vi) It is likely that this situation is to be found in most other outbreeding continuously distributed plant species occupying a wide range of habitats.

Evolutionary divergence among adjacent plant populations I. The evidence and its theoretical analysis

“…5) and Creed, Ford and McWhirter (1964) have discussed examples of evolutionary differentiation in Maniola jurtina which they term sympatric. But in their general pattern and in their details these examples are very similar to the general pattern in Agrostis tenuis (Bradshaw, 1959) and to the detailed patterns reported here. The case of Pinus monticola (Squillace and Bingham, 1958), quoted by Grant (1963) as being sympatric speciation, is similar.…”

“…In a species with more or less continuous distribution, there may be dines of varying slopes interspersed with rather sharp discontinuities corresponding to changes in the environment. Such a pattern of variation is perhaps best termed a graded patchwork and has been reported in Agrostis tenuis (Bradshaw, 1959), Eschscholtzia caljfornica (Cook, 1962) and other examples cited by these workers. As shown by Epling and Dobzhansky (1942) in Linanthus parryae, as well as by many other workers, the larger geographic subdivisions may give a greater degree of genetic divergence than the smaller microgeographic subdivisions.…”

Evolutionary studies on Maniola jurtina: the Isles of Scilly, 1958–59

“…Bradshaw (1952Bradshaw ( , 1959Bradshaw ( and 1960 has carried out an intensive survey of morphological and physiological differences in populations of a common grass, Agrostis tenuis Sibth. This outbreeding plant develops populations which vary greatly in habit and in ability to resist what are normally toxic levels of lead and zinc in the soil.…”

Evolution in closely adjacent plant populations III. Agrostis tenuis on a small copper mine

“…In a perennial grass species it seems reasonable to equate fitness and ultimate survival with tiller number and dry weight. Data of Bradshaw (1959) and Jowett (1959) suggest that density of vegetative tillers and density of reproductive tillers are closely correlated in metal tolerant and normal populations of A. tenuis. On this basis the coefficient of selection against tolerance on normal soils is 053.…”