SUMMARYSewall Wright's neighbourhood model indicates that the area containing a panmictic unit within a continuous and uniform array of organisms can be estimated by 4iro2 where a-2 is the parent-offspring dispersal variance measured around a zero mean and relative to a single reference axis passing through the population. The method has proved popular in studies on higher plants but the literature is confused as to how the two components of dispersal variance, for pollen and seeds, should be combined. It is argued that o2 o' ÷o-, where o and o are pollen and seed dispersal variances already corrected to give axial variances. The consequences of using different methods are compared and some puzzling aspects of the previous literature are discussed.

SUMMARYThe distribution of the dark keeled form of Lotus corniculatus in Western Europe is described. Although there is an association between the frequency of the dark keeled form and latitude in England and Wales, no differences between the dark keeled and light keeled forms in the winter survival or in seed production were found under experimental conditions. Furthermore, there appears to be no association between cyanogenesis in the leaves and keel colour, either on a per plant or on a population basis.

A survey has been carried out in Leeds, England, in the west Yorkshire industrial heartland, and in neighboring York, surrounded by agriculture, of melanic frequency in the moth species Biston betularia, Odontoptera bidentata, and Apamea crenata. All show a decline in melanics in the postindustrial environment, the first over almost the full range from nearly 100% to less that 10%, the others to smaller extents. Changes in several species over as great a magnitude and as wide an area must result from selection. The results are compared with others along a transect through northern England. The onset of response is progressively later from west to east. The rate of decline is lower at the extremes of the transect to west and east than it is in the center. We still do not have a clear picture of the causes of the changes. One major factor is likely to be selective predation, which is shown to be critically dependent on predation rate. As a consequence, differences in settling behavior between the species could account for different responses even if the species are attacked by the same predators.

The invasion of Great Britain by the gray squirrel from America has posed many interesting problems in ecology and conservation (interactions with the native red squirrel) and in forest management (the gray squirrel severely damages young stands of deciduous trees). Since 1973, mapped data on the annual distribution of the two species in state‐managed forests have been published by the Forestry Commission. Using the established methodology of annual ornithological index numbers, series of index numbers have been calculated to show the distributional dynamics of the two species. In Great Britain as a whole there has been rather limited change over the 15‐year period from 1973. However, this masks (1) the dramatic distributional decline in the red squirrel in Wales balanced by a modest expansion in Scotland, and (2) the substantial distributional increase in the gray squirrel in the small number of grid squares in Scotland in comparison with the relatively steady distribution in the large number of grid squares in England. A new index, measuring the annual replacement of one species by the other species, indicates little overall change in England and Scotland, but a net advance of the gray squirrel in Wales. A series of Markovian matrices predicted that the red squirrel would persist in forests in Scotland and in some areas of England, but would be rare in Wales. Over Great Britain as a whole, the gray squirrel is likely to occur in about twice as many forest areas as the red squirrel.

Keel-petal colour (dark versus light) in Lotus corniculatus L. has been scored for 125,503 plants at 1348 sites throughout the British Isles. The polymorphism shows strong and regular clinal change in morph frequencies. In all areas of western Britain and in southern England the majority of plants has light keels, particularly so in coastal regions. In eastern England, north of the River Humber, and in eastern Scotland dark-keeled plants predominate. Most off-shore islands conform to this pattern. Although the range in morph frequencies is virtually complete, monomorphism is unusual except in small samples, suggesting frequency-dependent selection in favour of the darkkeeled morph when rare. At least in Yorkshire, dark-keeled plants are. under-represented in the early stages of the flowering season; this is of doubtful evolutionary significance, although it is important in terms of sampling.