J. S. Jones

Male size and mating success in Drosophila melanogaster and D. pseudoobscura under field conditions

Abstract Wild mating male D. melanogaster and D. pseudoobscura were larger than randomly sampled males. That this was due to size and not a confounding effect of age was confirmed by release experiments with D. melanogaster of standard age; larger males were at a mating advantage with virgin and inseminated females. In both species larger males delivered more courtship and in D. pseudoobscura they won more aggressive encounters. These results on Drosophila in the field confirm and extend those previously obtained in the laboratory.

Large scale patterns of genetic differentiation at enzyme loci in the land snails Cepaea nemoralis and Cepaea hortensis

Samples of 231 populations of the land snails Cepaea nemoralis and C. hortensis from Britain, France, Switzerland and Spain were analysed for genetic polymorphism in six enzyme systems. These sibling species show similar levels of variation detected by electrophoresis, and have a generally similar degree of local divergence over homologous enzyme loci. As well as extensive local and regional differentiation, both species show large-scale changes in allele frequency across Europe. In C. hortensis there is a continuous gradient in allele frequency from northern Britain to northern Spain, while in C. nemoralis north-south clines on the continent of Europe are reversed in direction in Britain. There are few obvious correlations of allele frequency change at single loci with components of the environment. Patterns of allele frequency variation in 24 sympatric populations of the two species are generally independent of each other, although there is a positive association of the frequencies of alleles at a leucine aminopeptidase locus in the two species. The statistical significance of this association depends on a single locality, and there is little indication of shared patterns of allele frequency change which might reflect a common response to natural selection. In addition, there is no evidence that the extensive geographical change in C. nemoralis and C. hortensis is a precursor of speciation.

Climatic selection on body colour in Cepaea

There is geographical variation in body colour in the land snails Cepaea nemoralis and C. hortensis along a transect of 88 samples from the north of Scotland to the Pyrenees. Paler body colour is associated with higher mean daily maximum temperature in both species. Laboratory experiments show that dark-bodied snails heat up more rapidly and reach a higher equilibrium temperature than do pale snails when exposed to radiant energy. Climatic selection favouring pale body colour in warm areas and dark body colour in cold areas is probably responsible for the association with climate.

Ecological interactions between Cepaea nemoralis and Cepaea hortensis: competition, invasion but no niche displacement

The importance of coevolution in shaping communities is not clear. Experiments with shells marked with a paint which fades in the sun show that the land snail Cepaea hortensis (Mull.) is more exposed to daylight than is Cepaea nemoralis (L.), but that there is no change in this behaviour when snails are placed in mixed or single species populations and no evidence of an interspecific interaction that might be a precursor of coevolution and character displacement for this niche dimension. C. hortensis on the Marlborough Downs has invaded many C. nemoralis colonies and replaced others during the past 25 years, a range expansion that may reflect the competition between the two that has been observed in the laboratory. If the lack of interaction for the niche dimension studied here is typical, the relative abundance of C. hortensis and C. nemoralis may reflect a balance between invasion and extinction rather than coevolution. Key-words: Competition, invasion, niche displacement, coevolution, behavioural thermoregulation, community structure, land snail, Cepaea

Genotype-specific habitat selection and thermal ecology in Nucella lapillus (L.) (the dogwhelk)

Nucella lapillus is a marine gastropod found all around the British coastline. Populations frequently display great polymorphism in the shape and colour of the shells. Shell shape is related to wave exposure but the forces maintaining colour polymorphisms are less clear. Populations from the north coast of Cornwall are shown to exhibit genotype-specific microhabitat partitioning, with lighter colour morphs being relatively commoner in sunnier sites and on average spending more time in the sun than darker morphs. This type of habitat selection has been claimed to be important in the maintenance of genetic variation and may explain some of the colour polymorphisms seen.

Geographic structuring of molecular and morphological polymorphism in Pyrenean populations of the snail Cepaea nemoralis

To determine whether geographic patterns of variation in the frequency of shell polymorphisms in the land snail Cepaea nemoralis mark regions of extensive genetic differentiation, allele frequencies at six polymorphic enzyme-encoding loci were analyzed electrophoretically in 74 samples from the central part of the Spanish Pyrenees. Within a large ‘area effect’ for unbanded shells on the southern slope of the Pyrenees, there is no enzyme locus at which allele frequencies are homogeneous. Because this morphological area effect does not mark a region of pervasive genetic change, the genetic structure of populations in the region studied does not conform to recent models of ‘area effect speciation’. Factor analysis of allele frequencies at three loci controlling shell polymorphisms and six enzyme loci separated populations on the southern slope into two groups, one of which is similar to the group of populations on the northern slope. There is therefore considerable geographic structuring of both molecular and morphological polymorphism in this species.

Experimental Manipulation of Some Snail Populations Subject to Climatic Selection

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The genetic structure of some steppe populations of the snailCepaea vindobonensis (Pf.)

The snailCepaea vindobonensis, which is polymorphic for the number of bands on the shell and for intensity of band pigmentation, was studied in an area of steppe in Eastern Romania. There are marked geographical variations in phenotype frequency which, unlike the similar variation found in populations from mountainous areas, show no detectable associations with any easily identifiable component of the environment. These patterns of variation may be due to selection by cryptic ecological factors or to a balance between climatic selection and selection by the “genetic environment” of the population.

The effects of dispersion on frequency-dependent predation of polymorphic prey

Populations of coloured baits were presented to wild birds. When the two bait colours were randomly intermingled, the birds ate an excess of the commoner form. However, rarer prey were preferred if they were aggregated together. The fitness of alleles under apostatic natural selection may thus be related to spatial pattern as well as to frequency.

Gaps in Fossil Teeth: Saltations or Sampling Errors?

All evolutionists accept that there are gaps in the fossil record. To some, gaps are data: they mark sudden transitions, unlikely to be preserved, between one species and its descendants that have arisen by a brief genetic revolution within an otherwise stable lineage. However, recent studies of sediment gain and loss show that few fossil beds can be complete enough to give details of the process of speciation, so that most gaps are gaps, and nothing more. Differences in the perception of time and sampling completeness mean that paleontologists and geneticists see the rate and pattern of evolution in different ways. Just how misleading each view may be is seen by analysing the laws of life governing change in living populations using the limitations intrinsic to studies of past times; and by a new insight into the details of species formation now emerging from some remarkably complete primate fossil sequences.