Chiasma frequency variation with altitude in Cepaea hortensis (můll.).

Populations of Cepaea hortensis have been sampled from an area of 121 sq km on the north slopes of the Brendon Hills, Somerset, and analysed for chiasma frequency. Variation in chiasma frequency is due to variation in a single large bivalent--the A group bivalent. In 12 populations studied during 1973 no association between chiasma frequency and ecological factors could be detected. In particular, there is no evidence of a negative correlation of population density with chiasma frequency which is known to exist in populations of C. nemoralis within the area. In 1974, 17 populations, four of which were utilised in the 1973 analysis, were studied. Negative correlations of interstitial (but not total or terminal) chiasma frequency and between-cell variance with altitude were found. It is likely that the correlation of between-cell variance with altitude is due to the strong correlation of variance with chiasma frequency which exists in these populations. There is evidence from several populations that chiasma frequency is stable between years. The implications of the negative correlation of chiasma frequency with altitude and the lack of a correlation with population density are discussed.