Variation in Cepaea populations over 42 years: climate fluctuations destroy a topographical relationship of morph-frequencies

Abstract

Cepaea hortensis (Gastropoda: Pulmonata) populations in Hertfordshire, England, originally sampled in 1964-66, and sampled again in 1990, were sampled in 2007. Although the general pattern of shell colour and banding polymorphism remained stable, a significant trend for a reduction in the frequency of yellow shells in sites from valley bottoms, as observed in 1990, continued through to 2007. In both 1964-66 and 1990, populations in valley bottoms had higher frequencies of yellow shells than those on the sides, but this topographical relationship had effectively disappeared by 2007. The relationship had been explained in terms of low temperature extremes in valley bottoms in cold, anticyclonic conditions. Its decay is associated with much warmer winter conditions prevailing in the 3 years previous to both resurveys, compared with those before the original survey. The frequency of unbanded shells also increased over the period, but this change showed no topographical variation, and thus it is not possible to assign it to a particular cause. The low levels of variation between samples from the same habitat and the absence of present topographical relationships conceal the occurrence of powerful climatic selection, which is revealed only by repeated surveys. The conditions in which such changes might be detected are discussed.

Color polymorphism in a land snail Cepaea nemoralis (Pulmonata: Helicidae) as viewed by potential avian predators

“…Our study supports the hypothesis that predator visual selection in addition to other important factors like, e.g., climatic selection and habitat heterogeneity (e.g., Jones 1974; Cameron and Pokryszko 2008; Ożgo 2012), is one potential cause of Cepaea polymorphism evolution. Findings presented in this study warrant further investigation of the role of avian predation pressure on Cepaea nemoralis coloration.…”

“…This snail species exhibits genetic variability in shell color (yellow, pink, or brown) and banding pattern (zero, one, three, or five dark bands), creating a dozen or so morphs (Richards and Murray 1975). Many factors appear to influence spatiotemporal variability in Cepaea shell color including genetic drift, migration, climatic selection, habitat heterogeneity, and landscape structure (e.g., Jones 1974; Hutchison and Templeton 1999; Cameron and Pokryszko 2008; Le Mitouard et al 2010; Ożgo 2012). Selective predation by birds has long been hypothesized as one of the main forces shaping polymorphism in shell coloration of Cepaea nemorali s (e.g., Cain and Sheppard 1954; Allen 2004; Cook 2005; Punzalan et al 2005; Rosin et al 2011), but the exact mechanism behind this process remains uncertain.…”

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

“…Heat stress is very unlikely, and the direction of the association argues against it. Darker shells absorb more heat, possibly giving dark apices an advantage in cloudier winter conditions, analogous to frost hollows favouring darker shells in C. nemoralis and Cepaea hortensis (Bantock, 1980;Cameron and Pokryszko, 2008). The winter temperatures in Perth, however, are moderate (average minimum of 7.7 11C in July, the coldest month, without frost in most years), so it is uncertain how important this is.…”

Thirty-four years of climatic selection in the land snail Theba pisana

“…Likely mechanisms of selection are often clear in terms of selective predation or climatic selection (Jones et al, 1977;Clarke et al, 1978;Cain, 1983), allowing predictions of responses to changes in habitat or climate. Following the lead by Clarke and Murray (1962), several long-term studies have estimated rates of change and intensity of selection in snails, especially Cepaea nemoralis (see, for example, Murray and Clarke, 1978;Wall et al, 1980;Cain et al, 1990;Cameron, 1992Cameron, , 2001Cowie, 1992;Cook and Pettitt, 1998;Cowie and Jones, 1998;Cameron and Pokryszko, 2008;Ozgo and Kinnison, 2008). With few exceptions, these studies have compared morph frequencies at end points of intervals up to 50 years, rather than between successive years or generations.…”