Genetic variation in a cline in a living intertidal snail arose in the Neogastropoda over 100 million years ago.

To understand how species adapt and evolve it is necessary to appreciate the relationship between genetic variation and the environment. Here, the fossil record and molecular data from different lineages of the marine Gastropoda are used to understand the evolution of genetic variations found in the nuclear gene for mitochondrial malate dehydrogenase (mMDH; EC 1.1.1.37) in the living intertidal Muricid snail, Nucella lapillus. Assuming a molecular clock, DNA sequences of mMDH indicate that two variants found in N. lapillus, mMDH(9) and mMDH(10), may have arisen as long as 144 MY (million years) ago and at least prior to the evolution of the Muricidae approximately 112-90 MY ago. The Muricidae contain by far the greatest majority of the Neogastropoda specialized for life in the intertidal habitat. In N. lapillus the mMDH(9) and mMDH(10) variants covary with variations in other biochemically defined loci, inherited phenotypic traits (shell shape and physiology) and karyotype frequencies to differentiate two distinct nuclear haplotypes that are associated with different temperature environments. The variations in shell shape that are associated with the haplotypes of N. lapillus represent adaptations to temperature stress and similar variations occur in other related intertidal molluscs whose lineages are much older than Nucella, which arose around 25 million years ago. It is suggested that the divergence of the mMDH variants found in N. lapillus may reflect an ancient genetic event, such as a chromosomal mutation, perhaps involving variation in other linked traits that together became important in the subsequent evolution of the marine Gastropoda.