ABSTRACT
Molluscs have an excellent fossil record extending some 570 million years from the earliest Cambrian to the present and therefore may be expected to provide some of the best information about evolutionary processes that occur over long periods of time. For example, some molluscan lineages have undergone little structural (morphological) change in hundreds of millions of years, whereas others have evolved so rapidly that the events may be regarded as instantaneous in geological time. Although conservative lineages are characterised by their low diversities, the production of the gross morphological differences that lead to new higher taxa does not seem to require more than limited diversity in the ancestral group. It is therefore necessary to distinguish between the production of disparity (morphological difference) and the production of diversity (the number of lower taxa) when assessing rates of evolution. Macroevolution (the origin of new higher taxa) appears to depend largely on the production of disparity, whereas microevolution (the origin of new lower taxa) results mainly in increased diversity.
Molluscs exhibit many of the features long observed in the histories of other animal groups. The highest taxa appeared early in the history of the phylum when diversity was low (seven of the eight classes had evolved by the end of the Cambrian some 500 million years ago). Secondly, the oldest representatives of higher taxa are small and there has been a continuous increase in maximum body size over the past 570 million years. Thirdly, most higher taxa appeared long before their first substantial radiations. Fourthly, most higher taxa are recognised largely by hindsight after a period of diversification has occurred. Fifthly, rates of diversification vary between different higher taxa. Sixthly, accelerated rates of evolution occur under special conditions such as geographic isolation, the development of symbiotic relationships, or the removal of competition as a result of mass extinction.
