Factors that shape seed mass evolution

Abstract

We used correlated divergence analysis to determine which factors have been most closely associated with changes in seed mass during seed plant evolution. We found that divergences in seed mass have been more consistently associated with divergences in growth form than with divergences in any other variable. This finding is consistent with the strong relationship between seed mass and growth form across present-day species and with the available data from the paleobotanical literature. Divergences in seed mass have also been associated with divergences in latitude, net primary productivity, temperature, precipitation, and leaf area index. However, these environmental variables had much less explanatory power than did plant traits such as seed dispersal syndrome and plant growth form.correlated divergence ͉ plant traits ͉ growth form ͉ phylogeny ͉ seed dispersal A s seed plants diversified, they colonized a wide range of habitats and developed a range of growth forms and seed dispersal strategies (1-4). These changes were accompanied by major changes in seed size, a trait that is central to many aspects of plant ecology (5). The fossil record shows a particularly rapid period of change in seed size from 85 million years ago (Ma) until shortly after the Cretaceous-Tertiary boundary (65 Ma). During this period, angiosperms radiated out of the tropics, and they shifted from being predominantly small-seeded to having a much wider range of seed size strategies, with a larger mean size (2, 6). Present-day species have seed masses spanning more than 11 orders of magnitude, from the dust-like seeds of orchids up to the 20-kg seeds of the double coconut (5, 7).There are two main schools of thought on the factors most likely to have driven these changes in seed size. Tiffney (2,8) suggested that the radiation of mammals increased the availability of dispersal agents for large seeds and, therefore, allowed plants to radiate into a wider range of seed masses than had been possible previously. This hypothesis is consistent with correlations between large-seededness and animal dispersal in presentday species (9). Eriksson et al. (6) argue that it is more likely that a closure of canopies resulting from changes in climate around the Cretaceous-Tertiary favored species with larger seeds. The strong relationship between light environment and seed mass across present-day species (10-12) and the superior survival of large-seeded species when grown under deep shade provide some support for this hypothesis. Eriksson et al. (6) also note that changes in growth form around this time might have contributed to the changes in seed mass, because larger seeds are generally associated with larger plants (13).Here we map a large seed mass database, together with information on other plant traits and environmental data on to the seed plant phylogeny. We then used correlated divergence analysis (14) to determine which factors have been most closely associated with evolutionary divergences of seed mass. We believe this is the beginning of a ne...

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