THE CONTRIBUTION OF DISPERSAL ABILITY TO THE EXPANSION OF GEOGRAPHIC RANGE

The geographic range of genera plays an important roll in extinction. As such, it is important to understand how geographic range limits occur and change over time as well as what factors contribute to constructing geographic ranges. Dispersal ability is thought to limit geographic ranges, so that genera with low dispersal ability are expected to have smaller ranges than genera with high dispersal ability. We test this idea by comparing the pattern of geographic range expansion in two groups with demonstrably different dispersal abilities: clams and corals. The dominantly colonial life history of corals is associated with severely limited dispersal ability. Clams with both feeding and non-feeding larvae, in contrast, have relatively high dispersal ability. We decompose the contribution of dispersal to geographic range into two aspects: (1) the role of dispersal in limiting the initial expansion of geographic range and (2) the maintenance of geographic range over time by dispersal ability. We predict that dispersal ability will correlate with the earliest geographic range of a genus if dispersal is a significant limiter of geographic range and that high dispersal ability will help to maintain peak geographic range over longer periods of time than low dispersal. We find that clam and coral genera have indistinguishable geographic ranges in the time interval of their first occurrence in the fossil record, rejecting dispersal as limiting geographic range. Nor does dispersal ability play a role in the maintenance of geographic range. Clams and corals have indistinguishable patterns of geographic expansion over time. Dispersal ability is doubtlessly important on short timescales but our results suggest that the macroevolutionary significance of dispersal ability is minimal. Over geological timescales, all dispersal abilities are comparable.