Abstract
The geographic range sizes frequency distribution (RFD) within clades is typically right-skewed with untransformed data, and bell-shaped or slightly left-skewed under the log-transformation. This means that most species within clades occupy diminutive ranges, whereas just a few species are truly widespread. A number of ecological and evolutionary explanations have been proposed to account for this pattern. Among the latter, much attention has been given to the issue of how extinction and speciation probabilities influence RFD. Numerous accounts now convincingly demonstrate that extinction rate decreases with range size, both in living and extinct taxa. The relationship between range size and speciation rate, though, is much less obvious, with either small or large ranged species being proposed to originate more daughter taxa. Herein, we used a large fossil database including twenty-one animal clades and more than 80,000 fossil occurrences distributed over more than 400 million years of marine metazoans (exclusive of vertebrates) evolution, to test the relationship between extinction rate, speciation rate, and range size. As expected, we found that extinction rate almost linearly decreases with range size. In contrast, speciation rate peaks at the large (but not the largest) end of the range size spectrum. This is consistent with the peripheral isolation mode of allopatric speciation being the main mechanism of species origination. The huge variation in phylogeny, fossilization potential, time of fossilization, and the overarching effect of mass extinctions suggest caution must be posed at generalizing our results, as individual clades may deviate significantly from the general pattern.
Highlights
The extent of the geographical range is one of the most important biological attributes of a species
Albeit the quality of the fossil record in terrestrial settings may be very good in some instances (e.g., Laurin and Soler-Gijón, 2010) we focused on the marine realm, because of the usually finer and more continuous fossilization in marine sediments, and because the calculation of the speciation and extinction rates is more robust under better sampling (Foote, 2000)
We found that the degree of sympatry has no bearing on the proportion of small ranged species (SR: slope = −3.038, p = 2 ∗ 10−4; degree of sympatry: SR: slope = −0.021, p = 0.11). These results indicate that SR is a very good proxy to describe the degree of asymmetry of the range sizes frequency distribution (RFD) distribution
Summary
The extent of the geographical range is one of the most important biological attributes of a species. On the Speciation, Extinction, and Range Size one hand, large-ranged species may have greater chance to produce daughter species (Wagner and Erwin, 1995), either by peripheral isolation (Glazier, 1987; Rosenzweig, 1995), or microvicariance (Cracraft and Prum, 1988). Assuming most originations occur by allopatric speciation, the likelihood that a barrier is large enough to break a species’ range into smaller pieces is arguably low for the largest-ranged taxa (Gaston, 2003). This suggests that even under allopatry speciation probability might be a peaked function of species range size (Jablonski and Roy, 2003). It must be noted that since species with large range might be expected to live for longer (because of the positive effect of range size on survival), they will likely leave more descendants in the fossil record (Wagner and Erwin, 1995)
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