A “radiation” (sometimes called an “adaptive radiation”) is when a single ancestral species gives rise to a number of novel species, often in a fairly short (geological) period of time. Following this radiation event, it seems often to be the case that subsequent speciation is less common. In fact, many living clades that have only a small number of extant species have such radiations in their history. It is quite possible that the radiation event occurred for reasons local in time and space, such as a recent extinction leaving various niches open, or the presence of a particular adaptation suddenly enhancing fitness as it had not previously because of an ecological change.
But one basic question (among many) that needs to be addressed when thinking of these issues of macroevolutionary patterning is this: For a given clade, where we can presume that there is a great deal of competition among closely related species, is there a sort of maximum limit on how many species you can get? In other words, as a clad starts to diversify, does it fill up the available eco-morpho-nichey space, which would eventually slow down the rate of speciation because new species become less likely to arise?
The answer is a resounding: Probably!
Density-Dependent Cladogenesis in Birds, a paper just out in PLoS Biology, by Albert Phillmore and Trevor Price, examines this question. From the author’s summary:
It is probable that the number of species that a given region can support is limited; however, it is unclear whether the limit is approached sufficiently in nature such that the rate at which new species form slows down. Using the pattern of phylogenetic branching, a technique that estimates evolutionary relationships based on molecular data, we demonstrate that in large clades of birds, there is a decrease in the per-lineage probability of speciation as the number of species in the clade increase. We also show that this pattern can arise even if speciation and extinction occur randomly through time. This is because large clades are likely, by chance, to have rapidly speciated early in their history, and will relax back to the average speciation rate later on. We account for this effect, and we still find evidence that, as a clade grows to large size, the per-lineage probability of speciation declines. These results strongly suggest that speciation rates are slowed as environments fill up with competitors.
Simulations are a great way to get at evolutionary or ecological realities. Real life data are nice, but hard to get and often messed up with historical and particularistic oddness that comes along with real life. The authors use simulation techniques to show that the probability of a new species emerging goes down with the number of species that have already emerged in a given clade.
The authors test this model with a look at 45 real life clades of birds. The correlation is not overwhelming, but there is a negative correlation between clade size and the relevant statistic (which they call “Y”) of likelihood of a new species emerging within the clade.
The empirical results provide strong evidence for slowdown in speciation rate in large clades. The magnitude of slowdown seems to be quite large. For example, among the 22 clades with 15 or more lineages at 2 Mya, the median value for γ = −1.77, and the median clade size is 29, consistent with a slowing of speciation rate in the later stages of a radiation to 10%-50% of the initial rate…
The authors conclude that two factors primiarly cause the slowdowns of speciation seen in large clades. One is that I mentioned at the beginning: There is a sort of opportunity that is then realized at the beginning of a species radiation, followed by the effect demonstrated in this paper, whereby the niches are essentially filled. The other has more to do with the random nature of variation in species origin and extinction.
The overall importance of random processes as causes of slowdowns depends on the true extinction rate. If extinction rates are low, the importance of stochastic factors in generating slowdowns may have been underestimated. If, as seems likely, extinction rates approach the speciation rate, then constant birth-death models on their own cannot explain slowdowns. Instead, our findings of strong slowdowns provide support for nonrandom processes of species diversification through time.
Phillimore, A.B., Price, T.D. (2008). Density-Dependent Cladogenesis in Birds. PLoS Biology, 6(3), e71. DOI: 10.1371/journal.pbio.0060071