Limits to speciation inferred from times to secondary sympatry and ages of hybridizing species along a latitudinal gradient

Range expansions are critical to renewed bouts of allopatric or parapatric speciation. Limits on range expansions—and, by implication, speciation—include dispersal ability and permeability of geographical barriers. In addition, recently diverged taxa may interfere with each other, preventing mutual expansion of each other’s range into sympatry, because reproductive isolation is incomplete and/or ecological competition particularly strong. On the basis of geographical distributions and mitochondrial DNA phylogenetic information for 418 recently diverged species of New World birds, we estimate that secondary sympatry takes on the order of millions of years following population splitting and hence could impose an important limit on the rate of range expansion, thereby limiting further rounds of species formation. Average rates of achievement of sympatry have been faster in the temperate region (we estimate 1.7 million years to sympatry at 60°) than in the tropics (3.2 million years to sym...分布范围扩张对于异地物种形成（allopatric speciation）与邻域物种形成（parapatric speciation）的新一轮发生至关重要。对分布范围扩张的限制——由此也会间接限制物种形成——包括扩散能力与地理屏障的通透性。此外，新近分化的类群可能会相互干扰，阻碍彼此的分布范围扩张至同域分布（sympatry）状态，这是因为生殖隔离尚未完全建立，且/或生态竞争尤为激烈。我们基于418种新近分化的新大陆鸟类的地理分布数据与线粒体DNA（mitochondrial DNA）系统发育信息，估算得出次级同域分布（secondary sympatry）的形成大约需要数百万年的时间，其始于种群分化之后；因此这一过程可能会对分布范围扩张的速率构成重要限制，进而进一步制约物种形成的轮次。温带地区达成同域分布的平均速率（我们估算在60°纬度处仅需170万年即可形成同域分布）快于热带地区（需320万年以达成同域分布……）