T1 - Evolution and the latitudinal diversity gradient

In 1807, Alexander von Humboldt wrote “The nearer we approach the tropics, the greater the increase in the variety of structure, grace of form, and mixture of colors, as also in perpetual youth and vigor of organic life.” The increase in numbers of animal and plant species from the poles toward the equator is one of the most pervasive patterns of life on earth. Although known at least since the early 1800s, this pattern still lacks a consensus explanation (–). And although well documented in large, multicellular animals and plants, this pattern is reported to be relatively weak or absent in morphospecies of unicellular organisms (). The lack of apparent geographic pattern has been attributed to high abundances, frequent and long-distance dispersal, and low extinction rates (). This argument would suggest that bacteria, even smaller, more abundant, and more readily dispersed than protists (), would also show little or no latitudinal gradient of diversity.

Here we review two major hypotheses for the origin of the latitudinal diversity gradient.

Lennon JJ, Greenwood JJD and Turner JRG (2000) Bird diversity and environmental gradients in Britain: a test of the species‐energy hypothesis. Journal of Animal Ecology 69: 581–598.

KW - Latitudinal diversity gradient

Postglacial range expansion drives the rapid diversification of the genus Pleistocene glacial cycles are thought to have played a major role in the diversification of temperate and boreal species of North American birds. Given that coalescence times between sister taxa typically range from 0.1 to 2.0 Myr, it has been assumed that diversification occurred as populations were isolated in refugia over long periods of time, probably spanning one to several full glacial cycles. In contrast, the rapid postglacial range expansions and recolonization of northern latitudes following glacial maxima have received less attention as potential promoters of speciation. reported a case of extremely rapid diversification in the songbird genus as a result of a single continent-wide range expansion within the last 10000 years. Molecular data from 264 juncos sampled throughout their range reveal that as the Yellow-eyed Junco () of Mesoamerica expanded northward following the last glacial maximum, it speciated into the Dark-eyed Junco (), which subsequently diversified itself into at least five markedly distinct and geographically structured morphotypes in the USA and Canada. Patterns of low genetic structure and diversity in mitochondrial DNA and amplified fragment length polymorphism loci found in Dark-eyed Juncos relative to Mesoamerican Yellow-eyed Juncos provide support for the hypothesis of an expansion from the south, followed by rapid diversification in the north. These results underscore the role of postglacial expansions in promoting diversification and speciation through a mechanism that represents an alternative to traditional modes of Pleistocene speciation. Recent divergence with fastplumage evolution (since the LGM) has been recently documented in orioles, redpolls, bluethroats, yellow wagtails, and Yellow-rumped Warblers. However, given the higher number and marked distinctiveness of plumage morphs and the short amount of time involved, the Dark-eyed Junco complex represents an exceptional case of fast plumage diversification. Given the marked differences in plumage characters, sexual selection may have acted as the mechanism of divergence

Latitudinal Gradients in Species Diversity – Klaus Rohde

Mean geographical distance among lower latitudinal populations was not different from that among higher latitudinal populations (b). The linear regression indicated no significant relationship between latitudinal midpoints of the examined taxa and the standardized differences between higher and lower latitudinal population differentiations (R2=0.0006, p>0.05). Therefore, it is unlikely that the observed pattern resulted from bias in geographical distances between the sampled populations or from latitudinal midpoints of examined taxa.

latitudinal diversity gradient | elena suglia

The demise of the larger-bodied putative stem-group taxa near the end of the Eocene about 40 mya coincides roughly with the origin of Antarctic-breeding extant taxa ( and ) in the crown-group, and with the beginning of a general cooling in global climate. This was also approximately when the fish-eating cetaceans evolved, and it has been hypothesized they may have out-competed these larger penguins which probably relied on the same food source. Two abrupt cooling periods resulting in the formation of large ice sheets in Antarctica are associated with the diversification of penguin taxa. The first cooling occurred about 34-25 mya, when , and diverged from the older Antarctic genera. These latter ancestral lineages may have dispersed northward by the newly formed circumpolar current, judging from the occurrence of a fossil in New Zealand about 24 mya. As surface waters in the Southern Oceans continued to cool towards the middle Miocene, and the flow of the circumpolar current around Antarctica intensified, another rapid climate transition, the middle miocene climate transition (MMCT) and subsequent increase in Antarctic ice volume occurred between 14 and 12 mya. The MMCT was accompanied by a second bout of cladogenesis that gave rise to multiple species of extant penguins distributed at even lower latitudes, including tips of southern continents. If this scenario is run backwards in the future, continued global warming might be expected to drive temperate-adapted species out of lower latitudes towards their ancestral distribution, possibly causing multiple extinctions of existing species.