Review: Weir JT, Schluter D (2007) The latitudinal gradient in recent speciation and extinction rates of birds and mammals. Science, 315:1574-1576 + supplement.

Feature Paper: DOWNLOAD * Weir JT, Schluter D (2007) The latitudinal gradient in recent speciation and extinction rates of birds and mammals. Science, 315:1574-1576 + supplement.

Author Abstract: Although the tropics harbor greater numbers of species than do temperate zones, it is not known whether the rates of speciation and extinction also follow a latitudinal gradient. By sampling birds and mammals, we found that the distribution of the evolutionary ages of sister species — pairs of species in which each is the other’s closest relative — adheres to a latitudinal gradient. The time to divergence for sister species is shorter at high latitudes and longer in the tropics. Birth-death models fitting these data estimate that the highest recent speciation and extinction rates occur at high latitudes and decline toward the tropics. These results conflict with the prevailing view that links high tropical diversity to elevated tropical speciation rates. Instead, our findings suggest that faster turnover at high latitudes contributes to the latitudinal diversity gradient.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This is the third paper of our 12-week course in biogeography, and the second paper of week 2. As with the first paper this week, this paper deals with approaches meant to discover, map, and quantify patterns of diversity of various organisms in space. This week we'll discuss the latitudinal species diversity gradient, meaning the concept that there is a distinct non-random pattern of species accumulation and diversity as one moves latitudinally from the equator to the poles.

As the authors state, "the tropics possess many more species than temperate regions, yet the underlying causes of this latitudinal gradient in species diversity are poorly understood."

So here we are left with two points to discuss.

First, that there is a nonrandom gradient in species diversity, with the tropics (at the bare minimum the region bounded between the Tropic of Cancer (23º26' north of the equator) and the Tropic of Capricorn (23º26' south of the equator), but more realistically dependent upon climatic conditions as a result of upwelling (in the case of marine environments) or altitude and precipitation (in the case of terrestrial environments).

Second, not only does such a pattern exist (for the majority of species and taxonomic groups in nature, both for plants and animals, though there are some distinct exceptions), but this pattern is poorly understood and there is a lot of speculation among scientists as to why such a pattern exists. Over the coming weeks, we'll discuss some of these theories, but if you also recall our Week One lecture on historical biogeography, a quick summary of some of the more important theories was provided.

The paper this week deals with two terrestrial groups (birds and mammals) and tries to describe and explain their latitudinal diversity gradient. And while broad generalizations are almost never possible in biological sciences (sometimes, exceptions to the rule are the rule), the mere fact that most species studied show a similar pattern of diversity (aquatic and terrestrial) suggests that similar principles apply.

This is a lesson that one should learn when starting out in science: find principles that apply for one group of organisms and perform a similar study to determine whether such principles apply for other groups of organisms. If enough organisms follow the same pattern, then theories can be proposed and mechanisms underlying such patterns can be discussed.

In performing their study, the authors chose to look at speciation and extinction rates as a mechanism to "cause" the observed distribution patterns today. How did they do this since it is probably impossible to be present for a speciation or extinction event? The answer lies in genetics.

The authors looked at the "genetic distances of mitochondrial DNA from the cytochrome b gene" in 309 sister species pairs ("most closely related pair of extant species descended from an immediate common ancestor") of "New World birds and mammals." As mtDNA is passed down through females (mothers) only and is fairly conservative over time, mapping genetic mutations within the mtDNA of individuals and assuming (based on past studies for wide groups of organisms) a certain constant rate of random mutation allows one to determine the amount of time that two species have existed separately since they last shared a common ancestor (the point where no mutations in the mtDNA existed between individuals in each species examined.

One interesting finding for the authors was that age of divergence (and by proxy, speciation rate) differed latitudinally, with species near the equator having diverged up to "10 million years ago, with a mean age of 3.4 million years ago" and that "as distance from the equator increased, the upper limit and mean ages of sister species declined significantly" such that "at the highest latitudes, all of the sister species diverged less than 1.0 million years ago."

The authors point out that "this pattern of declining age with latitude is opposite to the pattern that would occur if faster rates of speciation had driven the buildup of Neotropical diversity, because the ages of sister species should be youngest where speciation rates are highest."

The authors then suggest that "it may be possible to extract information about speciation and extinction rates from the distribution of sister-species ages… because speciation and extinction can be inferred by the shape of the age distributions of sister species." The authors explored various mathematical models to fit speciation and extinction rates across a latitudinal gradient.

The authors found that "estimated speciation and extinction rates were lowest at the equator and increased significantly toward the poles." The authors note that "these results are surprising because the latitudinal gradient in estimated speciation rate is opposite to the gradient in net rate of diversification estimated by many studies to be highest in tropical taxa."

This finding is important because one theory meant to explain higher species diversity in the tropics is that speciation rates are higher in the tropics than in temperate latitudes and therefore, more species are "created" in the tropics as a result of faster speciation rates.

In reconciling their findings the authors suggest the following explanations:

"These quantitative estimates are based on the assumption that speciation and extinction can be approximated by a continuous birth-death process as latitude becomes higher or lower. Yet, we know that there have been fluctuations in the opportunities for speciation and extinction over the past few million years. For example, extensive climatic fluctuations that occurred at high latitudes during the late Pliocene and Pleistocene (2.5 Ma to present) may have concentrated speciation and extinction events in time, resulting in episodic species turnover. In contrast, the bursts of diversification in tropical faunas may predate the late Pliocene and Pleistocene, and the patterns observed today may be the result of a subsequent decline in diversification either because the geological processes that promoted diversification (e.g., formation of Isthmus of Panama, marine incursions, orogeny, and river formation) have slowed or because diversification rates declined as the number of tropical species approached a “carrying capacity." Given such variability, our estimates are best regarded as averages over the periods studied. These results suggest that extinction rates are greatest where species diversity is lowest [in Temperate regions for most species]. Whereas most efforts have aimed at identifying the geological, climatic, and ecological factors that might have elevated tropical speciation rates, our results suggest that both speciation and extinction vary with latitude and contributed importantly to the latitudinal diversity gradient."

As we look at other groups of organisms over the "semester" we'll see how well the authors' findings apply to other taxonomic groups, especially in a marine context.

Next we'll be looking at the biogeography subdiscipline of "panbiogeography" with the reading list noted below. As usual, I encourage downloading the articles in advance and reading them before the summaries.

Cheers and happy studies.