Saturday, March 13, 2010

Mr. Toad's wild ride

An evolutionary reconstruction of the world-wide radiation of toads speaks to the process of speciation.

What makes toads so successful? They arose sixty million years ago in South America from frogs that had existed for 200 million years before, and had spread all over the world by twenty million years ago. In our day, cane toads have become pests in Australia, arriving from Hawaii and showing their awesome evolutionary fitness.

A recent paper in Science (with comment) attempts to classify traits among toads and deduce which traits were present in those lineages that spread most rapidly, arriving at a suite of traits that promoted both radiation and speciation.

The distinction between toads and frogs is a bit hard to define. Proper toads are a phylogenetic lineage (Bufonidae). But many frogs which convergently evolved similar dry-tolerance traits are commonly called toads, while some rainforest toads (harlequin frogs) are called frogs. This paper sticks to the lineage-based definition, however.

Traits allowing toads to spread rapidly over the terrestrial globe are relatively obvious, involving independence from water- to live in drier areas outside rain forests; fat storage and large size- also for portability; secretion of toxins for protection; and high fecundity- large numbers of small eggs, with larvae that feed themselves rather than living off egg reserves.

The authors deduce these traits from current conditions- traits common in the most widely-distributed toads, relative to those toads with smaller geographic distribution. The authors then take this method back in time, creating a DNA-based family tree (phylogeny) of toads, aligning it with paleontological data for time estimates, and deducing which toads had which traits at which times in the past.

They use paleontological data and various statistics to estimate which toads had which traits back in the day. For instance, poison glands appear in toads shortly before their entrance into North America ~47 milion years ago- no toads from lineages branching prior to this point have them.

Their key conclusion is that those lineages that spread to new areas (entering North America, for instance, or Asia) shared a high number of these portability traits. Thereafter, these lineages generated a radiation of species, many of which were more specialized again and restricted in range. The figure shows what they are talking about, with color codes indicating the proportion of portability traits at each divergence.

Phylogenetic tree of toads, color coded with inferred range-expansion traits (click for larger size).
The authors conclude that toads have repeatedly evolved both range-expanding and range-restricting traits, and it is the range-expanding traits that led not only to range expansion (obviously) but also to speciation, since arriving in new continents and climates generates speciation (sometimes called range-edge speciation).

An important corollary is that lineage representatives at the range edge tend to both colonize new areas and to generate new species. "We hypothesize that these reciprocal effects [of range expansion, leading to better adaptation to diverse habitats] have caused the rapid global colonization of bufonids and produced the enhanced genetic drift at the expanding frontier, with consequent high levels of population differentiation and speciation."

Something similar happened in the human lineage. Absolute genetic diversity is highest in the ancestral homeland of Africa, hosting lineages far more ancient than those that migrated to other parts of the world. Yet migration to range edges and distant continents led to new traits and population differentiation, and may have involved range expansion traits as well. What have been traditionally been viewed as "races" would have become incipient species, had hundreds of thousands more years elapsed with sufficient isolation (as apparently was true for past hominid lineages, which ramify as more fossils are unearthed).

Perhaps our instinctive xenophobia is a related mechanism of speciation by which miniscule differences between virtually identical populations is psychologically enforced, preserving whatever small advantage or peculiarity a successful population embodies. This would operate in delicate tension with our other strong tendency to seek novel experiences, resources, and marriage partners in other lands.

  • Galbraith gets it- and writes an excellent primer on government spending and deficits.
  • Great pair of articles on Afghanistan in TNR. I'd reiterate that the Afghan people would probably welcome NATO replacement of Karzai and a guarantee of 10 years of federal administration.
  • Interesting notes on Jung and antisemitism.
  • A small reminder of what Republicans do with power.
  • Moral sentiments and morals.
  • Bill Mitchel quote of the week:
"But on more substantive matters, today I have been thinking about how much momentum the conservative lobby has at present and that history is being continually re-written to give these characters the oxygen they need to warp public opinion. We are now in danger of an even greater shift to the right in the coming years than was represented by the “neo-liberal” era. It is an ugly thought. But the macroeconomics is clear – if these ideas really take over the policy making process – then we will be facing a lengthy period of economic malaise."

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