Sunday, 27 April 2014

The diversification of Bees.

Bees (Anthophila) are generally accepted to have arisen during the Mesozoic, but estimates of exactly when vary considerably. The group are not well known in the fossil record (at least in part because many early palaeontologists tended to ignore Insect fossils in Mesozoic beds, happily destroying them in the quest for Dinosaurs and other large, glamorous Vertebrates), with the earliest putative Bee being Melittosphex burmensis, from 100-110 million-year-old Burmese Amber, which some palaeoentomologists  argue is a Crabronid Wasp (the group of Wasps most closely related to Bees), while the oldest universally accepted Bee is Cretotrigona prisca, a 65-million-year-old probable Stingless Bee from New Jersey Amber. Attempts to date the origin of the group using genetic molecular dating techniques have, to date, been equally problematic, with origin dates varying from 275 million years ago (early Permian) to 147 million years ago (Late Jurassic).

An artists impression of Cretotrigona prisca, from the Late Cretaceous, the oldest universally accepted Bee. Michael Rothman in Engel (2000).

This lack of knowledge about the timing of the origin and diversification of Bees presents difficulties for palaeoecologists trying to understand Cretaceous ecosystems. Between 78% and 94% of all modern Flowering Plants (Angiosperms) are pollinated by Animals, with Bees being one of the most important groups. The exact date of origin for the Angiosperms is equally obscure, but the group is known to have existed by the Early Cretaceous and undergone a dramatic radiation during the middle part of the period, so that Late Cretaceous floras are dominated by Flowering Plants. Logically it would be expected that such an outburst of diversity and ecological success would be accompanied by a similar rise in some associated group of pollinators, with Bees being the primary candidate, but to date evidence for this has been lacking.

In a paper published in the Proceedings of the Royal Society: Series B Biological Sciences on 30 January 2013, Sophie Cardinal of the Canadian National Collection of Insects at Agriculture and Agri-Food Canada and Bryan Danforth of the Department of Entomology at Cornell University, publish the results of a new genetic study of the phylogeny of Bees, which examines seven different gene groups and uses a relaxed molecular clock to calibrate this phylogeny from the fossil record (this assumes that any group must have originated before its first occurrence in the fossil record). Since Melittosphex burmensis is not universally accepted as a Bee it was not used in the study, and since Cretotrigona prisca is not universally accepted as a member of the modern Meliponini (Stingless Bees) it is treated only as a Bee, not a Stingless Bee.

Cardinal & Danforth produced a dated molecular phylogeny which suggested that the Bees arose in the Early Cretaceous, around 140 million years ago, and differentiated into the modern Bee groups (except the Leaf-cutter Bees, Megachilini) during the Middle Cretaceous, with a latest common ancestor of all modern bees living between 113 and 132 million years ago. This supports the idea that Bees underwent a major evolutionary radiation at the same time as Flowering Plants, and that the diversification of both groups was driven by the plant/pollinator relationship.

Time calibrated phylogeny of bees based on analysis 3 (age of the root node sampled from a normal distribution with a mean of 140 and a s.d. of five and tree constrained to have the same relationships as the MRBAYES produced tree at the subfamily and taxonomically higher levels). Horizontal bars indicate 95% HPD of estimated divergence times. Posterior probabilities are shown at the right of each node. Cardinal & Danforth (2013).

Cardinal & Danforth did not include trace fossils in their calculations, nevertheless they do observe that their results do confirm the existence of Halictid Bees by the Late Cenomanian, when putative burrows made by the group have been found in Arizona. Conversely their results suggest that the last common ancestor of all modern Leaf-cutter Bees (Megachilini) lived more recently than the Mid-Eocene Messel Shale, where putative damage to leaves caused by members of this group has been found. Cardinal and Danforth suggest that this does not rule out a Leaf-cutter Bee as the cause of this damage, as the group could potentially have existed before the last common ancestor of all its modern members.

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