The BeeHolder, July 2009.

the evolution of Hymenoptera (bees wasps and ants)

Flowering plants and bees evolved together. In the last issue of BeeHolder we examined the early evolution of bees. Here we look at the pressures which caused the flowering plants to evolve.

Raison d'eater cont :

The mutualistic relationship between plants and insects may have begun as long ago as 200 million years when the first flowering plants benefitted by the visits of foraging beetles. About this same time (during the Triassic period) the order Hymenoptera, to which bees belong, arose from either an off-shoot of the Mecoptera (represented today by the scorpionflies ) or the Neuroptera (fishflies, snakewings and lacewings). The earliest Hymenoptera were probably completely herbivorous and thus in direct competition with other plant-eating organisms. There was a strong evolutionary incentive to maintain any favourable random mutation that inevitably occurred and which eventually led to specialized life-styles assisting survival. Some hymenopteran species developed a larval stage which burrowed into the plant tissue and eventually developed special adaptations which regulated the growth of plant tissues stimulating gall formation. The galls offering both a food source and a protective defence against predators.

Adult females of some of these species developed the trait of using their ovipositor to cut slits in foliage or twigs into which eggs were laid (hence their name "sawflies”). Some 125 million years ago the flowering plants were enjoying a period of expansion due to the cooling climate of the era, against which their protected seeds gave them some defence. It was at this point that some sphecoid wasp species turned away from a predaceous existence to find nurture in the pollen and nectar produced by the flowering plants: they giving rise to the bees. Bees, as a group (the superfamily Apoidae), are distinguished from wasps in that they have plumose body-hairs, that is, branched or feathery hairs (rather than smooth hairs as seen in the wasps). Bees then derived all their food from floral sources while wasps were frequently carnivorous (scavenging on dead animals or attacking other insects, including bees).

This series of shifts in life-style, from external foliage-feeding to gall-forming to parasitism to pollen and nectar foraging, also provided the basic anatomical tools to allow the development of another adaptation that is almost uniquely Hymenopteran: eusociality. The "fortuitous" acquisition of certain behaviours, which are, in fact, adaptive in themselves, seems to have neatly predisposed the order for the development of eusocial existence. In an impressive example of evolutionary convergence, eusociality has arisen independently in the Hymenoptera at least eleven times and only once, in termites, among other insects. The preconditions that favoured the development of such eusocial behaviour include: parental care of offspring, including feeding and nest defence, mutualism, parental manipulation and indirect kin selection.

To offer some idea of the diversity and venerableness represented by the order Hymenoptera in general (of which over 100 thousand species have been described) and of the bees in particular: we should contemplate the fact that there are as many species of bees around today as there are individual honeybees in an average wild colony.