The Beeholder, April 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.

The flowering plants, or angiosperms, arose from another, older, division of seed-producing plants, the cone-bearers, or gymnosperms. In both cases the male and female sex cells are separated into distinct organs. For fertilization to occur, pollen, which carries the male germ plasm, must first be conducted to the female organs of the plant- - this, of course, is pollination. The gymnosperms produce air-borne pollen, as, most likely, did the first flowering plants. The success of air-borne pollen in pollination is dependent on the whims of wind and on the amount of pollen that a plant produces. So plants that tended to produce large quantities of pollen had a greater chance for competitive success. All this pollen represented a source of high energy lipids and proteins-- food-- to the insect world. Competition for food sources represents a major selective pressure and serves to mould the life history of an organism. Insects that were better able to exploit this resource, because of behaviour or physiology (form and function) appropriate to the task, had a better chance for survival and thus more of their offspring survived.

These insects, in their rummaging about for food, became the agents of pollination, as the pollen adhering to their bodies was transferred to the female organs of the plant. Thus, not only were the plants benefitted by increased pollination but the insects were helping to pave the way for an ensured supply of their food source. Eventually, both plants and insects became more and more specialized as a result of this relation. Many of the insects evolved behaviour and physiology completely dependent upon the cycles of flowering plants. Similarly, certain plants developed flower structures in which pollination was possible only with the intervention of an insect intermediary.Even the structure of pollen, itself, changed. Air-borne pollen, like that of the gymnosperms and some angiosperms, is generally smooth, small and light. Pollen that is transferred by insects or other animals usually has spines, ridges or an adhesive surface which aids in attaching to the animal vector.(5) Expanding this adaptive arsenal even further, some plants even developed certain organs, nectaries, that secreted a sugary liquid, nectar, at the base of the flower. This proved to be an adaptive advantage for the plant since the nectar, as a food source, was a further attraction to many insect species whose, now, increased rummaging promoted the success of pollination and seed-set even further. The lifestyles of flowering plants and of pollinating insects became forever intertwined.

To be continued next issue...