ORCHID EVOLUTION

Orchids evolved during the Late Cretaceous period, roughly 76 to 105 million years ago. This is much earlier than botanists once thought and makes Orchidaceae one of the 15 oldest angiosperm families, of which there are 416 in total. Few orchid fossils older than 20 to 30 million years have been found, and it was thought that orchids evolved relatively recently compared to many other groups of flowering plants. That they have a poor fossil record is not surprising because most orchids are herbs, which generally do not fossilize well, and their highly modified pollinia are difficult to recognize in the fossil record.

DINOSAUR DEPENDENCE

All five orchid subfamilies evolved before the end of the Cretaceous period, which means that orchids and dinosaurs overlapped. Considering the great diversity of orchid pollinators, we can only wonder if orchids managed to adapt to pollination by dinosaurs before the latter became extinct 65 million years ago. Vertebrates in general are uncommon orchid pollinators, and nearly all of those recorded are birds — direct descendants of the dinosaurs. There were many small species of dinosaurs, so it is possible that some visited flowers to collect nectar and, like many animals today, were deceived into pollinating orchids. Any orchids adapted to dinosaur pollination would have become extinct with their pollinator, and so are now lost to us.

DISTRIBUTION

The discovery that orchids were much older than previously thought was a result of the widespread sequencing of DNA that only became possible in the mid-1990s. This greater age makes a good deal of sense when it comes to understanding the geographic distribution of orchids. It was long assumed that orchids could have reached their current worldwide distribution relatively recently by long-distance dispersal of their small, almost microscopic seeds. Due to their dependence for food and minerals on the fungi with which they associate, orchids do not include food reserves or minerals in their seeds, unlike, for example, a bean in which the stored food and minerals make up the bulk of its much larger seed. Orchid seeds are, therefore, light and easily distributed by the wind, which theoretically could propel them over long distances. However, the longer an orchid seed remains aloft, the more the small embryo dries out, making most orchid seeds inviable before they can travel great distances. So, most orchid species have a limited distribution, even as constrained as a single mountain. Orchids have instead achieved their worldwide distribution by passively riding the continents, which at the time the plants evolved were much closer than they are today.

POLLINATION

Orchids are well known for elaborate pollination mechanisms that have evolved to achieve the mating of different plants, or cross-fertilization. Flowers of most plants, including orchids, contain organs of both sexes, but self-pollination is as generally undesirable in plants as it is in animals. Most plants, and orchids in particular, have evolved methods, often exceedingly complicated, to avoid self-pollination happening. This process has long fascinated scientists, including Charles Darwin, who studied pollination of orchids in detail and was so enthralled by the plants that his first book after publication of On the Origin of Species (1859) was entirely dedicated to orchids. The short title, Fertilization of Orchids, gave little hint of its main hypothesis, unlike its full and explanatory title, On the Various Contrivances By Which British and Foreign Orchids Are Fertilized By Insects, and On the Good Effects of Intercrossing (1862). Among the orchids studied by Darwin were a large number of tropical species provided by the then Director of the Royal Botanic Gardens, Kew, Sir Joseph D. Hooker.

POLLINATOR DECEPTION

Most orchids produce pollen in two to six tight bundles, called pollinia. These are often attached to ancillary structures that together are called a pollinarium, which attaches the pollinia to the pollinator's body, usually in a position that makes it difficult for the animal to remove them. Most orchids look as if they contain a reward for pollinators but few actually offer it. Some even produce long nectar spurs that are devoid of nectar. Rates of visitation by pollinating insects to such deceptive flowers are, understandably, low. Insects learn quickly to avoid these rewardless flowers, but they make the mistake often enough for it to be effective in a system in which a single visit can result in deposition of thousands of pollen grains, each fertilizing one of the thousands of orchid ovules produced by each flower. A rare mistake by a deceived pollinator is enough for the orchid to produce large numbers of seeds.

Darwin himself came to the conclusion that outcrossing, or pollination between unrelated plants, is so advantageous for most orchids that deceit and corresponding low rates of visitation are the general rule. Apparently, setting seeds in only a few flowers but guaranteeing that these are of high quality (due to cross-fertilization involving flowers on different plants) makes deceit a successful strategy. In this case, the cheating orchids have prospered, despite the fact that they so badly treat the insects upon which they depend. There is no mutual benefit for the orchid and its pollinators as there is in pollination systems with rewarding plants; the deceiving orchid could go extinct and the animal would only experience a slight improvement in its condition due to fewer floral visits without a reward. However, if the animal pollinating a deceitful orchid species becomes extinct, then the orchid also disappears or develops a method by which to self-pollinate its flowers, which has been known to evolve when an orchid species reaches an island without its pollinator accompanying it.

SEED PRODUCTION

The combination of delivery of whole pollinaria on a single visit and fertilization of a correspondingly large number of ovules in the ovary means that from a single pollinator visit a massive number of seeds can be produced. That many orchids, such as some species of Dendrobium, Epidendrum, and Oncidium, bear large inflorescences with hundreds of flowers may seem like an extreme waste of energy, but production of mature ovules ready for fertilization is delayed until pollination takes place, thus reducing energy inputs associated with these large numbers of flowers.

MIMICRY AND DECEIT

Deceit involving mimicry of other local plants that produce a reward for their pollinator is another common habit for orchids. Although not offering a reward itself, the orchid benefits from pollinators that fail to distinguish between a cheating orchid and the rewarding species, and so the former obtains a degree of pollinator...