Thursday, 31 July 2014

More Evolved Mimicry

This is my last blog for a while on the fascinating topic of mimicry in animals and plants. Previously I wrote about Batesian and Müllerian mimicry in Copycat Evolution and More Mimetic Evolution where a harmless species comes to resemble a harmful one and so gains protection from a predator which has learned to avoid the harmful one (Batesian), or two harmful species come to resemble one another and so gain from the evolutionary 'spade-work' of the other (Müllerian).

Now I'm going to look at another form of mimicry where (usually) a plant deceives another species into thinking it's something else, not to repel or avoid it, but to attract it. Almost invariably this improves pollen or spore distribution so it's not hard to understand how any improvement in this system gave the plant an advantage and so natural selection favoured it.

Perhaps the most famous are the orchids which resemble female bees or wasps to attract males of the mimicked species which try to mate with them. In doing so, the bees pick up a packet of pollen (a pollinia) which they deposit on the stigma of the next flower they try to mate with. There is also selection pressure on the orchid to resemble their 'victims' more closely and other species less closely otherwise there would be a good probability of the pollen being carried to the wrong species. So, different species of orchid tend to specialise on one type of bee or wasp.


Mirror Bee Orchid - Ophrys speculum
Bumblebee Orchid - Ophrys bomybliflora
Sombre Bee-orchid - Ophrys fusca

Yellow Bee-orchid - Ophrys lutea
Source: Wikipedia - Ophrys

Although many cases of plant mimicry don't appear to harm the duped victim particularly, and there are several instances where they are actually rewarded with food, in these examples of bee mimicry there is probably a cost to the duped male in that they waste sperm on the flower and waste reproductive effort which may be costly in terms of breeding success. So there is probably selection pressure on these bees to be able to distinguish the flower from the real thing. This in turn produces selection pressure on the orchid to perfect the mimicry, hence the often stunningly detailed result. The system is intimately bound up with the reproduction of both species so natural selection will be especially acute forcing the orchid to become as perfect a mimic as is possible but if it became so good that the bee species's survival became threatened then any further improvement might be counter-productive, so we would expect a dynamic to be produced in which bees are fooled often enough for the flowers to get pollinated but not so often that the bees breeding success is unduly affected.


Monkey-faced orchid - Dracula simia
As I mentioned in Copycat Evolution mimicry needs to be seen from the point of view and with the perception of the intended victim of the deception. It's easy to be fooled into thinking something which resembles something else is some sort of mimicry. This example is known as monkey-faced orchids because, to humans with our pattern recognition which tends to see faces in anything remotely resembling a face, it looks a bit like a monkey face. Monkeys play no part in pollination nor does the face act as some sort of deterrent as with Batesian mimicry, so the orchids derive no discernible benefit from this false mimicry.

I've seen this example held up as proof of intelligent design - how they like to find 'proof' everywhere. If anything, it's an example of pointless, stupid design since it serves no special purpose as the face of a monkey. It is a trick of human perception, like so many 'proofs' of intelligent design. It's also a cultivated variety, bred specially to look more like a monkey face for it's novelty value. Such is the 'proof' of intelligent design.

Above: Dendrobium sinense
Below: Vespa bicolor with pollinia.
Mimicry is not confined to visual mimicry. I pinched this especially devious example from the splendid website Why Evolution Is True. The hornet Vespa bicolor hunts bees which produce an alarm pheromone to warn other bees in a typical evolved act of altruism whereby genes sacrifice a few of themselves to ensure the survival of lots more copies of themselves in the other bees of which they are genetic clones - not that there is any conscious decision in this. The hornet has evolved the ability to detect this pheromone. How the orchid Dendrobium sinense has evolved to exploit this was revealed in a paper published a 2009:

Summary
Approximately one-third of the world's estimated 30,000 orchid species are deceptive and do not reward their pollinators with nectar or pollen. Most of these deceptive orchids imitate the scent of rewarding flowers or potential mates. In this study, we investigated the floral scent involved in pollinator attraction to the rewardless orchid Dendrobium sinense, a species endemic to the Chinese island Hainan that is pollinated by the hornet Vespa bicolor. Via chemical analyses and electrophysiological methods, we demonstrate that the flowers of D. sinense produce (Z)-11-eicosen-1-ol and that the pollinator can smell this compound. This is a major compound in the alarm pheromones of both Asian (Apis cerana) and European (Apis mellifera) honey bees and is also exploited by the European beewolf (Philanthus triangulum) to locate its prey. This is the first time that (Z)-11-eicosen-1-ol has been identified as a floral volatile. In behavioral experiments, we demonstrate that the floral scent of D. sinense and synthetic (Z)-11-eicosen-1-ol are both attractive to hornets. Because hornets frequently capture honey bees to feed to their larvae, we suggest that the flowers of D. sinense mimic the alarm pheromone of honey bees in order to attract prey-hunting hornets for pollination.



Conclusions
Orchids show a remarkable variation of floral forms and a high diversity in pollination systems. Nonrewarding flowers are widespread among Orchidaceae, and deceptive orchid species are well known for their specific pollination systems in which only one or a few animal species are attracted. Orchid species pollinated by social wasps are rare, and most of them offer edible rewards or are assumed to mimic food. The pollination system that we have found in the orchid D. sinense represents another fascinating example of chemical mimicry in deceptive pollination. By emitting volatiles indicating the presence of prey, the flower is capable of attracting its pollinator, the foraging social wasp V. bicolor.


On the subject of scent mimicry, a common form is to imitate the smell of dung or rotting flesh to attract dung beetles and flies. The rotting flesh smell of the flowers of the Stapelia are so realistic that blowflies actually lay eggs on them.

A flower which uses this deception is the arum lily (Arum maculatum) also known in the UK as the cuckoopint or Jack-in-the-pulpit and very common in woods and hedges. Using the highly specialised purple structure, the spadix, A. maculatum increases its metabolism to generate enough heat to vaporize a volatile odour substance which smells of rotting flesh. Flies which land on the slippery spathe slip down into the flower chamber at the base of the spathe and through a ring of downward pointing hairs which prevent their escape. They are kept alive with nectar from the female flowers and during the night the male flowers in a ring towards the top of the chamber, produce pollen which the insects transfer onto the female flowers as the try to escape.

Stinkhorn - Phallus impudicus
The timing of this whole sequence is coordinated by the male flowers which release a substance which initiates the production and vaporization of the special odour 6-8 hours before they mature. Once the female flowers have been pollinated the hairs trapping the flies shrink and shrivel up, releasing the flies unharmed.

Stinkhorn mushroom of the genus Phallus attract carrion-feeding flies which feed on the gelatinous covering of the cap which contains spores. These pass through the flies digestive system unharmed and so get dispersed widely.

My last example is one which I find amazing. It is that of the group of mosses of the genus Splanchnum which not only emit the smell of rotting flesh but also have structures resembling the flowers of flowering plants.

Yellow moose-dung moss - Splanchnum luteum
Given that flowering plants (Angiosperms) are assumed to have evolved out of the 'lower' plants such as mosses, liverworts and ferns, leading to the co-evolution of pollinating insects, it is intriguing to think that mosses have short-circuited that whole evolutionary process and now mimic flowers from Angiosperms in order to attract those insects that co-evolved to pollinate them.

Although having evolved as a group far earlier than the flowering plants did, this mimicry can only have evolved after both flowering plants and pollinating insects had evolved. And of course the group has diversified to produce several different flower-like structures to correspond with the preferences of different insects.

This is a beautiful example of an environmental opportunity being exploited by mindless, trial and error evolution and of the information carried by genes only having any meaning when interpreted by their environment. There could have been no possible adaptive advantage for mosses to evolve flower-like structures if there had not been both flowers and pollinating insects in their environment, just as there would be no advantage to orchids to resemble female bees in the absence of male bees, or stinkhorns, Stapelia flowers and cuckoopints mimicking rotting flesh in the absence of flesh to rot and flies to feed and lay eggs on it, or to smell of animal dung in the absence of animals and dung beetles.

There are of course other forms of mimicry which I haven't touched on in this series - maybe a future blog - such as camouflage where a predator is hidden by cryptic colouring or a potential prey resembles a leaf or dead twig, or even a bird dropping, or where the animal is almost invisible against a given background and can even change to resemble it, such as octopuses and chameleons.

All of this can be easily and readily understood as the product of 'selfish' evolution over time, evolutionary arms races, competition for resource and between predator and prey. There is no morality, no direction, no purpose and no sentimentality involved and yet beneficial alliance can and do evolve when it produces more descendants than conflict and competition.

None of this can be explained rationally using an intelligent design model involving an omnibenevolent, omnipotent and omniscient designer. The last thing biology is evidence for is intelligent design, and to dismiss it all as magic simply so you can claim to understand it from a position of ignorance and just so you can feel a nice warm glow of self-importance in the thought that somehow it's all there for you, is to fail to see the real wonder in life on Earth and to experience the real humility of finding out how you fit in with it all.


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2 comments :

  1. This is really a fascinating topic. You can look at the mimicry phenomenon from more than one angle. Here's another site dealing with, at least in part, the same sort of phenomena as you describe, Rosa: http://waynesword.palomar.edu/ww0602.htm .

    But at that site focus is more on homology and homoplasy.

    A quote from that site: I prefer to use the terms homology and homoplasy when dicussing the evolution of similar characteristics. Homology refers to similarity due to a common ancestor. Characteristics derived from a common ancestor are termed homologous. Homologous organs are similar in structure and embryonic origin but are not necessarily similar in function. Cactus spines are homologous to bud scales of an axillary bud. Seed-bearing carpels of flowering plants are homologous to leaves because of their similarity in form, anatomy and development. The bone structure in the wings of a bat is homologous to the forelimbs of humans and other mammals. For example, a bat's wing and whale's flipper both originated from the forelimbs of early mammalian ancestors, but they have undergone different evolutionary modification to perform radically different tasks of flying and swimming. The penis and clitoris are also homologous organs. The presence of homology is evidence that organisms are related. Birds, bats and insects are not closely related and their wings are not homologous. Although they are all used for flight, they evolved independently in distantly related animals and are structurally quite different. Unrelated species with flowers that smell like carrion is not an example of homology because they are not closely related. They belong to very different and distantly related plant families. Stinking flowers in unrelated plant families is a good example of homoplasy or convergent [or parallel] evolution.

    Read more about convergent or parallel evolution here: http://rosarubicondior.blogspot.se/2014/05/dna-shows-big-bird-evolution.html .

    Here's a quote from that blog article: [B]ig flighteless birds like the emu, ostrich, moa, the extinct giant elephant bird or Aepyornis maximus of Madagascar, and not so big New Zealand kiwi, which we collectively call the ratities, (...) may not be closely related at all and may have evolved from flying birds on at least six different occasions. Their similar appearance may simply be an example of convergent evolution where broadly similar environments produce broadly similar solutions.

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  2. And here's an article especially for you, Rosa: http://msutoday.msu.edu/news/2014/evolutionary-compromises-drive-diversity/ .

    Three quotes from that article: 1) Trade-offs, which are evolutionary compromises, drive the diversity of life, said Chris Adami, MSU professor of microbiology and molecular genetics.

    2) “Trade-offs in biology can take many forms, but in general, they imply that organisms cannot optimize all traits at the same time,” said Bjorn Ostman, post-doctoral research associate in the Adami lab. In other words, they can’t have it all.

    3) This reality is well known in the animal world. For example, in Madagascar, lemurs have evolved to eat different things, which ensures that there is enough food for all. Some lemur species eat fruit, but due to trade-offs, cannot eat leaves because their digestive tracts have become too short and cannot process the fiber. Other lemurs have long digestive tracts and can eat leaves, but they get sick from eating fruit because it ferments from staying in their guts too long.

    ReplyDelete

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