In The Blink Of An Eye (27 page)

Figure 5.5
Electron micrograph of a hair from
Lobochesis longiseta
, a bristle worm. The ridges are spaced about one micron apart, forming a diffraction grating that causes a spectral effect.

The upside-down flies fall within a group of small flies described by David McAlpine of the Australian Museum. David McAlpine also noticed similarities between and peculiarities in the behaviour of species from this group. Some plants have long, vertically upright leaves. These
leaves provide a home for the ‘McAlpine flies', which often rest on the leaves in a group, bodies oriented vertically. The gathering of the flies is an act of safety - there is, after all, much to be said for safety in numbers. Also the possibility of reproduction is enhanced if potential mates are close by and easy to find.

The flies involved in this story belong to many species, and collectively live in Africa, Madagascar, South-East Asia and Australia; the ancestral species became divided as the continents separated millions of years ago. In fact the ancestral fly
is
known: one specimen has been found preserved in amber in wonderful condition.

I borrowed the amber specimen from a museum in Göttingen, Germany. The amber has been fashioned into a neat block, about a centimetre square and a few millimetres deep, and is mounted on a glass microscope slide. Inside the amber are two flies, one the size of a large mosquito with big, perfectly preserved eyes, and a smaller specimen, which is the one of interest here. The smaller fly was described by the German biologist Willi Hennig, a man rather more famous for his development of a phylogenetic method - the main tool used to study evolution today. Unfortunately, the fly is orientated in a most inconvenient way. Along with inconsistencies in the amber, it can be seen only in a limited and distorted view, so it is not easy to say whether or not this ancestral specimen possesses reflective patches. To make matters worse, amber would affect the optical properties of many reflector types, such as diffraction gratings. We would need to see this fly in air, not in amber. And the rarity of this specimen has resulted in a ban on any potentially destructive handling, so an informative dissection is out of the question.

The living relatives of this amber specimen, however, do possess light reflectors - diffractive structures based on a system of hairs that appear silver. The hairs come in different shapes and sizes, and can be aligned differently, although always spaced evenly. The specific, microscopic characters determine the optical properties of the complete reflector, which vary from species to species. And a pattern emerged from a study of this variation.

The fly in amber evolved along two separate paths. Like the history of notched seed-shrimps, the evolutionary tree originating from the fly
in amber can be divided into two halves. On one side we have the right-way-up flies, and on the other side the upside-downs. But all have one thing in common - they reflect silver light upwards, towards the sky. This reflection probably acts as a signpost to other flies in the vicinity, to invite them to a gathering.

The right-way-up flies, orientated vertically on their host leaves, go about their business with heads facing the sky. They live in South-East Asia and Australia. Those (‘primitive') species of right-way-up flies with the oldest ancestors possess very inefficient reflectors, positioned between the eyes so that sunlight can be reflected back towards the sky. This reflective patch must have proved rather useful to the fly. It was not only passed on to the next species in the evolutionary line, but it was also improved upon. The physics of the reflector became more efficient, and, consequently, its visual effect became more striking. This trend can be traced through the entire evolutionary line of the right-way-up flies. The next species to evolve not only improved upon the physics of the reflector again, it also sprouted more reflectors over its body. The additional reflectors appeared only on other sky-facing parts of the fly, such as the front parts of the first pair of legs. A greatly speeded up film of evolution through geological time would reveal reflectors blooming from increasingly more parts of the upward-facing body. Furthermore, the reflections would appear increasingly brighter as the optical properties kept on improving via evolution. And exactly the same was happening, independently, in the other half of the evolutionary tree.

The upside-down-flies live in Africa, Madagascar and Australia. They are so called because a strange thing happened at the beginning of their history. As the ancestor represented in amber evolved in this half of the evolutionary tree, it turned upside-down. Still living on vertical leaves, and still with its body orientated vertically, it turned through 180° - and never looked back. The upside-down flies all face the ground, so that their rear ends point towards the sky. This could be explained by their occupation of slightly different plant species - plants with predatory spiders lurking near their leaf bases. So an upside-down fly could keep a lookout for dangers from below. The upside-down flies continued to aggregate, and probably also employed reflectors to call to
their friends. So how could they signal towards the sky when they are facing downwards? They simply ‘moved' their reflectors so that they faced the other way.

The upside-down flies have reflectors on the backward-facing parts of their bodies. And they evolved almost in tandem with their right-way-up counterparts. Again the reflective patches increased both in abundance and efficiency throughout the evolution of the upside-down group. There were, however, differences in the designs of reflectors between the right-way-up and upside-down flies. In fact it is the most recently evolved upside-down fly that owns the most efficient reflector. This champion upside-down fly lives, of course, in Australia, and from rudimentary beginnings it has evolved a type of reflector never before seen in the world of optics, let alone biology. This could even have been applied to human optical devices. But it is the evolutionary tale that is relevant to this book.

The group of McAlpine flies, like the notched seed-shrimps, has evolved with light as a major stimulus. We may be so bold as to say that light has driven the evolution of this group. And this second example of the influence of light on evolution is not the last. Back in the sea, light has been known to insert its influence on the evolution of a group of crabs.

The snapping, or pistol, shrimp has one small and one large crab-like claw. The large claw is the pistol, which fires an underwater bullet of sound so loud that it can be detected by passing submarines. In fact it can even interrupt their sonar. Sound can have its drawbacks because it is an omnidirectional signal - it is, as the word suggests, sent out in every direction. So not only does one reach a target organism, but also every other organism in the vicinity.

Another crustacean making sounds in the sea is the oval (swimming) crab. Although spending most of its time on the sea floor, it is equipped with swimming paddles on its rear legs to propel it through the water whenever required. Individuals aggregate on the sea floor to form
species groups, which are highly aggressive towards each other.

There are many species of oval crabs and the ancestral type, known from fossils, made sounds in prehistoric times. This ancestor possessed a file and pick that scraped together to make trademark music audible in ancient seas. This was probably the oval crab's means of attracting its own species for aggregation. And it was successful because those sounds can still be heard today, made by descendants of the ancestral species. In fact about half of the oval crab species living today employ a similar instrument. The oval crabs, nonetheless, have greatly increased their diversity by succumbing to a selection pressure other than that for sound production - sunlight.

The ancestral oval crab and the living musical species all have strong shells. Their shells are strong because they are composed of a stack of thin layers. In fact a cross section of their shells appears like a multilayer reflector, except that the layers are too thick to cause a reflection. Still, a stack of layers is stronger, tougher and more resistant to cracking than a continuous slab of the same material. Think of pieces of wood used in DIY that are composed of thin layers glued together, for instance; they are both strong and effective.

Although one group of oval crabs continued with their music-making, another group gradually lost the ability to make sounds, while progressively acquiring the ability to reflect light. Throughout the evolution of this colourful group, the picks and files gradually diminished until they vanished completely. But early on in the evolution of this group, a change took place in their shells - the composite layers became thinner and, to maintain the overall thickness of the shell wall, more numerous. While retaining their strength characteristics, the layers had formed into multilayer reflectors. Shells began to appear iridescent.

The first oval crab species to evolve iridescence retained some ability to make sounds, which was probably useful because only a small area of its shell had become iridescent. It was a shy flasher. But then the floodgates opened. The next species to evolve contributed a greater spectrum to the oceans - it was more extensively clothed in iridescence. And so on until the most spectacular marine animal of all had arrived on Earth - the majestic iridescent crab. This is a large crab, with a shell the size of a grapefruit, that gleams with brilliant iridescence
from every part of its body - shell, legs and claws. Imagine a crab made of the most spectacular opal. There would have to be great advantages to having such a bright attire, because the disadvantages are obvious, particularly the way the crab continually advertises its presence to predatory fish. Where seed-shrimps succeeded in concealing their iridescence when it was not required, oval crabs failed. But the iridescent advertising of the majestic iridescent crab is not as pointed as would at first appear to be the case because it has a trick up its sleeve - in its natural environment it can make itself invisible. Here lies an advantage of an iridescent signal - it is directional. Compare the explosion of a pistol to the flash of a torch in a bright environment. Unlike the explosion, the torchlight can only be detected when one looks directly at it. Either way, there really must be advantages in appearing brightly coloured, because the oval crabs that evolved iridescence also devolved their sound production. Time does tell. But these advantages could be confined to certain areas of the globe - the areas where the colourful oval crabs live. Maybe predatory fish have ‘bigger ears' in these areas, so it is best to keep quiet.

Again, the relevant conclusion to be drawn for the purposes of this chapter is that light has played a major role in the evolution of an animal group. Sunlight could be considered the driving force for the evolution of the iridescent half of the oval crab tree. And the momentum of evolution in the direction of the sunlight selection pressure never slackened.

I have dealt only with structural colours in this chapter because these can be represented by mathematical equations and granted efficiency values rather easily. But changes in pigments are also known to occur throughout evolution. Nudibranchs, or sea slugs - marine snails that have lost their shells (through evolution, of course) - demonstrate just how spectacular a pigment can be. Some of the most memorable underwater photographs seen in the coffee-table books on marine life are of sea slugs. But the taxonomy of sea slugs is problematic. Once their pigments
have broken down in preservative, and their colours have faded completely, many of them look extremely similar. It is their colours that separate them into species without the aid of dissections or genetic analyses. Their unmistakable colours provide warnings to predators. Different species have different predators, and their colours have evolved to suit. As predator vision changes or evolves, so do the sea slug colours. Hence light is a major selection pressure to the evolution of sea slugs.

There are many other examples of evolution driven by light. Light is not only a governing factor of animal behaviour at any one point in time, such as today, but is equally important in the evolution from today's ecosystem to that of the geological tomorrow. Light not only exposes an animal as conspicuous or camouflaged today, but can also drive the evolution of animals in the future. As inferred in Chapter 3, if an animal is not adapted to the light in its environment, it will not survive. And light is an exception among the stimuli because in most environments it is always there. One cannot ignore light. But equally important to this book is the issue of evolutionary dynamics. It is one thing to know
what
happens during the course of evolution, or the design of the evolutionary tree, but something altogether different to explain
why
it happens. In this chapter it has been demonstrated that adaptation to light can be the
why
of evolution. And the next questions to emerge are, of course, ‘Was light a selection pressure at the time of the Cambrian explosion?' and, if so, ‘How strong a selection pressure was it compared with others?'

The disparate subjects of colour and animal evolution have emerged as compatible. This chapter signals the dawn of a relationship that will mature as subsequent chapters unfold. The perseverance with colour is beginning to pay off, as clues begin to gather towards solving the Cambrian enigma that this book is attempting to understand. But these are still early days in the Cambrian trial, and evidence also needs to be sought from other avenues.

So far we have examined colour in living animals and predicted the course of colour evolution. But is there also real evidence of colour in the past? Can we return now to the fossils and hope to unearth their true colours? If so, this may be a step towards finding the answers for
the above questions about Cambrian light. Armed with our understanding of colour today, it is certainly worth a closer look at what was described in Chapter 2 as a void in palaeontology. In Chapter 6 I will attempt to start filling that void.