Seeing Further (25 page)

In modern genetic terms, not Darwin’s own, natural selection may be defined as the non-random survival of randomly varying coded instructions for how to survive. We see – and admire – the products, the
phenotypes,
of the successful instructions. The instructions are DNA and their most visible products are bodies that survive by doing something impressive such as flying, swimming, running, digging or climbing – all in the service of reproduction, which means they also tend to be good at attracting a mate and warding off rivals. An important part of the environment that each gene must exploit, if it is to ensure its survival in the form of copies of itself, is the other
genes
it encounters in the
genomes
of a succession of bodies – which, because of sexual recombination, means the other genes in the
gene pool
of the species. As a result of this, cartels of mutually supportive genes cooperate to build bodies that specialise in some particular method of surviving, such as grazing or hunting. Different cartels are the gene pools of different species, bound together by the remarkable phenomenon of sexual recombination – and separated from all other cartels, for it is part of the definition of species that they can’t interbreed. Occasionally, often through accidents of geography, gene pools find themselves subdivided for long enough to become sexually incompatible, and the subdivisions are then free to go their separate evolutionary ways as distinct species. Eventually, ‘separate ways’ can mean ‘very separate indeed’, for animals as different as vertebrates and molluscs originally split
apart as members of the same species. Successive branchings of this kind have given rise to hundreds of millions of species, over thousands of millions of years.

At least in sexually reproducing species, evolution consists of changes in gene frequencies in gene pools. I stipulate sexual reproduction, because without it we have no clear idea what ‘gene pool’ even means. Where there is sexual reproduction, the gene pool is the set of available alleles from which the individual members of a species draw their genomes – ‘draw’ as in a lottery, the lottery of sex. Each individual genome is like a shuffled pack of cards. The available cards to be shuffled are sampled from the gene pool. The statistical frequencies of these available cards change as the generations go by, and that is evolution. We can monitor evolution by measuring a sample of the phenotypes – the anatomy and physiology of typical members of the population. As the average phenotype changes – as legs get shorter, horns longer, coats shaggier, or whatever happens to be evolving at the time – it is tempting to see natural selection as a sculptor’s chisel, carving the bones and flesh of the animals themselves.

But if we want to talk chisels, a sharper representation of evolution sees them as working not on the bodies of animals but on the statistical structure of gene pools. As crests get longer, or eyes rounder, or tails gaudier, what is really being carved by natural selection is the gene pool. As mutation and sexual recombination enrich the gene pool, the chisels of natural selection carve it into shape. We observe the results in the form of changes in the average phenotype, and it is phenotypes that serve as the proxies for genes. As the external and visible manifestations of genes, they determine whether those genes are eliminated, or whether they persist in the gene pool.

Natural selection carves and whittles gene pools into shape, working away through geological time. It is an image that might have seemed strange to Darwin. But I think he would have come to love it.

A
S CELEBRATED IN THEIR DAY AS THE STATESMEN OF SCIENCE WERE THE GREAT ENGINEERS OF THE NINETEENTH CENTURY. HENRY PETROSKI EXPLAINS HOW THEY BUILT THESE AWESOME STRUCTURES AND WHY THEY ATTRACTED SUCH ACCLAIM.

One of the great engineering achievements of the nineteenth century was the expansion of the railways into an ever-widening network. Extending the right of way across major bodies of water naturally presented especially difficult problems for engineers, and so early railways often relied upon ferries at these locations. But this solution was not in keeping with the developing image of a fast and uninterrupted journey in a string of carriages pulled by a steam locomotive, and so bridges were built whenever possible. The most daring of these bridges, symbolic of the creativity, resolve, and integrity of the engineers that designed and built them, proved to be great engineering achievements in their own right, especially when the body of water to be crossed presented unique challenges, as it did at the Menai Strait.

This strategic strait, which separates the isle of Anglesey from the mainland of north-west Wales, was controlled by the Royal Navy, and so the Admiralty required that any bridge that was to cross it had to provide a
clearance of at least 400 feet horizontally and 100 feet vertically so that tall-masted sailing ships of the day could pass between its piers and beneath its roadway without hindrance. Furthermore, because of the importance of the strait, temporary supports were not allowed in the water during construction. This virtually ruled out the choice of an arch bridge, which traditionally required the use of an elaborate system of falsework upon which the arch was assembled until it was self supporting.

Thomas Telford had already been presented with this problem when he was charged with completing the highway that connected London and Dublin and thereby providing a reliable route for the delivery of, among other things, the royal mail. The Irish Sea could only be crossed by ferry. The ideal location for a terminal was at Holyhead, which is on the west side of the island of Anglesey.

To carry the road from London to Holyhead meant bridging the Menai Strait. Telford initially wanted a cast-iron arch, which in 1811 he proposed to support by cables from above and thereby not obstruct ship traffic during construction. This untried method would have worked, as would be proven a half-century later, but it was not to be tried first at Menai. Instead, Telford designed the only other then-known bridge type that could span the distance and provide enough headroom: a suspension bridge.

The Menai Strait Suspension Bridge was completed in 1826 and remains an aesthetic paragon of what can be achieved with the form. Telford’s early experience as a mason enabled him to design graceful viaducts and towers bracketing the main span, which was a record-shattering 580 feet. He employed wrought-iron chains that were tested before installation, and the completed bridge was a structural marvel of its time. Unfortunately, the wooden roadway of the bridge proved not to be as substantial as its stone towers and viaducts and iron chains. When the wind was especially unfavourable, the roadway was susceptible to being tossed about, and on occasion it was destroyed.

When the Chester & Holyhead Railway was being laid out, routing its tracks across the Menai Bridge seemed the natural thing to do. However, as the wind had demonstrated, the structure’s roadway was light and flexible, and this would not serve the purpose of the contemporary railway. As well as the possibility of the road being destroyed in a storm, there was also the problem of a heavy steam locomotive causing the roadway of the bridge’s main span to deflect so much that the engine would have had to climb out of a valley of its own creation. The engineer George Stephenson suggested decoupling the train of carriages from its locomotive and using horses to pull the carriages to the other side of the bridge, where they could be coupled to another locomotive for the continuation of the journey. This was not what engineers would call an elegant solution.

Stephenson’s son, Robert, had a different idea. It involved designing a bridge that relied on neither the arch nor the suspension principle. Stephenson identified a site about a mile south of Telford’s Menai Suspension Bridge, where a large rock formation divided the strait into two wide navigation channels. Since this natural formation, known as Britannia Rock, was a recognised and accepted obstacle to shipping, there could be no reasonable objection to constructing a tall stone tower upon it. Similarly
tall towers could also be erected outside the navigation channels on either side of the rock. Massive wrought-iron girders could then be installed at a sufficient height between these towers so that the vertical clearance was equal to that beneath the suspension bridge.

Robert Stephenson’s scheme was acceptable to both the railway company and the government, and so the detailed design and construction of the bridge was begun in the mid-1840s. Since no such structure had ever been designed, let alone built, it fell to Stephenson to organise what would today be called a research-and-development project. In order to keep the weight of deep girders exceeding 450 feet in length within acceptable bounds, it was decided early on that they should be hollow. At the time there existed no structural theory sufficiently advanced whereby the design of such girders could proceed by calculation alone. An experimental programme was thus embarked upon.

The experimentalist-engineer William Fairbairn, who had established a shipyard and had tested cast-iron beams years earlier, was responsible for conducting scale-model strength tests to establish the preferred shape and detailed design of the wrought-iron tubes. He began with small-scale models to compare the relative strengths of different shapes and arrived at the conclusion that a rectangular cross-section was the best. The model tubes were tested by hanging from their centre weights that represented the load of a heavy locomotive. Weights were added until the tube failed, which revealed the weakness of the structure and thereby provided guidance for how to modify it in the next model. By progressively increasing the scale of his models, Fairbairn was able to establish trends of behaviour, and from the experimental data the theorist Eaton Hodgkinson established an empirical formula by means of which he could extrapolate to the requirements for the full-size tube.

To build a full-scale model and test it to destruction would have been essentially to build the bridge itself. So, as is typical in the engineering of large structures to this day, there comes a point when judgment dictates that the model testing must end and the real thing begin. In order to keep the navigation channels of the Menai Strait unobstructed, the longest tubes were fabricated along the banks. When completed, the tubular beams were floated into position between the towers and lifted into place by means of hydraulic jacks. This critical stage in the construction sequence was accomplished in a relatively short period, during which ships used the channel on the other side of the rock.

Although there were some anxious moments in the floating and hoisting process, the tubular girders were finally in place by 1850. However, since they had not been tested at full scale, there remained legitimate questions as to how they would perform. Such heavy girders might deflect so much under their own weight that they would be noticeably bowed and so present to a steam locomotive little better a roadway than the flexible deck of the suspension bridge. In anticipation of this possibility, the towers had been deliberately designed to be tall enough to accept iron chains from which the weight of the tubes could be partially supported. If this were necessary, then the bridge would effectively be a suspension bridge with a very heavy roadway. However, the tubes proved to be sufficiently stiff so that no supplementary support was necessary. Thus, the height of the towers in the finished bridge appeared to serve no structural purpose, a condition that some structural critics have seen as a flaw of its form.