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But what exactly is evolution?
Evolution is the unifying force in modern biology; it ties together fields as disparate as genetics, microbiology and palaeontology. It is an elegant and convincing explanation for the staggering diversity of Earth’s nine million or so living species. Here is a primer to the basics.
Evolution has several facets. The first is the theory that all living species are the modified descendants of earlier species, and that we all share a common ancestor in the distant past. All species are therefore related via a vast tree of life. The second is that this evolution is driven by a process of natural selection – or the ‘survival of the fittest’.
Darwin argued that all individuals struggle to survive on limited resources, but some have small, heritable differences that give them a greater chance of surviving or reproducing, than individuals lacking these beneficial traits. Such individuals have a higher evolutionary fitness, and the useful traits they possess become more common in the population because more of their offspring survive.
Eventually these advantageous traits become the norm. Conversely, harmful traits are quickly eradicated as individuals that possess them are less likely to reproduce. Natural selection therefore works to create a population that is highly suited to its environment, and can adapt to changes.
Sex wars
When individuals compete for limited resources in their environment they are subject to ecological selection. However, useful traits are not only those that give a survival advantage, but also those that increase a plant or animal’s chance of reproducing. These traits are subject to sexual selection.
Sexually selected traits can make a male organism more attractive to females: the peacock’s tail for example. These are sometimes correlated to the health of an individual, and are therefore an honest badge of fitness. Another type of sexually selected trait gives males a physical advantage in out-competing other males for mates: the stag’s antlers for example. Sexual selection can even act at a molecular level.
FIGURE 2.1 The peacock is the poster child for the concept of sexual selection. Characteristics such as the peacock’s tail evolve because individuals possessing them acquire more mates than their competitors.
Birds are particularly known for showy ornaments that attract mates, but which also increase the chances of being spotted by predators. Other sexually selected traits include: lions’ manes, the plumage of great tits or budgies, grouse mating rituals, insect love tokens, the height of human males and human hair, intelligence and facial features. But natural and sexual selection are not the only factors that give rise to biological change. Random genetic drift is another factor. This could be called ‘survival of the luckiest’ (see Figure 6.2).
Species spawning
Although Darwin titled his book On the Origin of Species, speciation was one thing he could not explain. He called it the ‘mystery of mysteries’, and even a century and a half later the mechanism by which two groups of animals become genetically incompatible remains one of the greatest puzzles in biology.
We understand how Darwin’s Galapagos finches could have evolved from a single species – different populations became isolated and gradually adapted to different environments until they were no longer able to reproduce with each other.
This ‘allopatric speciation’ happens when a geographical change – a river changing course for example or a new mountain range – splits a species in two. Once separated, as happened to antelope squirrels on either side of the Grand Canyon in the US, the populations evolve independently, eventually becoming distinct and reproductively isolated.
However, speciation also occurs rapidly and without physical isolation of populations, which is far harder to explain, and researchers are still trying to pinpoint the exact biological mechanisms that underly it. Examples of this ‘sympatric speciation’ include the 13 species of Galapagos finch or Africa’s cichlid fish. These species adapt to different opportunities in the environment, and then cease to interbreed – perhaps due to some isolating mechanism. New species can also form through hybridization, such as sunflowers. (See ‘How new species are formed’ in Chapter 7 for more on this)
Like individuals in a population, species also struggle to survive, and most become extinct over time. Species can also die out in mass extinctions, such as the one that caused the demise of the dinosaurs. Today we may be in the throes of another mass extinction, caused by human over-exploitation of habitats.
Evolutionary scenarios
During his voyage on the HMS Beagle and throughout his life, Darwin gathered evidence that contributed to his theory of natural selection. In On the Origin of Species he presented support from the fields of embryology, geography, palaeontology and comparative anatomy. Darwin also found evidence for his theory in examples of convergent evolution, co-evolution and adaptive radiation.
Convergent evolution, is when the same adaptations have evolved independently in different lineages of species under similar selection pressures. Today we see convergent evolution in species as diverse as: shark and camels, shrimps and grasshoppers, flamingos and spoonbills, marsupial and placental mammals and bioluminescent sea creatures. We also see it in the ears and teeth of mammals.
Co-evolution is when the evolutionary history of two species or groups of species is intimately intertwined. Examples include: the co-evolution of flowering plants and pollinators such as bees, lizards and moths; pocket gophers and their lice; humans and intestinal microbes; and the war our immune systems wage with the pathogens that attack us.
Adaptive radiation is the rapid speciation of one ancestral species to fill many empty ecological niches. Adaptive radiations are most common when animals and plants arrive at previously uninhabited islands. Examples of adaptive radiation can be found in: the Galapagos finches, Australia’s marsupials, Hawaii’s honeycreepers and fruit flies, Madagascar’s carnivores and other mammals, New Zealand’s birds and the prehistoric flying pterosaurs.
How did the giraffe get its long neck?
Most people assume that giraffes’ long necks evolved to help them feed. If you have a long neck, runs the argument, you can eat leaves on tall trees that your rivals can’t reach. But there is another possibility: the prodigious necks may have little to do with food, and everything to do with sex.
The evidence supporting the high-feeding theory is ambiguous. Giraffes in South Africa do spend a lot of time browsing for food high up in trees, but research from Kenya showed that they don’t seem to bother, even when food is scarce.
Long necks come at a cost. Because a giraffe’s brain is around 2 metres above its heart, the heart has to be big and powerful. In fact, for the blood to reach the brain it has to be pumped at the highest pressure of any animal. So there must be a big payback to keep giraffes’ necks so long.
A competing theory is that the long necks are the result of sexual selection: that is, they evolved in males as a way of competing for females.
Male giraffes fight for females by ‘necking’. They stand side by side and swing the backs of their heads into each others’ ribs and legs. To help with this, their skulls are unusually thick and they have horn-like growths called ossicones on the tops of their heads. Their heads, in short, are battering rams. Having a long and powerful neck would be an advantage in these duels, and it’s been found that males with long necks tend to win, and also that females prefer them.
The ‘necks for sex’ idea is contentious, but also helps explain why giraffes have extended their necks so much more than their legs. If giraffes evolved to reach higher branches, we might expect their legs to have lengthened as fast as their necks, but they haven’t.
The problem for the sex idea is that it implies that female giraffes shouldn’t have long necks, and they plainly do. One suggestion is that giraffes’ necks may have begun growing as a way of eating hard-to-reach food, but that they were then ‘hijacked’ for mating purposes. Once the necks had reached a certain length, males could use them for necking and clubbing – and at that point sexual selection took over, driving the necks to their current extreme lengths.
Secret code
Darwin was able to establish natural selection, without any understanding of the genetic mechanisms of inheritance, or the source of novel variation in a population. His own theory on the transmission of traits, called pangenesis, was completely wrong.
It was not until the start of the twentieth century that the genetic mechanism of inheritance began to be revealed (see Chapter 3 for more on this).
Interview: Wonders of nature have been my solace in life
Alfred Russel Wallace (1823–1913) discovered the theory of evolution by natural selection independently of Charles Darwin and founded the science of evolutionary biogeography. His correspondence – from which the answers for this ‘interview’ are mined – is now available online: he tells us of his research, expeditions and enduring fascination for nature’s mysteries.
You are famously joint author, with Darwin, of the first paper describing the origin of species and natural selection, published in 1858. When did you first get the idea?
I began [in 1847] to feel rather dissatisfied with a mere local collection – little is to be learnt by it. I sh[ould]d like to take some one family, to study thoroughly – principally with a view to the theory of the origin of species. By that means I am strongly of [the] opinion that some definite results might be arrived at.
This desire led you to Brazil to collect birds, butterflies and beetles to try to discover what drives the evolution of new species. Were there any incidents on the voyage?
On Friday the 6th of August [1852]…the Captain (who was the owner of the vessel) came into the cabin & said “I am afraid the ship’s on fire. Come & see what you think of it.”
Despite that harrowing experience, you next undertook an eight-year expedition to the Malay Archipelago, where you discovered the invisible boundary between the animals of Asia and the Australian region, which would later be called the Wallace Line in your honour. What fascinated you most on that trip?
The Birds have however interested me much more than the insects, they are proportionally much more numerous, and throw great light on the laws of Geographical distribution of Animals in the East…As an instance I may mention the Cockatoos, a group of birds confined to Australia & the Moluccas, but quite unknown in Java, Borneo, Sumatra & Malacca…Many other species illustrate the same fact.
You have been famously good-natured about sharing the discovery of natural selection with Darwin…
I also look upon it as a most fortunate circumstance that I had a short time ago commenced a correspondence with Mr. Darwin on the subject of “Varieties”, since it has led to the earlier publication of a portion of his researches & has secured to him a claim to priority which an independent publication either by myself or some other party might have injuriously effected.
What did you and Darwin have in common?
In early life both Darwin and myself became ardent beetle-hunters. Both Darwin and myself had, what he terms “the mere passion of collecting”…Now it is this superficial and almost child-like interest in the outward forms of living things, which, though often despised as unscientific, happened to be the only one which would lead us towards a solution of the problem of species.
Do you feel your contribution has been overlooked?
The idea came to me, as it had come to Darwin, in a sudden flash of insight: it was thought out in a few hours – was written down with such a sketch of its various applications and developments…then copied on thin letter-paper and sent off to Darwin – all within one week.
I should have had no cause for complaint if the respective shares of Darwin and myself in regard to the elucidation of nature’s method of organic development had been thenceforth estimated as being, roughly, proportional to the time we had each bestowed upon it when it was thus first given to the world – that is to say, as 20 years is to one week.
How did you feel looking back on your life’s work, at the age of 89?
The wonders of nature have been the delight and solace of…life. Nature has afforded…an ever increasing rapture, and the attempt to solve some of her myriad problems an ever-growing sense of mystery and awe.
