One of the most important books for an ecologist to read must be Charles Darwin’s ‘On the Origin of Species’. Published in 1859, it was the result of years of developing ideas, experimenting and researching by Darwin. The previous year, the naturalist Alfred Russel Wallace wrote to him with the same concept of natural selection. After announcing their joint theory at the Linnean Society, Darwin was finally pushed into action to publish his ideas. The resulting book, whilst only thought of as an abstract by Darwin, is very interesting; ground-breaking, thought-provoking and still relevant today.
The book opens discussing ‘Variations under Domestication’ and the many examples of selective breeding of domestic animals. Looking at the different breeds of dogs, horses, pigeons and plants, the accumulative actions of selection become clear. Darwin noted how throughout history, many cultures and civilisations domesticated animals and plants for their own benefit, from the ancient Chinese to the Romans.
‘We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them…the key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him.’
So what of variations under nature? Questioning whether the principles of domestic selection applied to wild plants and animals, Darwin first wanted to establish whether they are subject to variation. Noting how individuals of any species possess slight individual differences, there must be the capacity for these characteristics to accumulate in a particular direction. He found that the most abundant species in the larger genera have a lot of variation and are most likely to evolve into other new species. Deeply appreciative of the natural world, Darwin argued how natural selection is much superior to ‘man’s feeble efforts.’ After all, human attempts at selective breeding have to start with an organic being already equipped with many well-adapted characteristics for their place in nature.
‘We see beautiful co-adaptations most plainly in the woodpecker and mistletoe… in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.’
In his ‘Struggle for Existence’ chapter, Darwin described the processes that regulate populations of species:
‘Every single organic being around us may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the number of species will almost instantaneously increase to any amount.’
Darwin studied many species including bumblebees and observed how red clover relied on bumblebees for pollination; concluding that red clover would go extinct if bumblebees did too. Another of his thoughts on the interconnectedness of different species was how bumblebees were more common in areas where cats were present due to their predation of mice, which themselves can predate bumblebee nests. If only bumblebee abundance today could be quite so simple!
In the chapter entitled ‘Natural Selection’, Darwin explains how slight modifications brought about by natural variation could tip the scales in favour of the individual if it was advantageous to it:
‘This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.’
He discussed interesting examples of wolves and deer; how the swiftest wolf would survive, and concerning flowers and bees; how each would become gradually modified to benefit the plant and pollinator. Darwin noted how there were no examples of species evolving beneficial characteristics for other species, unless they were of use to the individual in the first place.
‘Though nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving, it may be said, to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated.’
Darwin discussed the concept of ecological niches (though this term was yet to be invented) as he noted how it was advantageous for species in a region to be diverse, adapted to fulfil a particular role. Observing an area of turf 3 x 4ft in size, Darwin saw that it supported 20 species of plant belonging to 18 genera: a fitting example of how, though diversification of structure, so many different plants could co-exist in a small area. Only by occupying a specific niche can an abundance of species live so close together.
Throughout his book, Darwin stated numerous times how the evidence showed inheritance of characteristics through natural selection, refuting claims that species were independently created.
In ‘Laws of Variation’, Darwin discussed how natural selection would enhance or diminish certain organs for efficiency if they are/aren’t required. Examples include how a number of beetle species inhabiting Madeira have evolved to be flightless through disuse of their wings and cave-dwelling crabs have evolved eye-stalks without eyes. The steady accumulation of slight differences can add up beneficial modifications of structure, such as a bat’s wing.
In ‘Difficulties on Theory’ Darwin discussed various issues that have the potential to disprove his theory, including organs of extreme perfection (e.g. eyes) or organs that are of little importance. Anthropocentrism pervaded the way the natural world was seen in Darwin’s day: some naturalists believed that many parts of species anatomy had been ‘created for beauty in the eyes of man’. The idea that species must have some beneficial use for humans is still one held by many today.
Another obstacle for Darwin’s theory was the presence of distinct species, but no species that appear to be the transitional links between one and the other. The geological record is far from perfect: a tiny area of the earth’s surface has been geologically explored and the conditions required for preservation and discovery of good specimens are quite rare. Darwin compared the geological record to a large book where only short chapters and passages have been preserved across the vastness of time. He wrote with humility how we could struggle to grasp the vastness of time over which evolution takes place: we can struggle to appreciate the length of time of a couple of generations, let alone 100’s of millions of years.
Referring to the absence of these stepping stone species (e.g. the evolution of a horse from a tapir):
‘The new and improved forms of life will tend to supplant the old and unimproved forms.’
The numerous, transitional species will improve and evolve; outcompeting weaker forms and we will always have missing evidence due to the fragmentary geological record. Evidence of several stages of chalk formation showed distinct species, but all sharing characteristics showing them being closely related.
Explaining how non-native species can become invasive and dominant in other countries, Darwin stated:
‘Natural selection in each country must act chiefly through the competition of the inhabitants one with another and consequently will produce perfection or strength in the battle for life, only according to the standard of that country’.
‘Instinct’ is a fascinating chapter. Species such as cuckoos and cuckoo bees with their unusual adaptations to survive fitted into Darwin’s theory, as he saw they possess evidently successful methods of living and have not yet become extinct. He went into great detail to explore how instinct drives honeybees to create the honeycomb structure in the hive and how the design of this space-saving shape could have evolved from the simper, spherical design of bumblebee cells through the sections of natural selection.
Darwin went on to discuss how the production of new species consequently went hand in hand with the extinction of less favoured forms: ‘There is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.’
Of course, today our continued destruction and exploitation of the natural world hinders natural processes, meaning that more species are going and will continue to become extinct at our hands.
In exploring the means in which species disperse to colonise new islands, Darwin performed experiments with a number of seeds: He floated a number of seeds from many different plants in sea water for between 20 and 137 days, and found that a large proportion successfully germinated after immersion, concluding that around 15% of all plants could survive after being in seawater for a month. The same process could explain the similar communities of species found in bodies of freshwater far apart.
Darwin was fond of creating his own experiments to discover fascinating things. He once collected 3 tablespoons of mud from the edge of a pond and counted 537 individual plants growing from it: such is the abundance of life in the soil.
Looking at oceanic islands, Darwin discussed how the amount of endemic species is high. These island’s species bear a close affinity to those on the nearest continent, even though climate, soils and conditions of life are different. Here Darwin compared the Galapagos and the Cape de Verge archipelagos, observing how the theory of independent creation by God couldn’t be correct because despite similarity of size, climate and soils, the inhabitants are very different, resembling those of the nearest continents, America and Africa respectively: ‘All the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other.’
The finches of the Galapagos provided an excellent example of modification through natural selection:
‘What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern and should include the same bones, in the same relative positions?’
When classifying species, Darwin concluded that embryological characteristics can be of importance. Embryos of species within the same class can be very similar: mammals, birds and reptiles can look alike at early stages and in ‘The Descent of Man’, Darwin went on to show illustrations of human and dog embryos that bear very close resemblances: ‘The embryos of air-breathing mammals and birds possess branchial slits and arteries running in loops like those in a fish which has to breathe the air dissolved in water.’
In his conclusion Darwin stated:
‘More individuals are born than can possibly survive. A grain in the balance will determine which individual shall live and which shall die, which variety or species shall increase in number, and which shall decrease, or finally become extinct.’
‘Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products? What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure and habits of each creature, – favouring the good and rejecting the bad? I can see no limit to this power in slowly and beautifully adapting each form to the most complex relations of life.’
His arguments are so well thought out, challenging the commonly held views of the most eminent naturalists of the time. He admits our own inability to grasp the vast expanse of geological time across infinite numbers of generations, to explain how we need to expand our thinking to truly understand this almost incomprehensible theory. A true scientist, Darwin took great care to try and prove or disprove an idea with experiments, research and accumulated facts. He stated how those who prefer to stick to old, unexplained ideas than to new and numerous facts would reject his theory and how new enlightened naturalists could fully consider both sides of his arguments. His ideas were always well expressed, with a beautiful writing style, and although quite intense, ‘On the Origin of Species’ was a fascinating read.
‘It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.’