Ecology. Michael Begon
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that, in everyday speech, is most commonly used to describe the match between organisms and environment is: ‘organism X is adapted to’ followed by a description of where the organism is found. Thus, we often hear that ‘fish are adapted to live in water’, or ‘cacti are adapted to live in conditions of drought’. In everyday speech, this may mean very little: simply that fish have characteristics that allow them to live in water (and perhaps exclude them from other environments) or that cacti have characteristics that allow them to live where water is scarce. The word ‘adapted’ here says nothing about how the characteristics were acquired. For an ecologist or evolutionary biologist, however, ‘X is adapted to live in Y’ means that environment Y has provided forces of natural selection that have affected the life of X’s ancestors and so have moulded and specialised the evolution of X. ‘Adaptation’ means that genetic change has occurred.
Regrettably, though, the word ‘adaptation’ implies that organisms are matched to their present environments, suggesting ‘design’ or even ‘prediction’. But organisms have not been designed for, or fitted to, the present: they have been moulded (by natural selection) by past environments. Their characteristics reflect the successes and failures of ancestors. They appear to be apt for the environments that they live in at present only because present environments tend to be similar to those of the past.
evolution by natural selection
The theory of evolution by natural selection is an ecological theory. It was first elaborated by Charles Darwin (1859), though its essence was also appreciated by a contemporary and correspondent of Darwin’s, Alfred Russell Wallace (Figure 1.1). It rests on a series of propositions.
1 The individuals that make up a population of a species are not identical: they vary, although sometimes only slightly, in size, rate of development, response to temperature, and so on.
2 Some, at least, of this variation is heritable. In other words, the characteristics of an individual are determined to some extent by its genetic make‐up. Individuals receive their genes from their ancestors and therefore tend to share their characteristics.
3 All populations have the potential to populate the whole earth, and they would do so if each individual survived and each individual produced its maximum number of descendants. But they do not: many individuals die prior to reproduction, and most (if not all) reproduce at a less than maximal rate.
4 Different ancestors leave different numbers of descendants. This means much more than saying that different individuals produce different numbers of offspring. It includes also the chances of survival of offspring to reproductive age, the survival and reproduction of the progeny of these offspring, the survival and reproduction of their offspring in turn, and so on.
5 Finally, the number of descendants that an individual leaves depends, not entirely but crucially, on the interaction between the characteristics of the individual and its environment.
Figure 1.1 The fathers of evolution. (a) Charles Darwin. Detail from painting by John Collier 1883 (National Portrait Gallery RPG 1024). (b) Alfred Russell Wallace. Detail from photograph by Thomas Sims 1869, colourised by Paul Edwards, copyright G. W. Beccaloni.
In any environment, some individuals will tend to survive and reproduce better, and leave more descendants, than others. If, because of this, the heritable characteristics of a population change from generation to generation, then evolution by natural selection is said to have occurred. This is the sense in which nature may loosely be thought of as selecting. But nature does not select in the way that plant and animal breeders select. Breeders have a defined end in view – bigger seeds or a faster racehorse. But nature does not actively select in this way: it simply sets the scene within which the evolutionary play of differential survival and reproduction is played out.
fitness: it is all relative
The fittest individuals in a population are those that leave the greatest number of descendants. In practice, the term is often applied not to a single individual, but to a typical individual or a type. For example, we may say that in sand dunes, yellow‐shelled snails are fitter than brown‐shelled snails. Fitness, then, is a relative not an absolute term. The fittest individuals in a population are those that leave the greatest number of descendants relative to the number of descendants left by other individuals in the population.
evolved perfection? no
When we marvel at the diversity of complex specialisations, there is a temptation to regard each case as an example of evolved perfection. But this would be wrong. The evolutionary process works on the genetic variation that is available. It follows that natural selection is unlikely to lead to the evolution of perfect, ‘maximally fit’ individuals. Rather, organisms come to match their environments by being ‘the fittest available’ or ‘the fittest yet’: they are not ‘the best imaginable’. Part of the lack of fit arises because the present properties of an organism have not all originated in an environment similar in every respect to the one in which it now lives. Over the course of its evolutionary history (its phylogeny), an organism’s remote ancestors may have evolved a set of characteristics – evolutionary ‘baggage’ – that subsequently constrain future evolution. For many millions of years, the evolution of vertebrates has been limited to what can be achieved by organisms with a vertebral column. Moreover, much of what we now see as precise matches between an organism and its environment may equally be seen as constraints: koala bears live successfully on Eucalyptus foliage, but, from another perspective, koala bears cannot live without Eucalyptus foliage.
1.2 Specialisation within species
The natural world is not composed of a continuum of types of organism each grading into the next: we recognise boundaries between one type of organism and another. Nevertheless, within what we recognise as species (defined below), there is often considerable variation, and some of this is heritable. It is on such intraspecific variation, after all, that plant and animal breeders – and natural selection – work.
The word ‘ecotype’ was first coined for plant populations (Turesson, 1922a, 1922b) to describe genetically determined differences between populations within a species that reflect local matches between the organisms and their environments. But evolution forces the characteristics of populations to diverge from each other only if: (i) there is sufficient heritable variation on which selection can act; and (ii) the forces favouring divergence are strong enough to counteract the mixing and hybridisation of individuals from different sites. Two populations will not diverge completely if their members (or, in the case of plants, their pollen) are continually migrating between them and mixing their genes.
Local, specialised populations become differentiated most conspicuously amongst organisms that are immobile for most of their lives. Motile organisms have a large measure of control over the environment in which they live; they can recoil or retreat from a lethal or unfavourable environment and actively seek another. Sessile, immobile organisms have no such freedom. They must live, or die, in the conditions where they settle. Populations of sessile organisms are therefore exposed to forces of natural selection in a peculiarly intense form.
This contrast is highlighted on the seashore, where the intertidal environment continually oscillates between the terrestrial and the aquatic. The fixed algae, sponges, mussels and barnacles all tolerate life somewhere along the continuum. But the mobile shrimps, crabs and fish track their aquatic habitat as it moves; whilst the shore‐feeding birds track their terrestrial habitat. The mobility of such organisms enables them to match their environments to themselves. The immobile organism must match itself to its environment.
1.2.1 Geographic