(Cook & Saccheri, 2013).
reversing man‐made selection pressures
In the 1960s, industrialised environments in Western Europe and the USA started to change again, as oil and electricity began to replace coal, and legislation was passed to impose smoke‐free zones and to reduce industrial emissions of sulphur dioxide. The frequency of melanic forms then fell back to near preindustrial levels with remarkable speed (Figure 1.7). Again, there was transient polymorphism – but this time populations were heading in the other direction as pollution was declining.
It is heartening to note that sometimes the consequences of anthropogenic pressures can be reversed if appropriate action is taken.
1.3 Speciation
It is clear, then, that natural selection can force populations of plants and animals to change their character – to evolve. But none of the examples we have considered so far has involved the evolution of a new species. What, then, justifies naming two populations as different species? And what is the process – ‘speciation’ – by which two or more new species are formed from one original species?
1.3.1 What do we mean by a ‘species’?
biospecies: the Mayr–Dobzhansky test
Cynics have said, with some truth, that a species is what a competent taxonomist regards as a species. On the other hand, back in the 1930s two American biologists, Mayr and Dobzhansky, proposed an empirical test that could be used to decide whether two populations were part of the same species or of two different species. They recognised organisms as being members of a single species if they could, at least potentially, breed together in nature to produce fertile offspring. They called a species tested and defined in this way a biological species or biospecies. In the examples that we have used earlier in this chapter, we know that melanic and normal peppered moths can mate and that the offspring are fully fertile; this is also true of Anthoxanthum plants from different positions along the gradient at the Trelogan mine. They are all variations within species – not separate species.
In practice, however, biologists do not apply the Mayr–Dobzhansky test before they recognise every species: there is simply not enough time or resources, and in any case, there are vast portions of the living world – most microorganisms, for example – where an absence of sexual reproduction makes a strict interbreeding criterion inappropriate. What is more important is that the test recognises a crucial element in the evolutionary process that we have met already in considering specialisation within species. If the members of two populations are able to hybridise, and their genes are combined and reassorted in their progeny, then natural selection can never make them truly distinct. Although natural selection may tend to force a population to evolve into two or more distinct forms, sexual reproduction and hybridisation mix them up again.
1.3.2 Allopatric speciation
Allopatric speciation is speciation driven by divergent natural selection in distinct subpopulations in different places. The most orthodox scenario for this comprises a number of stages (Figure 1.8). First, two subpopulations become geographically isolated and natural selection drives genetic adaptation to their local environments. Next, as a byproduct of this genetic differentiation, a degree of reproductive isolation builds up between the two. This may be ‘pre‐zygotic’, tending to prevent mating in the first place (e.g. differences in courtship ritual), or ‘post‐zygotic’: reduced viability, perhaps inviability, of the offspring themselves. Then, in a phase of ‘secondary contact’, the two subpopulations re‐meet. The hybrids between individuals from the different subpopulations are now of low fitness, because they are literally neither one thing nor the other. Natural selection will then favour any feature in either subpopulation that reinforces reproductive isolation, especially pre‐zygotic characteristics, preventing the production of low‐fitness hybrid offspring. These breeding barriers then cement the distinction between what have now become separate species.
Figure 1.8 The orthodox picture of ecological speciation. A uniform species with a large range (1) differentiates (2) into subpopulations (for example, separated by geographic barriers or dispersed onto different islands), which become genetically isolated from each other. (3) After evolution in isolation they may meet again, when they are either already unable to hybridise (4a) and have become true biospecies, or they produce hybrids of lower fitness (4b), in which case evolution may favour features that prevent interbreeding between the ‘emerging species’ until they are true biospecies.
Darwin’s finches
The isolation of islands provides arguably the most favourable scenario for populations to diverge into distinct species. The most celebrated example is the case of Darwin’s finches in the Galápagos archipelago, a group of volcanic islands isolated in the Pacific Ocean about 1000 km west of Ecuador and 750 km from the island of Cocos, which is itself 500 km from Central America (Figure 1.9). At more than 500 m above sea level the vegetation is open grassland. Below this is a humid zone of forest that grades into a coastal strip of desert vegetation with some endemic species of prickly pear cactus (Opuntia). Fourteen species of finch are found on the islands. The evolutionary relationships amongst them have been traced by molecular techniques using microsatellite DNA that have confirmed the long‐held view that the family tree of the Galápagos finches radiated from a single trunk: a single ancestral species that invaded the islands from the mainland of Central America. The molecular data also provide strong evidence that the warbler finch (Certhidea olivacea) was the first to split off from the founding group and is likely to be the most similar to the original colonist ancestors. The entire process of evolutionary divergence of these species appears to have happened in less than 3 million years.
Figure 1.9 Many different species of Darwin’s finches have evolved on the Galápagos Islands. (a) Map of the Galápagos Islands showing their position relative to Central America; on the equator 5° equals approximately 560 km. (b) A reconstruction of the evolutionary history of the Galápagos finches based on variation
