Threespine sticklebacks, numerous species of disease-causing bacteria, and Darwin’s finches have all shown rapid evolutionary change in response to changing environments. Evolutionary ecologists use a variety of genetic and molecular approaches to study evolutionary change in these and other species. Gene flow, genetic drift, mutation, and natural selection can cause evolutionary change within a population, but natural selection is the only evolutionary process that can lead to adaptation. The benefits and costs of adaptations are environment-dependent and reflect evolutionary tradeoffs, so a trait may be beneficial in one environmental context and costly in a second. Natural selection may lead to speciation when genetic divergence is maintained either by physical barriers to gene flow, or by assortative mating of similar genotypes within a population. Evolutionary ecologists compare morphological, behavioral, and, most commonly, molecular characters in related groups of organisms, and use similarities in these characters to create phylogenetic trees that reflect evolutionary relationships.

Chapter one offers a constructive proposal of an Aristotelian-Thomistic model of metaphysics of evolutionary transitions, grounded in the categories of hylomorphism, virtual presence, disposition of matter, and accidental and substantial changes. The final part of this chapter concentrates on the classical principle of proportionate causation and the question of whether the proposed metaphysical model of evolutionary changes contradicts it.

Chapter seven addresses the difficulty of the theological interpretation of evolutionary biology in delineating a precise account of the concurrence of divine and contingent causes engaged in speciation. Invoking Aquinas’s famous distinction between God’s primary and principal causation and the secondary and instrumental causation of creatures, a constructive model of the concurrence of divine and natural causes in evolutionary transformations is offered.

Every textbook of biology will supply a number of ‘modes of speciation’, the ways in which new species evolve. But the issues in dispute among the biologists themselves are rather odd. The adoption of evolutionary theory by biologists has had a great impact on how species are understood. From the idea that kinds of living beings were created and at best had devolved to localised varieties, now species were the target of a ‘mechanical’ or ‘physiological’ explanation: they came into being. And under Darwin’s version of the evolutionary account (initially known as the ‘development theory’, since the Latin word evolutio means ‘development’), species were made from other, allied (which means ‘closely related’), species. The processes and causes of new species set up the ‘species question’ that Darwin and other naturalists were seeking to answer.

What are species worth? Do they have inherent value or are they just of value to human beings? Do they have rights? Does their integrity as species have moral worth, and do we have a duty to preserve them, or to modify them? Are species of utilitarian or instrumental value? These are the questions that the third great topic of philosophy seeks to answer: axiology – the values of things, and the duties they impose upon us as ethical, economic and aesthetic beings.

For a long time, species have been thought to be the index marker for healthy ecosystems, for undisturbed nature and for conservation, but the reasons why have varied considerably. National Parks developed from a desire to maintain potential sources of timber, game and hunting opportunities in the United States at the end of the nineteenth and the turn of the twentieth century, as demonstrated in Teddy Roosevelt’s book The Wilderness Hunter; An Account of the Big Game of the United States and Its Chase with Horse, Hound, and Rifle.

It’s not enough to just list the clusters in the living world. One also needs to group clusters together within larger clusters. This process is sometimes referred to as ‘ordering the world’, and is called taxonomy, from the Greek word for ‘order’, taxis. In traditional taxonomy, begun in the sixteenth and seventeenth centuries, and formalised in the eighteenth century by Carl Linnaeus, this meant that species were grouped together in groups called in Latin genera (that’s the plural; the singular is genus). As a result, Linnaeus gave each species a two-part name (a binomial): its genus name (which always has a capital initial) and its species ‘epithet’ (which is always in lowercase). So, our species binomial is Homo sapiens; we are the species sapiens in the genus Homo. It’s kind of like a street address – you have the ‘general’ name (the ‘street’) and the ‘specific’ name (the ‘house number’) (see Box 2.1)

There are several ‘enigmatic canid’ species in North America. One of them is the red wolf (Canis rufus, Figure 1.1), and another is the Great Lakes Wolf. Red wolves are seriously endangered, with a re-released population in North Carolina and breeding programmes being the last populations. Red wolves weren’t even studied closely until the 1960s, after having been hunted nearly to extinction in the nineteenth and twentieth centuries.

The title of this book is Understanding Species, and I have spoken at length about what we understand species to be and to mean. Now, though, I would like to ruminate for a bit on the ‘understanding’ part.

To understand something is not necessarily to have the One True Answer. Human knowledge, and especially its concepts, is in a state of flux at all times. Sometimes, this is because we are learning new things about what the concept refers to, as is the so-called rule in science (it sometimes isn’t). At other times it is because the concept no longer means anything (like ‘phlogiston’ in chemistry or ‘vital force’ in biology). But sometimes it is because the concept has been included into the ‘what everybody knows’ segment of culture. John Maynard Smith, a famous and influential British evolutionary biologist, called this the Bellman’s Theorem (from Lewis Carroll’s The Hunting of the Snark): ‘what I tell you three times is true’.

As I have noted, terms for species are at best polysemic (that is, they are a single word in a language with multiple and often incompatible meanings), and at worst species is a term with no meaning of any real scientific importance. Now we will consider several replacement concepts, and the evolutionary and genetic considerations that make them seemingly viable.

In Chapter 2 we considered the extent of the different definitions as applied to a simplified version of human evolution and genetics. One of those definitions included a historical aspect – monophyly.

If there is an issue in a science, philosophers will attend to it. This is not new, either. Since the rise of modern science in the seventeenth century, many if not most of the problems that philosophers have addressed or formulated have arisen out of science one way or another. Books on ‘the philosophy of botany’ or ‘the philosophy of natural history’ were published from the late eighteenth century onwards, although ‘philosophy’ meant knowledge in those days, and included scientific thinking. Nevertheless, science has always been a productive source of new problems for philosophy to chew on.

One of the things that is often said about the frankly catastrophic loss of biodiversity in the world today is that extinction is a natural process of the living world, and this is quite true. Extinction does not naturally occur at a constant rate, however. It ranges from near instantaneous (as when a 12-km-wide rock hits the planet, causing a Very Bad Day for most living things) to a slow background rate of extinction of species that have been reduced to a relic of past distributions and population numbers. So, when those who do not think we are in a catastrophic situation say, ‘Extinction is natural’, point out to them that the present scale of extinction is in global terms worse than a 12-km bolide, at least in geological terms, for the geological record doesn’t distinguish easily between a one-day catastrophe and a four-century one. Both are ‘sudden’ events in Deep Time. As E. O. Wilson wrote, in his book The Diversity of Life (1992)

There are, says Professor Julia Sigwart, an American mollusc specialist (malacologist), species makers and species users. The former are the taxonomists, and they identify, name and record species in technical journals and store the type specimens (the original specimen that ‘bears’ the name) in museums and other collections. There are way too few of these. The latter – well, that includes everybody, according to Sigwart. She notes in her 2019 book What Species Mean (chapter 3) that looking out of her window she sees species of tree, animal, bird and other living things, and that this knowledge involves two main steps: knowing that something is different from other similar (or related) things; and giving it a unique name to communicate and identify it to other users, for the taxonomists are also users of species. Knowing and naming species are related activities, but not the same.

Textbook histories are how most scientists learn about the past of the ideas and disciplines they employ, and any textbook will tell you that the idea of species goes back to the classical era if not earlier. In a way this is true, but textbook histories are written by scientists, not historians, and they often repeat untested or false ideas for reasons other than knowing the past. Often, history is something to be used as a way of establishing the in-groups and out-groups of science; in other words, history can be used as a weapon in the sciences. So, some critical revision is required.

Plato’s theory of Forms uses a closely related term ‘idea’ as well as eidos to denote ‘forms’, which are eternal and beyond the physical. Plato, as with philosophers since who are interested in kinds of things, used biological illustrations, such as ‘horse’, ‘human’ and ‘dog’, but he did not think actual horses, humans and dogs were species (or members of a class of things) because none of them, not even Socrates himself, were perfect examples of their forms.