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The Evolutionary Biology of Species
'Species' are central to understanding the origin and dynamics of biological diversity; explaining why lineages split into multiple distinct species is one of the main goals of evolutionary biology. However the existence of species is often taken for granted, and precisely what is meant by species and whether they really exist as a pattern of nature has rarely been modelled or critically tested. This novel book presents a synthetic overview of the evolutionary biology of species, describing what species are, how they form, the consequences of species boundaries and diversity for evolution, and patterns of species accumulation over time. The central thesis is that species represent more than just a unit of taxonomy; they are a model of how diversity is structured as well as how groups of related organisms evolve. The author adopts an intentionally broad approach, stepping back from the details to consider what species constitute, both theoretically and empirically, and how we
detect them, drawing on a wealth of examples from microbes to multicellular organisms.
detect them, drawing on a wealth of examples from microbes to multicellular organisms.
- GenresScience
284 pages, Hardcover
Published September 11, 2019
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April 6, 2026
I bought this book as an interested layperson, without realising that it was aimed at postgraduates and researchers. The result is that most of the technical details in the book went over my head. But I picked up the key facts and arguments from the introduction and conclusion to each of Barraclough’s chapters.
The book will be a “must” for those working professionally in this area, and as an amateur I cannot do otherwise than give it the five stars which it certainly appears to me to deserve. As an amateur, too, it is rather presumptuous of me to comment on the book – but I will venture to do so anyway.
In terms of defining what a species is, Barraclough does not get involved in the debate between the adherents of the Biological Species Concept (BSC) on the one hand and those of the Phylogenetic Species Concept (PSC) on the other. Rather, he puts forward his own definition of a species as “an independently evolving group of organisms that is genetically and phenotypically distinct from other such groups.”
Barraclough says that his definition applies to asexually reproducing species as well as sexually reproducing ones, whereas Ernst Mayr acknowledged that his BSC only applies to the latter. This is because Mayr’s BSC definition is that a biological species comprises “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” (Mayr’s BSC cannot be used in practice for fossils either, because the interbreeding test cannot be applied to them.)
According to the PSC on the other hand, a species is seen as the smallest cluster of individual organisms in an evolutionary tree in which all members of the cluster share at least one distinctive, inherited trait that is absent in other groups. This definition might be of great practical use, but Mayr’s supporters would say that it is just identifying the features of a species, whereas the BSC is defining what a species actually IS in nature.
One thing that both Mayr and Barraclough emphasise is that species are not just categories created by humans. They have a real existence in nature. (Whereas genera, families, clades etc are categories that scientists use to show how closely related various species are in evolutionary terms.)
Mayr has said that species exist in nature because “The isolating mechanisms of species are devices to protect the integrity of well-balanced, harmonious genotypes.” (Hybrids are usually infertile or less “fit”.) To be honest, I’m not sure whether or not Barraclough would agree with this. But what Barraclough does do is provide plenty of real, concrete evidence for the objective existence of species in phenotypic and genetic “clustering”.
One problem for defining species boundaries is the fact that hybrids do not always seem to fail. For example, we now know that there was some interbreeding between modern humans and Neanderthals. But most scientists would still see the Neanderthals as a separate species from Homo sapiens. We can only solve this conundrum by seeing speciation as a PROCESS. The longer two species have been separated and the further apart they have grown, the less likely there is to be successful hybridisation.
As for the speciation process itself, Barraclough states that it happens “when divergent selection and geographical isolation align.” (This ties in with the BSC’s requirement for reproductive isolation to take place in order for there to be a speciation event.) Barraclough does show, though, that there can be sympatric speciation, too – for example by allochrony.
Overall, a great book for the experts – and some food for thought for this amateur!
The book will be a “must” for those working professionally in this area, and as an amateur I cannot do otherwise than give it the five stars which it certainly appears to me to deserve. As an amateur, too, it is rather presumptuous of me to comment on the book – but I will venture to do so anyway.
In terms of defining what a species is, Barraclough does not get involved in the debate between the adherents of the Biological Species Concept (BSC) on the one hand and those of the Phylogenetic Species Concept (PSC) on the other. Rather, he puts forward his own definition of a species as “an independently evolving group of organisms that is genetically and phenotypically distinct from other such groups.”
Barraclough says that his definition applies to asexually reproducing species as well as sexually reproducing ones, whereas Ernst Mayr acknowledged that his BSC only applies to the latter. This is because Mayr’s BSC definition is that a biological species comprises “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” (Mayr’s BSC cannot be used in practice for fossils either, because the interbreeding test cannot be applied to them.)
According to the PSC on the other hand, a species is seen as the smallest cluster of individual organisms in an evolutionary tree in which all members of the cluster share at least one distinctive, inherited trait that is absent in other groups. This definition might be of great practical use, but Mayr’s supporters would say that it is just identifying the features of a species, whereas the BSC is defining what a species actually IS in nature.
One thing that both Mayr and Barraclough emphasise is that species are not just categories created by humans. They have a real existence in nature. (Whereas genera, families, clades etc are categories that scientists use to show how closely related various species are in evolutionary terms.)
Mayr has said that species exist in nature because “The isolating mechanisms of species are devices to protect the integrity of well-balanced, harmonious genotypes.” (Hybrids are usually infertile or less “fit”.) To be honest, I’m not sure whether or not Barraclough would agree with this. But what Barraclough does do is provide plenty of real, concrete evidence for the objective existence of species in phenotypic and genetic “clustering”.
One problem for defining species boundaries is the fact that hybrids do not always seem to fail. For example, we now know that there was some interbreeding between modern humans and Neanderthals. But most scientists would still see the Neanderthals as a separate species from Homo sapiens. We can only solve this conundrum by seeing speciation as a PROCESS. The longer two species have been separated and the further apart they have grown, the less likely there is to be successful hybridisation.
As for the speciation process itself, Barraclough states that it happens “when divergent selection and geographical isolation align.” (This ties in with the BSC’s requirement for reproductive isolation to take place in order for there to be a speciation event.) Barraclough does show, though, that there can be sympatric speciation, too – for example by allochrony.
Overall, a great book for the experts – and some food for thought for this amateur!
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