(i) Preamble

"Wallace and Darwin’s great insight [natural selection as a factor in speciation] only began the era of asking. The "mystery of mysteries" had been solved, at least in rough outline; then came the task of elaboration. The colleagues and successors of Darwin and Wallace have now been at it for more than a century and a quarter, and throughout most of that effort biogeography has been their paramount tool. The patterns of species distribution have provided clues about the ways in which species originate, change, and diverge, and the question how? has remained inseparable from the question where?….The linkage between geographical circumstance and evolutionary development is embodied in the very word "biogeography."" (from Song of the Dodo by D. Quammen).

An understanding of what constitutes a "species" and how they arise ("speciation") is fundamental to biogeography. Understanding these issues, however, is still a central and highly controversial problem in evolutionary biology. We need to understand aspects of species and speciation because:

The geographic distribution of species (and their attributes) is the fundamental observation unit in biogeography. Obviously, therefore, we must define what we mean by species in order to collect and understand the basic data in biogeography (their distributions)

The role (or not) of geography in speciation is one of the most controversial aspects of speciation and in evolutionary biology. As evolutionary biology is probably the central unifying theme of biology, geographic aspects of species formation are clearly an important unknown in biology

The geographic distribution of species have played (and play) important roles in trying top understand the process of speciation

(ii) What is a species?

Perhaps no other concept has "…remained so consistently controversial as the species concept…" (Mayr 1982)

Ernst Mayr made the statement above in his 1982 book, The Growth of Biological Thought. Nineteen years later, nothing has changed!!

Mayden (1997) summarized the many species concepts that have been proposed over the years and cam up with a list of 22 different concepts. Suffice it to say that there remain strong disagreements about what actually constitutes a species. Nothwithstanding the continuing debate, there are four major concepts that have received most of the attention and have been applied most widely.

The Morphological Species Concept (MSC). This is really the most "traditional" concept. From ancient times, naturalists and philosophers recognized "species’ by their general morphological characteristics and discreet distinctions from other species. Even today, probably most biologists expect species to be morphologically distinct. In most cases, in fact they are, but the MSC has some problems in that it: (i) is often unclear how "distinct" something must be to be declared a distinct species, (ii) many species show tremendous morphological diversity across their geographic ranges (so-called "polytypic species") and it is hard to "draw the line" as to where a new species should be recognized (this is where "subspecies" are often defined), (iii) the problem of "sibling species", i.e. species that are morphologically indistinguishable, but in many cases are evolutionarily distinct in other ways that suggests they are distinct species (see concepts below).

The Biological Species Concept (BSC). Probably the most widely accepted (and controversial) concept. It was defined by Mayr and Dobzhansky to represent "groups of populations of organisms that are actually or potential interbreeding, but that are reproductively isolated from other such groups." The key concepts here are in bold face. Dobzhansky tended to define reproductive isolation rather strictly, i.e. two groups were reproductively isolated if they never interbred or if they did the hybrids were infertile or sterile such that there was no gene flow between the species. Mayr had a slightly "looser" idea of reproductive isolation (that has been modified over time), i.e. no interbreeding, or if two "species" did interbreed the level of gene flow was low enough that it did not erode the genetic and evolutionary integrity of the two species. There are several (and often argued) potential weaknesses of the BSC:

It’s reliance on reproductive isolation can’t be applied to asexual life forms

Ditto for organisms known only from fossils

Ditto for allopatric populations (here one has to rely on the rather unsatisfying potentially interbreeding aspect of the definition

Many species of plants and animals hybridize and undergo some degree of introgression, including many organisms that by any other set of criteria we consider to be "good" species. By the BSC (at least for those taking the hard line) – they would not qualify as species.

3. The Evolutionary Species Concept (ESC). The ESC was initially formulated by GG Simpson in the 1950s and was later refined by E. Wiley. It considers those groups of individuals as separate species if these groups maintain their identities from each other over time and space and if each has their own, independent evolutionary fates and historical tendencies. The ESC emphasizes the recognition of independent evolutionary lineages as species and, therefore, tends to focus on realized differences rather than potential (say in the case of interbreeding potential between allopatric groups in the BSC) differences.

4. The Phylogenetic Species Concept (PSC). Cracraft (1983) first coined the term PSC. It has a few variants, but a phylogenetic species refers, essentially, to the smallest group of individuals that share some derived, diagnostic character inherited through a pattern of ancestry and descent. Naturally, the methods of phylogenetic systematics and the recognition of monophyletic groups is central to the PSC.

The bottom line is that there still is no general agreement on a single, all-encompassing definition for species (that’s what makes the whole thing so interesting!!). In general, most zoologists probably accept (albeit grudgingly at times) the BSC as a reasonable working concept for now-warts and all.

(iii) Speciation: a definition

Speciation is the splitting of a single, ancestral lineage into two or more species. It is a hugely spectacular (and controversial- what else!) phenomenon. The vast majority of the 28,000 or so species of fish have evolved within the last 350 million years, most land plants have only been around for less than 500 million years, and the millions of insect species and been around for only about 400 million years.

The exact mechanisms of speciation are varied and their relative importance are controversial. The following are clearly involved in some way or another:

Genetic drift, (2) natural and sexual selection, (3) gene flow, and (4) mutation. These are all evolutionary phenomena that act to drive evolutionary change (1, 2, and 4) or constrain divergence (3). The factors may act independently or in concert to influence rates and patterns of evolutionary divergence and speciation.

Below is a schematic of the major geographic (or lack of it!) modes of speciation.