SPECIES CONCEPT

I. Species Concepts Since antiquity biologists have felt a compelling need to posit an atomic unit by which diversity can be broken apart, then described, measured, and reassembled. But, are species really ‘real’? The species concept is … the grail of systematic biology. EO Wilson

“Not to have a natural unit such as the species would be to abandon a large part of biology into free fall… It would be to concede the idea of amorphous variation and arbitrary limits for such intuitively obvious entities such as America elms (Ulmus americana), cabbage white butterflies (Pieris rapae), and human beings (Homo sapiens). Without natural species, ecosystems could be analyzed only in the broadest terms, using crude and shifting descriptions of the organisms that compose them. Biologists would find it difficult to compare results from one study to the next. How might we assess, for example, the thousands of research papers on the fruit fly, which form much of the foundation of modern genetics, if no one could tell one kind of fruit fly from another?”

Species Concepts 1. Typological - Species are static, non-variable assemblages of organisms that conform to a common morphological plan Plato (platonic forms) and Aristotle the ‘archetype’ today’s ‘type specimen’? (ATTC) What about variation? Observed diversity within types represents imperfections in an eternal striving toward immutability and perfection

Typological Species Concept BUT, can variation really be viewed as imperfections around a true type? What about speciation? What about any developing variation through time? Concept doesn’t really work with evolution. Also, What are ‘type’ characteristics? Can we ever know the true identity of the archetype?

Species Concepts 2. Morphological – a species is a group of organisms with similar anatomical characteristics Derivative of ‘typological’, preferred by some plant taxonomists, and by some for asexually reproducing organisms (morphology still plays a role in Bergey’s Manual of Systematic Bacteriology) Useful for species concepts in the fossil record

Morphological Species Concept BUT: sometimes morphological criteria are arbitrary rely on ‘expert’ opinion for key traits (mayflies larvae…) What about sympatric species, morphologically indistinguishable, but are clearly different lineages? e.g., Bacillus species form identical colonies, live in the same environment, but are totally distinct according to sequence data. = cryptomorphic sibling species: where substantial genetic differences exist, but are not manifest in morphology What about sexual dimorphism: where male and female morphologies differ Concept may focus too much on the outcome of evolution at the expense of accommodating the mechanisms that underlie speciation

Species Concepts 3. Biological Species Concept Ernst Mayr (1904-2005) “species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups” (Mayr 1963)

The Biological Species Concept “A species is a population whose members are able to interbreed freely under natural conditions” “Searching for an anchor, willing to compromise in order to find some process shared by a large fraction of organisms, biologists keep returning to the biological species concept” EO Wilson, The Diversity of Life

Face spiders – all can interbreed, so all belong to one biological species, Theridion grallator.

Are species cultural artifacts, or are they real? Ernst Mayr, 1928 Traveled to New Guinea Found 136 species of birds recognized by Arfak people Matched exactly with European classification, with only 1 exception Similar classification across diverse cultures suggests that species are ‘real’

Fundamental challenge for all species concepts… “All the members of a given class are identical, and the class is forever absolute and unalterable… but …species are always evolving, which means that each one perpetually changes in relation to others”

“We must confront an even more serious conceptual difficulty for the biological species concept. The idea of the closed-gene pool has no meaning at all for the minority of organisms that are… parthenogenetic, producing offspring from unfertilized eggs. …Various microorganisms, fungi, plants, mites, tardigrades, crustaceans, insects and even lizards simply forgo the inconvenience and perils and gambler’s excitement of sexual reproduction” (EO Wilson)

The biological species concept and the dilemma of asexual reproduction…                               

How to solve the dilemma? “Asexual forms tend to maintain a remarkable integrity. The vast majority, even though freed from the evolutionary constraints of sexual compatibility, do not vary in all directions, do not fan out to create wide continuous variation and taxonomic confusion. The gene combinations of the organism are prone to exist in clusters, enabling systematists to place most specimens with ease. It is widely believed that the clustering is due to the lower survival and reproduction rate of vagrant intermediate forms. Only those organisms with anatomy and behavior close to the norm are able to do well…. In the end, however, the lines drawn by the biologist around such species must be arbitrary.”

“The vast majority of species are in fact sexual; they do exist as closed-gene pools. The biological species concept works very well in the study of local faunas and floras, such as the hawks of Texas and the mosquitoes of Europe and the primates of the Old World including Homo sapiens, and maximally so in well-demarcated communities on islands and isolated habitat patches, circumstances, in short, found in a large part of the real world.” (EO Wilson)

Biological Species Concept Species are genetically identified only through population gene pools, in constant flux because of adaptive genetic exchange This concept captures the significance of reproductive communities where specialized features prevent the dilution of the species gene pool through intra-specific genetic exchange BUT: microbes commonly take up ‘free’ genetic materials from the environment So species concepts that depend on reproductive isolation fail for prokaryotes

Species Concepts 4. Ecological Species Concept - A species is a group of organisms that occupy the same ecological niche Niche: The ecological niche of an organism depends not only on where it lives but also on what it does. By analogy, it may be said that the habitat is the organism's "address", and the niche is its "profession", biologically speaking. (Odum, 1959) Grinnell (1917) defined niche as all the sites where organisms of a species can live (where conditions are suitable for life). The ecological species concept captures the essence of the phenotype as an expression of genetics and environment

Ecological Species Concepts BUT difficult to recognize, because many organisms occupy different niches due to adaptation or developmental changes Beierinck – everything is everywhere… Does the ecological species concept provide much resolving power for microbes?