When is a species really a species? Even Darwin got it wrong.

When is a species really a species?  Even Darwin got it wrong.

https://theconversation.com/the-long-struggle-to-understand-species-from-pre-darwin-to-the-present-day-65962

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Tim Crowe, Tshifhiwa Mandiwana-Neudani, Potiphar Kaliba and Muthama Muasya

How many species of humans have existed?  Depending on the concept of species employed, there was – and still is - only one or as many as 17 species of Homo.  http://www.jstor.org/stable/10.1086/377663?seq=1#page_scan_tab_contents This is because taxonomy, the process involved with the discovery and classifying of species, has been contentious since time immemorial.  We as African Ph.D.-educated taxonomists, here show how taxonomists think about and do their work.  

Even pre-scientific ‘ethno-taxonomists’ had a “largely unconscious appreciation of the natural [biological] affinities among groupings of plants and animals … quite independently of [their] actual or potential usefulness or symbolic significance in human society".   https://www.google.co.za/?gfe_rd=cr&ei=v6zBV7LTN4up8weTwo6IBQ#q=Berlin%2C+B.+(1992).+Ethnobiological+Classification:+Principles+of+Categorization+of+Plants+and+Animals+in+Traditional+Societies  Our Afrocentric experience with plants and animals supports this.  The Akamba people from Kenya partition species relatively broadly: nzoka = snakes; nyunyi = birds; nyeki = grass-like plants.  In Malawi, the Tumbuka, Chewa and Lhonwe independently recognize the same ‘species’, partitioning them more finely than the Akamba, but still only down to the equivalent of biological genera.  http://www.biology-online.org/dictionary/Genus   In South Africa, the same is true for Northern Sesotho speakers for both trees and birds.  http://africanlanguages.com/sdp/ff/ 

The ‘bottom line’ is that all humans have an “innate” interest/ability in naming biologically meaningful entities.  Taxonomy thus vies for the title “oldest profession”.

In the beginning: ‘typological’ and Darwinian species

Before Darwin, nearly all scientists believed that life on Earth, including humans, was created by God +-6000 years ago, and had remained unchanged over time. http://anthro.palomar.edu/evolve/evolve_1.htm  Working on this premise, the “Father of taxonomy”, Carolus Linnaeus (1707–1778) http://www.famousscientists.org/carolus-linnaeus/, used morphology (overall internal/external physical form) to describe species, using a binomial, genus-species nomenclature.  http://www.dictionary.com/browse/binomial-nomenclature  He named us and you readers Homo sapiens in 1758.  The Linnaean system of classification remains the basis for naming all life forms.

In his On the Origin of Species, Darwin demolished the notions that life on Earth was created recently and species are immutable. http://darwin-online.org.uk/converted/pdf/1861_OriginNY_F382.pdf  However, he took no or an authoritarian position on what constitutes a species:

“There is no infallible criterion by which to distinguish species and well-marked varieties.”

“The opinion of naturalists having sound judgement and wide experiences

seems the only guide to follow”.

In short, Darwinian species are artificial constructs partitioning an evolutionary continuum.

We, like most 20-21^st Century neo-Darwinian evolutionary biologists, disagree.  Nevertheless, Darwin’s thinking, based on the premise of “descent with modification”, laid the foundations for an evolutionary concept of species that allows their placement in phylogenies (evolutionary genealogical trees).

Biological (Isolation) Concept

Pre- and Darwinian species concepts have limitations.  Some pre-Darwinian taxonomists (including Darwin) maintained that interbreeding between anatomically distinct, ‘good species’ only warrants their rank as subspecies (= races).  http://www.biology-online.org/dictionary/Subspecies ‘Proto-ecologists’ suggested that anatomically similar populations that differ in ecology/behavior form valid ‘cryptic species’.  https://www.researchgate.net/post/Why_are_cryptic_species_important_in_ecology_and_evolutionary_biology  

To deal with these concerns, early 20^th Century taxonomists adopted the reproduction-based Biological Species Concept (BSC).  BSC-species, irrespective of anatomical distinctiveness, are ‘real’, self-defining, “protected gene pools” separated by intrinsic pre-mating (e.g. male/female displays) and/or post-mating (e.g. embryonic death/offspring sterility) reproductive “isolating mechanisms”. http://www.evolution.berkeley.edu/evosite/evo101/VA1BioSpeciesConcept.shtml The major proponent of the BSC was Harvard Professor Ernst Mayr, often described as the “Darwin of the 20th century”.  http://www.famousscientists.org/ernst-mayr/

But, still not all taxonomists were satisfied with the BSC.  Since 1950, +- 25 rival concepts of species have emerged. https://ncse.com/library-resource/species-concepts-modern-literature

Here are the key competitors.

Specific-mate Recognition Concept (SRC)

Hugh Paterson turned the BSC on its head, maintaining that it was shared specific-mate recognition systems (genetic ‘lock-and-key’ anatomical compatibility of genitalia, sperm and eggs, mating behaviours/calls, pheromones, etc.) that unite species’ populations, rather than isolating mechanisms that separate them. https://books.google.co.za/books?id=D1PwAAAAMAAJ&q=specific-mate+recognition+systems+paterson&dq=specific-mate+recognition+systems+paterson&hl=en&sa=X&ved=0ahUKEwji9uPRkcXNAhUFBMAKHbXVA1AQ6AEIKjAA  Thus, according to the SRC, if males and females attempt to interbreed and fertilization occurs, they belong to the same species.

Phenetic Concept

Taking an arguably anti-evolutionary view, other taxonomists argued that species can be discovered/delineated mechanistically using multi-variate statistical procedures based on many, equally weighted (i.e. not favouring those involved with reproduction or other ‘important’ biological processes) quantitative measurements.  PC species are delineated according to an ‘empirically determined’, overall average level of morphological identity.  This approach was labelled disparagingly by Mayr as ‘phenetics’.  https://www.blackwellpublishing.com/ridley/a-z/Phenetic_species_concept.asp  Francis Thackeray uses a phenetic species concept to investigate the   species status of Homo naledi.   http://sajs.co.za/sites/default/files/publications/pdf/SAJS%20111_11-12_Thackeray_Sci%20Cor.pdf

Some molecular pheneticists insist that species may be delineated (regardless of morphological/behavioural/ecological evindence) using ‘empirically determined’ levels of divergence in DNA.  For example, advocates of DNA ‘barcoding’ maintain that vertebrate species are entities that differ by >2% in the mitochondrial gene, cytochrome c oxidase I (COI).  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC518999/

Ecological Concept

Some ecologically-minded taxonomists consider species to be morphologically distinct (even interbreeding) groups of populations commonly adapted to a particular set of resources and selective pressures (diet, fire and water uptake), called a niche.  http://www.blackwellpublishing.com/ridley/a-z/Ecological_species_concept.asp  They are less concerned with interbreeding and more interested in recognizing adaptive ‘solutions’ to environmental challenges.

Evolutionary/Phylogenetic Concepts

Finally, taxonomists intent on discovering distinct ‘tips’ of evolutionarily genealogical lineages (trees) have promoted ‘evolutionary’ and ‘phylogenetic’ species concepts.  http://sysbio.oxfordjournals.org/content/27/1/17  The former is nebulously defined: “a lineage of ancestral descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”.  The latter involves entities that possess a unique combination of evolutionarily novel and ancestral characters. http://www.encyclopedia.com/topic/phylogenetic_species_concept.aspx

How many species exist(ed)?

Depending on the species concept, different numbers of species are accepted.  By setting the outer limits of species to mate recognition, Paterson’s SRC accepts the fewest.  The BSC recognizes more species, but relies on the use of races/subspecies to ‘downgrade’ geographically diagnosable, hybridizing populations.  Its application resulted in a large drop in the number of birds species from  +-19 000 in the early 1900s to 8 600 in 1980) and a massive increase in taxonomically trivial races/subspecies. http://www.amerika.org/nature/subspecies-and-classification/  This disparity created enormous problems for conservationists who need biodiversity inventories to prioritize evolutionarily significant entities for conservation action.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560251/

The ecological species concept has not been applied broadly, but would probably recognize more species than the BSC, but fewer if subspecies come into play.

With the availability DNA evidence  http://www.pitt.edu/~jhs/articles/molecular_systematics.pdf and sophisticated phenetic/morphometric techniques developed since the 1980s https://www.researchgate.net/publication/247548080_Book_Review_Morphometrics_in_Evolutionary_Biology_The_Geometry_of_Size_and_Shape_Change_with_Examples_from_Fishes_Special_Publication_15_Fred_L_Bookstein_Barry_Chernoff_Ruth_L_Elder_Julian_M_Humphries, it is now possible to delineate an astronomical number of  evolutionary/phylogenetic ‘species’.  Some misguided/‘toxic-taxonomists’ have even used these ‘forensic’ techniques to distinguish five or more different ‘races’ amongst Homo sapiens.  http://theconversation.com/how-science-has-been-abused-through-the-ages-to-promote-racism-50629  

Palaeontologists working with fragmentary bits-and-pieces fossils frequently ‘over-split’ them into phenetic/phylogenetic species to emphasize morphological differences: hence the proliferation of ‘species’ of Homo.

What to do? -   Based on our “lived experience in Africa”, we prefer a novel concept, the Consilience Species Concept (CSC) incorporating useful features of those outlined above.  A CSC-species is a group of populations that is diagnosable using a suite of heritable, complementary, arguably independent characteristics (qualitative anatomical, behavioural, ecological, physiological and molecular genetic) that show consilient, multifaceted variation.  http://dx.doi.org/10.1111/j.1474-919X.1994.tb01081.x  The term consilience (a "jumping together" of knowledge) was coined by philosopher William Whewell. http://plato.stanford.edu/entries/whewell/   In simple terms: “If it walks like a duck, quacks like a duck, water ‘rolls’ off its back, has webbed feet and a flattened bill, it’s a duck”. 

Thus, academic, conservation and ‘citizen’ scientists http://www.birds.cornell.edu/citscitoolkit/about/definition should consider evidence from a range of independent sources and delineate species on the basis of consilient relationship and not on ability to interbreed (or not) or some arbitrary amount of difference in anatomy or DNA composition.  The CSC is superior to its competitors because, by design, it prevents the recognition of huge numbers of trivial entities and does not ignore evolutionarily significant ones because they interbreed.  Furthermore, it can be applied consistently to both sexual and asexually reproducing ‘species’.  

In a curious sense, our concept constitutes sensible, ‘decolonized’ African science.