Species as "Hypotheses"

Richard Pyle deepreef at BISHOPMUSEUM.ORG
Wed Jul 14 14:52:33 CDT 2004

Hi Nico,

Thanks for the feedback!

>    what you're doing (at least to some of us here) is throwing around
> terms like "hypothesis", "theory", "concept", "definition", with the
> expectation of a clarification, when it's clear that (1) you do attach
> a more or less particular meaning to each of them (e.g. to be a
> scientific hypothesis, something must have qualities A, B, C,...), and
> (2) you're not spending enough time to explain to us what that meaning
> is.

You are right, and I apologize.  I often accuse others of failing to be
explicit about the terms they use, so I now find myself guilty as charged.
So, some clarifications:

1. "Hypothesis"  This is actually the word I'm trying to solicit definitions
for.  Whenever I see someone assert that "a species in an hypothesis", I
assume that they are using the word "hypothesis" in the way that I was
taught the meaning of the word, which is its fundamental role in the
scientific method (see definition of "hypothesis" at:
http://www.ucmp.berkeley.edu/glossary/gloss1phylo.html).  Thus, my
interpretation of the statement, "a species in an hypothesis", is the
suggestion that a proposed "species" unit (presumably referring to set of
organisms that share some level of kinship) can somehow be falsified.  In
other words, the species can be tested for its validity.  My question to the
list was: "What is being tested?".  The general response I've received (both
on-list and off-list) is that a species is a testable hypothesis only in the
context of a specific species "concept" (*see comments below about the word
"concept").  My use of the word "concept" here is in the sense of
"Biological Species Concept", "Evolutionary Species Concept", "Phylogenetic
Species Concept", etc.  For example, suppose I describe a new species, and I
explicitly state in my description that I am describing this as a new
species according to the "Biological Species Concept".  My species
description then becomes a hypothesis something along the lines of the

"No individual organism within the circumscription of my new species will
ever produce any fertile offspring by mating with any individual organism
that falls outside the circumscription of my new species, in a natural
environment."  Stated more simply, no member of my new species will produce
fertile offspring when hybridizing with members of other species in nature.

This is a testable, falsifiable hypothesis:  the discovery in nature of a
single fertile hybrid progeny involving my new species and a different
species would falsify my hypothesis, and thus my new species would be

Similar hypotheses could be constructed and tested in accordance with other
species "concepts" (Evolutionary, various flavors of Phylogenetic, etc.)

So, basically, my original question has been answered:  When people assert
that "a species is an hypothesis", it is always understood that it is only
an hypothesis in the context of a specified objective "concept" of species
(Biological, Evolutionary, Phylogenetic, etc.).  My response to that is hard
to summarize in words, but boils down to "Why bother?" (in light of the
logical problems with each of these major species "concepts", and the lack
of universal acceptance of any one of them, and the practical difficulties
associated with testing each of them for each proposed species, and the
almost complete lack of explicitly stating which species "concept" is
implied whenever any taxonomic or other biological publication makes
reference to any species....etc., etc.).

It seems to me that a better statement than "a species is an hypothesis",
would be "a species can be treated as an hypothesis, provided that it is
explicitly framed in the context of a particular species concept" -- which
to me seems like a rather futile exercise because all of the major species
concepts (as I understand them) suffer either from issues of insufficient
objectivity in the concept definition, or from serious issues of
practicality (in terms of testing), or both. More fundamentally, given that
the prerequisite of "explicitly framed in the context of a particular
species concept" is so seldom satisfied (even in most taxonomic
publications, let alone other biological publications, specimen
determinations, and other instances where "species" units are cited.)

A lot of this goes back to the species as "real" vs. "artificial" units --
but I specifically didn't want to re-kindle those flames.

2. "Theory" -- As in "Theory of Evolution", "Theory of Plate Tectonics",
"Theory of heliocentric solar system", "Atomic Theory", "Special Theory of
Relativity", etc.  I was always taught that the word "Theory" is reserved
for only those hypotheses whose consilience of supporting evidence is so
overwhelming, that it becomes broadly accepted by science.  I only brought
up this word to ask whether some species hypotheses, being so well
established as such (e.g., Homo sapiens), ever achieve the level of a
"theory" in the way that other hypotheses aspire to become.  I wonder,
though, if in the case of Homo sapiens, there still may be some messy bits
about when the species began, historically....but we don't need to go there.

3. "Concept" -- this is another one of those unfortunate multi-use words in
taxonomy.  In point 1 above, my use of the word "Concept" was as in
"Biological Species Concept", "Evolutionary Species Concept", "Phylogenetic
Species Concept", etc.  Of course, there is another use of that word in
Taxonomy, which is a taxonomic "concept" (circumscription of organisms" as
contrasted with a taxonomic name (text string established in accordance with
Code rules, anchored to a primary type specimen, etc.)  In my posts on this
thread, I've used the word in both senses, and I apologize if it was not
clear which sense I was using it.  I'd be happy to clarify for any specific

4. "Definition" -- This one is a bit trickier.  When I say "species are
definitions", as contrasted with "species are hypotheses", what I meant was
that a "species" is asserted to have some particular scope by a taxonomist
(using whatever means available to define the scope, such as morphological
or genetic characters, geographic distribution, kinship relations among
organisms, rates of gene flow among populations, etc.)  This follows my own
preferred Species Concept (capital "C"), which is that a species is whatever
a taxonomist or community of taxonomists says it is. In other words, a
construct defined by humans (taxonomist humans). For instance, one
"definition" of the species "Centropyge fisheri" is all the individual
organisms that look like this:


...that live (or have lived in the recent past, or will live in the near
future) in the Hawaiian Archipelago, Johnston Atoll, and Ogaswara Islands.
We'll call this the "sensu stricto" definition. This definition would
exclude things that look like these:


...which live in other geographic regions in the Pacific and central Indian

Another "definition" of "C. fisheri" might include all of the populations
represented by all of the photo links above. We'll call that the "sensu
lato" definition of C. fisheri.

Yet another definition of this species might include this:


...from the western Indian Ocean.  We'll call this C. fisheri "sensu

The "species as hypothesis" perspective says that only one of these can be
logically correct (within the context of a given species concept), and the
three alternatives can somehow be tested to "discover" the correct one.  The
"species as definition" perspective is that all three of these notions of
"C. fisheri" are simply alternative conventions for where it makes the most
sense to draw species boundaries.  None of them can be tested to be "more
correct" than any other -- they're simply alternative definitions for the
same name.  These definitions can be followed or ignored, but they cannot be
"falsified" in any objective sense.

>    To think of a definition in a narrow, stipulative, necessarily
> truth-preserving way - a la PhyloCode - is but one way to think of
> definitions.

When I was discussing the merits of a multi-anchored taxonomic units
(analogous to Phylocode taxa), what I meant was that the biological scope of
such names have objective boundaries, and while they do not themselves
constitute testable hypotheses, they do provide an unambiguously objective
method for testing whether any particular organism belongs to the taxon or
not.  This is not true of Linnaean taxa, which have a single anchor (unless
you have an unambiguous method of determining the scope of organisms with
kinship affinities to the primary type that can be objectively included or
excluded from the taxon -- and I would maintain that no such method yet
exists, let alone attained widespread acceptance).

> Same is true for "a hypothesis must be falsifiable to be
> scientific."

If it's not falsifiable, then why call it an hypothesis?  Are there kinds of
scientific hypotheses that are not falsifiable? If so, shouldn't some other
word be used to refer to them, other than "hypothesis"?

>    The testable part of a species concept/hypothesis/theory/definition can
> be loosely seen as: you perceive there to be (and name) a natural
> entity,

And we now return to this idea of "natural", that we (over)discussed on this
list recently.  It's my turn to ask you to be more precise in your meaning
of the word of "natural" here.  Do you mean to suggest that one of the three
definitions for "Centropyge fisheri" (sensu stricto, sensu lato, and sensu
ultra-lato) is intrinsically more "natural" than the other two, and that the
accumulation of evidence will eventually point us to the "most natural"
grouping?  Or, are all three groupings equally "natural" (assuming that all
three are monophyletic), and the decision of where to draw the "species
line" is a purely subjective one?  If the latter, then I would describe the
three notions of C. fisheri as three different "definitions" of the species,
none of which is testable or falsifiable (except for monophyly -- which is
not what I was talking about); and therefore none of which could be thought
of as an "hypothesis" (by any definition of that term that I am familiar

> with certain (described, exemplified) boundaries, and whether
> by accepting that entity and its boundaries others can advance their
> knowledge, is an empirical question.

I'm not sure I follow here.  What is being tested?  The prediction that
future taxonomists will accept my species?  And if taxonomists sink it into
synonomy, the very act of doing so falsifies the hypothesis?  In other
words, is the hypothesis that is being tested: "Other taxonomists besides me
will find this clustering of organisms within a (named) species unit useful
for their work" ... ? I don't think that's what you're saying, but perhaps
you could clarify?

Many thanks,

P.S. Thanks to all who responded to my question (both on and off list).  No,
I wasn't just trying to "stir the pot"  -- I really wanted to understand
what people meant when they said that "a species is a hypothesis".  I
believe I understand now -- that the statement necessarily presumes that the
species is presented in the context of some "Species Concept" (capital "C");
one that allows for falsifiable predictions to be made.

Richard L. Pyle, PhD
Ichthyology, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at bishopmuseum.org

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