Re: NANFA-L-- Re: Species questions

Christopher Scharpf (
Mon, 14 Mar 2005 20:05:47 -0400

> But really, what is a species these days?

Here's something I've been noodling with to try to answer this question.
(Dave, Peter, please tell me if I'm making any sense, or if I'm full of it!)

Despite 250 years of naming species, biologists do not agree on what exactly
a species is. You'd think a concept so fundamental to the study of biology
would be clear and unified, but it isn't. Why? Because life is so diverse
that it resists human efforts to classify it into a covenient and
universally agreed-upon fashion. Fishes are no exception. As a result, a
list of native North American fish species compiled by one expert will
likely not match a list compiled by another. Nomenclatural changes will
account for some differences, but profound scientific and philosophical
disagreements on what constitutes a species will likely account for others.

For many non-biologists, the species concept is a no-brainer: A species is
any group of organisms that is morphologically distinct from another. This
was the concept employed by Linneaus when he got the practice of identifying
and naming species off to its official start in 1753. However, the criteria
for determining a sufficient level of morphological difference between
closely related species is subjective and arbitrary. There's no rule of
thumb for how different a species must be in order to be a different
species. A trait that's morphologically distinct to one expert may be
interpreted as the natural variation that can occur within a species by
another. Another problem with this species concept is the existence of
"sibling" or "cryptic" species. Cryptic species are usually closely related
species that look alike but have different DNA. In such cases the a species
concept based solely on morphology overlooks discrete forms and
underestimates biological diversity.

Clearly, a better definition of a species was needed, and for most of the
20th century biologists thought they had one. First proposed in the 1930s,
the Biological Species Concept (BSC) defines a species as a group of
interbreeding populations that under natural conditions do not interbreed
with other populations. In other words, a species is reproductively isolated
from other species. It seems straightforward -- a species breeds among
itself but can't breed with another species. But nature is rarely
straightforward. Some species do interbreed, or hybridize, under natural
conditions. For example, white sucker (Catostomus commersonii) and
largescale sucker (C. macrocheilus) hybridize where their distributions
overlap in the Columbia River basin but nevertheless maintain separate
genetic identities despite occasionally swapping their genetic material. And
as demonstrated among chubs of the genus Gila, hybridization is one way new
species can be formed. So if species can interbreed, what then, if anything,
constitutes a species?

Fast forward to the 1960s and the introduction of the Evolutionary Species
Concept (ESC). According to the ESC, a species is any independent
evolutionary lineage that maintains its identity over space and time from
other such lineages and has its own evolutionary tendencies and historical
fate. Let's say a river (undammed, unchanneled, and flowing in a pristine,
natural state) floods its banks and several individuals of a minnow species
are isolated in a nearby fishless spring. The minnows in the spring cannot
return to the river, and minnows in the river cannot enter the spring. The
spring minnows reproduce and over the years they begin to diverge slightly
from the parent population. Let's say their bodies tend to be rounder and
chunkier, and their fins tend to be shorter. If you looked-in-one spring
specimen you probably wouldn't be able to distinguish it from a river
specimen, but if you looked-in-100 spring specimens you notice an overall
tendency towards rounder, chunkier bodies and shorter fins. It's clear that
the spring form is evolving independently from the river form and
maintaining its identity. According to the ESC, this evolutionary
independence means the spring minnow constitutes a separate species. Many
evolutionary biologists like the ESC because it's a flexible concept that
can accommodate all units of biodiversity, including natural hybrids.
Conservation biologists like the ESC too; granting species status to
"special" populations can be a powerful tool in protecting the smaller and
lesser-known units of biodiversity that are often among the first to
disappear. One drawback is that it's a hypothetical concept rather than a
hands-on, operational one. How, for example, do you look into the future to
confirm that a population will continue to evolve separately (i.e., have its
own historical fate)? How do we know that the river will not flood again and
wash all the spring minnows back into the river from whence they came? The
ESC works great as a conceptual basis for viewing patterns in nature. In
fact, the ESC singles out not just the species, but the process of
speciation itself. But it's near impossible to actually use the ESC to
identify lineages over space and time. For that, biologists still need to
rely on other, more operational concepts like the BSC, and an even newer
concept, the PSC.

Developed in the 1980s, the Phylogenetic Species Concept (PSC) defines a
species as any group of organisms in which all individuals share a unique
derived (termed apomorphic) characteristic -- that is, any characteristic,
be it morpholgical, behavioral, or genetic -- not present in its ancestors
or relatives. Put another way, a species is the smallest discernible
self-perpetuating cluster of organisms. The degree or size of the difference
is not important. Just as long as a group of organisms is distinct in some
reliably discernible way, it qualifies as a species. A common objection to
the PSC is that it will result in an increase in the number of recognized
species. Proponents counter with a big "So what?" If the species are out
there, then they should be recognized. What's the advantage of placing an
arbitrary limit on the number of species that can be named? Any species
concept that conceals biodiversity reduces our ability to inventory,
understand, manage, and potentially benefit from this biodiversity.

So which species concept is the "right" one? Well, they all are, and none of
them are,-in-the same time. It's important to realize that species may be
more of an artificial tool of organization and convenience than it is an
actual natural entity. Species don't know that they're species and don't
always behave as such; they continue to evolve and find ways to sidestep the
rules a "species" is supposed to follow. To solve the species puzzle, Rick
Mayden proposed a hierarchical species concept with the ESC serving as the
primary concept and other species concepts serving as secondary ones. The
ESC, Mayden argued, has the greatest ability to account for the enormous
array of life on this planet, while secondary concepts like the BSC and PSC,
being more practical or operationally driven, serve as useful tools for
discovering and investigating species that are consistent with the primary
concept. In some ways, Mayden's idea is consistent with an oft-repeated joke
among taxonomists, and which for my dollar stands as the simplest, most
honest definition of a species: A species is what a competent taxonomist
says it is!

If the papers currently being published by competent taxonomists are any
indication, then it seems that the vast majority of ichthyologists are using
the PSC, with many new species being described. Most of these species aren't
"new" in the sense that their populations were previously undiscovered. It's
just that no one looked-in-them closely enough to discover just how diverse
they really are. The orangethroat darter is a case in point. The more
ichthyologists have looked-in-it, especially-in-the color differences
between breeding males, the more they realize that multiple species (under
the PSC) exist. When all is said and done, the "orangethroat darter" may
actually be 17 (or more) different species. (So don't mix your stocks; an
orangethroat darter from, say, Arkansas, is likely a different species than
an orangethroat darter from, say, Kansas.)

And then there's subspecies. Struggling with how to catalogue the
morphological variation between different populations of the same species,
taxonomists have long divided themselves into "splitters" and "lumpers."
Splitters like to name well-defined local populations as new species;
lumpers prefer to unite local variants into a single species. Eventually
biologists began to realize that many clearly identifiable geographic forms
were an important intermediate stage between local variants and "good"
species. An uneasy compromise was reached: give these "in between" forms a
trinomial (third name) and call them subspecies. Generally, subspecies fall
into two categories: local populations that differ genetically from each
other and do not interbreed because of a natural barrier, but probably would
interbreed if that barrier was removed; and local populations that differ
genetically from each other but do interbreed in a hybrid zone where their
populations overlap. Most contemporary taxonomists view subspecies as an
artificial construct. If anything, the category of subspecies is indicative
of a potential need for further taxonomic study. Quite often these studies
(usually following the ESC or PSC) present new data that justify the
elevation of subspecies to full species status. Apparently that's going to
happen soon with Etheostoma atripinne, which most sources list as as
subspecies of E. simoterum.

Chris Scharpf
Herring Run --> Back River --> Chesapeake Bay

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