Dunque essendosi il discorso incanalato verso disquisizioni sul merito dello status di Poecilia sp.Endler come specie a sè stante o popolazione molto particolare di Poecilia reticulata, vi riporto alcune mail da una mailing list sull'argomento scaturite da una discussione a riguardo.
Per ora vi metto il tutto in lingua originale (inglese) se avrò tempo cercherò di tradurre almeno il succo del discorso.
Neale Monks, studioso di sistematica dei fossili e socio British Livebearers Association
I'm coming to this as someone who did his PhD in systematics*,
cladistics of ammonites to be precise, and let me tell you that if you
think defining species of living animals is tricky, it's nothing
compared to doing the same with fossils!
One thing I learned is that systematists -- scientists who classify
different organisms -- can be divided into two groups, "lumpers" and
"splitters". Lumpers see variation as 'background noise' if you like,
so to take one example they might classify European bison and American
buffalo as a single species with some local variation. Splitters zoom
in on the differences, and create taxonomic groups for consistently
recognisable populations, so European bison and American buffalo would
be different species. Lots and lots of animals throw up these sorts of
problems: giraffes, cheetahs, tigers, wolves, and wild cats are all
animals with a lot of regional variation with regard to things like
colouration and markings, and hence have often been sometimes divided
into lots of species, and other times lumped into just one. Subspecies,
races, varieties and so on are all ways of side-stepping the problem,
keeping the coherence of the species while recognising the variation
within it. According to some systematists, we are the subspecies Homo
sapiens sapiens, as contrasted with Homo sapiens neanderthalis. Other
systematists consider Homo neanderthalis to be a species in its own
right.
What's this got to do with Endler guppies I hear you ask. Lots.
Endler guppies as known in the aquarium hobby may or may not be
genetically pure derivatives of the population found in a single lake
in Venezuela by John Endler back in the 1970s (I think). These wild
Endler guppies supposedly live in a slightly different ecological niche
than wild regular guppies, specifically slightly cooler water. In the
wild at least, the two fish don't overlap much, with one replacing the
other where their preferred habitats merge.
This, in and of itself doesn't make it a different species. Siberian
tigers and Sumatran tigers obviously inhabit completely different
habitats (tundra vs. rain forest) yet they are one and the same
species. They are physically different as well, Siberian tigers being
about twice the mass of a Sumatran tiger, as well as having longer fur.
So colours and size can vary dramatically within a single species.
So, in short: size, colour, and ecological preference, as well as
habitat, can vary within a species. None of these things define a
species.
In fact it is very difficult to define a species. A common, but flawed,
criterion (probably the one you learned in school) is that if one thing
can mate with another and produce fertile offspring, then they're the
same species. While this broadly is true, there are lots of exceptions,
so it isn't a perfect definition. Moreover, things like behaviour that
effectively isolate males and females of different species in the wild
might not work in captivity, so the fact that many fish hybridise in
aquaria doesn't mean they belong to a single species.
(By them way, as I understand it, the Endler guppies sold in fish
stores may (probably) have some regular guppy ancestors simply because
lots of people (and commercial breeders) aren't careful about
separating them. Moreover, for example, a retailer gets given a bunch
of fish that are hybrids between guppies and Endlers, there's a strong
temptation for him to sell them as Endlers because he'll get much more
money per fish.)
Anyway, without knowing more about Endlers, my take as a systematist is
that they don't look to me like a robust, clearly recognisable species,
and even if one fish taxonomist describes them as a new species, it
won't be long before another (a lumper) puts them back in with the
regular guppies. To me, someone who knows a little about taxonomy but
nothing about Endlers, they look like nothing more (or less) than a
geographical variation, a race or local population. Genetically
distinct, yes, but no more a different species than Native Americans
are from Native Australians.
Cheers,
Neale
Thue Grum-Schwensen dalla Danimarca, socio Poecilia Scandinavia
Thank you, Neale, for your very fine statement.
You are right about lumpers and splitters - and you have exactly that
discussion about Endlers. To day it is a fact, they are found them in
more than one lake in Venezuela as Morten write - and it`s a reasonable
question to ask, if they at all belongs to that lake (at least from the
start) - but that`s another discussion.
Most of the socalled Endlers, you can by in the shops, are mixed with
the normal guppy.
The division between the splitters and the lumpers you allready have in
this Endler-discission - and it is very difficult to define a species.
It is not possible to do it with the genes (and DNA) alone - and not by
morphological alone either. I think we agree about that.
I like to read your statement, while you are using the morphological
arguments - the scientist, I know just now is working on a description
of endler as a species for it self, is doing exactly the same - but I
can see, your conclusion isn't the same..
The genetics is very much the same for endlers and the ordinary guppy.
Especially for endlers found in rivers there are some differences in
habitat, and you havent any hybridisation with ordinary guppys - I am
told. Then you the differences betwen the ordinary guppys (where you
allready have a lot of geografical variations) and the endlers (and
maybe there is some geographycal variations here as well) - for examble
in sixe (but more than that).
I am not convinced about anything in that discussion - not yet at
least...I have no conclusion - while I simply dont know whats right or
wrong. But it`s very interesting - and I`ll follow the discussion..not
only because of the endler/guppy complex, but because of the
interesting discussion betwen splitters and lumpers...
I look forward to read the description (of Endler as a species for it
self), when it will be published..whether he`s rigtht or wrong..
Yours
Thue
Ancora Neale Monks
Using the ability to produce fertile offspring is not, on its own, a
useful criterion for determining a species. In genera that only
recently diversified, such as Rift Valley cichlids, species can easily
hybridise and produce fertile offspring. What separates one species
from another can be summarised thus:
1) Geographical isolation -- they physically cannot meet
2) Temporal isolation -- they can meet, but they do not breed at the
same time of the year, so do not mate
3) Behavioural isolation -- they can meet, and breed at the same time
of the year, but their mating behaviours are so different that they do
not mate
4) Physically isolated -- they can meet and mate, but their sexual
organs are incompatible
5) Genetic isolation -- they can mate and meet, but their genomes are
incompatible, and no fertile offspring are produced
In the wild, 1, 2, and 3 usually work reliably but they can be
short-circuited in aquaria. In cases where the species involved are not
physically and genetically isolated, then they can mate and produce
fertile offspring, even if they don't in the wild. Rift Valley cichlids
are the classic example of this: they share similar mating behaviours
and breed more or less all the time, so in captivity readily hybridise.
This doesn't diminish their status as true species though, because _in
the wild_ 1, 2, and 3 are plenty good enough to keep them separate.
That's the problem with the Endler guppy: is it (a) a variety of guppy
that breeds freely with other guppies or (b) a closely related species
that hybridises with them.
The other way of looking at things is to take the view from the
animal's perspective. Species are artificial constructs that reflect
our understanding of classification and phylogeny. The only unit that
matters in Nature is the population, the group within a certain
geographical region that can meet and mate with one another. The fact
that there are grey whales in the North West Pacific and another groups
in the North East Pacific doesn't matter to the whales themselves,
since the two don't meet, and probably haven't done for thousands of
years. They're still the same species, and if you took one of each and
bred them, they'd produce fine baby grey whales, but the fate of one
population neither helps nor hinders the other. From this point of
view, the Endler is something special and worth protecting.
Cheers,
Neale
A dire il vero una grande schiera di persone fa parte di coloro che sostengono sia l'una che l'altra tesi.
Tra quelli che pensano che P.endler sia una varietà di Poecilia reticulata c'è ad esempio Manfred Mayer (diverse descrizioni di pecilidi) , sull'altro lato c'è la persona (citata anche negli interventi) Prof. Fred Poeser di Amsterdam che dovrebbe avere pronta la descrizione di Poecilia sp.Endler entro fine anno secondo palesi, a detta loro, differenze morfometriche. Poeser non considera però i test genetici del DNA per le sue descrizioni ed è questa la critica + feroce che gli viene mossa, risultando i DNA di endler e guppy molto simili (vedi esempio tigre di sumatra e tigre siberiana nel primo intervento di monks).
ciao diego
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