[Taxacom] The 'reality' of species boundaries -- Once Again (UGHHH!)

[Stephen Thorpe]

[Richard Pyle wrote] So...at what proportion of hybridization does it stop being hybridization (between two different species), and start being freely inter-breeding morphotypes (within the same species)?  Is that threshold established through "real", "natural" values that exist independent of human subjectivity?

[reply] so, statistics is all just human subjectivity, is it? Funny, I thought the whole idea of statistics was to reveal objective facts about the world! But wait ... how could I have been so stupid? You have to choose a significance level! So, statistics is all just human subjectivity after all ...

[Richard] This is a COMPLETELY different situation

[reply] yes, indeed! Unusually (for me!), I was not intending it to be an analogy! It wasn't intended to be an example of the same form as your Centropyge example, but rather just another illustration of the paramount importance to species concepts of reproductive integrity issues.

[Richard] The "reality" (or not) of species boundaries depends on future events

[reply] Aha! Now we are getting somewhere! No, it depends on the way things are at present, and in principle could be determined right now, but in practice we need to "wait and see what happens". Analogy, whether or not there is a ticking bomb hidden in the building doesn't depend on whether it eventually blows up or not! But it might be the only way we can find out, if we can't work out where it is hidden!

[Richard] Right now, the proportion of hybridization between these two morphotypes in Centropyge is low enough that we would call them distinct species.  If this proportion changes in the
future (e.g., through alterations in oceanic current patterns and their effect on larval dispersal, resulting from global climate change) such that it *increases*, then today's populations retroactively become the same species.  But if the proportion stays the same or decreases in the future, then today's populations retroactively become different species

[reply] change "retroactively become the same species" to "were in fact the same species all along", and change "retroactively become different species" to "were in fact different species all along", and I would agree! :)

Stephen

________________________________________
From: Richard Pyle [deepreef at bishopmuseum.org]
Sent: Saturday, 12 September 2009 12:37 p.m.
To: Stephen Thorpe; 'TAXACOM'
Subject: RE: [Taxacom] The 'reality' of species boundaries -- Once Again (UGHHH!)

> By FREELY INTERBREED, I mean more than just a bit of
> hybridisation around the edges! If the Centropyge FREELY
> INTERBRED, then hybrids would take over the whole range and
> the whole darn lot would become one.

So...at what proportion of hybridization does it stop being hybridization
(between two different species), and start being freely inter-breeding
morphotypes (within the same species)?  Is that threshold established
through "real", "natural" values that exist independent of human
subjectivity?

> If you think that is
> (slowly) happening (and you are correct), then they are the
> same species. If you think the bulk of them are retaining
> integrity, with just a bit of mixing at the border (and you
> are correct), then they are distinct species.

You're approaching a point I had raised years ago in this same discussion on
this list (BTW, have you had a chance to read the archives yet -- as several
of us have recommended you do?).  The "reality" (or not) of species
boundaries depends on future events.  Right now, the proportion of
hybridization between these two morphotypes in Centropyge is low enough that
we would call them distinct species.  If this proportion changes in the
future (e.g., through alterations in oceanic current patterns and their
effect on larval dispersal, resulting from global climate change) such that
it *increases*, then today's populations retroactively become the same
species.  But if the proportion stays the same or decreases in the future,
then today's populations retroactively become different species.

Thus, today's "reality" is contingent upon future events. No?

> Many parasitic Hymenoptera are so sexually dimorphic that
> there are no morphological features to match up males with
> females. Why are they still males and females of the same
> species? Because they FREELY INTERBREED!

This is a COMPLETELY different situation. One hundred percent of male
morphotypes are born from female-morphotype mothers, and 100% female
morphotypes are sired by male-morphotype fathers.  If you had some
geographic regions where ONLY males existed and reproduced with each other,
and produced fertile offspring; and other geographic regions where ONLY
females existed and reproduced with each other, and produced fertile
offspring; and a few places where both males and females occurred together,
and reproduced with each other, and produced fertile offspring; then it
would be a meaningful comparison.

Aloha,
Rich