[Taxacom] Dispersal paradigm (was: Antarctic dispersal)

[Michael Heads]

Hi Richard,

You wrote: 'That you can find examples where ecologically disparate
organisms fall into repeated patterns of distribution across many different
taxonomic groups is not at all surprising to me, because I do not reject
vicariance as a mechanism of speciation...'

I agree with this, although Ken doesn't - he believes the molecular clock
dates and so thinks the geographic patterns are 'pseudopatterns' caused by
chance, not vicariance. So where are the patterns?

As Robert MacArthur wrote: 'To do science is to search for repeated
patterns, not simply to accumulate facts, and to do the science of
geographical ecology is to search for patterns of plant and animal life
that can be put on a map'.




On Thu, Jun 19, 2014 at 6:09 AM, Richard Pyle <deepreef at bishopmuseum.org>
wrote:

> Hi Michael,
>
> > You suggested that 'there are very, very few generalities we can make
> > concerning broad patterns in the history of evolution of life on Earth'.
> > One generality seems to be that the same distributions are, surprisingly,
> > repeated in groups with completely different ecology and means of
> dispersal.
>
> By using the word "generality", are you implying that *most* (nearly all?)
> species of life on earth are members of these groups with repeated patterns
> of distribution?  If so, then one would surmise that there are a finite set
> of patterns, and the vast majority of species fall into one of these group
> patterns.  That's certainly not what I see on coral reefs. What I see on
> coral reefs is that *some* (perhaps even "many", but DEFNITELY not "most")
> species fall into distributional patterns that are (more or less) repeated
> across many taxa with completely different ecology, etc.  The
> fit-to-pattern is not perfect, and the more precisely you define the
> patterns, the smaller the number of members you find within the groups.
>
> Is the proportion of species that fall into such patterns "surprisingly"
> high?  No, not really -- because in most cases (not all), they are very
> obviously correlated with vicariant events (e.g., one or more of the
> closing events for the Isthmus of Panama).  And this is why I (and
> virtually everyone I know who pays attention to biogeography) embrace
> vicariance as a mechanism for population isolation and subsequent
> speciation.  Just like we also embrace rare dispersal/colonization events
> as a mechanism for population isolation and subsequent speciation in the
> many cases that do not fall into one of the repeated patterns.
>
> This brings me back to my question:  are we debating the overarching
> dominance of one mechanism over another in the totality of all life on
> earth?  Or, are we just debating proportions here (which one tends to be
> more prevalent across all of life)? I'm not sure we're really in a position
> to debate the latter, because we don't even have good distributional
> knowledge of the majority of species that we know exist, and most
> rational/plausible estimates of total biodiversity on earth strongly
> suggest that the species we know exist represent a small minority of the
> total diversity of species on Earth.
>
> Now, if you want to confine arguments to particular groups in which we
> have a reasonably full understanding of both species diversity and
> distribution patterns, then that's an interesting debate.  But finding
> specific examples in different groups that are not so well understood to
> make a broader case about "general" patterns in nature sounds to me to be a
> lot like telling "just so" stories.  That you can find examples where
> ecologically disparate organisms fall into repeated patterns of
> distribution across many different taxonomic groups is not at all
> surprising to me, because I do not reject vicariance as a mechanism of
> speciation (really a mechanism of geographic isolation, which establishes
> conditions that may enhance subsequent speciation).  But others can find
> examples (across many ecologically disparate taxa, etc.) of organisms that
> do not fall into such consistent/repeated distributional patterns.  And
> those people would be equally incorrect to assert that their smattering of
> examples somehow "disprove" the role of vicariance in speciation.
>
> In a situation where we appear to be at the dawn of the sixth great
> extinction event on the planet, when reasonable estimates of total global
> biodiversity put us somewhere around 90% of it still being undocumented, it
> seems both premature and misplaced effort to focus on arguments about which
> class of mechanisms that set up conditions for speciation has a (slight?)
> edge over another in terms of explaining current and historical patterns of
> distribution.
>
> Don't get me wrong -- I think these kinds of conversations are extremely
> interesting, and I consider mechanisms that lead to speciation to be among
> the most fascinating topics of conversation in all of biology.  The part
> that gets old is the extensive hyperbolic debates where each side
> cherry-picks data to show how the other side are a bunch of fools; when the
> truly interesting pattern is heterogeny represented in nature (i.e., that
> some taxa follow patterns that strongly suggest vicariance, some species
> follow patterns that strongly suggest dispersal, and some follow patterns
> that don't fit either mode particularly well).
>
> Aloha,
> Rich
>
>
> Richard L. Pyle, PhD
> Database Coordinator for Natural Sciences
> Associate Zoologist in Ichthyology
> Dive Safety Officer
> Department of Natural Sciences, Bishop Museum
> 1525 Bernice St., Honolulu, HI 96817
> Ph: (808)848-4115, Fax: (808)847-8252
> email: deepreef at bishopmuseum.org
> http://hbs.bishopmuseum.org/staff/pylerichard.html
>
>
>
>
>


-- 
Dunedin, New Zealand.

My recent books:

*Molecular panbiogeography of the tropics.* 2012. University of California
Press, Berkeley. www.ucpress.edu/book.php?isbn=9780520271968

*Biogeography of Australasia:  A molecular analysis*. 2014. Cambridge
University Press, Cambridge. www.cambridge.org/9781107041028