[Taxacom] Dispersal paradigm (was: Antarctic dispersal)

[Richard Pyle]

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