[Taxacom] (no subject)

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Mon Dec 21 03:54:25 CST 2020


 But, what you don't seem to grasp is that a time slice (the present slice) of a continuum is still a real thing, just as a slice of cake is no less real than the whole cake.Stephen
    On Monday, 21 December 2020, 10:31:31 pm NZDT, Les Watling via Taxacom <taxacom at mailman.nhm.ku.edu> wrote:  
 
 Hi Rich,
I like your example, but I have one of my own from a slightly different
perspective.

As an undergrad at U Calgary who was thinking of becoming a geologist, I
took a paleontology course. One of the great experiences of my career. At
any rate, one of the field exercises was a trip to Lake Minnewanka in Banff
National Park. There is a trail that runs the length of one side of the
lake.

In that area all the rock strata are turned on their side so that when you
walk the trail you traverse 30 or 40 million (if I remember correctly)
years of the Devonian. Embedded in the rock are fossils of the colonial
coral Syringopora. Our job was to measure the diameter of the corallites
and estimate their density.

As you went along you could see the corals changing in size from small and
moderately spread out (i.e., separated by several mm) to larger and
adjacent.

In the lab we were shown specimens of the "end members" which had different
names, as you might imagine. But we were able to see the gradual change
from one to the other. Evolution in action (which in those days in Alberta
was not legal to teach in public schools, by the way).

Another quick example. When in grad school in California I got interested
in ostracods because they occur both as living things and as fossils. As it
happens, there is a Plio-Pleistocene deposit on the shores of Tomales Bay
in central California. In that deposit I found ostracod shells belonging to
several different species. One of those species I also found living in the
sediment in Tomales Bay. The shells were identical as far as I could see.

So, I have always had a tacit understanding that what we name as species
are merely populations isolated in time. Some of them will be changing
relatively rapidly and others, like some of the deep sea octocorals I work
on, have probably been unchanged since the Miocene... who knows...

As a practical matter we draw boundaries probably because we either don't
have the mental acuity or words in our language to talk about species as
continua. At least I don't.

Aloha and Mele Kalikimaka

Les
  Les Watling
Professor, School of Life Sciences
216 Edmondson Hall
University of Hawaii at Manoa
Honolulu, HI 96822
Ph. 808-956-8621
Cell: 808-772-9563
e-mail: watling at hawaii.edu



OK, I’ll bite.



Let’s pick something super basic – like maybe the isthmus joining North
America and South America.  I’m a fish nerd, so I’ll use a marine example
(but the same principles apply for terrestrial stuff).



So we start with an open water pathway between the two Oceans on either
side of what will eventually become the isthmus.  A single population of
fish that routinely and freely exchange genes with each other via
reproductive events exists across both ocean basins.  Any two individuals
of opposite sex within the population could mate and yield healthy viable
offspring.  By *any* definition, the population is a single species.



Geologic conditions change, and the water bridge between the two oceans
shrinks.  At some point in time, the land bridge is complete.  Shortly
before that, the last larvae or adult “individual” ( :-) ) traversed from
one ocean to the other, after which gene-flow between the two oceans fell
to zero.  The day after that happens, did the two populations in the two
separate oceans become distinct species?  I’m guessing most of us would
agree they were not (yet) different species – they were two isolated
populations of the same species.



Time passes.  Lots of time passes.  Through genetic drift or founder
effects or natural selection or whatever processes we want to conjure, gene
frequencies among the populations in each of the two oceans diverge.  The
extent to which the gene frequencies diverge is a function of time and gene
flow.  With the nice, clean, geologic barrier between the two populations,
we can safely assume gene-flow is zero. At what time/degree of
gene-frequency divergence do we stop calling them the same species, and
start calling them different species?  At the moment any measurable
differences in gene frequencies across the two populations can be
detected?  At the first sign of any phenotypic (aka morphological)
difference?  At the point where the morphological difference between he two
oceans is 100% (e.g., when the last individual with the spot on the fin on
the Atlantic side has died, and the last individual lacking the spot on the
fin on the Pacific side has died)?  At the point where viability of
Atlantic-Pacific cross-bred individuals is lower than the average viability
of Atlantic-Atlantic or Pacific-Pacific offspring?  At the point in time
where reproduction between any two individuals on either side of the
isthmus fails to yield any viable offspring?  There are gazillions of other
points in time where I could declare that the moment has arrived when we
should stop calling members of each of the two ocean populations the “same”
species, and start calling them “different” species.  And, of course,
different people would (reasonably) disagree on when that threshold has
been crossed.



So, my point is this:  even if populations can be split effectively
instantaneously, evolution is a gradual process, and both opportunity and
biochemical/genetic compatibility for gene exchange are not absolutes.  The
transition between one species and two species cannot be easily
pinpointed.  Therefore, there is always going to be (as Doug nicely noted),
some “fuzziness” in the boundaries between two sister species.  Our
nomenclatural system doesn’t accommodate this.  They either are, or are not
distinct/same species.  So, yes, John, assuming enough time passes with
zero gene-flow between the two Oceans, we can confidently expect the two
populations to diverge, and to reflect this divergence through genetics,
morphology and reproductive viability.  That’s definitely “natural”, and
the resulting “taxa” exist independently of our (human) interpretation of
them.  But once we start labelling taxa with names, we emphasize the
boundaries between them.  And as I’ve hope I’ve illustrated, those
boundaries are not so natural – they are more in the realm of artificial.



Darwin said it best:

“We must, however, in many cases, decide by a majority of naturalists, for
few well-marked and well-known varieties can be named which have not been
ranked as species by at least some competent judges.”



We can certainly discuss the naturalness of phylogenies – especially when
we represent them as tidy cladograms with clean bifurcations at nodes among
the branches.  But I’ve rambled on enough already that I’ll save the
dissection of that particular illusion for another rant.  But for now, at
least, I hope I’ve adequately explained my deliberately cryptic and
intentionally snarky remark that taxa may be “natural”, but the boundaries
between them not so much.



Aloha,

Rich
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