Britches and Niches: was - Cope's rule and bacterial evolution
- From: "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx>
- Date: Sat, 18 Feb 2006 17:25:15 -0500 (EST)
"William Morse" <wdmorse@xxxxxxxxxxxx> wrote in message news:dt5615$1bfp$1@xxxxxxxxxxxxxxxxxxxxxx
"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in news:dsl3qe$tnq
$1@xxxxxxxxxxxxxxxxxxx:
There is an old idea about mammal and dinosaur evolution known as
Cope's rule or Cope's law. Wikipedia has a nice explanation:
http://en.wikipedia.org/wiki/Cope's_law
Whether the rule stands up to scrutiny or not, it is interesting
because of its somewhat paradoxical combination of a direction
to micro-evolution superposed on a macro-evolutionary trend in
the opposite direction. Most species get bigger over time,
but big species tend to go extinct and it is the little species
which branch prolifically producing new species. So the
number of species in each size range stays constant.
Cute idea, even if it isn't really true.
Perhaps I am being naive, but as I understand it Cope's rule is more an
observation than a prediction. It is fairly easily explained:
Niches are characterized by available energy, so that small species will
have more individuals than big species.
The likelihood of a species surviving an extinction event is proportional
to the absolute size of the remaining population, but the likelihood of
an individual surviving an extinction event is independent of the
population size (i.e. the event kills 90% of the population of both large
and small species).
A change in size (in either direction) is a common way of invading or
creating a new niche - it is fairly easy to accomplish developmentally
yet it causes a large change in interactions with other species.
So what we expect to see is a regular loss of large species after
extinction events, followed by a recreation of the large species to fill
the otherwise vacant niches. Becoming smaller is another option to invade
or create a niche, but the small species all survived the extinction
event.
I found out about the following research on an ID website, but
it seems legit:
A. Mira, et al., "Deletional bias and the evolution of bacterial
genomes", Trends in Genetics 17 (2001), 589.
I haven't read it, but the idea seems to be a kind of Cope's rule
for bacterial evolution. But in this case, the bacteria aren't
trending larger, they are trending SIMPLER. Their genomes are
getting smaller. They are optimizing by discarding rarely needed
functionality and thereby gaining a competitive edge in a
narrowly specialized niche. But, in doing so, they risk eventual
extinction.
And of course, counter to this micro-evolutionary trend, there
is the macro-trend. It is the generalist bacteria with the
relatively large genomes which branch and produce new bacterial
species.
The same two options should exist for new niches - you can get smaller or
you can get larger. The problem for bacteria is that the larger option is
limited by competition with eukaryotes (and may also be limited by
physical and chemical factors based on cell organization - I suspect this
but don't know enough about cellular biology). Meanwhile the smaller
option requires specialization in order to gain a competitive edge. In
the case of bacteria, however, the populations are already so large that
survival of species during an extinction event is no longer related to
size. Survival is instead related to survival of the niche - and in this
aspect the generalist will have a survival edge. (Note that generalists
also have a survival edge among eukaryotes.)
Bill,
Your response raises some interesting implicit epistemological issues.
Gil's response speaks directly to epistemology. Hence the change in
direction and thread title, which is further explained by this parable:
A team of Martian scientists is observing a rural village on Earth.
They notice that the youngsters in the village tend to grow in size,
becoming too large for their britches. They also notice that when
the young reach a certain age, they frequently catch the first bus
out of town, join the navy, and leave their old wardrobe behind as
hand-me-downs for their younger siblings. Finally, they notice that
there is a continuing supply of new small youngsters.
The first Martian scientist has a hypothesis. All three observations
can be explained by the buses. The departure of the older Earthlings
leaves behind a 'britches vacuum', which induces younger Earthlings to
grow so as to fill them. And this vacuum, in turn, causes the apparent
spontaneous generation of very small Earthlings to fill the smallest
vacant britches.
The second Martian says that the first has cause and effect backward.
It is the spontaneous appearance of new small Earthlings which creates
a "britches pressure" forcing each cohort to the next larger size of
britches.
The third Martian (actually not ethnically Martian; she is a post-doc
from Titan) claims that the growth is caused by something internal to
the Earthlings; that the growth, the appearance of small Earthlings, and
the departure of large ones are actually three separate phenomena with
three separate explanations.
The other two both scorn this hypothesis, pointing out that it violates
the principle of explanatory economy. They also point out that if the
three phenomena are separate, that would mean that the apparent steady
state is something of a fluke. However, since the Martian scientific
tradition emphasizes respect for specialists, and since there is a
support team of epistemological specialists available in orbit, they
radio up for advice. "Is one hypothesis to be preferred?, they ask.
"What kinds of observations would support one hypothesis over another?
And is this all this hypothesis generation just 'speculation'?"
The parable ends at this point because the entire point of the story
is to invite you to put yourselves in the shoes (figuratively speaking)
of the Martian epistemologists. You may object that it is difficult
to do this since you already know which of the three scientists was
right. But this is no different from the usual situation in which an
Earthling scientist and generalist, attempting to do his own epistemological
thinking, tries to ignore his own preconceptions about the validity of
his own hypotheses and adopt an objective, neutral epistemological stance.
I tell this parable, in part, because the story I heard regarding Cope's
rule is that the long-term growth of the size of the typical organism
in a population is driven by things like sexual competition among males
or territorial exploitation of resources - not by "niche vacuums" or
"niche pressure".
So, what advice would I give as an epistemologist? Well, I would tell
the Martians to seek data as to whether the apparent 'steady state' is real,
or just an assumption. I would point out that the 'vacuum' and 'pressure'
hypotheses can probably only be distinguished internally by looking
at situations in which the steady state is disturbed, so that pressures
or vacuums are particularly large. I would urge that they investigate
more closely whether there is indeed a law of "conservation of britches"
as they seem to assume. And I would quote the great Earthling scientist
Lewontin to point out that all three hypotheses would benefit from
further attention to issues of mechanism. But I would not like to call
the hypotheses 'mere speculation'. To the contrary, I would urge the
scientists to continue to generate these hypotheses, and even to elaborate
them, because the whole question of what kind of observations to make
can only be driven by hypothesis.
The epistemological issues here interest me mostly because I see the
same kinds of issues arising in my area of greatest interest - OOL.
What kind of weight should be given to claims of explanatory economy
in evaluating competing hypotheses? However, our cynical postdoc
from Titan might well point out that I am not really interested in
achieving epistemological objectivity. I just want to know how to
structure my arguments so as to convince others to my viewpoint.
.
- References:
- Cope's rule and bacterial evolution
- From: Perplexed in Peoria
- Re: Cope's rule and bacterial evolution
- From: William Morse
- Cope's rule and bacterial evolution
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