cjm@reed.UUCP (Chris Marshall) (04/07/89)
Bryan, I've been following your discussion for a week or so and have a
problem with the question you are addressing. The original question of
"why do birds have beaks? is analogous to many other questions
regarding a variety of character traits in an array of organism. You
pointed this out with your giraffe example of "why do giraffes have long
necks". The problem with this sort of question is that the question is
not clear enough. Are you refering to what evolutionary processes caused
beaks to become prevalent in birds, or are you discussing what current
environmental cirumstances exist such that beaks are a favorable trait?
the two question are quite different, and have I think resulted in the
general dismay that other readers have voiced concerning the discussion
in general. Is it truely possible to show that beaks are an adaptive
trait which arose due to specific selective forces favoring them;
specifically flight? An example of what I'm trying to say can be seen
in the case of artic animals such as the arctic fox.
In looking at the arctic fox one notes that its ears are much smaller than
its more temperate cousins. From this one might be led to the question of
why do arctic foxes have small ears? To address this issue the investigator
can then proceed to make observations, do experiments and such in order to
examine the roll of the smaller ears within the arctic environment. He/She
may come to the conclusion that the small ears grant the fox an advantage in
cold environments because of reduced heat loss. So, then is it correct to
assume that small ears arose over evolutionary time because of natural
selection's favoring of small ears in foxes? The answer is no, this does
not necessarily follow as the small ears could have arisen for a variety of
non-adaptive factors and are subsequently maintained in the population because
of the proximate effects of the environment. However, all is not lost, for
studies investigating ear size in other organisms also show that reduced ear
size is correlated to colder conditions (arctic rabbits and wolves show a
similar phenomenon). This tends to give more support to the claim that
small ears are an adaptive trait, but still doesn't prove it. The question
then arises as to how much evidence needs to be given in order to show that
a trait is adaptive?
Obviously your disscussion has shown that very little evidence needs
to be given in order to "prove" the adaptive nature of beaks for many
people. In fact many people are happy to accept just about any adaptationist
story about any organism, including humans. Thus, we have a large mass of
fun nature stories concerning "why this" and "why that". I am not, I have
to state, undermining adaptation as a fundamental evolutionary process,
however, I think that many cases, such as birds and beaks, it is
not possible to provide the answer to the quetion of whether or not the trait
is adaptive.
In the above case of ears, there are a vast array of organisms which live in
both cold and warm environments. Similar comparisons, regarding beaks,
are not as easy in your case. Comparison to bats is close to this, but
there just isn't enough other animals to compare with.
Another approach is to compare all the animals with small ears and
check for environmental similarities...such as your comparison to parrot
fish and turtles. However, these are somewhat unconvincing, for in the case
of small ears, they may also be prevalent in other environments for different
reasons.
A good article to read would be Gould and Vrba (1982) "Exaption--a missing
term in the science of form", Paleobiology, 8(1):pp4-15. It goes through
this issue as well.
I wish you luck in finding out the definitive answer to your question...but
in truth I think it will ultimately depend on what you consider to be
"definitive". Maybe a better question would be: "Why are humans so
obsessed with the question of why birds have beaks?"
please respond via mail- (I tried to mail this letter to you but somehow
it bounced back)
-chris marshall