Subject: Re: the significance of a wing bar
Date: Apr 27 15:24:48 1995
From: wrightdb at pigsty.dental.washington.edu - wrightdb at pigsty.dental.washington.edu


>>And David Wright wrote:
DW>>" Suppose the common ancestor of warblers and
^^^^^^^ [dw emphasis]
>>vireos had wing bars. If one species of vireo and one species of
>>warbler lost wingbars, all the descendant species of those ancestral
>>populations would lack wingbars. Explaining two losses of wingbars as
>>resulting from drift sounds a lot more plausible than trying to explain
>>independent losses in dozens of species, which is the assumption
>>implicit in raw comparisons of how many have them vs. how many do not.
>>And those that do have wingbars? That may require only one event -- the
>>acquisition of wingbars in the common ancestor."

DP>But in fact vireos and warblers didn't have a common ancestor, and, if
>you look at books on birds of different parts of the world, you will see
>again and again that some species of a family have wing bars, and some
>don't. Common ancestry can't stand up as a hypothesis here at all, as far
>as I can see. I was using vireos and warblers as an easily understood
>example, but examples abound, many independent gains and/or losses of
>wing bars.

<snip>

Vireos and warblers most certainly *do* share a common ancestor (unless
you want to invoke parallel spontaneous generation... [I am not claiming
that vireos + warblers constitute a monophyletic group, just that they
share characters inherited from a common ancestor]). Whether or not that
common ancestor (some passerine) really had wingbars I don't know, but
that was not my point. My hypothetical example (note use of word
"suppose") was simply a reminder that the occurence of a trait in N
species does not require N events to account for it, even though many
studies in evolutionary ecology and the like continue to make that
spurious assumption (e.g., any study that statistically contrasts the
number of species that have a trait vs the number that lack a trait
without demonstrating evolutionary independence).


DP>>David, further:
DW>"So crests in birds such as jays may well be the product of sexual
>>selection on males, even though they are present in both sexes. (Along
>>the lines of Burt Guttman's suggestion, if these crests incur little
>>cost in females, there will be little pressure for females to "lose"
>>them, and that slight negative selection will be swamped by sexual
>>selection for the crest in males, and they could be retained by females
>>indefinitely despite having no function in females)" [oriole example
deleted]

DP>But studies on Steller's Jays have shown that *both* sexes use them in
>signalling "moods." That seems the more parsimonious explanation to me.
>David, can you think of a bird species in which males use their crest but
>females don't?

What I was trying to point out here is the interesting (at least I think
so) situation of a case where it is not only *conceivable* that some
organisms will acquire traits that are nonadaptive (or even maladapative),
but where it is *expected* that this it will happen -- sexual selection on
polygenic, autosomal traits.


Most studies of such characters as crests, bright colors, etc. focus on
males and ignore females (with a few exceptions, e.g. Parakeet Auklets).
I'd be very interested in a reference to the Steller's Jay work you
mentioned. I don't have any crest examples, but in Pinyon Jays females
had a strong preference for brightly colored males, while male selection
of females was not influenced by brightness of female plumage (ref not at
hand, but can be dug up). What we really need to better understand these
traits are studies across species that track the use of a character from
its origin through successive lineage splits and modifications of the
character. This means choosing a clade that has both the requisite
character variation and a cladogram with which identify evolutionary
transformations in relevant characters (these *transformations* are the
evolutionary *events* that we need to explain).


David Wright
dwright at u.washington.edu