Dear All - This has been a most interesting thread. Jim, from Brazil,
has given me permission to publish some of his comments, and I find them
very perceptive, and skeptical, which is what all scientists need.
Also, Steve has commented that composites just don't produce red petaled
flowers, and if composites flower in the fall, this lends itself to
having yellow, blue and white flowers out there, but not red. All good
observations.
What we really need is a way to determine quantitatively flower
color predominance (how should we define it: number of flowers/meter
squared, area of flowers/meter squared, or some other metric?). Then,
one could actually determine if this pattern is real. Perhaps we just
are misled by the abundance of goldenrods and other flower colors are
out there, but humans miss them. To bugs, maybe the fields look some
other color! Just some random thoughts.
Howie Neufeld
From Jim:
If Stephen J.Gould were alive today, he would roll his eyes. First, the
sampling was not carried out in a systematic matter. So, it could be that
1) There is indeed NO predominance of yellow flowers in the fall -
thus, nothing to explain.
2) One or a few species of closely related plants flower in the fall.
So, it is not a predominance of species with yellow flowers, but rather
an abundance of one, or a few, species who, because they are closely
related, have the same flower type.
3) The question is really, why do other colored flowers bloom at other
times of the year?
And, finally, nobody mentioned bees in their adaptive explanations.
But, it could just be that humans are biased, and we think we see more
yellow flowers because they grab our attention. Again, until the data
are in, we don't know if there is a pattern even!
And, to comment on a few "explanations":
1. Perhaps the fall migration of hummingbirds, which prefer red flowers,
selects for other colors at this time, and perhaps yellow.
"I like this one, but why should this select for yellow preferentially?"
What, the ABSENCE of hummingbirds SELECTS for something? This is the
first time I have ever heard that by being absent, an organism can
select for some pattern in another organism....
2. Yellows predominate in the Composites, which one person said
originated largely in South America, and perhaps drives their fall
flowering. Increasing xeric conditions may
also select for this group of plants.
"Perhaps, but then why did the Composites settle on yellow flowers?"
Bees.
3. Shorter day lengths and solar angles may select for yellow flowers at
this time of the year.
How in the world does daylength select for flower color, and since
daylength is similar in both the spring and the fall, why don't we have
yellow flowers at both times of year (if indeed we have yellow
predominately at ANY time of the year)?
4. It could be that human disturbance has created habitats that favor
fall flowering plants, which just happen to have mostly yellow flowers.
That begs the question - why do plants with yellow flowers AND that
flower in the Fall, like human disturbed habitats? Boy, this one really
needs data...
5. In the fall, pollinators may be relatively scarce, and perhaps yellow
flowers do a better job at attracting them.
Bees - they are never scarce. But, this kind of reasoning is suggesting
that all colors were vying for success but the other colors lost out.
If so, this is not even testable, because they lost and we can't find
color in fossils.
6. Color predominance may be regional - several people pointed out that
different colors dominate in their particular area.
I would say, until the data are in, I am not even sure that a pattern
truly exists. And then, with the data, some hypotheses should arise.
Further comments from Jim:
Glad to see you took my comments well. Sometimes the net makes "humor"
or "friendly skepticism" seem like "sarcasm" or downright obnoxious.
Remember, the phylogentic argument may include an overabundance of one
or few related species. So, the number of species with yellow flowers
may actually be less than that of other colors, but there are many more
individuals of the few species with yellow flowers.
The daylength issue, if it includes climate and seasonality, is no
longer a daylength issue. I mean that, once we start putting
restrictions on the argument, the argument is really about all the
restrictions, and not the variable of interest. Do I explain myself well?
By the way, while hummingbirds may like red flowers, you would be
surprised to see the number of colors they actually visit. I live in
southern Brazil, and have 14 species of hummers that visit my place, and
they will visit any flower, anywhere at any time. They learn, so the
same individuals do not necessarily come back all the time, but, you
will see hummers visit any kind of flower, at any kind of year, even
when they have my red hummingbird feeders available. Even flower that
are supposedly without nectar! And besides, yellow flowers, composites,
usually don't have nectar anyway, or not much. Also, hummers migrate
and so would visit any available flower during migration - why don't we
have an abundance of red flowers during hummer migration?
Bees. Is it that before the winter, bees are storing up as much pollen
as they can gather, and flowers with abundant pollen are more available
then? What, the plants like the bees and produce more just for them?
Or, do they need to attract more bees to insure pollination during that
time? Or, is it merely that annual plants flower before they die, and
the cycle is just that, germinate in the spring, die at the first freeze
(seeds overwinter, of course). So, is it that annual plants tend
towards yellow flowers?
I am still skeptical - I am not sure there is a pattern. What if the
pattern were that there is always the same number of yellow flowers
available, but during the spring, other color flowers are more
abundant? The pattern would be to explain the other flowers, not the
yellow ones. After all, the habitat of plants with flowers of different
colors are different anyway.
The disturbance hypothesis assumes that the yellow flowers are being
seen in or near distrubed areas - an observation bias. Perhaps people
go out to the countryside less in the fall, and stare out the window,
wishing it were still summer, and they see all the yellow flowers that
they forgot they planted around the house. During the summer, they are
elsewhere more, seeing the flowers of elsewhere....
Regardless, data are necessary for two of the the first steps of the
scientific method - Observed pattern, hypothesis generation and testing.
Sure, you can share!
Jim
From Steve:
Having seen the replies, I could not resist making one more comment.
Call me the "non-adaptationist scrooge" if you will. One thing no one
has mentioned is that a likely reason why most of the composites in the
Asteraceae we see flowering in the East right now are yellow (and I
would also argue blue and white, e.g., Aster and Boltonia), but not red,
is that this group lacks the floral structure/arrangement to support
bird pollination. So, red ray flowers most likely were not common EVEN
IN THE ANCESTORS of this group. In other words, the lower proportion of
red flowers in the fall (in the East, at least) may largely be the
result of a phylogenetic constraint within the Asteraceae subfamily of
composites, which represents a large proportion of the plants flowering
right now in the East.
Sounds like a good question for a class.
--
Dr. Howard S. Neufeld, Professor
Department of Biology
572 Rivers Street
Appalachian State University
Boone, NC 28608
email: [EMAIL PROTECTED]
departmental webpage: http://www.biology.appstate.edu/faculty/neufeldhs.htm
personal webpage: http://www.appstate.edu/~neufeldhs/index.html
Tel: 828-262-2683
Fax: 828-262-2127
