Subject: News:  Darwinian Evolution

http://www.newswise.com/articles/BIRDS.TUL.html
University of Tulsa
24-Oct-98

TU Biologist Sees Darwinian Selection in Action


 To Charles Darwin, evolution through natural selection occurred
over countless generations. To Charles Brown, a University of Tulsa
biology professor, natural selection took place literally overnight
in 1996 as he witnessed the starvation death of half the cliff
swallows he has studied in Nebraska for 17 years.

"The population in our study area, which is a region of about 100
miles along the Platte River, fell by 53 percent during a six-day
cold spell in May," explains Brown. "Mortality was probably in the
vicinity of 30,000 birds, but about 28,000 survived."

Basically, Brown found that the survivors were larger and more
symmetrical than those that died, and those who lived through the
bad weather have apparently passed on the genes for larger size to
their offspring.

Brown's findings are presented in the October issue of Evolution
in the article "Intense Natural Selection on Body Size and Wing and
Tail Asymmetry in Cliff Swallows During Severe Weather."

"This event is an example of evolution in contemporary time," says
Brown. "When we talk about evolution, we're talking about gradual
changes that have occurred over millions of years, so it's rare to
be able to go out and demonstrate natural selection in the time
frame of this study. In this case, essentially overnight."

Brown, his wife, Mary, a research associate in TU's biological
sciences department, and student assistants spend the summers along
the Platte River observing, trapping, and banding swallows to
determine life span, migration patterns, sex and health. They have
banded more than 100,000 birds. The research represents the longest
running study of coloniality in vertebrates. The goal is to uncover
the advantages and disadvantages of group living.

On May 24, 1996, began six days of cold weather, with highs ranging
from 40 to 50 degrees, and some rain daily. As a result, flying
insects were not active, which sharply curtailed the food supply
of the insect-eating swallows, who normally catch their prey in
flight at about 200 feet above the ground.

"They can go four days without food, and they can eke out a living
if it is in the 50s, but dry. But when you put rain on top of it
-- that's what was so bad," Brown says. By the fifth and sixth day,
when lack of food took it's toll, the researchers' instincts took
over, asking: What traits do the survivors have that are absent in
the ones who succumbed?

They collected more than 1,800 dead cliff swallows and measured
their wings, legs and beaks. They measured the same features on
about 1,000 survivors. Many of the birds sampled -- dead or alive
-- had been banded. "They were ones that we knew something about;
we had their history. We had 10-year-old birds that were dying."

The survivors were larger skeletally, including bigger beaks and
bigger legs. "They probably were able to hold more fat going into
the bad weather, which would help cope with food scarcity.
Surviving swallows had shorter wing and tail feathers, but Brown
has no clear explanation for that difference.

Among the dead birds, the right and left sides did not match in
size as closely as those who lived. The difference in lengths was
about 5 percent.

Brown says some researchers believe that symmetry is a reliable
indicator of an individual's "quality" -- meaning overall vigor,
including disease resistance. "In addition, there is a great deal
of interest now in asymmetry as a cue for mate choice," he says.
"It is believed that many wild animals, particularly females, will
pick symmetrical males, although the reason is not clear. But this
case provides a good example of why a female would want a
symmetrical male."

Presumably evenly formed individuals are survivors who will pass
those symmetry qualities to their young. "Our research actually
provides rationale for this widely held view that symmetry is
important," says Brown. "This is the first example of natural
selection for high symmetry in the wild."

"We've looked at the offspring of the 1996 survivors, and they are
also big and symmetrical," Brown points out. "So these traits
appear to be genetically based."

The same patterns were seen among samples of 25 barn swallows that
survived and 21 that perished in the study area.

They also found that the survivors tended to be younger birds. "We
suspect that the older birds had arrived earlier and were probably
further into nesting at the time the event occurred, and therefore,
they probably had fewer fat reserves; they had used up more energy
in laying eggs and incubating eggs. Many birds are real fat when
they first arrive on the breeding grounds, and that's believed to
be so that they can endure some bad weather. The younger birds
probably arrived more recently, and they still had some of these
reserves."

Temperature and rainfall records since 1875 for the study area show
only two similar events, in 1967 and 1996, "and that really
indicates to me that we witnessed a very rare event." A similar
weather spell occurred in 1992, but relatively few birds were
involved.

Brown, who has served as a curator of ornithology at Yale's Peabody
Museum of Natural History, wrote "Swallow Summer," a journal-type
book published this year about his passion for birds and the
personal challenges of scientific research. He and his wife wrote
the book "Coloniality in the Cliff Swallow: The Effect of Group
Size on Social Behavior."

The field work is funded by the National Science Foundation and the
National Geographic Society. The Browns met when he was in graduate
school in Princeton University. She was his first assistant, and
the project became his dissertation project.

# # #

Note to editor: Brown can be reached at TU by calling (918)
631-3943 or by e-mail at: charles-brown@utulsa.edu . He can provide
slides.

Contact: Andrea Sharrer, Office of University Relations, The
University of Tulsa Phone at (918) 631-2309 E-mail:
LAW_AS@centum.utulsa.edu