Bio A - Evolution
Meaty Chickens
Consumers demand bigger breasts
This has become a world of chickens. It’s Planet Pollo. It’s an astonishing triumph for
the descendants of the humble creature called the red jungle fowl (Gallus gallus).
Before World War II, chicken was fairly expensive, hardly a staple in the
developed world. The political promise of “a chicken in every pot” seemed far-fetched.
But breeders realized that if they could hybridize corn, they could do a similar thing with
chickens and today chicken is the number one meat in the U.S.
The demand for processed chicken parts along with a taste for breast meat drives
the commercial industry. In the 80’s, 10 percent of a typical chicken’s weight was breast
meat, but that has risen to 21 percent, says John Hardiman, geneticist with Cobb-Vantress
in Siloam Springs, Arkansas. “I think for sure we’re going to see 30 percent, which is
probably a little bit closer to where the turkeys are these days,” Hardiman says.
His company is one of a handful of major firms that still raise breeder chicks for
the commercial Market. Billions of chickens come from a few original populations.
Cobb’s big product is a hybrid chick with lots of breast meat, high livability, and
excellent feed efficiency.
But with all those chickens coming from a limited number of breeders, you can
imagine the potential for the chickens to lose genetic diversity. That, in turn, could invite
a biological disaster. Pathogens could sweep through the chicken industry.
So far chickens have managed to retain a surprising amount of genetic diversity.
Therein lies a possible insight into the way genes evolve.
Paul Siegel, a Virginia Tech poultry geneticist, started an experiment in 1957 in
which he plucked the largest and smallest chickens from an original hatch. He bred them
in two separate lines, such that his large chickens over time became larger, close to four
pounds after eight weeks, while his small ones became ever more petite, with an eightweek weight of less than half a pound. From a a common genetic origin he would up,
after nearly half a century and 47 generations of chickens, with a ten-fold different in
size.
His biggest fear never materialized: “We should have run out of genetic
variation,” he says.
“We haven’t. Genetic progress seems to level off for a couple of generations and then
responds to selection again.”
Siegel’s theory: “I think the mutation rate is a lot higher than we’d thought.” We
tend to think of mutation as bad, but in Siegel’s experiment it rescues the chickens from a
genetic dead end. The biggest limitation is physiological: His big chickens will eat
Bio A - Evolution
themselves to death if their feed isn’t restricted, and some of his littlest chickens don’t eat
enough and can die from anorexia.
One thing’s for sure, though: When they turn into chicken nuggets, they are going
to look exactly the same.
- Joel Achenbach
- Washington Post Staff Writer
Meaty Chickens:
1. What trait is being selected for in this article?
2. Why is this “artificial” selection as opposed to Natural Selection?
3. What evidence do we have that the selection process is working? What specific data
does the article give you about the trait?
4. What fear do the geneticists have regarding the selection of chickens?
5. What accounts for the fact that genetic diversity is still present in the chicken
population?
Bio A - Evolution
Evolution of Antibiotic Resistance
Forty or fifty years ago, thanks to antibiotics, scientists thought medicine had all but
eradicated infectious agents as a major health threat. Instead, the past two decades have
seen an alarming resurgence of infectious diseases and the appearance of new ones.
Today, the AIDS virus, tuberculosis, malaria, diarrheal diseases and other infectious
agents pose far greater hazards to human existence than any other creatures.
This upsurge of infectious disease is a problem we have unwittingly created for
ourselves. The rise of rapid, frequent, and relatively cheap international travel allows
diseases to leap from continent to continent. Inadequate sanitation and lack of clean
drinking water are another factor. A third is the "antibiotic paradox" -- the overuse of
the "miracle drugs" to the point that they lose their potency.
Whenever antibiotics wage war on microorganisms, a few of the enemy are able to
survive the drug. Because microbes are always mutating, some random mutation
eventually will protect against the drug. Antibiotics used only when needed and as
directed usually overwhelm the bugs. Too much antibiotic use selects for more resistant
mutants. When patients cut short the full course of drugs, the resistant strains have a
chance to multiply and spread.
In some countries, such as the United States, patients expect and demand antibiotics
from doctors, even in situations where they are inappropriate or ineffective. Our
immune systems will cure many minor bacterial infections on their own, if given the
chance, and antibiotics have no effect on viral infections at all. Every time antibiotics
are used unnecessarily, they add to the selective pressure we are putting on microbes to
evolve resistance. Then, when we really need antibiotics, they are less effective.
While drug companies race to develop new antibiotics that kill resistant microbes,
scientists are urging patients and doctors to limit antibiotic use.
That means not asking for penicillin when all you have is a cold, since colds are caused
by viruses that are not affected at all by antibiotics. It means taking all the pills that are
prescribed, even if you're feeling better. Physicians have to resist prescribing the
strongest and most broadly effective drugs unless the disease absolutely requires it. If
society adopts these measures rigorously, the drugs may regain at least some of their
lost "miracle" powers.
1. What is antibiotic resistance?
Bio A - Evolution
2. You have a bad strep infection in your throat, why do you want to make sure you
complete your entire prescription of antibiotics, even after you feel better?
3. Why is it not a good idea to prescribe someone antibiotics when they have a cold or
other viral infection?
4. Many people say that humans are no longer subject to natural selection from other
organisms. Know what we know about bacteria, viruses and disease, propose how we
still may be under selection pressures?
5. What is one idea you have that may help slow the rapid emergence of antibioticresistant bacteria?