Transcript Evolution and Sex - Blue Valley Schools

Evolution and Sex
Quote of the Day
“The sight of the peacock’s tail makes me sick.”
Charles Darwin
What’s so disgusting about the peacock?
Darwin’s Dilemma
• Many species have characteristics
that seemingly should be
eliminated by natural selection.
• Bright coloring and attentiongetting behavior tend to attract
predators.
• Large body size or highly
exaggerated physical features
require a large amount of energy
to produce.
Darwin’s Solution
• Sexual Selection: the theory that competition
for mates between individuals of the same
sex drives the evolution of certain traits.
• Seemingly harmful traits can evolve when the
sexual attractiveness of the trait outweighs
the problem it presents for survival.
• Whereas natural selection is sometimes
called the “survival of the fittest,” sexual
selection can be thought of as the “survival of
the sexiest.”
Causes of Sexual Selection:
Male Competition
• Males have lots of sperm and often
reproduce with several different females.
• Generally, males compete for the opportunity
to mate with females.
Size Differences
• Larger, stronger, and tougher males are
typically more successful in competitions for
females and more likely to be able to defend
their offspring.
• Over time, great size differences can evolve
in species when males compete for access
to females.
• Examples: baboons, lions, moose, crocodiles
Example: Elephant Seals
• Males (bulls): 4000-6000 lbs, up to 20 ft long
• Females (cows): 1300-2000 lbs, ~10 ft long
Causes of Sexual Selection:
Female Choice
• Females have fewer eggs and invest more
time and energy in reproduction.
• Generally, females hold out for the best
genes and/or the most helpful mates.
Exaggerated Physical Features
• When females choose a
trait that is closely linked
to successful genes, that
trait spreads throughout
the population over
generations.
• Sometimes these traits
can become highly
exaggerated, as seen in
the peacock.
Exaggerated Physical Features
in Males
Lion
Frigate bird
Pheasant
Elk
Comparison of Natural and Sexual Selection
What is
selected for?
Who or what
does the
selecting?
Natural Selection
The best
survivors
Climate,
Predators,
Competitors,
Disease
Sexual Selection
The best
reproducers
Members of the
opposite sex
Runaway Sexual Selection
• When males with a specific trait mate with
females that prefer that trait, the trait and the
preference for it become genetically linked.
• This can lead to a "runaway" process, causing
the male trait to become more and more
extreme until the survival disadvantages of
the trait outweigh the sexual attractiveness.
• This “runaway sexual selection” may
eventually lead to extinction of a species.
• The Fate of the Irish Elk?