Natural Selection
How have organism’s adapted to
their environments over time?
The Four Tenants of Natural
Selection…
1. OVERPRODUCTION: Organism’s produce more offspring
than their environment can support. Why?
2. HERITABLE VARIATION: Offspring vary in their appearance
and function, some of these variations are heritable.
3. COMPETITION: Offspring must compete for survival, food
and mates (reproduction), living space, etc.
4. SURVIVAL OF THE FITTEST: Offspring who have the highest
fitness for their environment will live longer and leave more
offspring than those less suited for the environment.
Can we see natural selection in
one generation?
• No…typically not; although some examples
show otherwise.
• This process occurs very slowly, over many
generations
• Natural selection can only be observed as
changes in the characteristics or behaviors
in populations over time.
Sexual Selection…
The male peacock… why waste so much energy producing feathers?
Overproduction & Heritable
Variation in peacocks
• How many offspring do they produce?
• 6-8 per year are hatched… not that many survive
• What kind of variation could we see in male peacocks?
• Different color patterns
• Different feather sizes
• Size of bird
• Is this variation in feather size heritable?
• Genes control how large feathers will get…
• Offspring from a peacock with large feathers?
• Offspring from a peacock with short feathers?
Competition & Survival of the
Fittest…
• Peacocks have competition for food, survival & reproduction.
• Who will reproduce the most?
• Largest feathers = more reproduction
• If those with large feathers reproduce more, what will the
population start to look like over many generations?
• Larger feathers, on average.
• SEXUAL SELECTION – the natural selection of secondary sex
characterizes in males is WORTH the energy they spend
because it increases levels of reproduction – makes them
more fit.
Other interesting secondary
sex characteristics….
Natural Selection…
Caterpillars… why would they want to look like bird poop?
Overproduction & Heritable
Variation in butterflies…
• How many offspring do butterflies produce?
•
•
•
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1000+ offspring … not that many survive
Remember energy is neither created; nor destroyed
Invest the energy at beginning (make lots of eggs) or
Invest the energy at the end (care for the young
• What kind of variation could we see in larva (caterpillars)?
• Different color patterns
• Different sizes
• Diet
• Is this variation in color patterns heritable?
• Genes control color patterns in all organisms…
• Offspring from a butterfly (caterpillar) that was green as a larva?
• Offspring from a butterfly (caterpillar) that was brown and white as a
larva?
Competition & Survival of the
Fittest…
• Caterpillars have competition for food, survival &
reproduction.
• Who will survive the longest?
• Best camouflage = less likely to be seen by predators = longer
survival = more reproduction
• If those that look like bird poop survive longer, what will the
population start to look like over many generations?
• Like bird poop!
• NATURAL SELECTION – the natural selection of characteristics
that allow an animal to blend in with their environments –
makes them more fit.
Other examples of selection for
survival…
Natural Selection…
Cheetahs… why can they run so fast?
Overproduction & Heritable
Variation in cheetahs…
• How many offspring do cheetahs produce?
• 5-6 offspring per year … not that many survive
• What kind of variation could we see in cheetahs?
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•
•
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Size of the heart
Length of the legs
Color patterns
+ many others
• Is this variation in length of legs heritable?
• Genes control color length of limbs in all organisms…
• Offspring from a cheetah with long legs?
• Offspring from a cheetah with short legs?
Competition & Survival of the
Fittest…
• Cheetahs have competition for food, survival & reproduction.
• Who will survive the longest?
• Those that can run fast enough to catch prey to survive = longer
survival = more reproduction.
• If those that look run faster eat more, what will the population
start to look like over many generations?
• Fast runners!
• NATURAL SELECTION – the natural selection of characteristics
that allow an animal to get more food with their environments
– makes them more fit.