Download PGS: 454 – 458

Unit 3: Evolution
Content Outline: Evolution of Populations (3.1) – Part 3
I.
Natural Selection (A.K.A. “Survival of the Fittest”)
A. The offspring that do survive will pass on, by reproduction, their useful variations/genes to its offspring
that will help the species better survive in their environment because the “weak” traits will eventually be
eliminated by nature.
B. Overproduction
1. Overproduction is a species’ ability to produce more offspring than the environment can support.
a. Most offspring will not survive to adulthood due to:
i. Predation.
ii. Limited resources.
iii. Disease.
(Example: Clownfish can lay 600-1500 eggs per clutch and green sea turtles lay 75-200 eggs per
clutch.)
C. Variation
1. Variation is the difference in traits present within a given species or population.
2. These options are essential to surviving in a changing environment. If there is no variation or
“options”, then a species is limited to what is available; even if it is weak or unfavorable.
3. Variations only come into existence with a change in the organism’s DNA,
a. This change is known as a mutation. These mutations are rare and randomly occur in
gametes (sex cells) because these cells are normally not exposed to the environmental
stresses an organism may encounter in their existence.
i.
Mutations mostly occur in somatic cells because these cells are exposed to the
environmental stresses.
ii.
Some mutations are helpful to a species, but most are harmful and therefore the
cell or organism usually dies.
D. Competition
1. Competition occurs because there is an overproduction in a species’ population.
2. Resources available (food, nesting sites, mates, etc.) are limited in number and because of this,
not every individual will be able to survive and reproduce.
E. Adaptation
1. An adaptation is a specific variation that is or becomes a common trait within a population
because it helps the species better survive in their environment.
(Examples: Clownfish are naturally slow swimmers because of their rounded caudal fins
that help them make quick darting movements. Over time, they have become resistant to
the stings of the sea anemones, in which they live, because they secrete a mucus which
coats and protects the body. This allows the fish to hide from predators and provides it
with a safe place to live.)
2. Adaptations are preserved over time as the preferred variation is inherited by offspring. This is
natural selection.