Transcript Natural Selection and Evolution

Natural Selection and
Evolution
Five Principles of Natural Selection
• Organisms produce more offspring than can
survive and they compete for limited resources.
• Random variations in traits occur among individuals
of a population.
• Variations are heritable (may be passed on to the
next generation through reproduction).
• Individuals with helpful variations are more likely to
survive and pass on their traits.
• Over time, offspring with these helpful variations
make up more and more of the population.
Organisms produce more offspring
than can survive.
• All organisms produce more
offspring than can survive.
• Survivors continue to pass
their traits on to future
generations.
Random variations occur among
individuals of a population.
• Organisms have various traits.
• Among the members of a
population, individuals have
differences in their traits.
• These differences are called
variations.
Variations Continued
• Measure any trait in a population of
organisms, and random differences will be
noted.
• Variations may be helpful to survival,
harmful, and neutral. It is all a matter of
random chance.
Variations are Heritable
• Heritable means they may be passed on
to the next generation from parent to
offspring.
• Variations that cannot be inherited do not
contribute to evolution (i.e. Lamark).
Individuals with helpful variations are
more likely to survive and pass on their
traits.
• Helpful variation such as stronger
muscles, long legs, or good camouflage
help an individual survive.
• Survivors have a better chance at
reproduction.
• Unhelpful variations…
Over time, offspring with helpful
variations make up more and more
of the population.
• Less useful variations place an individual
at a disadvantage.
• These individuals are more likely to
succumb to disease, starvation, or
predators.
• “Beneficial” variations lead to better
chances and are passed on to offspring.
Variations & Natural Selection
• Given a trait such as leg length in zebras,
variations will occur among individuals.
• Some zebras will have longer legs, others
average, others still will have shorter than
average.
• Assuming leg length is related to speed,
individuals with longer legs will be better
equipped to outrun predators and survive.
• Their offspring will inherit longer legs and
survive better than short legged zebras!
So what is Natural Selection?
The tendency of individual with
helpful variation to survive and
reproduce more often that
individuals with less helpful or
harmful variations.
Adaptation
• An adaptation is any variation that makes an
organism better suited to its environment.
• It is a noun not a
verb!
• A species has
various adaptations,
but individuals
cannot adapt by
changing their traits.
How do adaptations arise?
• Natural selection works on variations within each
generation. Consider bears…
• Long ago, some bears living in the north had
thicker coats than average (and some had
thinner)
• Thick coat bears had a survival advantage and
produce more surviving offspring than thin coat
bears.
• Over many generations, thicker coats become
more common until they are a trait that is fixed in
the species.
• Thick coats are now an adaptation – polar bears
have them!
What does it mean to be well
adapted?
• When the traits of a species are well
matched to the environment it is said to be
well adapted.
• This organism is fit, and has a high
degree of fitness.
• Fitness is a measure of an how well an
organism is adapted to its environment,
and its chances of surviving and
reproducing successfully.
Fitness
The Big Ideas
Genes, Mutations Variations, and Heritability
• Genes – segments of DNA that control traits.
• Mutations – mistakes made when DNA is copied
before meiosis or mitosis.
• Mutations change genes and thus change traits.
• Changed traits are really just variations.
• Since variations are from DNA – they’re heritable.
• Natural Selection favors helpful variations that
were created by lucky mutations – EVOLUTION!
Mutations Lead to Variations in
Traits
Geographic Isolation
• For a new species to arise, a group of
individuals must be reproductively
isolated from its parent population.
• Often this happens due to geographic
isolation.
• Isolation allows natural selection to slowly
change the traits of one population while
the other population does not.
Geographic
Isolation &
The Grand
Canyon
Kaibab Squirrel (North Rim)
Albert’s Squirrel (South Rim)
What if isolation does not occur?
• Without isolation, individuals of both
populations will interbreed – swapping
genes in the process.
• This will keep the traits of the two groups
more or less together
• A new species cannot forms, since all of
the individuals exchange genes (and
traits) amongst each other.
The speed of evolution…
• Sometimes evolution may
occur due to minute
changes over many
millions of years.
• This is called
gradualism.
• Camel & whale evolution
are two good examples.
These organisms
changed over millions of
years leaving many
intermediate forms.
• Other times evolution
may occur in much
shorter periods of time
with little change before
or after that period.
• This is Punctuated
Equilibrium.
• When a sudden change
in the environment
causes heavy natural
selection pressure – new
traits can arise quickly.
• Antibiotic resistance is a
good example.