The Theory of Evolution
by Natural Selection
Part 2: Natural Selection
7 Points to Remember:
1.
2.
3.
4.
5.
6.
7.
Overproduction
Competition
Variation
Adaptation
Natural Selection
Inheritance
Speciation
1. Reproduction
• Every species is capable
of producing far more
offspring than are
needed to maintain a
stable population.
•
Yet, generally, populations
of organisms are stable
within an environment.
•
What are some ways that
species can reproduce?
2. Competition
• Living space and food
resources are limited,
so offspring from each
generation must
compete in order to
live.
•
•
•
They compete with each
other, and with other
species for the resources.
Only a fraction will survive
long enough to reproduce.
These environmental
pressures are what helps
keep population sizes stable.
2. Competition - 2
5. Competition - 3
“Survival of the Fittest”
•
MEANING: The idea that species adapt and change by natural selection with the
best suited mutations becoming dominant
ORIGIN: This expression is often attributed to Charles Darwin and although it
appears in the fifth edition of his Origin of Species (1869), it is there attributed to
Herbert Spencer:
“The expression often used by Mr. Herbert Spencer of the survival of the fittest is
more accurate..”
Spencer had published The Principles of Biology in1864. In that he referred to
‘survival of the fittest’ twice:
•
•
–
–
•
"This survival of the fittest, implies multiplication of the fittest."
"This survival of the fittest... is that which Mr. Darwin has called 'natural selection, or the
preservation of favoured races in the struggle for life'.“
By 'fittest', of course, Spencer and Darwin did NOT have in mind the commonly
used meaning of the word now, i.e. the most highly trained and physically
energetic. The 'fittest' referred to here are those animals which are the most
suited to their environment, i.e. those which are best fitted to survive.
3. Variation
• Individuals of any
species vary in their
characteristics. These
differences are
variations within a
species.
•
•
What produces these
variations?
What form of reproduction
mixes genetic material
from two parents?
3. Variation - 2
How do you account for the variation in a zebra’s stripes?
How do you account for the regularity in a zebra’s stripes?
3. Variation - 3
People around the globe have taken advantage of those
variations, and selectively bred organisms of domestic plants
and animals to accumulate the characteristics they were
looking for.
There is a big difference between selective breeding and
natural selection (can you explain the difference?), but the
result is the same – gradual change in characteristics over
time.
4. Adaptations
• An adaptation is any
characteristic that
improves an
organisms’ chance of
survival and
reproduction in its
environment.
•
•
What molecule codes for
an organism’s
characteristics?
How could that molecule
vary from individual to
individual?
4. Adaptations - 2
What adaptations make these organisms well-suited to
their environments?
4. Adaptations - 3
•
What is the difference
between:
–
–
adapting to one’s
environment
and
having an adaptation to that
environment?
5. Natural Selection
•
The environment (nature)
selects for organisms most
suited for each environment
by eliminating the unfit.
Adaptations that are not
suitable for an environment
are weeded out.
•
Organisms most suited survive &
reproduce.
Selective Pressures
•
–
–
–
–
Climate/Weather
Resource availability
Predation
Sexual Selection
Source: http://evolution.berkeley.edu
5. Natural Selection - 2
•
The fittest survive to pass on
their DNA to the next
generation. (“Survival of the
fittest”)
•
Offspring inherit these better
characteristics and as a
whole the population
improves.
5. Natural Selection - 3
IMPORTANT POINTS #1
• Individuals do NOT
evolve.
• Populations--and
therefore species--do.
5. Natural Selection - 4
IMPORTANT POINTS #2
•
•
•
•
Natural Selection does not
move in a pre-determined
direction!
Organisms don’t “get” what
they “need” to survive.
Organisms don’t get to
choose their DNA or
characteristics that would
help them survive.
The changing earth
determines what will and can
survive.
Source: http://sci.waikato.ac.nz
5. Natural Selection - 5
5. Natural Selection - 6
•
The environment (nature)
selects for organisms most
suited for each environment
by eliminating the unfit.
Adaptations that are not
suitable for an environment
are weeded out.
•
Organisms most suited survive &
reproduce.
Selective Pressures
•
–
–
–
–
Climate/Weather
Resource availability
Predation
Sexual Selection
6. Inheritance
• Those best adapted to
their environment will
survive to reproduce.
• They pass on their
genetic information
(DNA) to the next
generation.
6. Inheritance - 2
•
The environment
(nature) selects for
organisms most suited
for each environment
by eliminating the
unfit. Adaptations that
are not suitable for an
environment are
weeded out.
•
Organisms most suited
survive & reproduce.
Selective Pressures
•
–
–
–
–
Climate/Weather
Resource availability
Predation
Sexual Selection
6. Inheritance - 3
• Lamarck thought that a
trait acquired during an
organisms life could be
passed on.
–
Example: Giraffes that
stretched their necks to
reach higher branches for
food would pass the longer
necks to their offspring.
• We now know this is
wrong.
6. Inheritance - 4
6. Inheritance - 5
•
•
•
•
•
•
Darwin did not know anything about DNA or
genes or genetics or inherited characteristics.
He inferred everything about it from his
careful observations from nature.
Darwin's theory of descent with modification
was accepted by most scientists worldwide
within ten years of its publication in 1859.
However, his theory of natural selection was
widely criticized, and by the turn of the 20th
century was widely considered to be dead.
However, the work of Gregor Mendel a
Czechoslovakian monk, who discovered the
foundations of what we now call genetics,
provided a mechanism by which Darwin's
theory could be revived and expanded.
Gregor Mendel published his work with peas in
1865 (Six years after Darwin’s Origin of
Species)
7. Speciation
•
•
•
•
Over many generations,
favorable adaptations
gradually accumulate in a
species and bad ones tend to
disappear.
Eventually, accumulated
changes become so great,
the result is a new species.
Formation of a new species is
called “Speciation”
Speciation takes many,
many, many generations to
occur.
7. Speciation - 2
• Speciation may occur due
to:
– Geographical isolation
– Reproductive isolation
– Genetic drift
Genetic Drift
• Genetic drift—along with natural
selection, mutation, and
migration—is one of the basic
mechanisms of evolution.
In each generation, some individuals
may, just by chance, leave behind a few
more descendants (and genes, of
course!) than other individuals. The
genes of the next generation will be the
genes of the “lucky” individuals, not
necessarily the healthier or “better”
individuals. That, in a nutshell, is
genetic drift. It happens to ALL
populations—there’s no avoiding the
vagaries of chance.