Transcript Evolution and Natural Selection

Evolution and Natural
Selection
Biology
Evolution
• Idea put forth by others before Charles
Darwin, but Darwin is credited with putting all
of the prior knowledge together and it make
real sense.
• A change in the characteristics of a
population from one generation to the next;
gradual development of organisms from other
organisms since the beginning of life.
Evolution
• Common Ancestry
– Shared descent
– Do all organisms
share a common
ancestor?
– Research and
shared
characteristics
between all of life
seem to agree with
this theory
Evolution by Natural Selection
• Natural Selection
– The mechanism of
evolution
– Individuals that have
physical or behavioral
traits that better suit their
environment are more
likely to survive and will
reproduce more
successfully than those
that do not have such
traits.
– These successful
characteristics must also
be shared within the
population.
Adaptations
• An inherited trait
that has become
common in a
population because
the trait provides a
selective advantage
Population
• All of the individuals of a species that
live in a specific geographical area and
that can interbreed.
Change over Time
• Gradualism
– Gradual change in a
population over a
long period of time
that leads to new
species.
– Does this type of
evolution occur?
Change over time
• Punctuated
Equilibrium
– Periods of rapid
change in
population genetics
that are separated by
periods of little or no
change
How did Darwin visualize
these changes?
• Fossils
– Trace or remains of an
organism that lived long
ago, preserved in
sedimentary rock.
• Paleontologist
– Scientists who study
fossils
• So was Darwin an early
paleontologist?
What do we know now that
Darwin didn’t?
• We now know that earth is about 4.5
billion years old
• Organisms have inhabited Earth for
most of its history
• All organisms living today share
common ancestry with earlier, simpler
life-forms.
Relatedness
• Vestigial structures
– Structures that are
present in only a
rudimentary capacity
compared to other
related organisms.
Relatedness
Relatedness
Relatedness
Relatedness
• Homologous
Structures
– Structures that share
common ancestry
– A similar structure in
2 organisms can be
found in the common
ancestor of the
organisms.
– The limb bones of
mammals
Species Formation
• What is a species?
– A group of closely
related organisms that
mate naturally
– Examples:
• Humans, dogs, cats
– Not Examples:
• Mule (Infertile offspring
of a male horse and
female donkey)
Homo sapien
Speciation
• Process by which
new species form.
• Divergence
– Accumulation of
differences between
groups
Speciation
Variation
Isolation
Natural Selection
Divergence
New Species
Population
• All organisms within a species that live
in a specific area that interbreed.
3 Key Features of Populations
1. Size
2. Density
3. Dispersion
Population Size
Small vs Large
• Small Population
– Most likely to
become extinct
– Disturbances can
dramatically affect
this population
– Inbreeding
• Better chance for
non-beneficial traits
being passed on
• Large Population
– Many possible mates
– More variation
– Can better survive
disturbance
– Less chance of
inbreeding
Population Density
• Number of individuals in a given area
• City vs farming community
– More people in a city
– More diversity of individuals in a city
Population Dispersion
Checked vs Unchecked
Populations
• Checked Population
growth
– Typical of most
natural populations
– Limiting factors
cause populations to
stabilize
– Carrying Capacity
• Unchecked
Population Growth
– Humans follow this
curve
– So do bacteria
– Limiting Factors play
less of a role
Population Density Factors
• Density-dependent
– Disease
– Competition for
resources
– Predation
• Density-independent
– Climate
– Weather
– Pollution
Growth Rate and Population
Size
• Birth Rate vs Death Rate for Humans
http://www.sonic.net/~evolve/wp/human_ecolog
y/worldpopulation_2004a.gif
Population Genetics
• How many of you remember alleles?
–
–
–
–
A = dominant, a = recessive
AA = homozygous dominant
Aa = heterozygous
aa = homozygous recessive
• The frequency of alleles in a population can
be explained by the Hardy-Weinberg Principle
– Allele Frequency=how often alleles occur in a
population
Hardy-Weinberg Principle
• The frequency of alleles in a population
do NOT change unless evolutionary
forces act on the population.
• Stable population?
• Also, the Hardy-Weinberg Principle
holds true if there is no inbreeding
p2 + 2pq + q2 = 1
p+q=1
Using Hardy-Weinberg
What does p and q mean?
p2 + 2pq + q2 = 1
p+q=1
p= dominant alleles
q= recessive alleles
p2 = frequency of homozygous dominant
q2 = frequency of homozygous recessive
pq = frequency of heterozygous
Hardy-Weinberg Example
What information do we
• For a disease
know?
caused by a
2 = 0.25
p
recessive allele,
2
p
=
?
=
p
 .25  .5
calculate the
number of carriers in q = ? = 1 p  1 .5  .5
2
2
.5
 .25
q =?=
a population if
2pq = ?
p2 = 0.25. Use

Hardy-Weinberg to  p + q = 1

solve this problem.
Now can we find out 2pq
or the frequency of
heterozygotes?
Hardy-Weinberg Continued
p = .5
p2 = .25
q = .5
q2 = .25
Now determine the number of heterozygotes
2pq = 2 *.5 * .5 = .5
What does this mean?
p  2 pq  q  1
2
2
.25  .5  .25  1
Evolutionary Forces that
Control Populations
1.
2.
3.
4.
5.
Mutation
Gene flow
Genetic drift
Nonrandom mating
Natural selection
Mutation
• GREAT SOURCE
OF VARIATION
ANT MAKES
EVOLUTION
POSSIBLE
• Change in genotype
which leads to
change in
phenotype
Gene Flow
• Migration of individuals
into and out of a
population
• Immigration: individuals
move into a population;
adds new alleles to a
population
• Emigration: individuals
move out of a
population; takes away
alleles from a
population
Nonrandom Mating
• Preference to mate
with others that live
nearby or are
attractive in some
way to that
individual
• Examples:
HUMANS, birds
Genetic Drift
• Small populations
• The number of
alleles is so small
that loss of one
individual can effect
the whole population
• Natural disasters,
drought
Natural Selection
• The frequency of
certain alleles increases
or decreases depending
on the effect those
alleles have on survival
and reproduction.
• One of the most
powerful agents to
changes in the genetics
of populations
Polygenic Traits
• Traits due to interactions of multiple
genes on multiple chromosomes
• Examples: skin color, height
Normal Distribution
• Based on the fact
that most traits
occur on an average
basis in nature. In
other words, being
too different can
lead to you being
dead in nature.
Directional Selection
• Natural selection
favors a particular
phenotype so the
frequency shifts in
one direction
Stabilizing Selection
• This type of
selection makes the
frequency of the
most common
alleles more
common.