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Transcript
Mechanisms
of Evolution
Mechanisms of Evolution
There are several:
1. Natural Selection
2. Gene Flow
3. Genetic drift
4. Mutations
5. Non-random mating
Artificial Selection
 Domesticated breeds have not
always been in their current
form. This change has been
achieved by repeatedly
selecting for breeding the
individuals most suited to human
uses. This shows that selection
can cause evolution.
Genetic Variation
 individuals in a species carry different
alleles (An allele is an alternative form of
a gene (one member of a pair) that is
located at a specific positionon on a
specific chromosome.
 Any change in gene (and allele)
frequencies within a population or species
is Evolution
 Allele Frequency – proportion of gene
copies in a population of a given allele
1. Natural Selection:
 Affects variation in a population as
the better adapted (more fit)
individuals to their environment
survive and reproduce, passing on
their genes to the successive
generations increasing the
frequency of favourable alleles in
the population.
 Nature “selects” which organisms
will be successful
 Imagine that green beetles are
easier for birds to spot (and
hence, eat). Brown beetles are a
little more likely to survive to
produce offspring. They pass
their genes for brown
coloration on to their
offspring. So in the next
generation, brown beetles are
more common than in the
previous generation.
Natural Selection
Dark Pepper Moths
 http://www.youtube.com/watch?v=LyRA8
07djLc&feature=related
4 Steps of Natural
Selection:
 1. In nature , more offspring are
produced than can survive.
 2. In any population, individuals have
variation.
 3. Individuals with advantageous
variations survive and pass on their
variations to the next generation.
 4. Overtime, offspring with certain
advantageous variations make up
most of the population
2. Gene Flow:
 Is the movement of alleles into or
out of a population (immigration or
emigration).
 Gene flow can introduce new
alleles into a gene pool or can
change allele frequencies.
 The overall effect of gene flow is
to counteract natural selection by
creating less differences between
populations.
 Example:
 Plant pollen being blown into a
new area
 Gene flow is what happens when two or
more populations interbreed. This
generally increases genetic diversity.
Imagine two populations of squirrels on
opposite sides of a river. The squirrels on
the west side have bushier tails than those
on the east side as a result of three
different genes that code for tail
bushiness. If a tree falls over the river and
the squirrels are able to scamper across
it to mate with the other population, gene
flow occurs. The next generation of
squirrels on the east side may have more
bushy tails than those in the previous
generation, and west side squirrels might
have fewer bushy tails.
Gene Flow
Some individuals from a population of brown beetles might
have joined a population of green beetles.
That would make the genes for brown beetles more frequent
in the green beetle population.
3. Genetic Drift
 The change in allele frequencies as a
result of chance processes.
 These changes are much more pronounced
in small populations.
 Directly related to the population
numbers.
 Smaller population sizes are more
susceptible to genetic drift than larger
populations because there is a greater
chance that a rare allele will be lost.
 Imagine that in one generation, two brown
beetles happened to have four offspring
survive to reproduce. Several green
beetles were killed when someone stepped
on them and had no offspring. The next
generation would have a few more brown
beetles than the previous generation—but
just by chance. These chance changes
from generation to generation are known
as genetic drift.
 In a population of 100 bears,
suppose there are two alleles for
fur color: A1 (black) and A2 (brown).
A1 has a frequency of .9, A2 a
frequency of .1 (1.0 = 100%). The
number of individuals carrying A2 is
very small compared to the number
of individuals carrying A1, and if
only fifty percent of the population
survives to breed that year, there's a
good chance that the A2s will be
wiped out.
Examples of Genetic Drift
 A) The Founder Effect:
A founder effect occurs when a new colony
is started by a few members of original
population.
 Small population that branches off
from a larger one may or may not be
genetically representative of the
larger population from which it was
derived.
 Only a fraction of the total genetic
diversity of the original gene pool is
represented in these few individuals.
 For example, the Afrikaner
population of Dutch settlers in
South Africa is descended mainly
from a few colonists. Today, the
Afrikaner population has an
unusually high frequency of the
gene that causes Huntington’s
disease, because those original
Dutch colonists just happened to
carry that gene with unusually high
frequency. This effect is easy to
recognize in genetic diseases, but of
course, the frequencies of all sorts
of genes are affected by founder
events.
Examples of Genetic Drift
 B)
Population Bottleneck:
 Occurs when a population undergoes an
event in which a significant percentage of a
population or species is killed or otherwise
prevented from reproducing.
•The event may
eliminate alleles
entirely or also
cause other
alleles to be overrepresented in a
gene pool.
Bottleneck = any kind of event that reduces the population
significantly..... earthquake....flood.....disease.....etc.…
 An example of a bottleneck: Northern elephant
seals have reduced genetic variation probably
because of a population bottleneck humans
inflicted on them in the 1890s. Hunting reduced
their population size to as few as 20 individuals at
the end of the 19th century. Their population has
since rebounded to over 30,000 but their genes
still carry the marks of this bottleneck. They have
much less genetic variation than a population of
southern elephant seals that was not so intensely
hunted.
4. Mutations
 Are inheritable changes in the genotype.
 Provide the variation that can be acted
upon by natural selection.
 Mutations provide the raw material on
which natural selection can act.
 Only source of additional genetic material
and new alleles.
 Can be neutral, harmful or beneficial( give
an individual a better chance for survival).
 Antibiotic resistance in bacteria is one
form.
 Mutation is a change in DNA the hereditary
material of life. An organism’s DNA affects
how it looks, how it behaves, and its
physiology—all aspects of its life. So a
change in an organism’s DNA can cause
changes in all aspects of its life.
 Somatic mutations occur in nonreproductive cells and won’t be passed
onto offspring.
 For example, the golden color on half of
this Red Delicious apple was caused by a
somatic mutation. The seeds of this apple
do not carry the mutation.
 The only mutations that matter
to large-scale evolution are
those that can be passed on to
offspring. These occur in
reproductive cells like eggs
and sperm and are called germ
line mutations.
 A single germ line mutation can
have a range of effects:
1. No change occurs in
phenotype.
2. Small change occurs in phenotype.
3. Big change occurs in phenotype. Some really important
resistance in insects are sometimes caused by single
mutations1. A single mutation can
also have strong negative effects for the organism.
Mutations that cause the death
of an organism are called lethals
—and it doesn't get more negative than that.
Causes of Mutations
 DNA fails to copy accurately.
 External influences can create
mutations.
 Mutations can also be caused by
exposure to specific chemicals or
radiation.
5. Non-Random Mating
 In animals, non-random mating can
change allele frequencies as the
choice of mates is often an
important part of behavior.
 Many plants self-pollinate, which is
also a form of non-random mating
(inbreeding).
Sexual reproduction results
in variation of traits in offspring
as a result of crossing over in
meiosis and mutations
Genetic shuffling is a source
of variation.
Sexual selection occurs when certain
traits increase mating success.
There are two types of
sexual selection.
– intrasexual selection: competition among males
https://www.youtube.com/watch?v=Ez7RUSCUhzk
– intersexual selection: males display certain traits to
females
https://www.youtube.com/watch?v=vR6gVWPke1Q
Modes of Selection
Stabilizing Selection, extreme varieties from both
ends of the frequency distribution are eliminated.
The frequency distribution looks exactly as it did in
the generation before
Directional Selection - individuals at one end of the
distribution of beak sizes do especially well, and so
the frequency distribution of the trait in the
subsequent generation is shifted from where it was
in the parental generation
Diversifying (disruptive) Selection - both extremes are
favored at the expense of intermediate varieties. This is
uncommon, but of theoretical interest because it
suggests a mechanism for species formation without
geographic isolation
The Hardy-Weinberg
Theorem
• Used to describe a non-evolving
population.
• Shuffling of alleles by meiosis and
random fertilization have no effect on
the overall gene pool.
• Natural populations are NOT expected
to actually be in Hardy-Weinberg
equilibrium.
42
The Hardy-Weinberg
Theorem
•Deviation from Hardy-Weinberg
equilibrium usually results in
evolution
•Understanding a non-evolving
population, helps us to
understand how evolution
occurs
43
Hardy-Weinberg Theorem
1. Large population. The population must be large to minimize random
sampling errors.
2. Random mating. There is no mating preference. For example an AA male
does not prefer an aa female.
3. No mutation. The alleles must not change.
4. No migration. Exchange of genes between the population and another
population must not occur.
5. No natural selection. Natural selection must not favor any particular
individual.
Hardy-Weinberg Theorem
 In each cell, chromosomes come in pairs, so the
possible combinations are AA, Aa, and aa.
 Since A is dominant, people with AA or Aa in their
cells will be right-handed.
 AA or Aa - dominant phenotype. (Aa - carriers of
recessive trait. AA - non-carriers).
 aa - recessive phenotype (left-handed people).
 Homozygote is an organism with pair of identical
alleles (in this case AA or aa).
 Heterozygote is an organism with two different
alleles (Aa).
Hardy-Weinberg Theorem







p = frequency of A
q = frequency of a
Note: p+q = 1 = 100% total number of alleles.
p² = frequency of AA
q² = frequency of aa
2pq = frequency of Aa because
Note: p² + 2pq + q² = 1 100% total number of
cases.
Examples:
1. Suppose there are 30 students in a class and 3 are lefthanded. How many right handed students carry a
recessive allele?
Solution: The frequency of aa is 3/30 students.
So, q² = 3/30 = 0.1. Hence, q = .316.
From that we get that p = 1 - q = .684.
To find how many students carry a recessive allele, we need to
find the frequency of recessive allele 2pq. 2pq = 2 x .684 x
.316 = .432 = 43.2%
So, the number of right-handed students who carry a recessive
allele is 43.2% of 30 =.432 x 30 = 13 students.
Note: The frequency of right-handed students (AA or Aa) is
27/30 = 0.9 = p² +2pq = 1 - q².
2. Experiments have been performed in which
you find that 41% of the population displays the
recessive trait. Determine the percentage of
the population that are carriers of the
recessive trait? That are homozygous?
3. If the frequency of the recessive allele is .32,
what percentage of population display the
dominant phenotype?