Transcript File - hs science @ cchs

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 position 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=LyRA
807djLc&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.
The Founder effect
Island
The
Mainlan
d
The Founder effect
Population
Island
The
Mainland
The Founder effect
The
Mainland
A few
individuals
colonise a
new isolated
area
There may be a
higher frequency of
one allele in the
founder population
just by chance
This allele needn’t have
been very common in the
original population
The island population
grows
Island
…after a few generations
The green allele may
be lost completely if
individuals fail to
leave offspring
carrying it
Island
…after a few generations
The green allele may
be lost completely if
individuals fail to
leave offspring
carrying it
Island
…after a few generations
Mutations
may occur
creating
new
alleles
Island
…after a few generations
The new
allele
becomes
more
common
Island
The 2 populations now
look very different!
The
Mainland
Island
Some examples of the
founder effect in action…
The Amish people,
Pennsylvania
The Fugates of Kentucky
The Amish People

200 in founding population

Within community marriages

Recessive conditions are common

Haemophilia

Dwarfism (1/14 carry the gene)

Still births/infant deaths

Physical deformaties
‘The Royal Disease’

Haemophilia
The Tsars
The Fugates

Small founding population

Mountain communities

2 of the founders were carriers
of a recessive allele

Blue skin!
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.
EX. Cheetahs
Cheetahs

10,000 years ago all but 1 species had died out

Severely threatened

Very low genetic diversity resulting in poor sperm
quality among males

Females forced to breed with close relatives

Inbreeding generally decreases
the fitness of a population
(an inbreeding depression)
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.
Similarities and differences between the
founder effect & bottlenecking
Similarities
Both are followed by genetic drift
which results in changes in allele
frequencies
Initially genetic diversity is lost in both
systems
Both involve a small number of
individuals breeding with each
other, both may involve inbreeding
among close relatives
Both may result in a new population
which carries alleles that are unlikely
to be a true representation of the
original group
Differences
In Bottlenecks individuals are killed,
reducing the choice of mates, in the
founder effect individuals are
ecologically separated
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 non-reproductive
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 phenotypic changes, like DDT
resistance in insects are sometimes caused by single
mutations.
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
behaviour.
 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
–
intersexual selection: males display certain traits to
females