Transcript Mechanisms of Evolution

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 that are located at specific
positions on a specific chromosome
• Allele - alternative form of a gene (one
member of a pair).
• 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,
how many times does a gene occur
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 favorable 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:
• 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
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
Examples of Genetic Drift
• 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
http://www.nytimes.com/1985/09/17/science/loss-of-gene-diversity-is-threat-to-cheetahs.htm
l
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.
 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:
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 mutations1.
A single mutation can also have strong
negative effects for the organism.
Mutations that cause the death of an
organism are called lethal — 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