Download Lesson 15c1 Genetic Variation PPT

Genetic Variation (c)(15)(C)
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The combination of
meiosis and fertilization
results in each offspring
having one copy of each
gene from each parent.
Consequently, children
tend to resemble their
parents and their siblings.
However, meiosis results
in genetically diverse
sperm and eggs which,
together with random
fertilization, results in
genetic diversity of the
zygotes and children
produced by the same
mother and father.
Variation and Evolution
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Give a definition of Variation
Variation is the range of differences that
there are between individual organisms.
Variation and Evolution
Variation can be within species
(Think of all the differences between individual humans)
These are different varieties of the same species
Variation and Evolution
Or between species:
Gorilla
Proboscis monkey
Variation and Evolution
Variation can be
CONTINUOUS,
have any value between two
extremes.
Examples:
Height of human
Body mass of cats
Trumpet length of daffodils
Leaf width of shrub
Length of bacteria
All these features
show normal
distribution (see graph)
Variation and Evolution
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Variation can be
DISCONTINUOUS , in this
case there are only a few
possible categories that the
characteristic can fall into.
EG:
Flower color in a single species
Human blood group or
ear lobe type
Bacteria that are or are not
resistant to an antibiotic
Variation and Evolution
Genetic variation
Each tomato is different because it
comes from a plant with different
alleles, that code for different
characteristics
Environmental variation
These plants are genetically identical,
the flower color difference is due to
the effects of soil pH.
Variation and Evolution
GENETIC
Differences that are due
to the DNA inside the
cells of the organism
rather than the effect
that its surroundings
have on it.
ENVIRONMENTAL
Differences that are due
to the external
environment. For
example availability
of water, nutrients,
light, prevailing
winds.
Meiosis creates genetic variation
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During normal cell growth, mitosis produces
daughter cells that are identical to parent cell.
Meiosis results in genetic variation by mixing of
maternal and paternal chromosomes and crossing
over.
No daughter cells formed during meiosis are
genetically identical to either mother or father.
During sexual reproduction, fusion of the
unique haploid gametes produces truly unique
offspring.
Why do cells divide by
Meiosis?
To reduce
chromosome number
Allows for
Chromosome
Variety
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N egg
N
Sperm
2N baby
Meiosis is how haploid gametes
are produced
Mitosis = ASEXUAL reproduction
Meiosis = SEXUAL reproduction
Each gamete produced by meiosis is
genetically unique.
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The Origins of Genetic Variation
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Offspring of sexual reproduction are genetically
different from their parents and from one another.
Independent Assortment
of Chromosomes
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In independent assortment, every chromosome pair
orients independently of the others during meiosis.
Random Fertilization
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The human egg cell is fertilized randomly by one
sperm, leading to genetic variety in the zygote.
Crossing Over
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In crossing over,
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Homologous chromosomes exchange genetic information.
Genetic recombination occurs.
How Accidents During Meiosis
Can Alter Chromosome Number
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In nondisjunction,
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The members of a chromosome pair fail to separate during
anaphase.
Gametes with an incorrect number of chromosomes are
produced.
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The result of nondisjunction
Down Syndrome
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Down Syndrome
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Is a condition where an individual has an extra
chromosome 21.
Is also called trisomy 21.
Evolution Connection
New Species from Errors
in Cell Division
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Errors in meiosis may have been instrumental in the
evolution of many species.
Polyploids
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Are new species.
Have more than two sets of homologous chromosomes in
each somatic cell.
The Gene Pool
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Members of a species
can interbreed & produce
fertile offspring
Species have a shared
gene pool
Gene pool – all of the
alleles of all individuals
in a population
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The Gene Pool
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Different species do
NOT exchange
genes by
interbreeding
Different species
that interbreed often
produce sterile or
less viable offspring
e.g. Mule
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Adaptations
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Many of the differences between species are
present because they help the individuals to
survive.
We call these differences ADAPTATIONS.
You should understand that there are Adaptations
to:
ANATOMY or body FORM,
PHYSIOLOGY or body FUNCTION
BEHAVIOUR
Variation leads to ?
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Explain the consequences of the four observations made by
Darwin in proposing his theory of natural selection.
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What are the 4 observations that Darwin made?
1. Offspring generally appear similar to parents.
2. No two individuals are identical. (Why not?)
3. organisms have the ability to produce large numbers of
offspring
4. Populations in nature tend to remain fairly stable in
size.
SO what are the consequences? Evolution
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Variation and Evolution
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Because individuals over produce, yet populations
remain stable COMPETITION must take place.
In COMPETITION there are “winners” and
“losers”. Winners are better adapted with more
useful characteristics, they are more able to survive
and breed, so pass on their beneficial alleles.
Over time when this happens continually,
accumulated changes can give rise to a new
species.
Variation and Evolution
Outline how variation, adaptation and
selection are major components of evolution.
Differences in organisms
How do they arise
Where do these changes happen
How can they be passed on
Modern Synthesis Theory
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Combines Darwinian
selection and
Mendelian inheritance
Population genetics study of genetic
variation within a
population
Emphasis on
quantitative characters
(height, size …)
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Natural Selection
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Variation in organisms characteristics are caused by the
organisms having different alleles of genes.
Some differences enable the organism to survive better
(compete more successfully)
The ones with beneficial alleles survive, breed and pass on
their alleles to the next generation
Those without beneficial alleles die before they reproduce,
so their alleles are less likely to be passed on.
The beneficial alleles increase in frequency in the population
and may eventually produce a a new species.
Modern Synthesis Theory
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TODAY’S theory on evolution
Recognizes that GENES are responsible for
the inheritance of characteristics
Recognizes that POPULATIONS, not
individuals, evolve due to natural selection &
genetic drift
Recognizes that SPECIATION usually is due
to the gradual accumulation of small genetic
changes
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Microevolution
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Changes occur in gene pools due to
mutation, natural selection, genetic drift, etc.
Gene pool changes cause more
VARIATION in individuals in the
population
This process is called MICROEVOLUTION
Example: Bacteria becoming unaffected by
antibiotics (resistant)
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Selective Pressures
Competition for food, water
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Predation
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Disease
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Physical and chemical factors
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Competition for mates
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Competition for space, nesting sites, territory etc
An allele that helps an organism compete better is “selected
for”, increases in frequency
An allele that fails to help an organism survive is usually
“selected against”, decreases in frequency
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Natural Selection
Darwin proposed that Natural
Selection could lead to new
species being produced over
time.
This was his explanation for
the large number of different
species of finch found in the
Galapagos Islands.
Each species has adaptations
to its body, beak and feet that
allow it to feed on specific
types of food in specific areas.
Evolution of a new species
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Speciation occurs when there is a
“Reproductive Barrier”, some thing that stops
members of an original population passing
their alleles freely among all members, this
prevents free gene flow. The barrier may be a
geographical barrow = Allopatric speciation.
The barrier may be behavioural, biochemical,
or anatomical = sympatric speciation.
Allopatric Speciation
Sympatric Speciation
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When there is NO geographical barrier
Initially individuals in the original species
may have variations that can be divisive of
the population.
E.g. in birds some might have slightly:
different shaped beaks, shorter wings, better
nocturnal vision, different color plumage or
courting behavior, etc
Speciation in the Genus
CANIS
Evidence for Evolution in brief
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Fossil record – simplest organisms in oldest rocks, old
species extinct, new species develop. Reptile/bird link, horse
evolution, human evolution.
Biochemical – Similar biochemistry of all living things,
closely related species have similar biochemistry, the longer
ago they diverged the more different the biochemistry.
Amino acid sequences in cytochrome c (used in respiration)
Regulation of DNA and RNA production.
DNA differences are greater between species which are not
closely related.
Darwin’s 4 Observations
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Offspring generally appear similar to their parents.
No two individuals are identical
Organisms have the ability to
produce large numbers
of offspring
Populations in nature
tend to remain fairly stable in size.
Darwin’s Evidence
(From fossils)
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In the past, there were species
different from those present
today.
Old species died out.
New species have arisen.
The new species are often similar to the older
ones in the same place.........
Armadillos and Glyptodonts
Darwin felt more modern
species had variations that
meant they were
Better adapted to
the environment.
In Summary: Causes for Genetic
Variation and Evolution
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Genetic Drift
- the change in the gene pool
of a small population due to chance
Natural Selection
- success in reproduction based on heritable traits
results in selected alleles being passed to relatively
more offspring (Darwinian inheritance)
- Cause ADAPTATION of Populations
Gene Flow
-is genetic exchange due to the migration of fertile
individuals or gametes between populations
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Causes Continue
 Crossing Over
 Exchange of segments of chromosomes
during meiosis
 Mutations
 a change in an organism’s DNA
 Mutations can be transmitted in gametes to
offspring
 Non-random mating
 - Mates are chosen on
 the basis of the best traits
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