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SPECIATION
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http://en.wikipedia.org/wiki/Image:Speciation_modes.svg
Geographic illustration of SPATIAL ASPECTS OF SPECIATION:
allopatric speciation - physical barrier divides population (Geographic isolation)
peripatric speciation - small founding population enters isolated niche
parapatric speciation - new niche found adjacent to original one
sympatric speciation - speciation occurs without physical separation
SYMPATRIC SPECIATION
Sympatric speciation does not require large-scale geographic distance to reduce gene flow between parts of a
population. Ecological factors in a sympatric population can cause speciation. Merely exploiting a new niche
may automatically reduce gene flow with individuals exploiting the other niche. Since they do not interbreed,
over time, due to mutations and genetic drift, their allele frequencies would change and each population evolve
into different species. There would be differences in their physical structure and behaviour which leads to
reproductive isolation. ie. they cannot recognise each other and do not interbreed.
ALLPOPATRIC SPECIATION
If a population is split geographically over a long period of time, they evolve into two different species. Each
population is subjected to different selection pressures and due to mutations and genetic drift, allele frequencies
might change which give rise to changes in genotypes and phenotypes.
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APOPTOSIS -Apoptosis is the term given when programmed cell death (PCD) occurs in multicellular
organisms
Why should a cell commit suicide?
1. Programmed cell death is as needed for proper development as mitosis is.
Examples:
The resorption of the tadpole tail at the time of its metamorphosis into a frog occurs by apoptosis.
2. The formation of the fingers and toes of the fetus requires the removal, by apoptosis, of the tissue between
them.
3. The sloughing off of the inner lining of the uterus at the start of menstruation occurs by apoptosis.
4. The formation of the proper connections between neurons in the brain requires that surplus cells be
eliminated by apoptosis
2. Programmed cell death is needed to destroy cells that represent a threat to the organism.
Cells infected with viruses -One of the methods by which cytotoxic T lymphocytes (CTLs) kill virus-infected
cells is by inducing apoptosis
Cells of the immune system- As cell-mediated immune responses wane, the effector cells must be removed to
prevent them from attacking body constituents. CTLs induce apoptosis in each other and even in themselves.
Defects in the apoptotic machinery is associated with autoimmune diseases such as lupus and rheumatoid
arthritis.
Cells with DNA damage -Damage to its genome can cause a cell to disrupt proper embryonic development
leading to birth defects to become cancerous.Cells respond to DNA damage by increasing their production of
inducer of apoptosis.
Cancer cells -Radiation and chemicals used in cancer therapy induce apoptosis in some types of cancer cells.
What makes a cell decide to commit suicide?
The balance between: the withdrawal of positive signals; that is, signals needed for continued survival, and
the receipt of negative signals.
Withdrawal of positive signals-The continued survival of most cells requires that they receive continuous
stimulation from other cells and, for many, continued adhesion to the surface on which they are growing.
Examples of positive signals:
growth factors for neurons
Interleukin-2 (IL-2), an essential factor
for the mitosis of lymphocytes
Examples of negative signals
increased levels of oxidants
damage to DNA by these oxidants
ultraviolet light
x-rays
chemotherapeutic drugs
Process of apoptosis
1.External signals are received by death
receptors located in the cell membrane.
2. Apoptosis can also be initiated following
intrinsic signals that are produced following
cellular stress which may occur from exposure
to radiation or chemicals or to viral infection.
3. Intrinsic signals initiate apoptosis via the
involvement of the mitochondria. Mitochondria
play an important role in amplifying the apoptotic signalling from the death receptors. They initiate the
formation of caspase enzyme formation.These enzymes breakdown cellular component and also activate other
degradative enzymes such as DNases, which begin to cleave the DNA in the nucleus.
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4. The cell begins to shrink and forms blebs or blister on their membranes. As cells continue to shrink they
package themselves into a form that allows for their removal by phagocytes.
5. Signals sent by apoptotic cells attract phagocytes which migrate to the site and engulf the cell fragments.
Advantage of apoptosis
Apoptosis confers advantages during an organism's life cycle. For instance during the development of
the fetus in the mother, the differentiation of fingers and toes occurs because cells between the
fingers apoptose with the end result that the digits are separate.
As more new cells are made each day by mitotic division, old and damaged cells die dur to apoptosis.
This process maintains equilibrium of cell number. Approximately between 50 billion and 70 billion
cells die each day due to apoptosis in the average human adult.
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Mendelian Genetics: Revision Table of Inheritance
Pattern of inheritance
Expected outcome
Monohybrid heterozygous cross
3:1
Monohybrid test cross
1:1
Codominance
1:2:1
Lethal gene
2:1
Dihybrid heterozygous cross – genes
not linked
9:3:3:1
Dihybrid test cross
1:1:1:1
Dihybrid heterozygous cross – genes
linked
3:1
Dihybrid test cross – genes linked
1:1
Dihybrid test cross – with crossover
high : low : low : high
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Patterns of Inheritance
Autosomal Dominant
Does not skip generations.
Either sex may be affected.
Affected individual must carry at least one dominant
allele.
At least one parent must have the trait
Autosomal Recessive
Can skip generations.
Either sex affected.
Affected individuals are homozygous recessive.
If parents are carriers there is 25% cgance of
producing an affected child
Sex-linked Dominant
Does not skip generations.
Affected males pass on the trait to all daughters.
Affected females pass on the trait to both sexes
Sex-linked Recessive
Can skip generations.
More common in males.
Affected mothers pass trait to all sons.
Only if father has the trait and mother is c carrier one
of the daughters can inherit the trait
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TYPES OF EVOLUTION
different ancestors
DIVERGENT EVPLUTION
CONVERGENT EVOLUTION
PARALLEL EVOLUTION
Divergent Evolution
Divergent evolution occurs when change from a common ancestor increases as time passes, producing distinct
species. The finches from the Galapagos Islands and honeycreepers from the Hawaiian Islands are examples of
the outcome of divergent evolution. The beaks of each of these bird species have evolved with time to be suited
to the foods they eat, beak size being influenced, respectively, by the seeds on the Galapagos Islands, and by the
requirements of nectar gathering on the Hawaiian Islands. The beaks of the species would be considered to be
homologous structures.(Similar structures, different functions)
Convergent Evolution
If two unrelated organisms have similar environments they may evolve similar structures or features that offer
them some survival advantage. For example, consider the dolphin and the shark. One is a mammal and one is
a fish, but both have a similar appearance. They have evolved from very different ancestors, but they have come
to resemble each other because of their common environment. The selection pressures of this environment
produced flippers and fins with a streamlined body shape for rapid movement through the water.
Although their structures are similar (analogous)
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