Classification, Selection and Evolution I

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Classification, Selection and Evolution I
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Learning outcomes         Explain how variation
is produced in sexually producing
organisms         Explain why variation caused
by genes can be inherited but variation caused by
the environment cannot         Explain how all
organisms can potentially overproduce        
Describe how different selection pressures may
act on individual organisms with different
alleles, so affecting the frequencies in
population         Explain, with examples, how
environmental factors can act as stabilising or
evolutionary forces of natural selection
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Sexual Reproduction 1.    Independent assortment
of chromosomes (and alleles) during meiosis 2.   
Random mating within a species is it? 3.   
Crossing over between chromatids of homologous
chromosomes 4.    Random fertilisation of
gametes Mutation
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Phenotypic variation 1 to 4 reshuffle existing
alleles to produce new, unique combinations new
phenotypes.  Gene Mutation Can create completely
new alleles new genotypes. Mutations are most
often recessive so only cause new phenotypes
generations later when two descendants of the
original mutant interbreed.  Mutation in somatic
cell ? usually no effect as only one cell, which
will be destroyed by bodys defences. If mutation
affects cell division ? tumour, a rapidly
dividing lump of cells. Benign tumours usually
cause no harm (possible blockage) Malignant
tumours can spread to other parts of the body and
are cancers. Mutations in cells, which form
gametes, will be passed on to future generations
if the gamete takes part in fertilisation. Environ
mental differences cause variation but these are
not passed on to offspring.
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Overproduction All living organisms can produce
many young in a lifetime Population is usually
held in check by Biotic factors, such as
predators, competition for food, disease
or Abiotic factors, such as water supply,
nutrient availability. If a new phenotype is
introduced into an environment, growth may be
unchecked by natural environmental factors and
numbers increase exponentially. (e.g. rabbits in
Australia had plentiful food and few predators)
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Natural Selection If many offspring are born but
few can survive which will live and breed and
which will die? Luck plays a part but if there is
variation in the population, the individuals with
any survival advantage will pass their genes on
to their offspring. E.g. white
rabbits.   Selection pressures affect which
phenotypes live or die and which alleles
inherited by offspring. Natural selection is the
effect of selection pressures on the frequency of
alleles in the population.
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Evolution Stabilising selection keeps thing the
way they are best adapted for a specific
environment. Directional selection causes change
due to a new allele or new selection
pressure. Evolution is the result of directional
selection causing some alleles to have a better
chance of survival than others. Over many
generations populations may change.  Why does
evolution happen faster in bacteria than in
animals?  Antibiotic resistance in
bacteria Single allele determines
phenotype Variation of resistance Transfer of
plasmids New selection pressure
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Biston betularia peppered moth
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