Download Genetics - Natural Selection File

Population genetics
 Species: a group of individuals that are more or less alike
and are able to interbreed under natural conditions to
produce fertile offspring.
 Population: is a group of individuals of the same species
living together in a particular place at a particular time.
 Gene pool: all the genotypes of individuals capable of
reproducing in a population are pooled together. This is
referred to as gene pool- a collection of all their genotypes.
Population genetics
 Allele frequency: in studying the gene pool geneticists are
interested in how often each allele of a gene occurs in the
population. These are called allele frequencies of that
population. Allele frequencies are determined by keeping
tabs on frequencies of characteristics.
 Half the alleles from each parent are passed on to the next
generation. How the alleles are passed on in reproduction
can determine the characteristics in future populations.
 We can now look at the process of natural selection in
terms of frequencies of alleles in the gene pool of a
population.
 Within a particular gene pool, the frequencies of the
alleles have changed over time.
Darwin’s theory
 Not all individuals in a population survive to maturity to
reproduce and pass on their genetic code.
 The birth rate exceeds the survival rate
 Reason: there is a ‘struggle for existence’ (DARWIN)
 Factors affecting survival:
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Climate
Availability of resources: food, water, shelter
Competition with other animals
Predation & disease
Characteristics of the organism, itself: - some individuals have
better resistance to disease, strength, speed etc.
Chance
Struggle of the fittest
 ‘Struggle of the fittest’ refers to organisms who have
alleles that are adapted to a change in the environment.
Fittest refers to reproductive fitness; the ability to
reproduce offspring with alleles that allow them to survive.
 Changing environments put ‘selective pressure’ on
populations, resulting in altered gene frequencies (i.e.
evolution).
Speciation – formation of a new species
 The formation of a new species results from the
effects of:
1)
2)
3)
variation
Isolation
Natural selection
Speciation – formation of a new species
1. VARIATION
 Variation refers to the genetic differences between
individuals.
Causes of variation:
 Sexual reproduction produces new combinations of genes:
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Independent assortment of genes into gamete during meiosis
Crossing over during meiosis
Random mating
Chromosomal errors
Mutation produces new alleles of existing genes.
Speciation – formation of a new species
2. ISOLATION
 Many barriers prevent breeding between distinct groups of
organisms.
 Amongst humans barriers may be:
 Geographical:
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distance/mountains/swamps/ocean/desert
 Socio-cultural:
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social position
laws
language
religion
Speciation – formation of a new species
 If exchange of genetic information (gene flow) is restricted
between adjacent groups, then populations will be
separated into different breeding groups. Their gene pools
become separated.
 In time the populations may become so different in
behaviour, physical appearance or functioning that
interbreeding becomes impossible.
Speciation – formation of a new species
3. NATURAL SELECTION
 Natural selection acts on the variation present in a
population
 Isolated groups will be subject to different environmental
stresses.
 Favourable genotypes tend to be selected while
unfavourable genotypes tend to be eliminated resulting in a
change in the frequency of some genes in the gene pool for
a population. It is said that the population has adapted to a
change in the environment by changing the frequency of
alleles in the gene pool.
Speciation – formation of a new species
 This will result in different variations becoming common
in different regions, depending on the environment. (e.g.
cold climates tend to favour the survival of short, stocky
people ENDOMORPHIC body shape (small surface area to
volume- better able to retain body heat)
 People living in hot climates often have a tall, slender
body form ECTOMORPHIC body shape
(large Surface area to volumebetter able to lose heat).
Speciation – formation of a new species
 One of the best examples of natural selection is the English
Peppered Moth. This moth is whitish with black speckles
and spots all over its wings. During the daytime, Peppered
moths are well-camouflaged as they rest on the speckled
lichens on tree trunks. Occasionally a very few moths have
a genetic mutation which causes them to be all black.
Black moths resting on light-coloured, speckled lichens are
not very well camouflaged, and so are easy prey for any
moth-eating birds that happen by. So, they never get to
reproduce and pass on their genes for black colour.
 With time, nature would select the white to survive
and reproduce.
Speciation – formation of a new species
 However, with the Industrial Revolution, many factories
and homes in British cities started burning coal. Coal
creates a lot of black soot and pollution. Since lichens are
extremely sensitive to air pollution, this caused all the
lichens on city trees to die. Also, as the soot settled out
everywhere, this turned the city tree trunks black.
 This enabled the occasional black moths living in the cities
to be well-camouflaged so they could live long enough to
reproduce, while the “normal” speckled moths were
gobbled up.