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Reasons for variation: Random assortment

Random assortment of chromosomes during meiosis = large number of possible combinations of chromosomes.


Variation: Crossing over

Of chromatids during meiosis = parts of chromatids attaching to different chromatids - changed sequence of alleles.


Variation: Non-disjunction

1 or more members of chromosomes pair fail to separate during meiosis.


Variation: Random fertilisation.

Infinite numbers of possible combinations of alleles in the offspring.


Variation: Mutations

Permanent changes in the DNA of a chromosome = totally new characteristic.

Changing the allele frequency.


Natural selection?

Process by which species become adapted to its environment - selection of favourable, strong genes

Having a survival advantage, passing their characteristics onto future generations


Random genetic drift?

Random, non-directional variation in allele frequencies
Due to chance events
Most powerful in small, isolated populations
Religious groups


Example of random genetic drift

- USA, originally from Germany
- no marriage outside their group, no interbreeding (religious beliefs)


Founder effect

When a small group moves away from homeland, establishing a new community that tends to expand

Their allele frequencies do not reflect the gene pool of the original population



Gene flow = movement from one population to another

If immigrants to certain country bring alleles that aren't already in the population, frequency of alleles of that gene are altered


Two main barriers to gene flow?

Geographical - oceans, mountain ranges, deserts
Sociocultural - language, education and social/economic status


Genetic disease?

Allele causing inherited disease (fatal) would eventually fade from population as those with the allele die and therefore don't pass on their gene.


Examples of genetic diseases: Tay-Sachs

Hereditary disease, occurring most frequently in those with Jewish descent
Worldwide: low, 1 : 500,000
Ashkenazi Jews: high, 1 : 2,500


Tuberculosis and Tay-Sachs

Heterozygous carriers of Tay-Sachs have an increased resistance to tuberculosis
As of discrimination, Jews were found in ghettos (isolated) which therefore began to decrease the spread or the disease as heterozygous carriers survived and reproduced


Examples of genetic diseases: Thalassaemia

Anaemia resulting from defects to haemoglobin, high frequency in people of a Mediterranean descent

Only marrying each other, isolating themselves


Examples of genetic diseases: Sickle cell anaemia

Mainly occurs in those of black African ancestry
Results in sickle shaped RBC's
Unable to carry as much O2 so they stick and clot together


What's the difference when you only have the sickle cell trait?

Mild sickling
Has a certain advantage: providing resistance to malaria


What is evolution?

Gradual change in the characteristics of a species overtime


Darwin's theory: based on three observations

1. Variation - variation within a species, but similar characteristics within a family

2. Birth rate - all living organisms reproduce at a greater rate than their available food supply (overcrowding)

3. Nature's balance - although birth rate is high, number of species remain relatively similar (survival of the fittest, struggle for existence)


Struggle for existence and survival of the fittest

Darwin concluded their must be a struggle for existence and those with characteristics best suited are more likely to survive (survival of the fittest)


Examples of natural selection in humans:

Short body, long limbs
Long body, short limbs

Long body, short limb: smaller surface area, smaller heat loss in cold environments (Inuits)

Short body, long limb: increased surface area, larger heat loss in hot environments (Black Africans)


Examples of natural selection in humans:

Sickle-cell anaemia

People with the sickle cell trait have a resistance to malaria
They survive, reproduce, therefore their characteristics are passed on


Speciation steps

1. Variation
2. Isolation
3. Selection
4. Speciation


Speciation steps 1. Variation

Range of variation (mix) in an ordinary population
Sharing common gene pool


Speciation steps 2. Isolation

Barrier is formed (e.g. geographical)
Divides population in half, no interbreeding
Forming a separate gene pool


Speciation steps 3. Selection pressures

Change in gene frequencies
Leads to the evolution of separate subspecies


Speciation steps 4. Speciation

Changes in gene frequencies may be great enough to prevent interbreeding between the two populations
Two species now exist


Gene flow?

Whole bunch of genes come in, changing the make up of a population


6 steps of evolution through natural selection:

1. Variation within species.
2. Ability to reproduce then they can raise to maturity.
3. Excessive birth rate/limited resources ➡️ struggle for existence, survival of the fittest.
4. Individuals with suited characteristics survive better ➡️ survival of the fittest.
5. Favourable characteristics are passed down.
6. Proportion of alleles that produce favourable characteristics gradually increases.


Summary of natural selection, 6 steps (remember)

Variation within a species
More offspring produced than what can survive
Excessive birth rate, limited resources -> struggle for existence
Individuals with favourable characteristics pass on their traits to future generations -> survival of the fittest
Favourable characteristics increase in a gene pool