Download Note 21

South Tuen Mun Government Secondary School
Biology Revision Note 21
EVOLUTION
(I) Evidence of evolution
 Fossil record : fossils (the preserved remains or traces of the living organisms in the past) were found in
the sedimentary rock. Deeper sedimentary rock provided fossil records which showed organisms living at
an earlier age as compared to those at higher layers. The records showed that a series of changes from
simple life forms into more complex life forms, suggesting that living organisms evolved over millions of
year.
 Limitation of fossil record – there are gaps (no fossil found) between intermediate forms of life, called
missing link. This is because
 fossil cannot be formed when dead bodies decay before fossil formation; the environmental condition
is not suitable; soft parts were difficult to form fossil.

fossil was located in inaccessible area or was damaged.
 DNA sequencing – the more similar the base sequence of DNA, the more closer two species are related
(II) Theory of evolution – Charles Darwin
 In a population, there is always genetic variation among different individuals. As they have genetic
variation, they show variation in their phenotypes. [Note : genetic variation may arise from independent
assortment, crossover during meiosis, random fertilization and mutation; only mutation can give new
alleles in a population.]
 In a natural habitat, the resources are limited. Therefore, different individuals of the same species compete
for resources. This is known as struggle for existence.
 As some individuals can gain more resources in the habitat, they possess phenotypes that can survive
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better, they are said to have favourable / advantageous variation; while those that cannot survive are said to
have unfavourable / disadvantageous variation. This is known as survival of the fittest.
The natural environment selects those phenotype which can survive better. The individuals with these
favourable variation can grow to maturity and reproduce. They pass their genes to the next generation.
This is known as natural selection.
After millions years of changes in the natural environment and natural selection, living organisms evolve
into individuals that are different from their ancestors.
An example of natural selection :
Bacteria A is a species that cause a disease to human. It is killed by an antibiotic X. Due to mutation, a few
individuals of A develops an antibiotic resistant allele (the allele can make a gene product which prevent
the bacteria from being killed by the antibiotic X).
When a person is infected by a population of bacteria A, the doctor applies antibiotic X. The antibiotic X
provides a natural environment to select individuals of bacteria A that have the antibiotic resistant allele.
They cannot be killed by antibiotic X. They survive better, grow to maturity and reproduce. They pass the
favourable genetic variation to the next generation. Eventually the whole population of bacteria A is
antibiotic resistant and thus antibiotic X cannot cure the disease.
Speciation
 A species is a population of individuals that can interbreed to produce fertile offspring.
 When a population of a species is isolated into two groups, the two groups do not interbreed with each
other. There is no gene flow between them.

The two groups may undergo different mutations and different natural selections at different times.
Eventually the two groups become so different that they are not able to interbreed. Two species are
formed.
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Isolation mechanisms include
 geographical isolation – the two groups do not interbreed due to a geographical barrier e.g. river, hill
etc.
 ecological isolation – the two groups do not interbreed because they live in a different region of the
same habitat.
 seasonal isolation – the two groups do not interbreed because their sex organs mature at different
seasons.
 behavioural isolation – the two groups do not interbreed because they have different courtship
behaviour.
 mechanical isolation – the two groups do not interbreed because their sex organs are incompatible.
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postmating isolation – the two groups have no gene flow because their offspring formed is not viable
or is not fertile.