5.4: Evolution
Evolution is the cumulative change in the heritable
characteristics of a population.
SPECIES AND
POPULATIONS EVOLVE
INDIVIDUALS DO NOT
EVOLVE
5.4.2: Fossil record
A fossil is any physical evidence about a dead organism. Some fossils are only
fragments of bone, teeth or shells. Amber fossils sometimes contain intact
bodies of insects and small amphibians. Rock fossils show complete details of
external structures.
5.4.2: Homologous structures
Structures derived from the same body part of a common
ancestor are called homologous structures. One example of a
homologous structure is the pentadactyl limb, which is an
appendage comprised of five bones.
5.4.2: Selective breeding
Selective breeding is the process used by breeders to
develop a plant or animal over time with desired
characteristics.
5.4.3: Overpopulation
STATE: Populations tend to produce more offspring than the
environment can support
5.4.4: Struggle for survival
Populations tend to produce more offspring than the environment can
support; and this over-production of offspring results in a struggle for
survival.
The ‘fittest’ individuals are those with the best genes; in other words, those
with the most favorable heritable variations. The ‘fittest’ individuals are the
most likely to survive long enough to reproduce and pass on their genes;
and the ‘weakest’ individuals are more likely to die young and not pass on
their genes.
N09/4/BIOLO/SP2/ENG/TZ0/XXExplain how
sexual reproduction can lead to variation in a
species.
[3]
allows characteristics from both parents to appear in offspring;
crossing over (during prophase 1) changes chromosome composition;
produces gametes which are all different;
random chance of which sperm fertilizes ovum;
greater variation (resulting from sexual reproduction) favours survival of species through natural
selection; [3 max]
Accept independent assortment during meiosis from AHL.
M08/4/BIOLO/SP2/ENG/TZ1/XX+ (b) Outline how sexual reproduction can give rise to genetic
variation in a population.
meiosis;
crossing over;
independent assortment;
sexual reproduction/fertilization/recombination;
mutations;
environmental; [2 max]
5.4.5: Variation within a species
STATE: Members of a species show variation
5.4.5: Sexual reproduction
Variation exists between members of one species, which means that some
individuals are better suited for survival than others.
The sources of variation are:
1) mutation, which creates new alleles in the first place;
2) meiosis, which enables each parent to produce millions of different gametes
(each with a unique combination of chromosomes); and
3) sexual reproduction, fertilisation and mate selection
Individuals that are better suited to
changes in the environment
survive and pass on their genes for
surviving
5.4.7: Natural Selection
Deduction 1:
Populations produce more offspring than the environment can support.
Deduction 2:
The over-production of offspring results in a struggle for survival and nature
selects the ‘fittest’ individuals.
Deduction 3:
The ‘fittest’ individuals survive long enough to reproduce and pass on their
genes; the ‘weakest’ individuals die young and fail to pass on their genes.
Thus natural selection leads to the increased reproduction of individuals with
favorable heritable variations.
Charles Darwin
Alfred Wallace
5.4.8: Evolution in Response to Environmental Change
Name of population: The tuberculosis bacterium
Characteristic under evolution: Resistance to the antibiotic Rifampicin
Environmental change: Exposure to an inadequate dose (or inadequate duration)
of the antibiotic
Response 1: When patients receive an inadequate dose (or inadequate duration)
of the antibiotic then some of the bacterial population may survive. Each
bacterium that is killed by the antibiotic has a particular allele that codes for the
particular protein that the antibiotic targets. And conversely, each bacterium that
survives the antibiotic must be lacking the particular allele that codes for the
particular protein that the antibiotic targets.
Response 2: When a surviving bacterium divides it passes on its antibioticresistant allele to its two daughter cells. Having inherited the antibiotic-resistant
allele, the two offspring survive, reproduce and increase the antibiotic-resistance
gene in the population. After several generations the population can become
resistant to the antibiotic.
Natural Selection
DDT Resistance in Anopheline
Mosquitoes
The malarial parasite is spread by
anopheline mosquitoes
The spread of malaria can
be controlled by
controlling mosquito
numbers
DDT
One way of controlling
mosquito numbers is to use
an insecticide like DDT
Not every mosquito will be killed each time
we spray
Some will survive to repopulate the area, so…
…we must spray frequently.
Random mutation may produce mosquitoes which are
resistant to the effects of DDT…
…these are more likely to survive and pass on their
genes to the next generation
NOTE
A resistant mosquito does not need to be totally
immune to the effects of DDT…
… it may just be able to survive higher does of
DDT than ‘normal’ mosquitoes.
The next generation contains more resistant
mosquitoes
Again, they are more likely to survive to reproduce, so…
…the proportion of the population which is resistant to
DDT increases
With each successive generation…
…the proportion of the mosquito population which is
resistant to DDT increases.
Eventually,…
…the whole population may consist of resistant mosquitoes
Spraying with DDT produces the selective pressure which
favours the resistant mosquitoes.
Because they can resist the effects of DDT, the resistant mosquitoes are said to
have a selective advantage
It may not be able to increase the dose of DDT used:
- higher doses may be dangerous
to humans
- higher doses may be too damaging to other wildlife
Using higher doses of DDT will also produce the selective
pressure which will favour mosquitoes with even higher
levels of resistance
Explain how natural selection can lead to
evolution using antibiotic resistance in bacteria
as an example. [9]
M11/4/BIOLO/SP2/ENG/TZ2/XX
members of a population of the same species show variation;
some organisms are more likely to survive due to selective advantage / survival of
the fittest;
some organisms have a reproductive advantage;
these variations may be genetically controlled/heritable;
these genes are most likely to be passed on to offspring;
this can change the characteristic of the population;
bacteria can normally be killed with antibiotics;
antibiotics impose a selection pressure;
if a few bacteria have natural resistance to the antibiotic they will survive;
if the resistance is heritable they will pass it on to their offspring;
they will reproduce/evolve to form bacterial colonies resistant to the antibiotic;
example of organism selected by use of antibiotic;
(e.g. MRSA bacteria /
resistant TB bacteria) [9 max]
M08/4/BIOLO/SP2/ENG/TZ1/XX+ (c) Explain two examples of the evolution of specific
populations of organisms in response to environmental change.
[5]
[5]
[8]
Two examples required. Award [4 max] for each example.
Example 1
name of population and characteristic: e.g. antibiotic resistance in bacteria (for
example Gonorrhea);
environmental change: exposure to the antibiotic;
response 1: antibiotic-sensitive bacteria die / antibiotic-resistant ones survive;
response 2: pass on antibiotic-resistance gene/inherited trait;
drug-resistance gene transferred to other bacteria (by plasmids);
over time resistance in the population is increased;
Example 2
name of population and characteristics: Galapagos finches, some with strong/big
beaks and some with small beaks;
environmental change: wet years with abundant small seed change to drought
years with only large seeds (as a prevalent food source);
response 1: small beaked finches die leaving primarily finches with strong/large
beaks / vice versa;
response 2: finches with large beaks survive to pass gene/inherited trait to
offspring;
over time large beaks predominate; [8 max]
M06/4/BIOLO/SP2/ENG/TZ1/XX Explain briefly how natural selection could lead to evolution. [3]
species produce more offspring that can survive;
populations will show variation;
individuals with favourable traits will survive;
some variations are inherited;
favourable (inherited) variations will increase in numbers; accept
converse answer
environmental changes will provide further selection;
sustained selection of favourable traits will result in evolution / results
in
speciation; [3 max]
Evolution or speciation must be addressed in order to receive full marks.
N.B. Examples are acceptable only if they support marking points given
above.
M05/4/BIOLO/SP2/ENG/TZ2/XX Discuss the theory of evolution by natural selection.
species produce more offspring that can survive;
populations will show variation;
individuals with favourable traits will survive;
some variations are inherited;
favourable (inherited) variations will increase in numbers; accept converse answer
environmental changes will provide further selection;
sustained selection of favourable traits will result in evolution / results in
speciation; Evolution or speciation must be addressed in order to receive full marks.
N.B. Examples are acceptable only if they support marking points given above.
[8]