Download Sympatric speciation

Higher Biology
Unit 1: DNA and the Genome
4. Transfer of genes and evolution
Notes
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Transfer of DNA
There are two types of transfer
1) Vertical
2) Horizontal
1) Vertical
This is where DNA is passed from parent to offspring.
Sexual
Asexual
Two parents
Variation
Fertilisation required
One parent
No variation
Fertilisation not required
2) Horizontal
The plasmid is transferred from 1 cell to another of a different species.
Eukaryotes are not capable of carrying out horizontal gene transfer.
However, bacteria and viruses can transfer genetic material horizontally into the
genomes of eukaryotes.
Horizontal is faster than vertical however there is no guarantee that the
plasmid will be taken up successfully
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Types of horizontal transfer
Transformational
The DNA plasmids / fragments from a bacteria are taken up from the environment
by another bacteria.
Conjugation
A bridge forms between two different bacteria and a DNA plasmid / fragment is
passed across
Transduction
A virus containing genetic material is transferred from one bacteria to another
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Examples of horizontal transfer
Bacteria to bacteria
Example: MRSA
The plasmid for antibiotic resistance is transferred from one bacteria to another. This
can result in superbugs which are resistant to many / all antibiotics
For info:
MRSA is a type of bacterial infection that is resistant to a number of widely used
antibiotics. This means it can be more difficult to treat than other bacterial
infections. The overuse of antibiotics in recent years has played a major part in
antibiotic resistance and superbugs. This includes using antibiotics to treat minor
conditions that would have got better anyway and not finishing a recommended course
of antibiotics. (www.nhs.co.uk)
Virus to Human Eukaryoate
Example: Herpes
The DNA of the herpes virus can be transferred to the genome of humans.. This causes
cold sores.
For info:
The herpes virus or "cold sore virus" – is highly contagious and can be easily passed
from person to person by close direct contact. After someone has contracted the virus,
it remains dormant (inactive) for most of the time. However, every so often the virus
can be activated by certain triggers, resulting in an outbreak of cold sores. These
triggers vary from person to person but can include fatigue and an injury to the affected
area. (www.nhs.co.uk)
Bacteria to Plant Eukaryoate
Example: Bacteria Eukaryoate
The bacteria agrobacterium transfers its own DNA into the genome of a wounded plant
cell.
For info
Agrobacterium tumefaciens is a remarkable species of soil-dwelling bacteria that has
the ability to infect plant cells with a piece of its own DNA. When the agrobacterial DNA
is integrated into a plant chromosome, it effectively uses the plant's own cellular
machinery to ensure the proliferation of the agrobacterial population.
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Evolution
A gradual change in organisms over generations as a result of changes to the frequency of gene alleles.
Non random selection in evolution
(a) Natural selection is the non-random increase in frequency of DNA sequences that increase
survival.
Steps in natural selection
 Members of a species reproduce at a rate that creates more offspring than the
environment can support
 As organisms reproduce, m_________ can occur in their offspring. These mutations can
sometimes be a good thing.
 This mutation could enable an organisms to have favourable characteristics. This is known
as having a s_____________ a
_____
.
 An organism with a selective advantage will have a higher chance of survival and is
therefore more likely to reproduce.
 The genes / mutation / favourable characteristic may be passed on. The offspring of this
m_______ may also contain the same genetic information.
 The original members of the species may not survive and therefore not reproduce.
 After a long period of time / many generations / hundreds of years, only the mutant will
have survived.
 This species has now said to have e_______________ .
Example: DDT
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(b) Sexual selection is the non-random increase in frequency of DNA sequences that increase
reproduction.
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Sperm produced by make animals are much smaller and far more numerous than the egg
produced by the females. Males therefore have the resources to produce sufficient sperm to mate
with many females.
 On the other hand, females have to invest a large proportion of their resources to produce fewer
eggs.
It is a better strategy to be selective and to choose one male of high quality. Therefore male often
find themselves in competition for females.
It operates in two ways:
1) Male-to-male competition Males compete aggressively with one another for territories and
access to females. The largest, strongest, and most aggressive males with the best weapons
successfully mate with the females and pass the alleles for these characteristics on to the next
generation.
2) Female choice - Where the male is unable to control access to females the onus is on the female
to select a male that she considers to be of high quality based on the traits that he displays.
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May take the form of ‘ornaments’ (bright plumage) in male birds.
For the female (peahen) it is important that she chooses a robust male who will father
strong, healthy offspring with a high survival rate.
Her choice of male is based on the condition of his plumage and the quality of his
display.
The peahens can use such showy displays as an indicator of a male's quality or fitness,
because the male has been able to survive even with the added cost of an otherwise
useless and sometimes dangerously conspicuous display.
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Outcomes from selection
There are differences in outcome as a result of directional, disruptive and stabilizing selection.
Directional
The example of industrial melanism is an example of directional selection. This is when natural
selection favours one extreme of continuous variation, in this case dark moths over lighter ones.
Disruptive
This is when natural selection favours both extremes of continuous variation.
If there is an area with dark trees covered in areas of light lichen, natural selection will favour both
types - those with lots of melanin, which will camouflage against the dark trees, but also the type with
very little melanin, which will camouflage against the light coloured lichen.
Moths with intermediate melanism will not be favoured as they will not be successfully camouflaged in
either environment.
Stabilising
This is when natural selection favours an intermediate state of continuous variation. An example of this
is clutch size in birds. Birds that lay too many eggs have an increased chance of losing offspring to
starvation.Birds which lay too few have a decreased chance of these birds surviving and passing their
genes on. Nature favours clutch sizes of an intermediate number.
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Random selection in evolution
Genetic drift –
The random increase and decrease in frequency of sequences This is particularly common in small
populations where the random loss of individuals with specific DNA sequences results in a significant
change in the frequency of genes in the population and further generations.
This leads to a non-representative sample of the alleles of the whole population being passed on.
Some alleles may be over-represented and some under-represented.
Unlike natural selection it normally fails to improve the population’s ability to adapt to the
environment.
Genetic drift may even cause an allele to disappear completely from a small population thereby
reducing genetic variation and driving the population towards uniformity.
Genetic Drift can be caused by:
1) A chance event
2) Natural mutation
3) Colonization founder effect - A founder effect occurs when a small population breaks away
from the main group and forms a new colony. Due to the size of the small population, the
group may have reduced genetic variation and a non-random sample of the alleles in the
original population.
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e.g It is thought that North America was first populated by a small group of Asian people who
migrated across the land bridge which is now the Bering Strait and became isolated from the rest
of the human Mongoloid race.
e.g The founder effect is thought to account for the differences in the percentages of the
population possessing certain blood groups
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Speciation
Speciation
The generation of new biological species by evolution as a result of isolation, mutation
and selection.
Species
The process
1. A population are separated by a barrier.
2. These barriers can be geographical, behavioural or ecological.
3. Mutations can occur and sometimes the mutant can have a selective
advantage due to different selection pressures.
4. Over many generations the sub-populations may adapt and eventually
become reproductively isolated. This results in a new species.
5. Over many generations the sub-populations may adapt and eventually
become reproductively isolated. This results in a new species.
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Allopatric speciation
Allopatric speciation is when new species arise due to isolation of a population by geographical
barriers. Features such as rivers or mountain ranges isolate populations of animals and plants.
Movement of land-masses by continental drift led to geographical isolation millions of years ago.
Taken from BBC Bitesize
Sympatric speciation
Sympatric speciation is when new species arise despite occupying the same geographical area.
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Ecological barriers - although groups are not geographically isolated from each other they may
be isolated by occupying different habitats or breeding areas, pH and salinity.
Behavioural barriers – a population may carry out complex mating rituals that may create a
barrier to reproduction. Different timings, locations or mating dances may result in members
of a population, who are not geographically separated, not being able to mate with each
other.
Such barriers are rarely entirely complete and so there are often hybrid zones in regions where the
ranges of closely related species meet.
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Gene Flow / Pool
A gene pool is the total of all the different genes in a population. This gene pool can be
altered by mutations, natural selection, gene migration and non-random mating. These
processes can result in speciation.
Interruption of the gene flow also results in speciation. This can be seen in the
following diagram:
POP
A
POP
B
POP
C
POP
D
All 4 populations belong to the same species because each population can breed with
its neighbour.
If however B became extinct, then the gene flow would be interrupted and separate
species would be formed.
POP
A
POP
C
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POP
D