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Selection and
Key Word Mind Map
• Organisms that belong to the same
species are able to breed together to
produce fertile offspring
Natural Selection
• Hardy-Weinberg principle cannot be true if individuals of a
particular genotype are –
• More likely to die before reproducing
• Unable to grow sufficiently well to reproduce successfully
• Unable to attract a mate
In all cases, organisms with one particular genotype will
reproduce less successfully than others in the
population, and leave fewer, if no, offspring. There is a
differential reproductive success between the
genotypes in the population
Natural Selection
1. Within a population, a characteristic has more than one
phenotype. These phenotypes result from mutation that
leads to genetic variation in the genotypes and alleles
controlling this characteristic
Natural Selection
2. There is different reproductive success between different
phenotypes, as some of these phenotypes will be better
adapted to survive in their environment. Some phenotypes will
be selected against.
Natural Selection
• Organisms with a greater reproductive success leave more
offspring than those with less reproductive success
Natural Selection
• Organisms with greater reproductive success will pass their
favourable allele to their offspring. As a result, the frequency
of this allele and these genotypes will increase in the
population, ie natural selection has occurred
Natural Selection
Directional Selection
• Acts against one of the extremes in a range of phenotypes. As
a result one phenotype becomes rare, and an alternative
phenotype becomes more common.
Directional Selection
• The upper graph represents in the
frequency of phenotypes in a population.
The graph has a normal distribution with
a fairly large standard deviation, this is
before natural selection has occurred.
• The second graph is a frequency
distribution of the same population after
natural selection has occurred. The
standard deviation of this curve is less
than the upper curve and its mode has
shifted to the right on the x-axis.
Natural selection has caused a genetic change in this population,
favouring organisms with a characteristic towards the upper
range of the frequency distribution
Stabilising Selection
• Stabilising Selection acts against both extremes in a range of
phenotypes. As a result the variation about the mode is
• After selection, the mode is in the same position: This is the
most advantageous phenotype. Stabilising selection has
reduced the variation about this modal value.
• Formation of a new species by natural selection is known as
• Geographic Isolation:
• This can happen when a species are physically separated into
• It could be due to a mainland population being separated from a
population on an island
• A creation of a volcano and therefore the two populations are
unable to reproduce
This prevents gene flow within the popuation
• There are many different species of fruit fly. The differences
between them are so small, you may not be able to tell one
species from another. But the flies can!
• Before mating, male and females undergo a courtship ritual
• The whole sequence, including the response from the female
is controlled by genes and is species-specific
• If the male does not have the correct courtship dance, the
female does not allow him to mate with her
• If the fruit fly is homozygous recessive for wing length, it will
not kill him, however his courtship dance will not attract a
female, and he will be unable to pass on his alleles of the gene
for wing length.
• His mutated gene for wing length will not be passed on.
However, suppose these genetic changes had occurred not in
an individual but in an entire population that was isolated
from another population of the same species – This will be
conditions in which speciation can occur
Geographic Isolation
• Geographic isolation will lead to a population of a species
being isolated, and as a result there will be no GENE FLOW to
neighbouring populations
• Gene mutations occur at a constant and low rate, some are
beneficial and result in increasing the organisms reproductive
success. This mutation will therefore be passed on.
• An accumulation of mutations can occur, which could mean
that if the population was reintroduced to the original
population, that it would be unable to reproduce. This has
resulted in SPECIATION
• This can occur if two populations
have been separated
The conditions for each population
will now be slightly different
As a result selection may occur in different
directions, some alleles maybe favoured
by one population but selected against in
An experiment was conducted with fruit flies. One
population was split into two and each was fed on
different food. After many generations the two
populations were placed together and it was
observed that they were unable to breed together.
1. What evidence shows speciation has occurred?(1)
2. Explain why the experiment resulted in speciation.
3. Suggest two possible reasons why members of the
two populations were not able to breed together (2)
The new species were unable to breed with each other (1)
Populations were isolated and fed on different foods (1)
This caused changes to allele frequencies between the populations (1)
Which made them reproductively isolated and eventually resulted in
speciation (1)
Seasonal changes (1)
Mechanical changes (1)
Behavioural changes (1)