2.3.12 Natural Selection

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Transcript 2.3.12 Natural Selection

2.3.12 Natural Selection
Objectives: Explain the consequences
of the four observations listed by
Charles Darwin in proposing his theory
of Natural Selection
Define the term speciation
Darwin’s 4 Observations
1. Offspring tend to resemble their
2. All individuals display variation
3. All organisms have the potential to
produce large numbers of offspring
4. Despite this populations tend to
remain fairly constant
Darwin’s Conclusions
Most offspring do not survive to breed
Only the best adapted (fittest)
individuals survive to pass on their
Over time and with the correct
circumstances a number of changes
may give rise to new species
This is the formation of a new species
from an existing one
It has been scientifically observed in
bacteria, fruit flies (Drosophila) and
some domesticated organisms
Relies upon strategies to reproductively
isolate organisms and enough
generations to render them unable to
produce viable offspring
Allopatric Speciation
Relies on physical barriers like
mountains or islands to separate
Island species are the best
studied examples
e.g. Galapagos finches, tortoises,
Sympatric Speciation
This is speciation of organisms within the
same habitat
This could be due to e.g. organisms
undergoing biochemical changes,
preferring different food species or
different mates
Often species that will not mate with
each other in the wild will do so under
laboratory conditions e.g stickleback &
cichlid fish studies
Other types of speciation
Peripatric: New species form in peripheral
sections of the main population e.g.London
Underground mosquito, Culex pipiens
Parapatric: Similar species in separate
habitats do occasionally overlap yet tend
not to interbreed e.g. many gull & warbler
Both of these types of speciation depend on
ecological niches to operate successfully