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Natural Selection
Mrs.jackie
Natural Selection
• 1858- Charles Darwin
and Alfred Rusell
Wallace published a
theory of evolution by
natural selection
Theory of Natural Selection
• Individuals within a species are all slightly different
from each other. These differences are called variation.
• IF the climate or food supply changes, some of these
variations may be better able to survive than others.
• If one variety lives longer than others, it is also likely to
leave behind more offspring.
• If some offspring inherit the variation that helped the
parent survive better, they too will live longer and have
more offspring.
• In time, this particular variety will outnumber and
finally replace the original variety.
Theory of Natural selection
• Also called Survival of the fittest
• Thomas Malthus in 1798, suggested that
– The size of the human population would outstrip
the rate of food production.
– The number of people will be regulated by famine,
disease and war.
Darwin apply Malthus ideas to other population of
living organisms
Darwin´s ideas
• He observed that
– Animals and plants produce vastly more offspring than
can possibly survive to maturity.
– Therefore there must be a struggle for survival
Example:
A pair of rabbits had eight offspring which grew up
and formed four pairs, eventually having 8 offspring
per pair, in four generations the number of rabbits
stemming from the original pair would be 512.
The population of rabbits however remains more or
less constant, therefore have failed to survive the
reproductive age.
Competition and selection
• There will be a competition between the
members of the rabbit population for food,
burrows and mates
• If food is scarce only the healthiest rabbits will
survive and breed
• Competition does not necessarily involved
direct conflict
– Able to run faster from predators
– Better camouflage
Competition and selection
• If the offspring inherit the advantageous
characteristics of their parents, they may give
rise to a new and more adapted race.
• This new variations are called survival value
• Selection pressures- better adapted varieties
are selected by the pressures of the
environment
• For natural selection to be effective, the
variations have to be heritable.
The peppered moth example
• The common form is speckle but there is a variety
which is black.
• The black variety was rare in 1850 but in 1895 in
the Manchester area its number had risen to 98%
• Observations showed that the light variety was
concealed better than the dark when they rested
on tree-trunks covered with lichens.
• IN Manchester Pollution had caused the death of
lichens and the darkening of tree trunks
• The dark variety was able to camouflage from
birds.
The peppered moth example
• The selection pressure: predation by birds
• Adaptive variation: dark color.
Sickle cell anemia
Sickle-cell anemia
• A person with this condition has inherited both
the Hbs and Hbs allele for defective hemoglobin
resulting in the destruction of red cells
• IN many African countries sufferers have a low
change of reaching reproductive age.
• Selection pressure remove the homozygous
recessive from the population
• However the heterozygous HbA Hbs have
virtually no symptoms of anemia but have the
advantage of been resistant to malaria.
• The selection pressure of malaria favors the
heterozygous over the homozygous HbA HbA
Artificial selection
• When humans breed animals or plants with
specific characteristics
– Examples: dogs, crop plants, cows.
Homework
• What features of a bird's appearance and
behavior do you think might help it compete
for a mate?
• What selection pressures do you think might
be operating on the plants in a lawn?