Download Natural Selection

DOMESTIC ANIMALS
AND PRODUCTION
NATURAL SELECTION
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I. What is evolution?
• A change in the genetic makeup of a
population with time.
• It is a two-step process
– Development of genetic variability in the
population.
– Natural selection of those variations which
are favorable for survival.
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Example through lecture
• Let’s say a population of black
spotted cattle--BB-- eventually
turn into a population of white
spotted cattle--bb-- over a
period of years.
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II. Genetic variability
• Assuming that a population starts out with
the same genetic makeup, all new genetic
material in that population will arise by
mutation.
– A mutation is a random inheritable event.
– Let’s take our black spotted cattle as an example;
for some reason the chromosomes that code for
black spots are mutated.
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• This mutation Changes the BB to Bb.
Now with the b gene in the herd, White
spotted cattle can arise.
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• Variation in the chromosomes also
occurs when an egg and a sperm
unite. Both carry unique traits from the
parents into one individual, forming an
individual which varies from either
parent.
– This variation is normally a random event.
– In our example, let’s say two mutated
animals mate with one another. Let’s look
at their offspring.
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B
B
BB
(black)
b
Bb
(black)
b
Bb
(black)
bb
(white)
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III. Natural selection
• In a population certain individuals may be
genetically better suited for the
environment than others.
• If the environment is such that it favors
these individuals then these are the
individuals that will survive to have
offspring and pass their genetic makeup to
their offspring.
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• As long as the environment does
not again change then these
individuals will become the
“normal” population.
• The sequence of events that leads
to a certain characteristic being
“selected” be the environment is
called natural selection.
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Let’s design a little scenario to
demonstrate natural selection
• It has been 50 years since the first mating of
our mutated cattle. Over the years the once all
black spotted herd is now about 75% black
spotted and about 25% white spotted.
• A new disease has also mutated during this
time period. A disease known as Bovine Black
Spotitis. The disease is not fatal, but any black
spotted bull or cow is sterile.
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• The disease spreads quickly through the
herd and within six months time all
black spotted cattle are sterile.
• What will eventually happen to the herd?
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Answer
• Only the white cattle will be able to
reproduce, eventually producing a
completely herd of white spotted cattle.
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IV. Use of genetic variability and
natural selection in producing
improved livestock.
• As livestock managers, we can take advantage of
natural genetic variability in livestock.
• Braham cattle (Bos indicus) are know for their
heat tolerance, due to the fact they have adapted to
the hotter climate of Asia and Africa. Bos taurus
(European cattle) does not have this characteristic.
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• By intentionally mating Brahman cattle to
European (like the Angus) we take the role
of “natural selectors” by determining the
matings that will take place. (we could call
this unnatural selection)
– This mating produces the “Brangus” breed.
• It has better carcass characteristics than the
Brahman.
• It has better heat resistance than the Angus.
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– A similar mating between
Brahman and Shorthorn
cattle has produced the
Santa Gertrudis Breed.
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