Download 8.1 Natural Selection

 Evolution occurs when natural selection acts on the
genetic variability within populations.
 Genetic variation occurs by chance through genetic
mutations and recombination.
 BUT, natural selection does NOT occur by chance –
the environment favours certain individuals over
others.
Evolutionary Change without
Selection
 Sometimes change in the genetic makeup of a
population is not influenced by the traits of
individuals.
 Genetic Drift
Genetic Drift
 When the genetic makeup of a population changes by
chance.
 The chances of passing on any particular allele is
random
 Therefore, it is possible for one allele to occur more
frequently than another, without the allele being
naturally selected for
Activity
 If you flip a coin 10 times, what should your expected
results be?
 Do it! Did you get the expected 1:1 ratio.
 Would your results be closer to the expected
probability if you flipped the coin 100 times?
Genetic Drift
 The smaller the number of individuals in a population
the greater the influence of genetic drift.
 As a result, alleles can completely disappear over a
number of generations.
 The loss of an allele means a reduction in genetic
diversity.
Bottleneck and Founder Effect
 If a population were to undergo an extreme reduction
in size, this would lead to a loss of genetic diversity
and what is known as a bottleneck.
 A population bottleneck may be the result of genetic
drift or natural selection. Which one do you think plays
a bigger role?
Bottleneck
 Ex: If initially a population contained 10 000
individuals, there is likely lots of different alleles and
variation in this population.
 If the population drops to 50 individuals, many of
those alleles will be lost.
 If the population is allowed to recover, those 50
individuals are the “founders” of the new population.
(FOUNDER EFFECT)
Founder Effect
 Future generations will only have the alleles that were
present in the founder and there will be a lack of
genetic diversity in the resulting populations.
Cheetahs are an example of a species that has undergone
a population bottleneck and a founder effect.
Why is lack of Diversity bad?
 Lack of diversity makes a population susceptible to
diseases and catastrophic events.
 Essentially inbreeding, which leads to the expression
of harmful homozygous recessive traits that are usually
rare and not expressed.
 Can lead to the extinction of a species.
Diversity and the Founder’s
Effect
 Populations under the founder effect will demonstrate
a lack of genetic diversity in the years that follow the
population bottleneck and will be vulnerable for
extinction.
 With time, after several generations, mutations will
build up in the gene pool and genetic diversity will
increase.
 Evolution
 http://www.youtube.com/watch?v=GhHOjC4oxh8
 Natural Selection
 http://www.youtube.com/watch?v=0SCjhI86grU