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Chapter 22

Chapter 22: Mechanisms of Evolution

Biological Sciences
Course Code
Kamini Persaud

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Sanchika K
March 31st 2011
Biology II
Chapter 22: Mechanisms of
Controversial book by Thomas Malthus
The human population grows exponentially, we should over populate the planet but we
dont why.
oFamine or disease are all things that are natural that are necessary to keep the
human population in tact
Certain individuals have to die otherwise there will be over population
If individuals die who would they be
oThere has to be variation in population
Variation in a population
oSome individuals are better suited to their environment than others
Natural selection
oIs the whole process in which some individuals are more better than others
oThe mechanism in which evolution occurs
What facts form the base of our understanding of evolution?
For a characteristic to evolve the variation must be heritable
Genotype- the genetic makeup of an organism

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Sanchika K
March 31st 2011
Biology II
othe genetic coding for a trait
Phenotype- the physical expression of an organisms genes
oPhysical appearance
Character- a particular feature of the individuals phenotype
Trait- a specific form of a character
What are the mechanisms of evolutionary change?
When do you see evolutionary change?
For any evolution there has to be something different
Mutation- the origin of genetic variation
ois random
omost are harmful or neutral but could be advantageous
mutation happens at a low rate (hence why evolution is slow)
Gene flow- from the migration of individuals and movements of gametes between
o New allele get added to gene pool and get changed
Genetic Drift- from random changes in allele frequencies
o One mates with the same kind
oYou can increase or decrease the proportion of a certain type
oNot as likely to happen in a large population
oBottleneck effect-if population are reduced to a small number of individuals,
reduced the genetic variation
usually natural disaster
eg. Hunting one type of animal and the rest mate with each other and that
species dominates

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Sanchika K
March 31st 2011
Biology II
oFounder effect- almost the same as bottleneck
One species leave and move to a new region
In the new population they will have the same phenotypes
Non-random mating-when individuals choose mates with particular phenotypes
Natural selection can act on characters with quantitative variation in 3 ways:
Stabilizing selection-preserve average phenotype
Most the time happens
One mean value of a trait which does the best
Best suited in the environment will last
As time goes on, its always going to be there
oEg. Human birth weight there is always an optimal birth weight, even if there is
something over weight or under weight
Rates of evolution are slow because natural selection is usually stabilized around the
optimal birth rate
Directional selection- favors individuals that vary in one direction
The environment changes a certain trait becomes optimal
Eg. Species that differ in how well they can see in the dark
oIf the species live in the dark , over generation individuals that are best able to see
in the dark survive the best and reproduce
Geographic regions
Global warming
Your group moving into a different environment
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