Download The Evolution of Populations The Evolution of Populations

The Evolution of Populations
What is Evolution?
A change over time in the genetic
composition of a population
Human evolution…
The gene pool
ƒ Is the total aggregate of genes for a particular trait in a population
ƒ Consists of all genes for that traits in all individuals of the population
e.g. population: 500, two alleles: Red (R) and White (r). Total: 1000 genes for flower color in the population
Example: alleles frequencies over time
ƒ Each allele has a given frequency (proportion) in the population.
R Æ 320*2 + 160 = 800 = 0.8
r Æ 20*2 + 160 = 200 = 0.2
Genotypes
20
rr
320
RR
160
Rr
Alleles frequencies
50 YEARS FROM NOW
R Æ = 0.6
r Æ = 0.4
Alleles frequencies
TODAY
Voila!
EVOLUTION!
The Evolution of Populations
What is a Population?
A group of individuals of the same species living in a given environment
peacocks
Species: Organisms that
share a more or less distinctive
form and are capable of
breeding and having fertile
offspring
female
Charles Darwin…
(still evolving)
Evolution through
natural selection theory:
1859
male
What is the time scale we ‰ Many and Many generations in general, million of years
‰ Microorganisms and viruses can have a rate of evolution
must consider to see
visible in a few generations
evolutionary changes?
Why?
short generation spans and sometimes also a high mutation rate
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Before Darwin’s ideas
1) The idea of fixed species
2) Lamarck and adaptive evolution
ƒ Animals develop
the characteristics
according to the
environmental
needs (neck
elongates with the
appearance of
taller trees)
ƒ These
characteristics
can be passed
to the next
generation
ƒ WRONG!
Evolution
•Change in gene (allele) frequencies in a population
A+a = 0.65+0.35= 1
That lead to change in the expressed characteristics we observe. These changes happen
due mainly to…
Natural Selection
Individuals whose genetic combination
is favored have a better chance to
survive, reproduce, and pass their
genes on to the next generation. The
environment is the selective agent
Fitness
Apache pocket mouse (lives on lava)
Rock pocket
mouse (lives on
white sand)
ƒ Reproductive success of each individual
within the population.
ƒ Individuals with those genes will be less
predated and will reproduce more than the
others
ƒ The fitness is determined by how well
the characteristics of the individual match
the demands of the environment in which
it lives
ƒ Genes for white and dark fur were favored
in each population of mice and this led to
changes in the allele frequencies
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Theory of natural selection (Darwin):
1) Genetic variation: Individuals within a species differ from each other
2) Inheritance: Offspring are similar to their parents
3) Excess of reproduction: More offspring are generally produced than
those to survive to maturity. Factors like predation, disease and
competition take place
4) Population size remains fairly constant: if environmental conditions
do the same
Important!
ƒ Natural selection do not affect the genes directly but indirectly by
selecting those phenotypes that better match the environmental
requirements for the species
ƒ Natural selection operates at the individual level, but the evolutionary
changes that produce operates at the population level.
Factors that lead to Evolution
Changes in the genetic composition of a population
1- Small population size: Genetic drift
Bottleneck effect
ƒ The population is severely reduced in number due to chance Æ a natural disaster, etc.
ƒ Genetic diversity is also reduced. Mating among the remaining individuals will not
replace the lost genes
Parent
population
50 %
50 %
Bottleneck: drastic
reduction caused for
example by a change
in the environmental
conditions
First
generation
after
bottleneck
25 %
75 %
Survival
individuals
3
A4
migration
Original population
Founder effect:
A5
•Some individuals migrate and become the
founders of a new, isolated population.
•Only part of the original gene pool is
present in the new population
New population
Founder effect in the case of Flores man?
Flores
island
An adult pygmy and
an adult Australian
Founder effect in the case of the Lake Erie water snake?
The Lake Erie water snake is
limited to the islands in the
western section of the Lake Erie
Northern water snake
Lake Erie water snake
The difference in color
patterns is related to
different allele
frequencies for color
pattern
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3- Mutations
ƒ Point mutations (only one base is
changed in the sequence)
ƒ Chromosomal aberrations (Major
change in the DNA at the level of
the chromosomes)
ƒ For only one individual mutations
may not appear relevant
ƒ But the cumulative impact of
mutations when considering the
entire population can be significant!
4- Natural Selection
CANADA
MAP
AREA
Beaufort Sea
Porcupine
herd range
N
TE OR
RR T H
IT WE
O S
RI T
ES
•
Fairbanks
Fortymile
herd range
ALASKA
YUKON
ƒ Genetic exchange due to migration
of fertile individuals or gametes
between populations
ALASKA
2- Gene flow = Migration
•
Whitehorse
The two caribou populations
share the same area in part of the
year, but they rarely breed with
members of the other population
ƒ Individuals whose genetic combination is favored for
the environment a better chance to survive and pass
their genes
# of individuals
Modes of Natural Selection
Evolved
Original population
population
(a) Directional selection
Shifts the overall makeup of the
population by favoring variants at one
extreme of the distribution. In this case,
darker mice are favored because they
live among dark rocks and a darker fur
color conceals them from predators.
Original
population
Which one may be related to…
Reduces variation?
Important in speciation?
Environment changes?
Phenotypes (fur color)
(c) Stabilizing selection
(b) Disruptive selection
Favors variants at both ends of the
distribution. These mice have
colonized a patchy habitat made up
of light and dark rocks, with the
result that mice of an intermediate
color are at a disadvantage.
Removes extreme variants from
the population and preserves
intermediate types. If the
environment consists of rocks of
an intermediate color, both light
and dark mice will be selected
against.
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