Download Ch13_How Populations Evolve The Evolution of Populations The

Evolution and Natural Selection
The Evolution of Populations
What is Evolution?
A change over time in the genetic
composition of a population
Human evolution…
The gene pool
 Is the total aggregate of genes for a particular trait in a population
 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
 Each allele has a given frequency (proportion) in the population.
R  320*2 + 160 = 800 = 0.8
r  20*2 + 160 = 200 = 0.2
Genotypes
20
rr
320
RR
160
Rr
R  = 0.6
Alleles frequencies
50 YEARS FROM NOW r  = 0.4
Alleles frequencies
TODAY
Voila!
(Micro) 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
female
share a more or less distinctive
form and are capable of
breeding and having fertile
offspring
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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Evolution and Natural Selection
Before Darwin’s ideas
1) The idea of fixed species (Aristotle)
2) Lamarck and his theory of Inheritance of Acquired Characteristics
 Animals develop
the characteristics
according to the
environmental
needs (neck
elongates with the
appearance of
taller trees)
 These
characteristics
can be passed
to the next
generation
 Not very
convincing,
right?
Evolution
• Is a Change in genes frequencies in a population
A+a = 0.65+0.35= 1
That leads to changes in the expressed characteristics we observe.
These changes happen due mainly to…
Darwinian Fitness
 Refers to reproductive success of each
individual within the population.
 The fitness is determined by how well the
characteristics of the individual match the
demands of the environment in which it lives
Natural Selection
Is a process in which organisms
with certain inherited
characteristics are more likely to
survive and reproduce. The
environment is the selective
agent
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Evolution and Natural Selection
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
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.
Theory of Evolution by 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.
4)Differential survival: Somme variations are more likely to survive tan others
Darwin’s Cultural and Scientific Context
 The Origin of Species challenged the notion that the Earth was:
 Relatively young (Bible: Earth is only be 6,000 years old)
 Populated by unrelated species (species were fixed, did not change)
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Evolution and Natural Selection
Sexual Selection
Ch 4
 Was Darwin’s ingenious idea that allowed explaining why we see obvious gender
differences in many animal species
 Traits can get very exaggerated as long as they denote better fitness
Peacocks
In evolutionary
terms, fitness is
used to indicate
reproductive success
Elephant Seals
EVIDENCE OF EVOLUTION
 Biological evolution leaves observable signs.
 Five of the many lines of evidence in support of evolution…
 The fossil record
 Biogeography
 Comparative anatomy
 Comparative embryology
 Molecular biology
 Is the ordered
sequence of fossils
as they appear in
rock layers
 Paleontologists
have discovered
many transitional
forms that link past
and present
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Evolution and Natural Selection
 Five of the many lines of evidence in support of evolution…
 The fossil record
 is the study of the geographic
distribution of species that first
 Biogeography
suggested to Darwin that today’s
 Comparative anatomy
organisms evolved from ancestral
 Comparative embryology
forms
 Molecular biology
 Is the comparison of body structure
between different species
 Confirms that evolution is a
remodeling process
 One example is the distribution of marsupial
mammals in Australia
Homology
 Is the similarity in structures due to
common ancestry
 Homologous structures have same
ancestry, but have different function at
present and are morphologically (form)
different as well
 Two or more organisms evolved from one original design,
which leads to  leads to divergent evolution
 Each organism adapted to different ecosystems
example: forearms in different mammals
Adaptive radiation:
 Is a case of divergent evolution characterized
by a rapid increase in the number of closely
related species.
 e.g. the colonization of a new environment
(e.g. first reptiles that colonized the land)
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Evolution and Natural Selection
Analogy
Cactus,
American
desert
 Morphologic similarity and/or function
but different origin, the opposite to
homologous!
Involves the development of similar
characteristics in organisms of widely
different origins
 leads to Convergent evolution
 Unrelated species become more and more
similar in appearance as they adapt to the
same kind of environment
Cactus,
South
America
desert,
Andes
mountains
Euphorbia,
African desert
 Absence of leaves to prevent water loss
 Spiny structures. Some of these are modified
twigs and others modified leaves (different
origin). These discourage herbivores from eating
them
 Swallows, some bats, and dragonflies evolved
from different ancestor to survive by hunting
insects while flying. All of them have…
 Wings
 Good eyesight or hearing
 Great agility and speed in flight
 Five of the many lines of evidence in support of evolution…
 The fossil record
 Biogeography
 Early stages of development in
different animal species reveal
 Comparative anatomy
additional homologous
 Comparative embryology
relationships
 Molecular biology
 The hereditary background of
an organism is documented in:
 Its DNA
 The proteins encoded by
the DNA
 Evolutionary relationships
among species can be
determined by comparing:
 Genes
 Proteins of different
organisms
 For example, pharyngeal pouches appear on the side
of the embryo’s throat, which:
 Develop into gill structures in fish
 Form parts of the ear and throat in
humans
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Evolution and Natural Selection
Early 1900’s: Population genetics
Reconciled Darwin’s and Mendel’s ideas
Modern synthesis
(1940’s)
1. Populations + Genetics = Darwin’s + Mendel’s ideas
2. Populations are the units of evolution
3. Natural Selection is the selective agent
4. Gradualism is the way populations evolve
Main postulates
Gradualism: Charles
Darwin’s view of
evolution.
Species slowly
accumulates
changes and finally
become a different
descendant species
Other factors that contribute to Evolution
Changes in the genetic composition of a population
Genetic drift: The “bottleneck effect”
 Takes place more often when the population size is small, so it can produce huge
fluctuations in the genes frequencies.
 The population is severely reduced in number due to chance  a natural disaster,
etc.
 Genetic diversity is also reduced, as 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
The population
recovered after
bottleneck
25 %
75 %
Survivors
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Evolution and Natural Selection
Gene flow = Migration
 Genetic exchange due to migration
of fertile individuals or gametes
between populations
MAP
AREA
Non random mating
(Sexual selection)
 Only those with the best genes get to
reproduce!
 Which eventually leads to some
traits becoming more common than
others
•
Fairbanks
Fortymile
herd range
•
Whitehorse
example: the two caribou populations
in Alaska share the same area during
part of the year, but they rarely breed
with members of the other population
example: birds of paradise
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