Download Mechanisms of Evolution PPT

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: I can analyze and evaluate how natural selection
produces change in populations, not individuals;
I can analyze and evaluate how the elements of
natural selection, including inherited variation,
the potential of a population to produce more
offspring than can survive, and a finite supply of
environmental resources, result in differential
reproductive success (7C); I can analyze and
evaluate the relationship of natural selection to
adaptation and to the development of diversity in
and among species; and I can analyze and
evaluate the effects of other evolutionary
mechanisms, including genetic drift, gene flow,
mutation, and recombination
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Evolution is the change in the development of
organisms over a long period of time.
Evolution can occur due to any of the following
mechanisms:
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Natural selection
Genetic drift
Gene flow
Mutation
“The only thing constant in life is change”
◦ ― François de La Rochefoucauld
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(1809- 1882) An English scientist
Considered the father of evolution.
It took him years of observation and study to
develop his theory of natural selection.
Wrote the book, On the Origin of Species by
Means of Natural Selection
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He began in 1831 at age 22 when he took a
job as a naturalist on the English ship HMS
Beagle, which sailed around the world on a
five-year scientific journey.
Darwin studied and collected biological and
fossil specimens at every port along the
route.
Darwin and Natural Selection
• On the Galápagos Islands, Darwin studied many
species of animals and plants that are unique to
the islands but similar to species on the
mainland.
• These observations led Darwin to consider the possibility
that species can change over time, especially if exposed
to different environments where they must adapt to
different things.
• He became curious about possible relationships
among species because of similarities he saw.
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One of the things Darwin observed is that in
nature, the traits of individuals vary in
populations.
Give some examples of variations of traits in
populations.
Darwin hypothesized that
there was a force in nature
that picked the traits which
are better for survival in a
species.
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He also observed that individuals
struggle to compete in changing
environmental conditions.
What do individuals compete for?
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What is a population?
What is the difference between populations
and individuals?
What is natural selection?
Organism – any living thing
Species – an organism that can
successfully reproduce and
produce viable offspring
Population – Same species in the
same location at the same time
Community – Different populations
in the same place at the same time
Ecosystem – Communities and the
surrounding abiotic (nonliving)
factors
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A group of individuals that actually or potentially
interbreed in nature. A species is the biggest gene
pool possible under natural conditions.
Scientists group organisms according to their
similarities.
The most similar organisms belong to a species.
Members of the same species can mate and produce
fertile offspring.
Ex: Humans belong to the species Homo sapiens.
Natural Selection –
 the process by which traits or alleles become
more or less frequent in a population,
depending on the advantage or disadvantage
they confer on the survival and reproduction
of the organism
 occurs when certain organisms with certain
variations survive, reproduce, and pass their
variations to the next generation.
 Also known as “survival of the fittest”
 Proposed by Charles Darwin
1. In any population, individuals have
variations in traits.(Ex. coloring, speed, size)
2. There is differential reproduction (fitness).
Organisms produce more offspring than
can survive….to ensure survival of the
species. Not all organisms will live to
reproduce. (Ex. eaten by a predator)
2. Fitness – REPRODUCTIVE SUCCESS includes an organism’s ability to survive,
find a mate, produce offspring — and
ultimately leave its genes in the next
generation.
3. There is heredity.
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Some variations are more beneficial than others.
The individuals with those beneficial traits will be the
ones that survive, reproduce, and pass those favorable
variations on to the next generation.
Individuals without those favorable traits are less likely
to survive and reproduce.
4. The more advantageous trait becomes more
common in the population.
• After generations, organisms with the favorable
variation make up most of the population.
• The allele for the unfavorable traits may
eventually cease to exist.
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Those organisms best adapted to the
environment will survive and pass on
their favorable traits.
Environments are affected by limiting
factors -Any factor (physical or
biological) which regulates the welfare
of an organism
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Examples of limiting factors are disease,
competition, predation, environmental
change, etc.
Graph of survival
curves ("survival
function estimates")
of children without
any sickle cell
genes (HbAA),
children with sickle
cell trait (HbAS),
and children with
sickle cell disease
(HbSS). Those who
had the sickle cell
trait (HbAS) had a
slight survival
advantage over
those without any
sickle cell genes
(HbAA), with
children with sickle
cell disease (HbSS)
faring the worst.
Reference: Protective Effects of the Sickle Cell Gene Against Malaria Morbidity and Mortality. Aidoo M, Terlouw DJ, Kolczak MS,
McElroy PD, ter Kuile FO, Kariuki S, Nahlen BL, Lal AA, Udhayakumar V. Lancet 2002; 359:1311-1312.
http://www.cdc.gov/malaria/about/biology/sickle_cell.html
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Which traits of each organism shown below
would allow for natural selection?
Example of Natural Selection –
“survival of the fittest”
Why do you think more and more bad bacteria
(like the kind that cause sinus infections) are becoming
resistant to anti-biotics?
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How do you think natural selection will affect this species
of moth? both the same species, just have a body color
variation)
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Evolution and natural selection are not the
same thing.
Natural selection results in differential
reproductive success.
Differential reproductive success is a
situation in which traits that help a species
survive change, and the resulting biological
changes are passed on to future generations.
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Three types of Natural Selection
Directional
◦ Allele frequencies shift to favor
individuals at one extreme of the
normal range
 Only one side of the distribution
reproduce
 Population looks different over
time
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Stabilizing
◦ Favors individuals with an
average genetic makeup
 Only the middle reproduce
 Population looks more similar over
time (elim. extremes)
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Disruptive (aka Diversifying)
◦ Environmental conditions favor
individuals at both ends of the
genetic spectrum
 Population split into two groups
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Stabilizing Selection
◦ Intermediate forms of a trait are
favored
◦ Favors average individuals in a
population
◦ Alleles that specify extreme forms
are eliminated from a population
◦ EX: Birth Weight and Clutch Size
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Example: human birth weight. Babies of low weight lose
heat more quickly and get ill from infectious disease
more easily, whereas babies of large body weight are
more difficult to deliver through the pelvis
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Disruptive Selection
◦ Both forms at extreme
ends are favored
◦ Intermediate forms are
eliminated
◦ Bill size in African
finches
I can determine the result of disruptive
selection …
• In disruptive selection, individuals with both extremes of a
trait’s variation are selected for.
• This results in eventually having no intermediate form of a
trait, and leading to two separate species.
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Directional Selection
◦ Shift in allele frequency in a
consistent direction
◦ With directional selection,
allele frequencies tend to
shift in response to
directional changes in the
environment
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Phenotypic Variation in a
population of butterflies
I can determine the result of
directional selection …
• Directional selection occurs when natural selection
favors one of the extreme variations of a trait.
• This type of selection can lead to rapid evolution of
a population.
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Examples of Directional Selection:
Peppered Moths: as the environment changes, so
do the traits that are fit for the new environment.
In the case of the moths, the forests changed
from light to dark and selection moved in the
direction of darker moths
Antibiotic Resistance
Pesticide Resistance
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Tortoise neck length
◦ Short grasses, for short-necked tortoises
◦ Tall grasses, for long-necked tortoises
◦ No grasses for average-necked tortoises, so over
time, they are selected against
Disruptive Selection
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Lizard body size:
◦ Large lizards are easily seen by predators, but
smaller lizards cannot run as fast to escape the
predators
◦ Mid sized lizards are most fit in the environment,
so they survive and reproduce more often,
changing the allele frequencies in the population
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Anteater tongue length:
◦ Anteaters with long tongues are most fit because of
the depth of the nests of the termites they eat.
Type
Directional
Selection
Disruptive
Selection
Stabilizing
Selection
Definition
Result
Type
Directional
Selection
Disruptive
Selection
Stabilizing
Selection
Definition
A process of natural selection in
which a single phenotype at one
extreme of the phenotypic range is
favored
Result
The population's trait
distribution shifts
toward an extreme.
Type
Definition
Result
Directional
Selection
A process of natural selection in
which a single phenotype at one
extreme of the phenotypic range is
favored
The population's trait
distribution shifts
toward an extreme.
Disruptive
Selection
A process of natural selection that
favors individuals at both extremes
of a phenotypic range
Can cause such
differences among a
species that the
variation leads to new
species (SPECIATION)
Stabilizing
Selection
Type
Definition
Result
Directional
Selection
A process of natural selection in
which a single phenotype at one
extreme of the phenotypic range is
favored
The population's trait
distribution shifts
toward an extreme.
Disruptive
Selection
A process of natural selection that
favors individuals at both extremes
of a phenotypic range
Can cause such
differences among a
species that the
variation leads to new
species (SPECIATION)
Stabilizing
Selection
A process of natural selection that
tends to favor genotypic
combinations that produce an
intermediate phenotype
It reduces phenotypic
variation. Natural
selection tends to
remove the more severe
phenotypes.
MAINTAINS STATUS
QUO
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If different populations develop widely
different traits, they may gradually become
different species. In this way, natural
selection contributes to the development of
diversity in and among species.
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Industrial revolution
◦ Pollution darkened tree trunks
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Camouflage of moths increases survival
from predators
Directional selection caused a shift away
from light-gray towards dark-gray moths
Fig. 18.5, p. 287
An adaptation is any variation in traits that
aids an organism’s chances of survival in its
environment.
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Examples are:
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Thorns on plants
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Special colorings for an organism
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Enhanced night vision
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Mimicry is an adaptation that enables one species
to resemble another species.
In one form of mimicry, a harmless species has
adaptations that result in a physical resemblance to a
harmful species.
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Another subtle adaptation is camouflage, an
adaptation that enables species to blend with their
surroundings.
Because well-camouflaged organisms are not easily
found by predators, they survive to reproduce.
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Limits to adaptation:
◦ A change in the environment can only lead to adaptation
for traits already present in the gene pool
◦ Reproductive capacity may limit a population’s ability to
adapt
 If you reproduce quickly (insects, bacteria) then your population
can adapt to changes in a short time
 If you reproduce slowly (elephants, tigers, corals) then it takes
thousands or millions of years to adapt through natural
selection
◦ Most individuals without trait would have to die in order
for the trait to predominate and be passed on
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When faced with a change in environmental
condition, a population of a species can get
MAD:
◦ MIGRATE to a more favorable location
◦ ALREADY be adapted
◦ DIE
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Natural selection can only act on inherited
alleles already present in the population—
do not think that the environment creates
favorable heritable characteristics!
Soooo….how do new alleles arise??????
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Changes in the structure of the
DNA
Adds genetic diversity to the
population
May or may not be adaptive
◦ Depends on the environment!
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The change in a POPULATION’S genetic makeup
(gene pool) over time (successive generations)
◦ Those with selective advantages (i.e., adaptations),
survive and reproduce
◦ All species descended from earlier ancestor species
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Small genetic changes in a population
such as the spread of a mutation or the
change in the frequency of a single
allele due to selection (changes to
gene pool)
◦ Not possible without genetic variability in a
pop…
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Inherited variation (diversity in gene
frequency)
If the genetic traits are favorable for
reproduction and survival of the population,
those traits will be more frequent in the
population through successive generationsthe genes will appear more in the
population’s gene pool – the gene frequency
will increase.
SPECIATION
• The evolution of new
species is called
speciation.
• This occurs when members of
similar populations change so
much from each other that they
no longer interbreed to produce
fertile/viable offspring.
SPECIATION
with a physical barrier
• In nature, physical barriers
can break large populations
into smaller ones.
• Geographic isolation occurs
whenever a physical barrier
divides a population and over
time they change and become two
different species.
SPECIATION
without a physical barrier
• Some speciation occurs while the
organisms still exist in the same area.
•Behavioral, Temporal, Mechanical
Northern
population
Early fox
population
Spreads
northward
and
southward
and
separates
Arctic Fox
Different environmental
conditions lead to different
selective pressures and evolution
into two different species.
Southern
population
Gray Fox
Adapted to cold
through heavier
fur, short ears,
short legs, short
nose. White fur
matches snow
for camouflage.
Adapted to heat
through lightweight
fur and long ears,
legs, and nose, which
give off more heat.
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new species may form when one population of
a species becomes reproductively isolated from
another population of the same species.
 Over
time, evolutionary mechanisms occur that
alter the gene pool of the isolated population so
that it is no longer reproductively compatible
with the original population.
 Speciation
is a lineage-splitting event that
produces two or more separate species.
 Since
being a member of one species is
defined by the ability to successfully
reproduce, speciation (the formation of a
different species) must involve an inability to
successfully reproduce.
Two way reproductive isolation occur:
Prezygotic mechanisms
Postzygotic mechanisms
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Temporal Isolation: Species reproduce in different
seasons or at different times of the day.
Geographical Isolation: Physical barriers (e.g., rivers,
oceans, mountains) prevent the mixing of
populations.
Behavioral Isolation: Species differ in their mating
rituals (e.g., differing bird songs, mating colors,
dances, pheromones).
Mechanical Isolation: Body structure prevents mating.
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Hybrid inviability: Embryological Arrest: Hybrid
embryos often do not develop properly; no viable
offspring is created.
Hybrid Sterility: Infertility: Hybrid offspring might
grow to viable adults, but these are infertile and
cannot produce further offspring
 This shuffling, along with sexual reproduction,
leads to variation within populations. This variation
leads to selection, which ultimately leads to
evolution.
◦ (Donkey + Horse = Mule; Mule is sterile.)
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Current debate:
Does speciation happen gradually or rapidly?
Or both?
◦ Gradualism
◦ Punctuated equilibrium
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Gradual change over
long spans of time
assume that big
changes occur as the
accumulation of many
small ones develop over
time.
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Rate of speciation is
not constant
◦ rapid bursts of change
◦ long periods of no change
◦ species undergo rapid
change when they 1st bud
from parent population
Time
Types of Change
• Divergent evolution is when species that are similar
and closely related become increasingly different from
each other. THEY DIVERGE
This is also called Adaptive Radiation….because it has to do with ADAPTING to
different environments and RADIATING out into different species.
Types of Change
• When distantly-related organisms evolve to become
more similar, it is called convergent evolution.
• Convergent evolution occurs when unrelated
species occupy similar environments in different
parts of the world.
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Changes in a population’s gene pool over
time.
◦ Genetic variability within a population is the catalyst
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Four Processes cause evolution
◦ Mutation (random changes in DNA—ultimate source
of new alleles) [stop little]
 Exposure to mutagens or random mistakes in copying
 Random/unpredictable relatively rare
◦ Natural Selection (more fit = more offspring)
◦ Gene flow (movement of genes between pop’s)
◦ Genetic drift (change in gene pool due to
random/chance events)
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Gene Flow
◦ Flow of alleles
 Emigration and immigration of individuals
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Genetic Drift
◦ Random change in allele frequencies over generations
brought about by chance
◦ In the absence of other forces, drift leads to loss of
genetic diversity
 Elephant seals, cheetahs
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Genes move with individuals when they move
(emigrate or immigrate) into and out of a
population…and it changes the gene pool
Neighboring
Population
Original Population
After Migration
Many Generations
Later
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Genetic Drift – the random fluctuation in allele
frequencies over time, due to chance occurrences
alone
It is more significant in smaller populations
It increases the chance of any given allele
becoming more or less prevalent when the
number of individuals is small
Founder Effect
Original Population
After Lightning
Many Generations
Later
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Magnitude of drift is greatest in small
populations
of two or more forms of a gene that
• Alleles: One
code for different versions of the same trait.
•Genotype:
The genetic combination of two
alleles, which decides what a
person’s trait will be (ex. Bb)
• Phenotype:
The physical expression of a genetic
trait (ex. Bb means Brown eyes)
B=brown
b=blonde
Picture all of the alleles of a population as being together
in a large pool called a gene pool.
The percentage of any specific
allele in the gene pool is called
the allelic frequency.
Look around the room….
What is the gene pool for hair
color???
What is the allelic frequency
of people with blonde hair?
GENETICS AND EVOLUTION
• Evolution occurs as a population’s genes and their
frequencies change over time.
•This can take millions of years for a species to change
GENETICS AND EVOLUTION
• A population in which the frequency of alleles
remains the same over generations as being in
genetic equilibrium.
• A population that is in genetic equilibrium is NOT
evolving or changing.
“Evolution is simply a change in frequencies of alleles in the gene
pool over time.”
The definition of evolution was developed in the early
20th century by
Godfrey Hardy, an English mathematician, and
Wilhelm Weinberg, a German physician.
Hardy
Weinberg