Transcript Causes of Evolution

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Evolution – A scientific theory that states that
living species are descendants of ancestral
species that were different from present day ones
(the genetic changes in a population over
generations)
◦ Process by which the frequency of heritable traits in a
population changes from one generation to the next
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Scientific Theory – a well-supported explanation
for some aspect of the natural world that
includes many observations, inferences, and
tested hypotheses (it is not “just an opinion or
belief”)
◦ Main difference between scientific law and theory is that
a law says that something does occur while a theory
attempts to explain why it occurs.
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Phenotype = physical structure determined by
genotype
Genotype = individual’s genes
Gene = enough DNA to make 1 polypeptide (1
part of a protein)
◦ Genes can code for morphology, physiology, or behavior
◦ Ex. Hair color
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Allele = one of several (or many) varieties of a
gene
◦ Ex. Black or blonde hair
◦ Evolution is change in frequency of these alleles
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Like Darwin, he also believed populations change
over time
BUT, his beliefs are no longer supported
Lamarck’s ideas on evolution:
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2.
Use and disuse
Inheritance of acquired characteristics
(Ex. Bodybuilder parents
Bodybuilder baby)
(1) Descent with Modification
 Basically, the process of evolution
 Living species descended with changes
(modifications) from prior species
 Species must be able to change over time
 Ex. Darwin’s Finches
(will discuss later)
(2) Natural Selection “Survival of the fittest”
 Individuals possess alleles (genotypes) that generate
traits (phenotypes) that enable them to cope more
successfully in their environment than others
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Fitness -(1) Survive Longer, (2) Reproduce More
(3) Pass on traits to next generation
Favorable traits are “adaptations” while unfavorable
are “maladapations”
Occurs in populations, not in individuals
◦ Note: It is not essential to survive longer. The longer life
simply gives more time to produce more offspring. Ex.
Evolution would favor someone who was 20 and had 4 kids
rather than someone who is 80 with 1 kid.
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Populations possess an enormous
reproductive potential
Population sizes remain stable
Resources are limited
Individuals compete for survival
There is variation among individuals in a
population
Much variation is heritable
Only the most fit individuals survive
Evolution occurs as favorable traits
accumulate in the population
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13 different species of finches on the Galapagos Islands
Each species has a beak that is best adapted for a certain
kind of food that is found on the different islands
Observed that beak size increased by 10% in one year
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Natural selection does not cause new
phenotypes. It only “selects” phenotypesalready present in the population that
maximize fitness.
If the environment changes and there is not a
suitable phenotype to survive in the new
environment then the species will go extinct.
There is no amount of “trying” that can help
you evolve.
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The next few slides are all species observed
by Darwin on the Galapagos Islands that led
to him create his theories of descent with
modification and natural selection.
Video
Video
Galapagos Tortoise
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Lamarck
“Acquire” new traits
though use or disuse
“Acquired” traits are
passed on to offspring
Evironment creates a
need for certain
“acquired” traits
Eventually get a new
species
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Darwin
Inheritable traits
already exist
Organisms with “new”
and “different” trait can
pass it on to offspring
Certain traits are
“selected” by nature
because the organism
is better able to survive
Eventually get a new
species
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The study of evolution on its smallest scale.
Microevolution – change in allele frequencies
in a population over generations
◦ (Remember that evolution does not occur in one
individual or during one lifetime)
Ex. Darwin’s Finches
Peppermoths
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Although Darwin understood that populations of organisms
change over time, he never understood how these heritable
traits were passed on. Gregor Mendel’s work on genetics
(which was published a few years after Origin of Species)
provided this mechanism
Usages:
◦ Average heterozygosity – used to quantify gene variability
◦ Geographic variation – differences in genetic composition
of separate populations
 Clines – a graded change in a character along a
geographic axis
Mutations - give rise to completely new characteristics
(and is the ultimate source of new alleles). All other
contributions to variation simply rearrange existing alleles to
new combinations
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MUTATION: a change in a nucleotide sequence of an
organism’s DNA (a change in a gene)
◦ Remember that a gene is enough DNA to control 1 trait
Remember that mutations are random and are typically
deletorious. On the rare occasion when mutations produce
favorable traits, a population’s gene pool (all of the genes
in a population) is changed because of the introduction of
this new, favored trait. The population will then evolve
through natural selection to show a greater percentage of
the new, beneficial trait.
Two Locations of Mutations:
(1) Changes in genes in somatic (body) cells:
can affect the organism itself, but cannot be
passed to sexually produced offspring.
(2) Changes in genes in gametes (egg and
sperm): do not affect the parent organism
itself, can be passed to sexually produced
offspring
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Mutations may result in a trait that improves an
organism’s chances for survival, so that organism
would be more likely to reproduce. Then the
favorable mutation would be passed on to
offspring.
This results in evolution.
Mutations occur very rarely (and are almost
always bad), so populations evolve slowly.
NOTE: EVEN THOUGH EVOLUTION GENERALLY
OCCURS SLOWLY…IT SOMETIMES CAN OCCUR
RAPIDLY IF THE NEW MUTATED TRAIT IS
STRONGLY FAVORED OVER ALL OTHER
PHENOTYPES PRESENT IN THE POPULATION
2. Meiosis (and sexual reproduction) - gives us
variation in a population that creates a range of
phenotypes that can be acted upon by natural
selection.
◦ Crossing over
◦ Independent assortment of chromosomes
◦ Random fertilization
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3. Diploidy – presence of 2 copies of each
chromosome (can “hide” recessive allele from
natural selection)
4. Balanced polymorphism - maintenance of
different phenotypes in a population
◦ Heterozygote advantage – heterozygous condition bears
greater selective advantage than either homozygous
condition
 Ex. Malaria in Africa
◦ Frequency-dependent selection (minority advantage) –
least common phenotype has selective advantage
 Ex. Predator and search image
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5. Neutral Variation
Not all variation is acted upon by evolution
For example, the differences in fingerprint
patterns among humans represent neutral
variation
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Adaptive Evolution – natural selection increases
the frequencies of alleles that enhance survival
and reproduction. As the proportion of
individuals that have favorable traits increases,
the match between a species and its environment
improves
It may seem that through natural selection 1
particular allele would be “chosen” for each locus
if that allele made its host have greater fitness.
Even so, there are variables that maintain the
variation of phenotypes in an environment.
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Selection can act only on existing variations
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Evolution is limited by historical constraints
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Adaptations are often compromises.
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Chance, natural selection, and the environment interact
◦ “Ideal” trait may not be available and new advantageous alleles do not
arise on demand.
◦ Everything present today has to arise from an ancestor. We simply
have their structures that have been adapted to new environments.
(Can’t just decide to develop new beneficial trait)
◦ A seal spends part of its time on rock. It would be able to walk better
with legs, but would swim worse. The multi-faceted style of a species
lives may make a trait beneficial in one area but not another.
◦ Environment changes year to year so natural selection doesn’t have
time to perfect traits
◦ Chance events such as fires or species being wind-blown to islands
may mean that the pioneer species arriving at the island are not as fit
as those left behind thus the “starting ingredients” with which
evolution to act on are not as good.
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1) Natural selection
The environmental conditions consistently
causing the favoring of certain alleles over
others causes adaptive evolution (evolution
that results in a better match between
organisms and their environment)
We’ll study natural selection at greater length later
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2) Genetic Drift – changes in gene pool
due to chance (has greater effect on smaller
populations)
◦ Ex. Flipping a coin
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Examples
◦ 1) Bottleneck effect – drastic decrease in population
size may leave only certain alleles available. These
alleles will quickly be perpetuated.
◦ 2) Founder effect –When a few individuals colonize
a new area (same reasoning as bottleneck)
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Summary of Genetic Drift
1. Genetic drift is significant in small populations
2. Genetic drift can cause allele frequencies to
change at random
3. Genetic drift can lead to a loss of genetic variation
within populations
4. Genetic drift can cause harmful alleles to become
fixed
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Gene flow
Introduction or exit of alleles in a population
whenever individuals enter or leave.
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Like mutations, gene flow can introduce new alleles into a
population.
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Even so, mutation is the only mechanism that introduces
completely unique alleles
Once these new alleles are introduced to a population, natural
selection is then free to act to increase or decrease the
frequency of the allele through adaptive evolution
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Notice that all 3 causes for evolution (natural
selection, genetic drift, and gene flow) all
have 1 similarity – they change the GENE
POOL of the population.
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When the allele frequencies in a population
remain constant from generation to generation,
the population is said to be in genetic (or HardyWeinberg) equilibrium.
It can be determined whether a population is
evolving by comparing the actual allele
frequencies at a particular locus to the allele
frequencies which would occur in HW
equilibrium.
If there is no difference then the population is
not evolving. If there are differences then the
population is evolving, and we can attempt to
determine why.
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Allele frequencies for each allele (p,q)
Frequency of homozygotes (p2, q2)
Frequency of heterozygotes (pq + qp =
2pq)
p + q = 1 (All alleles sum to 100%. True for
traits with 2 alleles.)
p2 + 2pq + q2 = 1
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All individuals sum to 100%
◦ p2 = # of pp homozygotes
◦ 2pq = # of heterozygotes
◦ q2 = # of qq homozygotes
*Give verbal explanation of each variable
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A plant population consists of 84% plants
with red flowers and 16% with white flowers.
Assume the red allele (R) is dominant and the
white allele (r) is recessive. (Hint: p would
represent the dominant allele and q would
represent the recessive allele in the HW
equation).
1. Find the allele frequency for p.
2. Find the frequency (or percentages) of individuals
with the homozygous dominant and heterozygous
condition).
3. Is this population in HW equilibrium?
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No mutations
Random mating
No natural selection (trait is selectively neutral)
Extremely large population size (so no genetic
drift)
No gene flow (population is isolated from other
populations)
Basically, anything that causes evolution / is a source for variation cannot
occur.
Note: A population can be evolving at some loci but
remaining in HW equilibrium at other loci
Additional usage: estimating the percentage of a population carrying the
allele for an inherited disease (as long as these 5 criteria are met)
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1) Directional Selection
Selection that favors one extreme phenotype
(physical type)
This causes this particular extreme
phenotype to become much more common
(the look of the population is pushed in 1
direction)
Common when a population’s environment
changes or when members of a population
migrate to a new (and different) habitat.
Ex. Peppered Moths
2) Disruptive Selection
 Selection that favors both extreme
phenotypes (physical type)
 This causes extreme phenotypes to become
much more common (the look of the
population shows extreme differences)
 Ex. Darwin’s Finches
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3) Stabilizing Selection
Selection against the extreme phenotypes
(physical type)
This causes average phenotype to become
much more common (the look of the
population is very similar and average)
Ex. Human weight at birth
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Artificial selection – Natural selection that
“isn’t natural” and is made to occur by
humans
◦ Several species have evolved in certain ways due to
the influence of humans.
◦ Ex. Dog breeding, Crops
Sexual Selection - natural selection in which
individuals with certain inherited characteristics are
more likely than other individuals to obtain mates.
◦ Causes sexual dimorphism -marked differences
between the two sexes in secondary sexual
characteristics, which are not directly associated
with reproduction or survival
 Ex. peacocks
Types of Sexual Selection
1.
Intrasexual selection – individuals of same sex compete directly for mates
of the opposite sex
 Ex. Male patrolling group of females and fighting off other males
 Leads to evolution of antlers, horns, large stature or musculature
2.
Intersexual selection – Mate choice
Generally, female’s choice depends on showiness of the male’s
appearance or behavior. Females make a greater energy
investment in producing offspring so they increase their fitness by
increasing QUALITY of their offspring by choosing superior males.
Males, on the other hand, contribute little energy to the
production of offspring and thus increase their fitness by
maximizing the QUANTITY of offspring produced.
Often, the trait that improves reproductive success is
disadvantageous as well in that it makes them more
easily visible to predators. If benefit outweighs risk, then it
will be passed on at greater frequency
 Females are trying to select bright, showy males because it
indicates “good genes”
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1) Molecular Biology
All living things use DNA and RNA.
The genetic code amongst all living things is
universally the same. (In other words, the fact
that the RNA codon AUG gives the amino acid
MET in humans is seen in every other living
thing as well)
This fact more than any other (at least to me)
indicates that all living things originated from
a common ancestor.
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Molecular Biology – Continued
The closer 2 different species DNA is to one
another, then the closer those 2 are related.
Which 2 species are closer related?
Species A: AACTGGCTTA
Species B: AACTAACCCG
Species C: TACTGGCTTA
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The remains or traces of organisms that have
once lived on Earth.
Fossil Record: the history of life on Earth,
based on fossils that have been discovered
The fossil record shows how organisms have
changed over time and shows that the life on
Earth is about 3.5 billion years old.
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Extinctions occur when there are major
changes in the environment and species do
not have the adaptations to survive.
The fossil record helps scientists to discover
relationships between different groups of
organisms and determine common ancestors.
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The Earth is divided into
layers called strata.
Generally speaking, the
lower the strata = the
older the layer of Earth
(each new layer
becomes stacked on the
older layer below it)
This means if two
fossils are found, the
fossil found in the
lowest layer would be
the oldest
There are also
techniques for
determining the
absolute (or relatively
exact date) of a fossil’s
age
Convergent evolution –
process by which different
species evolve similar traits
(SAME FUNCTION, DIFFERENT
ANCESTOR)
Example: Birds, bats, and
moths have wings, but they
did not evolve from a recent
common ancestor.
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Caused by:
◦ living in similar habitats /
performing similar
functions
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Structures with closely related function but
do not come from the same ancestral
structure
◦ Same function, different structure
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Example: Birds, bats, and moths have wings,
but they did not evolve from each other.
Divergent evolution- build up of differences
between groups which can lead to the
development of a new species
 In other words, two different species that
evolved from the same ancestor.
 Caused by populations of the same species:
1. moving to two different environments or..
2. specializing in different areas of the same
environment
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Structures in different species that originated
from common ancestor
May have different function but similar
structure
Evidence for Evolution –
Vestigial structures – no longer have a use,
but may have had a use in evolutionary
history.
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Ex: human tail bone (coccyx) made of 4 fused
vertebrae that resemble the bones in an
animal’s tail.
Other examples: the appendix, and ear
muscles!