Evolution by Natural Selection

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Proximate & Ultimate
Causation
The focus of all science…
Why something happened…
What Caused the why?
• A proximate cause is an event which is closest
to, or immediately responsible for causing,
some observed result.
• This exists in contrast to a higher-level
ultimate cause (or distal cause) which is
usually thought of as the "real" reason
something occurred.
• In Biology, first we locate the proximate cause
(Why something occurred) but we eventually
want to know the ultimate cause.
Airplane Crash
• An airplane crashed over the gulf of Mexico.
– Proximate Cause: The airplane was not able to
move fast enough to force air under its wings in a
way that it creates air-bouyancy. The plain lost
altitude due to gravity and eventually crashed.
– Ultimate Cause: Faulty electrical wiring caused an
electric short in the engine. This caused the
engine to fail. Without engine propulsion the
plane could not create the speed required and
crashed
Ultimate cause = proximate cause?
• In most situations, an ultimate cause may
itself be a proximate cause for a further
ultimate cause.
– Why did the electrical short occur.
• Proximate: It was not cleaned by the electrical
engineer prior to takeoff
• Ultimate: The electrical engineer was not adequately
trained to work with the plane model that crashed
– This often leads to another proximate and ultimate cause…
Proximal & Ultimate (Distal) Causation
(Ecology)
To give a biological example:
Why does the peacock have such long
tail feathers?
If… Then….
• From an evolutionary view point, scientists have
come to the consensus that for a trait to be
visible in a population, its benefit (fitness
increase) must outweigh any detriment (risk) to
its fitness
• The benefit must also have an ultimate causation
for how it increases fitness…
• A peacock is clearly slowed down by the
heavyweight tail feathers, and they expend a
massive amount of energy creating and carrying
the feathers
• If the feathers are detrimental, Then what
ultimately causes them to be beneficial?
Evolutionary Rule #1
(Form has a function)
• Every trait that we see commonly throughout
a population must have a positive fitness
affect.
– This also, but to a lesser affect is usually true for
even most uncommon traits within a population.
– It is rarely true of traits that are rare or no longer
seen within a population
Evolutionary Rule #2
(Changing Positive)
• Traits that have a positive fitness affect only
do so because of the environmental pressure
presently at that time
– If the selective pressure changes, it also often
changes the affect of the trait
– This can cause a positive trait to become negative
or a negative trait to become positive
Evolutionary Rule #3
(Selfish Gene)
• Genes have an innate ability to increase their
likelihood of being passed on to the next
generation.
• This controversial concept, devised by Richard
Dawkins, gave research based reason for why
organisms seem to be obsessed with passing on
and attaining the best genes possible
• Although this rule is still in its infancy, it seems to
have the ability to explain selective pressures (like
the transition to asexual reproduction) that were
previously “unexplained”
The Selfish Gene
• It is also important to remember, when
considering the selfish gene, that natural
selection happens as often within a
population as it does between them
• When trying to decide the evolutionary
reason for a trait, often times the selective
pressure is intraspecific
• IE: Sexual selection and mate competition
• IE: Y killer genes & genes that benefit only one
sex
Evolutionary Rule #4
(The red queen)
• As organisms change due to natural selection,
so do the pressures that are on the organism.
Therefore, the evolution of fitness and
selective pressures could be said to coevolve.
• The term is taken from the Red Queen's race in
Lewis Carroll's book, Through the Looking-Glass.
• The Red Queen said, "It takes all the running
you can do, to keep in the same place."
• This rule is the focus of the evolutionary arms
race
Evolution by Natural Selection
Round II
Now it is time to dive into the theory
that defines Biology
Natural Selection
• Darwin used the term “Descent with
Modification” to describe evolution
• He meant that organisms ‘descend’ from
other organisms
• One species ‘descends’ or comes from another
species, by a means called natural selection
• Natural selection is the motor that drives
evolution
• Darwin explained Natural selection to have 4
rules
Natural Selection
• 1. The individual organisms that make up a
population vary in the traits they possess,
such as their size and shape.
– Which means that each individual organism in a
population have very distinct traits such as tall &
short; blue eyes vs. brown eyes
Natural Selection
Natural Selection
• 2. Some of the trait differences are passed on
to offspring.
– For example, Tall parents may tend to have tall
offspring
– This means the trait (tall) has some genetic basis
and is inheritable
Natural Selection
Natural Selection
• 3. Only a subset of the offspring that are
produced in each generation survives long
enough to reproduce, and of the individuals
that reproduce, not all produce the same
number of offspring. Thus, some individuals
in the population produce more offspring than
others do.
– Non-random selection is present
– Only those who survive are allowed to reproduce
& not all that reproduce do it the same way
Natural Selection
Natural Selection
• 4. The subset of individuals that produce the
most offspring is not a random sample of the
population. Instead, individuals with certain
traits are more likely to produce the greatest
number of offspring in a given environment.
The individuals with these traits are Naturally
Selected
– It is Not the number of offspring you produce that
gives your genes the best chance of survival, it is
WHICH genes you have that decides your chances
Natural Selection
• Therefore, it is through the passing on of the
BEST genes that offspring is able to survive
• Because the offspring are able to survive, they
are able to reproduce
• Once they reproduce, they pass on the good
genes to their offspring, which are able to
survive and repeat the process
• The parents that are Fit are able to
successfully pass on half of their good genes
• Bad gene combinations fail…
Natural Selection
• The reason natural selection is called the
‘engine’ that drives evolution is because it is
this selection that causes change
• As a specific trait is chosen for it begins to
become dominantly seen in a community
• Sometimes that change begins to separate the
different members of a community
– Especially if there is habitat separation of species
• If this occurs, you may begin to get new
species over time
• The new species would have ‘evolved’ from
the original species
Evolutionary Rule #5
The big NO-NO!
• The problem with natural selection is that
everyone thinks that it gives a new species
what it needs to survive
• However, that is not at all the case
• Natural selection merely chooses for a trait
that is already there
• It DOES NOT give a trait
• Over time, it may make a trait better or more
distinct, but the trait needed to already be
there in the first place
The big NO-NO!
Natural Selection
Tools of Evolution
• Evolutionary Biologists have used several
common techniques for proving and mapping
evolution:
– Fossils
– Vestigial Traits
– Structural Homologies
– Embryological Homologies
– Tissue (cellular) Homologies
– Comparative Evolution
Fossils
• We can to some degree use fossils to map
evolution
• However, there are large gaps in the fossil
record that to the common person seem
strange
• It is often forgotten when viewing the many
dinosaur fossils found in museums that the
dinosaurs existed for MILLIONS of years, and
that is the only reason why there are a lot of
fossils of them
• Most species are lost without ever being
fossilized
Whale Evolution
Vestigial Traits
• One of the main forms of evidence for
evolution is the presence of Vestigial organs
• This is an organ that is either incompletely
developed or reduced and has no apparent
use
• Often these organs are remarkably similar to
other organs in similar species
Vestigial Traits
Vestigial Traits
•
•
•
•
•
•
•
•
•
•
The Wings on Flightless Birds
Hind Leg Bones in Whales
Body hair and goose bumps on humans
Human tailbone
Blind fish and subterranean organisms
Human wisdom teeth
Sex organs of dandelions (clones)
Fake sex in whiptailed lizards
Male breast tissue and nipples (Mirroring?)
Human appendix (plant/undercooked meat
digestion)
Evolutionary Rule #6
The other big NO NO!
• Evolution is NOT LINEAR! (it is not a straight
line!!)
• Evolution states that things evolve from
COMMON DESCENT or common ancestors
• Evolution by no means has ever claimed that
humans are descendants of monkeys, only that
they shared a common ancestor
• Only the most fit organism filling a niche will
survive, therefore nearly all “ancestral organism”
have been replaced by transitional forms
Structural Homologies
• Any two morphological traits (morphology is
the study of the form or shape of an organism
or part thereof) that are similar between
species
• The observable similarities between species
• This has become the leading evidence for
divergent evolution (the evolution we
commonly discuss)
• Common structures help to give evidence of
ancestral organisms and common ancestry
Structural Homologies
Embryological Homologies
Divergent Evolution
• Divergent Evolution is what we commonly
think of when we think of evolution
• One organism is similar to another organism
because it SHARES a common ancestor
• This is also called the process of adaptive
radiation
Divergent Evolution
Convergent Evolution
• Sometimes, organisms can exhibit
homologous traits with another species yet
have NO recent relation (they don’t recently
have a shared ancestor)
• This is called Convergent Evolution
• This occurs when natural selection favors
similar solutions to the problems posed by a
similar way of making a living
• The traits are called Analogous structures
Convergent Evolution
Evolutionary Rule #7
Violating Hardy-Weinberg
• For evolution to be occurring in a population,
the Hardy-Weinberg principle must be
violated at some point
• Dr. Hardy & Dr. Weinberg were charged with
statistically showing that evolution occurs
within a population
• They came up with a list of assumptions that
would only occur in a non-evolving population
Proving Evolution
• One issue that evolutionary biologists always
dealt with is the slow process of evolution in
animals
• But realize at a genetic level, evolution is
much more observable
– When a mutation occurs it is visible in an
individual.
– If it the mutation is successful, it will become
quickly visible within a population
– If the population is separated and genetic flow is
prevented, evolution will speed up
– This is how you can observe SPECIATION
Proving Evolution
• However, it is not always easy to sequence the
genes in a population, and it is definitely NOT
cheap
• How can you prove evolution, affordably???
– The same way you prove if your independent variable
had an affect on your experiments…
• USE YOUR NULL HYPOTHESIS
• That was the idea used by G. H. Hardy & Wilhelm
Weinberg who published independently in 1908
Hardy-Weinberg Principle
• Hardy & Weinberg analyzed what happens to
the frequency of alleles when many
individuals in a population mate and produce
offspring
– This was a new way of thinking about genes; up
until this time the fusion of gametes in Punnett
squares was the only observation of gene
frequencies
• Rather than thinking of genes in an individual,
they considered all genes in a group of genes
called a population gene pool
The H-W Principle on the surface looks
similar to a Punnett square, but deals
with multiple individuals and offspring
Hardy-Weinberg Equilibrium
• The Hardy–Weinberg principle states that both
allele and genotype frequencies in a population
remain constant
• —that is, they are in equilibrium—from
generation to generation unless specific
“disturbing influences” are introduced.
• In this equilibrium, there is NO evolution by
natural selection
– The population WILL NOT CHANGE if it is in HardyWeinberg equilibrium
• It is in the “disturbing influences” that evolution
and natural selection exist
“disturbing influences” =
H-W Assumptions
• The disturbing influences mentioned are what
are called the H-W assumptions, that MUST
be true for a population to remain unchanged
– No selection is occurring
– No genetic drift is occurring or random allele
frequency changes
– No Gene Flow
– No Mutations
– Mating is random within a population
If… Than…
• IF the Hardy-Weinberg principle is violated in
any of its 5 assumptions, THAN evolution is
occurring in a population.
• This theory has been proven at a genetic level,
a gene frequency level, and even at a
population level
• Therefore, by proving or disproving H-W
Equilibrium, you can prove or disprove
evolution’s presence… and existence…
Other uses of H.W.P.
• The Hardy-Weinberg Principle also allows us
to determine the potential of allele
combination if allele frequency is known
– For instance, if you found out that you were a
carrier of Cystic Fibrosis you could use H.W.P. to
determine the probability of finding a mate that is
a C.F. carrier
Natural Selection
• Often times genetic variation seems to be
selected for in a population.
• Statistically speaking, it is usually a
heterozygous population that has the highest
level of fitness
– This is called the heterozygous advantage
– This advantage causes genetic variation in a
population
• This shows the benefit of sexual selection
3 Types of Natural Selection
• Directional selection is when allele
frequencies change in one direction
– This will change the average value of a trait
3 Types of Natural Selection
• Stabilizing Selection is when the selection
causes NO CHANGE in the average value of a
trait over time, BUT can reduce the genetic
variation.
3 Types of Natural Selection
• Disruptive selection is the selection that
eliminates the phenotypes near the average
value and favors the extremes
Evolutionary Rule #8
Intergenomic Conflict Theory
• Traits that we see within a population are the result of gene
conflict
– A gene, by definition, if the result of genes that were successful
at the expense of other genes
• Genes can be thought of as individuals striving to get ahead
– A gene on its own is one v.s. many
– Genes working together (gene cooperation of a chromosome)
has a way to allow gene specialization
• Gene fate is based on the quality of the team
– Sexual reproduction is a way for a good gene to get off of a bad
team
• It is possible that our “want to have sex” is derived by the wants of our
genes
Natural or Sexual
• We have already discussed natural selection in
its entirety… the idea that the fittest
organisms survive
• SO WHY DO ADAPTATIONS THAT DO NOT
PROMOTE FITNESS EXIST?????????
– This idea not only perplexed Darwin, but it was
the main attack used by those who disagreed with
Darwin’s legendary theory…
Sexual Selection
• The fourth type of selection is not a direct form
of natural selection, but rather it is the selection
that occurs when a phenotype is favored in order
to increase mating…
• This is called Sexual Selection and was also a
theory published by Darwin later in his life
– It was called his last great theory
• Sexual selection often conflicts with natural
selection in that some phenotypes that are
favored may not increase fitness
• THIS IS STILL A “NATURAL SELECTION”
Sexual Dimorphism
• As the name implies, sexual dimorphism is any
trait that differs among different sexes of the
same species
– Mains in some lions
– Tails on peacocks
– Antlers on deer
• Sexual dimorphism is generally caused by
sexual selection
Evolutionary rule #9
The Sexy Son
• Sexual selection is as much of pressure within a
species as intra-specific pressure is from one
species to another
• The Sexy Son is a theory that states that sexually
attractive parents tend to make sexually
attractive offspring
• Therefore, an organism that contains genes that
are sexually appealing to their mate has the best
chance of producing successful offspring
• As it was stated by Weatherhead and Robertson
in 1979, it was claimed to be the only reason for
female selection
Artificial Selection
“Human Selection”
• Selection that is driven by human force is
known as artificial selection.
– When we choose which organisms survive
– When we choose which organisms interbreed
• Artificial selection is a major part of
agriculture and game hunting
• Humans are changing the rules of selection
– Transgenic organisms and GMOs
– Hybrids
– Vaccinations and medications
Random Selection
• When an organism is being preyed upon for
no apparent reason.
– If a predator eats (and is able to eat) the first thing
it sees
• If mating is random and there is no adaptive
benefit or reason in mate selection
Genetic Drift
• Genetic drift is the indirect loss of alleles through
random selection.
• It causes allele frequencies to drift up and down
randomly over time.
– Also called Sampling error
• Because it causes a change in the population because of
“blind luck”
• It is totally random with respect to fitness
• It is most pronounced in small populations
• Overtime, it can lead to the random loss or
fixation of alleles
• The smaller the population the larger the threat
of Genetic Drift eliminating important alleles
Bottlenecking
• Population bottlenecking: is the drastic
reduction in size of a large population to a
small population
– Often causes
• Genetic Bottlenecking: the drastic loss of
alleles from a divers gene pool.
– Can be caused by natural and unnatural events
– Is a major problem in wild populations today
Mutations Role
• We know that evolution is limited by the
available genes and mutation is the only way
to get new genes
• Without mutation, the big picture of evolution
will stop
• However, positive mutation is a remarkably
slow process
• Only 1 in 5,000 offspring will carry a mutated
gene
• Only 1 in 4 of those will show a phenotypic
change from the mutation
• Consider a human population of 195,000.
• 5,000 offspring are born per year
• At the end of year #1, there are 400,000 copies
of each gene in the 200,000 person pop.
• Only 1 of them is a new allele from mutation
• =1/400,000 or .0000025
• At this rate, it would take 4,000 years for a 1%
allele frequency change
• Mutation is a slow factor of evolution…
– There are some things that speed up mutation
Evolutionary Rule #10
The Island Theory
• This was the motivation behind E.O. Wilsons
island theory
– This theory suggested that evolution is far more
rapid on islands because gene flow is restricted
and selective pressures are increased
– Beneficial mutant genes are far more likely to
dominate in an island population
– Islands do not have to be surrounded by water!
• Earthquakes, the formation of mountains, and many
anthropogenic (human) creations
The Island Theory
• Wilson also used the island theory to
understand the levels of diversity on islands
– Depending on the size of the island and the
distance of the island from the mainland it can be
predicted how many different species will be
found there
– Remarkably, even with a reduced gene pool,
islands can, over time, lead to tremendous levels
of specialized speciation
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