Download Evolution Part 2

Evolution Part 2
By the end of this class you should understand:
• How genetic variation leads to natural
selection and adaptive radiation
• The different effects that environmental
factors can have on populations
• The nature of sexual selection and sexual
competition
• How genes can flow from one population to
another and how speciation occurs
Darwin’s Finches
• The famous example of natural
selection that led Darwin to
make sense of the diversity of
life was the finches on the
Galapagos islands
– They were clearly very similar but
had different beaks on different
islands
– The beaks allowed them to eat
different things
Struggle for Existence
• In an ordinary population, there is a
“struggle for existence” (Darwin’s
own words) that keeps populations
at about the same size from year to
year despite rapid reproduction
– Darwin calculated that two breeding
elephants with no death except from
old age would sire a population of 15
million in 500 years
• When the first finches arrived at
this island, they found no predators
and no competition and so they
didn’t have the same struggle
Genetic Variation
• Genetic variation is normal in
all populations thanks to
mutations that make recessive
and codominant alleles
– This includes for important
things like beak shape
• Normally, the only food the
bird has a chance at getting is
the food that the bird’s beak is
already specialized to get
– There’s a fierce competition for
all the foods!
Brave New World
• When a population arrives at a new, untapped area, the population
will grow quickly, which means many unusual mutations that
normally get weeded out quickly will instead become part of the
population
– Slightly thicker and slightly more curved beaks
• These finches will happen to find other foods are easier to get with
their unusual beak!
– All random chance!
Adaptive Radiation
• Anytime a population
suddenly finds a lack of
competition, the
variations that are
normally weeded out
expand into new species
• This is known as adaptive
radiation
– Very different from
regular radiation
• Can you think of another
example of this?...
Permian Extinction!
• The end of the Permian era saw a
mass die-off of most large
animals and almost all marine life
– Lystrosaurus went extinct ->
• In the late Permian and early
Triassic, dinosaurs evolved, at
first represented by only a few
species
– Coelophysis ->
• By the Jurassic era, dinosaurs had
undergone adaptive radiation to
a multitude of species!
– Behold! ->
Adaptive Radiation in Action
Danger and Opportunity
• It seems that the most species
changes happen when the
environment changes the most
• When the environment is stable,
the most successful organisms
resemble their successful
parents, and the species becomes
more specialized
• Catastrophes are also
opportunities for new species to
emerge and dominate
– Mammals did this too!
Quantifying Opportunity
• A species cannot grow a new
feature simply by trying, the
mutations must exist in the gene
pool already
• Many mutations are lethal
mutations or neutral mutations
but occasionally one appears that
makes a different phenotype
– Whether it’s beneficial or not
depends on the environment!
“Beneficial” Mutation
• One organism’s beneficial
mutation is another organism’s
harmful mutation
– It all depends on the environment!
• In the 1800s, when the Industrial
Revolution hit England, a lot of
the trees with light-colored lichen
became soot-covered and the
lichen died
– The light-colored Peppered Moth
that camouflaged on the tree had
a new problem
Brain Check!
• What environmental pressures were acting on
this moth?
• Which of these pressures might be altered by
having the trees change color?...
The Birds!
• Birds are monstrous creatures
to insects, and they hunt
primarily by sight
• Being visible to birds is a
serious threat for these moths
• Thanks to genetic variation,
there was already a rare
recessive allele that caused the
moth to be dark instead of light
– Guess who suddenly had the
advantage!
Change in Allele Frequency
Gene Pool Changes
• When organisms with one allele become more common
than those with another allele, this is called a change in
allele frequency
• If these changes are in response to the environment
(which they usually are) it is referred to as microevolution
• If the changes are random, it is referred to as genetic drift
More on the Peppered Moth
• The light-dark moth saga is an example of
directional selection, when one attribute (being
lighter or darker) is strictly better than the other
in a given environment
• Once pollution controls went into effect, the light
moths became more prevalent again
• Evolution happens at the speed of reproduction,
so bacteria evolve super fast, insects evolve fast,
and mammals evolve slowly
Directional Selection
• Directional Selection is what
happens when one
particular environmental
factor changes that benefits
one part of the population
more than another
– Camouflage changes and
one particular predator is a
serious threat
• In reality, there are usually
many threats with many
possible solutions
– Not all predators hunt by
sight!
Disruptive Selection
• Another type of selection is
disruptive selection, when
having more or less of a
particular trait is favored
– One example is the African
Seedcracker, where large or small
bills are helpful when food is
scarce
• Currently these two
phenotypes interbreed freely
Speciation
• The African Seedcrackers are liable to ultimately
undergo speciation (one species becoming two)
– Why? The large-bills that mate with the small-bills will
mostly have intermediate-bill babies that will die off
when food is scarce
• If any large-bill birds develop a genetic preference
for mating with other large-bills, their offspring
will have two traits:
– Large bills
– A preference for mating with other large-bills
Sexual Selection
• Animals do not typically mate
randomly but instead choose their
mates intentionally
– This choice can be driven by
“preference” which can be genetic in
nature
• This means if some trait will result in
more viable offspring, ultimately
more animals will prefer to mate
with animals with this trait
– This is because they actively chose to
mate with animals that had this trait
– This is known as sexual selection
Brain Check!
• What are things that are considered
“attractive” in humans?
• Why might we humans be undergoing sexual
selection for these traits?
Additional Sexual Selection
• Peacocks are well-known to
have massive tails which are a
hindrance to surviving in the
wild
– The ladies LOVE a survivor
– Also requires good health to
maintain all those feathers
symmetrically
• This is an example of sexual
dimorphism
– Many animals exhibit traits
found only in one gender,
almost always sexual selection
Sexual Dimorphism!
Fun Fact!
• The larger owl is in fact the female!
• Larger males seem to be selected for
with larger harems
• Birds of prey are one of the few
species that are naturally
monogamous, and the females are
larger in most of these cases
– No, humans are NOT naturally
monogamous
• Chimpanzees have a similar gender size
ratio and the dominant male mates with
all the ladies in the group
– No I am NOT encouraging you to cheat on
your significant other
Sexual Priorities in Mammals
• Consider that males can pass on their genes to as
many offspring as they can father
– Male goal: mate as much as possible
• Females, on the other hand, grow the next
generation inside their bodies and also literally
provide food for the babies out of their own body
– Female goal: mate with a successful and healthy male
– Additional goal: mate with a male who will actually
stick around and help
This explains a lot of cultural problems:
Bottlenecks and Founder Effect
• Additional force in
evolution is a bottleneck
– A bottleneck is when a
population loses many
individuals in a catastrophe
of some kind and alleles are
lost in the process
• Another version is the
founder effect: only a few
organisms make it to a new
island
– The resulting population
only has as much diversity
as those founders
Gene Flow
• In large populations, genetic drift (random variation
of allele frequency) and bottlenecks are not really a
problem
• In smaller populations they can be an issue, but
often organisms travel from one area to another and
mate in the new region
– Referred to as gene flow
– This maintains genetic diversity
Lack of Gene Flow
• If two populations become isolated (no more
gene flow), the allele frequencies will begin to
diverge from each other
– Especially if the environments are different!
• This ultimately will result again in speciation
– In particular this is known as allopatric speciation,
where the organisms are physically separated
– The previous example of African Seedcrackers
would be a case of sympatric speciation, where
the organisms are still in close proximity
See you in lab!