Evolution Part 2

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Evolution Part 2
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By the end of this class you should understand

• How genetic variation leads to natural selection
  and adaptive radiation
• How allele frequencies in a population may be
  altered by the environment
• The different effects that environmental factors
  can have on populations
• The nature of sexual selection and sexual
  competition

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Darwins 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

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Struggle for Existence

• In an ordinary population, there is a struggle
  for existence (Darwins 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 didnt have the same struggle

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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 birds beak is
  already specialized to get
• Theres a fierce competition for all the foods!

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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!

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Adaptive Radiation

• Anytime a population suddenly finds a lack of
  competition, the variations that are normally
  weeded out expand and
• This is known as adaptive radiation
• Very different from regular radiation
• Can you think of another example of this?...

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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 -gt
• In the late Permian and early Triassic, dinosaurs
  evolved, at first represented by only a few
  species
• Coelophysis -gt
• By the Jurassic era, dinosaurs had undergone
  adaptive radiation to a multitude of species!
• Behold! -gt

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Adaptive Radiation in Action
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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!

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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 its beneficial or not depends on the
  environment!

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Beneficial Mutation

• One organisms beneficial mutation is another
  organisms 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

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Brain Check!

• What environmental pressures were acting on this
  moth?
• Which of these pressures might be altered by
  having the trees change color?...

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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!

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Change in Allele Frequency
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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

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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

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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!

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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

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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

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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

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Brain Check!

• What are things that are considered attractive
  in humans?
• Why might we humans be undergoing sexual
  selection for these traits?

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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

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Sexual Dimorphism!
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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

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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

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This explains a lot of cultural problems
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Gene Flow

• In large populations, genetic drift (random
  variation of allele frequency) tends to even out
  over time
• 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

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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

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See you in lab!