Evolution of Populations

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Evolution of Populations
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• When Darwin developed his theory of evolution, he
  did not understand
• how heredity worked.
• This left him unable to explain two things
• a. source of variation
• b. how inheritable traits pass from one
  generation to the next

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• In the 1940s, Mendels work on genetics was
  rediscovered and scientists began to combine
  the ideas of many branches of biology to develop
  a modern theory of evolution. When studying
  evolution today, biologists often focus on a
  particular population. This evolution of
  populations is called microevolution.

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• 2. Vocabulary
• population group of individuals of the same
  species living in the same area that breed with
  each other.

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• 2. gene pool combined genetic info. for all
  members of a population

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• 2. allele one form of a gene

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• 2. relative frequency of an allele times an
  allele occurs in the gene pool compared to other
  alleles (percent)

Example Relative Frequency 70 Allele B 30
Allele b
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• 3. Sources of Variation
• a. mutations any change in DNA sequence
• Can occur because of
• mistakes in replication
• environmental chemicals
• May or may not affect an organisms phenotype

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• 3. Sources of Variation
• b. Gene Shuffling recombination of genes that
  occurs during production of gametes
• Cause most inheritable differences between
  relatives
• Occurs during meiosis
• As a result, sexual reproduction is a major
  source of variation in organisms.
• Despite gene shuffling, the frequency of alleles
  does not change in a population. Explain why this
  is true.
• Similar to a deck of cards no matter how many
  times you shuffle, same cards (alleles) are
  always there.

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• 4. Gene Traits
• A) Single gene trait controlled by single gene
  with two alleles
• Examples widows peak, hitchhikers thumb,
  tongue rolling

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• (4. Gene Traits)
• B) Polygenic trait controlled by 2 or more
  genes, each with 2 or more alleles
• Examples height, hair color, skin color, eye
  color
• Most human traits are polygenic.

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• Do the following graphs show the distribution of
  phenotypes for single-gene or polygenic traits?
  Explain.

type single gene why? Only two phenotypes
possible Example tongue roller or non-tongue
roller
type polygenic why? Multiple (many) phenotypes
possible Example height range 4feet to 9 feet
all
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• 5. Natural selection acts on phenotypes, not
  genotypes.
• Example in a forest covered in brown leaves,
  dirt and rocks which mouse will survive better
  brown or white?
• Brown, more hidden.

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• 5. If brown is dominant can the a predator tell
  the difference between
• Mouse with highest fitness will have the most
  alleles passed on to the next generation.
• White mouse will have low fitness

BB Bb
?
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• 5. Which mouse will have the lowest fitness?
• White, bb (recessive)
• Will the fitness of BB and Bb differ? Why?
• No, Both BB and Bb have the same fitness
  advantage of being brown

BB Bb
?
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• 6. Three ways in which natural selection affects
  polygenic traits.

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• a. Directional Selection individuals at one end
  of the curve have higher fitness so evolution
  causes increase in individuals with that trait
• Individuals with highest fitness those at one
  end of the curve
• Example Galapagos finches beak size

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Directional Selection (page 398)
Directional Selection
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• b. Stabilizing Selection individuals at the
  center of the curve have highest fitness
  evolution keeps center in the same position but
  narrows the curve

Individuals with highest fitness near the center
of the curve (average phenotype) Example human
birth weight
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Stabilizing Selection
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• c. Disruptive Selection individuals at both
  ends of the curve survive better than the middle
  of the curve.
• Individuals with highest fitness both ends of
  curve
• Example birds where seeds are either large or
  small

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Disruptive Selection (pg 399)
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The Process of Speciation

• The formation of new biological species, usually
  by the division of a single species into two or
  more genetically distinct one.

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Three Isolating Mechanisms Isolate species
forming subspecies and perhaps causing
speciation.

1. Geographic Isolation
2. Behavioral Isolation
3. Temporal Isolation

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1. Geographic Isolation

• Two populations separated by a geographic
  barrier river, lake, canyon, mountain range.

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Example 10,000 years ago the Colorado River
separated two squirrel populations.

• Kaibab Squirrel Abert Squirrel

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This resulted in a subspecies, but did not result
in speciation because the two can still mate
if brought together

• Kaibab Squirrel Abert Squirrel

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2. Behavioral Isolation

• Two populations are capable of interbreeding
  but do not interbreed because they have different
  courtship rituals or other lifestyle habits
  that differ.

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Example Eastern and Western Meadowlark

• Eastern and Western Meadowlark populations
  overlap in the middle of the US

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Example Eastern and Western Meadowlark

• Male birds sing a matting song that females like,
  East and West have different songs. Females only
  respond to their subspecies song.

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3. Temporal Isolation

• Populations reproduce at different times

January
1 2 3 4 5 6 7 8 9
10 11 12 13
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Example Northern Leopard Frog North American
Bullfrog

• Mates in Mates in
• April July

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Conclusion

• Geographic, Behavioral and Temporal Isolation are
  all believed to lead to speciation.

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However

• No examples ever observed in animals
• A couple examples that may demonstrate speciation
  exist in plants and some insects.

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Genetic Drift

• random change in allele frequency that occurs in
  small populations

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• The results of genetic crosses can usually be
  predicted using the laws of probability. In small
  populations, however, these predictions are not
  always accurate.
• a. Founder effect allele frequencies change due
  to migration of a small subgroup of a population
• Example fruit flies on Hawaiian islands

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Two phenomena that result in small populations
and cause genetic drift

1. Founder Effect
2. Bottleneck Effect

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Founder effect

• allele frequencies change due to migration of
  a small subgroup of a population

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Founder Effect Fruit Flies on Hawaiian islands
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2. Bottleneck effect

• major change in allele frequencies when
  population decreases dramatically due to
  catastrophe
• Example northern elephant seals
• decreased to 20 individuals in 1800s, now
  30,000
• no genetic variation in 24 genes

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Bottleneck Effect Northern Elephant Seal
Population

• Hunted to near extintion
• Population decreased to 20 individuals in 1800s,
  those 20 repopulated so todays population is
  30,000
• No genetic variation in 24 genes

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Bottleneck Effect
Original population
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Bottleneck Effect
Catastrophe
Original population
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Bottleneck Effect
Catastrophe
Surviving population
Original population
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Another picture to illustrate bottleneck effect
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