Population Genetics

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Population Genetics
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You may have heard that...

• Red hair will eventually die out because its
  recessive...
• Brown eyes are dominant so they will take over...
• Little toes will go away because we dont use
  them...
• Humans will get bigger brains because we need
  them...

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But is that how it works?
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Genetics Populations

• Evolution Change in the gene frequencies of a
  population across generations.
• Red hair and blue eyes are inherited traits. If
  red hair or blue eyes are to die out, what
  would cause this kind of change in the genetics
  of a population?

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Hardy Weinberg

• Hardy Weinberg asked the opposite question
  Under what conditions would there be NO change in
  the genetics of a population?
• Conditions of NO change large population, random
  mating, no migration, no mutations, no selection.

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Population size Random events have a larger
effect on smaller populations.
(a) Population size 10,000
(b) Population size 4
In the large population, allele frequencies
remain relatively constant.
frequency of allele A
frequency of allele A
In the small population, one allele may
become extinct in a few generations.
generation
generation
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A drastic change in population size decreases
genetic diversity in later generations.
A bottleneck event drastically reduces the size
of the population.
The gene pool of a population contains equal
numbers of red, blue, yellow, and green alleles.
After the population grows and returns to its
original size, blue alleles predominate red and
green alleles have disappeared.
By chance, the gene pool of the
reduced population contains mostly blue and a few
yellow alleles.
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Non-random mating
Mate preference is non-random. Even in the same
species, many animals will prefer mates that
resemble themselves.
Many living species have ways to compete for
mates. In some populations, only a few
individuals can mate.
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Sexual selection, the result of non-random
mating, selects for traits that secure mates
though some attractive traits are drawbacks in
other ways.
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Migration Migration can remove genes from
populations, add genes to a population, or create
a small population with different gene ratios
than the population it derives from.
Founder effect and polydactyly in the Amish.
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Mutations Small mutations creep into populations
over time. Some are helpful, some harmful, and
many are neutral but may be beneficial or harmful
later. Mutations increase genetic variation in a
population.
original DNA sequence
substitution
nucleotide pair changed from AT to TA
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Selection Selection occurs when a trait or set of
traits gives a survival advantage or a
disadvantage.
Selection in favor trait helps individuals
survive and have offspring.
Selection against trait interferes with
survival and reproduction.
Traits increase in a population if they cause
individuals to have more offspring. Traits
decrease in a population if they kill off
individuals before they can reproduce.
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(c) DISRUPTIVE SELECTION
(a) DIRECTIONAL SELECTION
(b) STABILIZING SELECTION
Smaller-than-average and larger than- average
sizes favored.
Larger-than-average sizes favored.
Average sizes favored.
BEFORE SELECTION
percent of population
AFTER SELECTION
Average phenotype does not change
phenotypic variability declines.
Average phenotype shifts to larger size over time.
Population divides into two phenotypic
groups over time.
range of a particular characteristic (size,
color, etc.)
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• Is there any population on the planet where all
  Hardy-Weinberg conditions (large population,
  random mating, no migration, no mutations, no
  selection) holds true?
• What must be true of all populations of living
  things?

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Using what you know about population genetics,
reassess these claims

• Red hair will eventually die out because its
  recessive.
• Brown eyes are dominant so they will take over.
• Little toes will go away because we dont use
  them.
• Humans will get bigger brains because we need
  them.