Download Population Genetics

Population Genetics
You may have heard that...
• Red hair will eventually die out because
it’s recessive...
• Brown eyes are dominant so they will
take over...
• Little toes will go away because we don’t
use them...
• Humans will get bigger brains because
we need them...
But is that how it works?
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?
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.
Population size
Random events have a larger effect on smaller populations.
In the large population,
allele frequencies remain
relatively constant.
generation
(b) Population size = 4
frequency of allele A
frequency of allele A
(a) Population size = 10,000
In the small population,
one allele may become
extinct in a few
generations.
generation
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.
By chance, the gene
pool of the reduced
population contains
mostly blue and a
few yellow alleles.
After the population
grows and returns to
its original size, blue
alleles predominate;
red and green
alleles
have disappeared.
Non-random mating
Many living species have
ways to compete for mates.
In some populations, only a
few individuals can mate.
Mate preference is nonrandom. Even in the same
species, many animals will
prefer mates that resemble
themselves.
Sexual selection, the result of non-random mating,
selects for traits that secure mates — though some
attractive traits are drawbacks in other ways.
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.
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 A–T to T–A
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.
(a) DIRECTIONAL
SELECTION
Larger-than-average
sizes favored.
Average sizes
favored.
(c) DISRUPTIVE
SELECTION
Smaller-than-average
and larger thanaverage sizes favored.
percent of population
BEFORE
SELECTION
(b) STABILIZING
SELECTION
AFTER
SELECTION
Average phenotype
shifts to larger size
over time.
Average phenotype does Population divides into
not change; phenotypic
two phenotypic groups
variability declines.
over time.
range of a particular characteristic (size, color, etc.)
• 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?
Using what you know about population genetics,
reassess these claims:
• Red hair will eventually die out because
it’s recessive.
• Brown eyes are dominant so they will
take over.
• Little toes will go away because we don’t
use them.
• Humans will get bigger brains because
we need them.