Chapter 16
Population Genetics and
16-1 Genetic Equilibrium
Terms to Know
Population genetics – study of evolution
from a genetic point of view
 Microevolution – evolution at the genetic
 Bell Curve – most members of the
population have similar traits; only a few
are at the extremes
– Can SHIFT over time!!!
Variations of Traits Within a
Causes of Variation
1. Mutations
2. Recombination (crossing-over and
independent assortment)
1. (during meiosis – see pic!)
– Random pairing of gametes
– Which sex cells combine successfully
The Gene Pool
Gene pool = the total genetic information
available in a population
 Allele Frequency = number of a certain
allele / total number of alleles in a
– Calculated by … # of allele A
total # of A and a
What is the allele frequency?
Half a population of four o clocks are red/
half are white.
 What is the frequency of the r allele?
– 0.50 or 50%
The Gene Pool
Phenotype Frequency = # of individuals
with a phenotype / total # of individuals
within the population
The Gene Pool
Phenotype Frequency = # of individuals with a
phenotype / total # of individuals within the
– # red flowers/ Total # of individuals
Hardy – Weinberg Genetic
Ideal hypothetical population that is not
evolving (ie not changing over time)
5 criteria (must be met)
No net mutations occur
No one enters or leaves the population
The population is large
Individuals mate randomly
Selection does not occur
Why might a population never be in HW genetic equilibrium???
Pg 320 Q’s (1-5)
– Answer in complete sentences!
– Turn in when completed!
Read pg’s 321-325 and answer the Q’s
16-2 Disruption of
Genetic Equilbrium
2. Gene flow – the process of genes
moving from one population to the next
- Immigration
- Emigration
3. Genetic Drift – allele frequencies in a
population change usually because the
population is small
4. Nonrandom mating – mate selection is
influenced by geographic proximity, mates
with similar traits, and sexual selection
5. Natural Selection – some members are
more likely to survive and reproduce
- stabilizing selection
- disruptive selection
- directional selection
Types of Selection
Stabilizing selection individuals with
average forms of a
trait have the highest
– Ex. Large lizards will
be spotted by
predators; small
lizards can’t run fast
enough to get away
from predators
Types of Selection
Disruptive Selection –
individuals with either
extreme of the trait has
the greatest fitness
– Ex. White moths on white
trees cannot be seen;
Dark moths on dark trees
cannot be seen; medium
colored moths will be seen
on both trees (eaten by
Types of Selection
Directional selection –
more extreme form of
a trait has the
greatest fitness
– Ex. Anteaters with the
longest tongues will
get the most food
16-3 Formation of Species
The Concept of Species
Speciation – the process of species
 Morphology – internal and external
appearance of an organism
 Species – morphologically similar and can
interbreed to produce fully fertile offspring
Morphological vs Biological Species
2 competing concepts (ideas) on how to
determine different species
– Morph = based solely on appearance
– Bio = based on who can reproduce with who
successfully (not useful for extinct or asexual
– What do we use today?
 Mix of BOTH!!!
Isolation and Speciation
Geographic isolation – physical
separation of members of a species
– Ex. River dries up into several small pools;
fish diverged enough to be considered
separate species
Reproductive Isolation – species
become genetically isolated
– Ex. The offspring of interbreeding species
may die early or may not be fertile
– Ex. Different mating times
Rates of Speciation
Gradualism – speciation occurs at a
regular, gradual rate
 Punctuated Equilibrium – sudden,
rapid change then long periods of no
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