Chapter 16
Population Genetics and
Speciation
16-1 Genetic Equilibrium
Terms to Know
Population genetics – study of evolution
from a genetic point of view
 Microevolution – evolution at the genetic
level
 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
Population

Causes of Variation
1. Mutations
TAGATC -> TAAATC
2. Recombination (crossing-over and
independent assortment)
(during meiosis)
3. 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
population

– Calculated by … # of allele B
total # of B and b
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
– # red flowers/ Total # of individuals
video
Hardy – Weinberg Genetic
Equilibrium


1.
2.
3.
4.
5.
Ideal hypothetical population that is not
evolving (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???
16-2 Disruption of
Genetic Equilbrium
Mutation
2. Gene flow – the ability of genes (alleles)
to be shared among members of a
species (immigration and emigration)
3. Genetic Drift – change in allele
frequencies due to chance, usually
because the population is small
1.
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
fitness
– 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
birds)
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
The Birth and Death of Genes
16-3 Formation of Species
The Concept of Species
Speciation – the process of species
formation
 Morphology – internal and external
appearance of an organism
 Species – morphologically similar and can
interbreed to produce fully fertile offspring

Morphological vs Biological Species
Concept

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
organisms)
– 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

Allopatric speciation- when a new species
arises as a result of geographic isolation
– Gene flow stops
– Gene pools separate
– Most likely to occur in small pop.
– Becomes reproductively isolated

Reproductive Isolation – species
become genetically isolated
– Barriers to successful breeding between
groups in the same area
– May arise from disruptive isolation
– Ex. Different mating times

Sympatric speciation- occurs when 2
groups become reproductively isolated in
the same geographic area
– Reduce gene flow by exploring new niche
Rates of Speciation
Gradualism – speciation occurs at a regular,
gradual rate
 Punctuated Equilibrium – sudden, rapid
change then long periods of no change
 video

Ring Species
video