Gene pools and speciation occur as populations become isolated over time. Isolation can be geographic, temporal, or behavioral. Isolated populations diverge genetically as allele frequencies change within each population. Speciation can be gradual, occurring over long periods, or abrupt. Polyploidy, where organisms gain extra sets of chromosomes, has led to speciation in plants like onions through reproductive isolation of populations with different ploidy levels. Selection also changes allele frequencies, with directional selection favoring one trait, stabilizing selection favoring intermediate traits, and disruptive selection favoring extreme traits.

1. 10.3 Gene Pools and Speciation
Essential idea: Gene pools change over time.

2. Understandings
Statement Guidance
10.3 U.1 A gene pool consists of all the genes
and their different alleles, present in
an interbreeding population.
10.3 U.2 Evolution requires that allele
frequencies change with time in
populations.
Punctuated equilibrium implies long
periods without appreciable change and
short periods of rapid evolution.
10.3 U.3 Reproductive isolation of populations
can be temporal, behavioral or
geographic.
10.3 U.4 Speciation due to divergence of
isolated populations can be gradual.
10.3 U.5 Speciation can occur abruptly.

3. Applications and Skills
Statement Utilization
10.3 A.1 Identifying examples of directional,
stabilizing and disruptive selection.
10.3 A.2 Speciation in the genus Alliumby
polyploidy.
Many crop species have been created to be
polyploid. Polyploidy increases allelic
diversity and permits novel phenotypes to
be generated. It also leads to hybrid vigor.
10.3 S.1 Comparison of allele frequencies of
geographically isolated
populations.

4. 10.3 U.1 A gene pool consists of all the genes and their different alleles,
present in an interbreeding population
Speciation
• A species a group of individuals
who produce offspring after
mating. This make individual of
that species reproductively
isolated from other species.
• A gene pool is the set of all genes,
in an interbreeding population.
http://data1.whicdn.com/images/63849/large.jpghttp://arkansasagnews.uark.edu/monarchs95.jpg

5. 10.3 U.2 Evolution requires that allele frequencies change with time in
populations.
If the allele frequencies of a population are not in equilibrium then the
frequencies are changing or evolving. The following processes facilitate evolution
by either adding or removing genetic variation from a population:
• Mutation
• Migration (Gene Flow)
• Genetic Drift
• Unequal Mating and/or Fertilization Success (Sexual Selection)
• Unequal Viability (Natural Selection)
Gene pool: The collection of genes in a population
Because diploids have only two versions of each gene, each has only a small fraction
of possible alleles in a population
Genotype: The genetic makeup of an individual at a given locus, taking into account the
two possible alleles
Genotype frequency is the proportion of a given genotype in the population
Allele frequency refers to the proportion of a particular allele, such as A or a
Phenotype: the traits of an individual
Phenotype frequency is the proportion of a given phenotype in the population
Phenotype frequency is influenced by the dominance characteristic of an allele

6. 10.3 U.2 Evolution requires that allele frequencies change with time in
populations.

7. Frequencies add up to 1.0
e.g. — a population has two alleles, A and a with A is dominant over a
The allele frequencies must sum to 1.0
(frequency of A) + (frequency of a) = 1.0
The genotype frequencies must sum to 1.0
(frequency of AA) + (frequency of Aa) + (frequency of aa) = 1.0
The phenotype frequencies must sum to 1.0
(frequency of AA and Aa phenotype) + (frequency of aa phenotype) = 1.0
Imagine 2 alleles, A and a
p is the frequency of A q the frequency of a
So, p + q = 1
The mathematical equivalent of a random mating can be given by multiplying this
relationship by itself
Therefore, (p + q)2 = 1 = p2 + 2pq + q2
p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa
Given this condition, we can always work out the frequencies of each allele in a sexual
population.
10.3 U.2 Evolution requires that allele frequencies change with time in
populations.

8. 10.3 U.2 Evolution requires that allele frequencies change with time in
populations.
• Evolution is the cumulative change in allele frequency or heritable characteristics in
a population over time
• The cumulative change can occur as a result of genetic changes and/or selective
pressures which favor certain heritable characteristics over other less favorable
characteristics
• These populations have to be reproductively isolated, thus preventing gene flow
between populations
P equals the dominant gene
Q equals the recessive gene

9. 10.3 S.1 Comparison of allele frequencies of geographically isolated
populations
• Cod fish have a gene that codes for an
integral membrane protein called
pantophysin.
• Two alleles of the gene, PanIA and PanIB,
code for versions of pantophysin, that
differ by four amino acids in one region of
the protein.
• Samples were collected from 23 locations
in the North Atlantic (numbered 1–23 in
each pie chart), on the map to the right.
• The frequency of an allele can vary from
0.0 to 1.0.
PanIA light grey sectors of the pie charts show
the allele frequency for the PanIA gene
PanIB black sectors show the allele frequency
for the PanIB gene.
• The biggest difference in allele frequency
occurs in the Cod fish isolated at the two
extremes of the map.

10. 10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.
• Reproductive isolation of populations occurs when barriers or
mechanisms prevent two populations from interbreeding, keeping
their gene pools isolated from each other.
• There are different types of reproductive isolation including temporal,
behavioral, and geographic

11. How and why do new species originate?
• Species are created by a series of
evolutionary processes
– populations become isolated
• geographically isolated
• reproductively isolated
– isolated populations
evolve independently
• Isolation
– allopatric
• geographic separation
– sympatric
• still live in same area
10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.

12. 10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.
Temporal isolation
• Species that breed during different
times of day, different seasons, or
different years cannot mix gametes
– reproductive isolation
– sympatric speciation
• “same country”
Eastern Spotted Skunk (Top Right)
& Western Spotted Skunk (Bottom
Right) overlap in range but Eastern
mates in late winter & Western
mates in late summer
http://upload.wikimedia.org/wikipedia/
commons/f/f2/Spilogale_putorius_(2).jp
g
http://upload.wikimedia.org/wikipe
dia/commons/9/98/Spilogale_gracil
is_amphiala.jpg

13. 10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.
Behavioral Isolation
• In most animal species, members of the two sexes must first search for each other
and come together.
• Unique behavioral patterns & rituals isolate species
 identifies members of species attract mates of same species
 courtship rituals, mating calls
 reproductive isolation
Blue footed boobies mate
only after a courtship display
unique to their specieshttp://upload.wikimedia.org/wikipedia/commo
ns/a/aa/Bluefooted_Booby_Comparison.jpg

14. So…what is a species?
Western MeadowlarkEastern Meadowlark
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.

15. 10.3 U.3 Reproductive isolation of populations can be temporal,
behavioral or geographic.
Geographic Isolation
Species occur in different areas
– physical barrier
– allopatric speciation
• “other country”
Harris’s Antelope
Squirrel inhabits the
canyon’s south rim
(L). Just a few miles
away on the north
rim (R) lives the
closely related
White-tailed
Antelope Squirrel

16. 10.3 A.1 Identifying examples of directional, stabilizing and disruptive
selection.
• If no selection occurs to a population (for
whatever means), population doesn’t
change with succeeding generations.
• If selection pressure is applied then those
not receiving selection pressure tend to
predominate…
 Stabilizing: the extremes are selected
against; center stays same and grows
in numbers
 Directional: one tail of the distribution
is selected against and the opposite
tail grows in numbers
 Disruptive: a mid-group is selected
against; the tails are allowed to
predominate and grow compared to
middle
As an example: in Humans we have
selected for a babies birth weight. This
protects the mother and the babies
health.

17. 10.3 A.1 Identifying examples of directional, stabilizing and disruptive
selection.
Directional Selection:
• Selection that removes
individuals from one
end of a phenotypic
distribution and thus
causes a shift in the
distribution towards
the other end.
• Over time, the favored
extreme will become
more common and
the other extreme will
be less common or lost.

18. 10.3 A.1 Identifying examples of directional, stabilizing and disruptive
selection.
Stabilizing Selection:
A type of selection that removes
individuals from both ends of a
phenotypic distribution, thus
maintaining the same distribution
mean. This occurs when natural
selection favors
the intermediate phenotypes.
Over time, the intermediate
states become more common and
each extreme variation will
become less common or lost.
Same mouse example where
medium colored fur is favored
over dark or light fur color.

19. 10.3 A.1 Identifying examples of directional, stabilizing and disruptive
selection.
Disruptive Selection:
• Removes individuals from the
center of a phenotype. This
occurs when natural
selection favors both ends of
the phenotypic variation.
• Over time, the two extreme
variations will become more
common and the intermediate
states will be less common or
lost.
• This can lead to two new
species.

20. 10.4 U.4 Speciation due to divergence of isolated populations can be
gradual.
• Speciation can occur gradually over long periods of time, with several intermediate
forms in between species leading to today’s current species. This can be seen in
some of the more complete fossil records, like the whale or the horse.
• In some species, large gaps were evident for certain species in the fossil record. This
imperfections in the fossil record, maybe the result of transitional species have not
been discovered yet or abrupt speciation.
http://www.sivatherium.narod.ru/library/Dixon/pics_01/p0010_e.gif

21. Gradualism
• Gradual divergence over long spans of
time
– assume that big changes occur as
the accumulation of many small
ones
10.4 U.4 Speciation due to divergence of isolated populations can be
gradual.
http://cnx.org/resources/22b17901c8ce6510b03e2f89df0bc072/graphics1.png

22. 10.3 U.5 Speciation can occur abruptly.
Punctuated Equilibrium
Species remain stable for long
periods of time (several million
years) interrupted by periods of
significant change, during which
time a new species may evolve.
 rapid bursts of change
 long periods of little or no
change
 species undergo rapid change
when they 1st bud from
parent population

23. 10.3 U.5 Speciation can occur abruptly.
http://static.skynetblogs.be/media/130852/12.11.jpg
Over 75% of all life on Earth was lost during the late
Devonian mass extinction which took place about 375-359
million years ago

24. 10.3 U.5 Speciation can occur abruptly.
https://evolutionliteracy.files.wordpress.com/2014/09/t
rilobites-evolution-literacy-g-paz-y-mino-c-photo.jpg
Over 97% of all life on Earth was lost during the
End-Permian mass extinction which took place 252
million years ago

25. 10.3 U.5 Speciation can occur abruptly.
http://www.gohobby.com/wp-
content/uploads/2012/11/Velociraptor-
Jurassic-Park.jpeg
Over 50% of all life on Earth was lost during the Triassic
mass extinction which took place 201 million years ago

26. 10.3 U.5 Speciation can occur abruptly.
https://evolutionliteracy.files.wordpress.com/2014/09/t
rilobites-evolution-literacy-g-paz-y-mino-c-photo.jpg
Over 80% of all life on Earth was lost during the end
Cretaceous. The mass extinction took place 65 million
years ago

27. Gradualism
Punctuated Equilibrium
10.3 U.5 Speciation can occur abruptly.

28. 10.3 A.2 Speciation in the genus Alliumby polyploidy.
• Polyploidy organisms contain more
than two pairs of the same
chromosomes.
• A likely advantage is it allows
for additional raw materials (i.e.
DNA, genes) for evolution. Every
gene is theoretically free to
evolve without substantial negative
effect.
• Polyploidy plants tend to be
larger. The reproductive organs and
fruit, in particular, are usually
enlarged in polyploidy. The likely
mechanism for this is simple: more
DNA results in a larger nucleus,
which results in larger cells,
especially in the reproductive
organs. http://www.vims.edu/newsandevents/top
stories/_images/diploid_triploid_250.jpg
Oysters

29. 10.3 A.2 Speciation in the genus Alliumby polyploidy.

30. 10.3 A.2 Speciation in the genus Alliumby polyploidy.
• The genus Allium comprises monocot
flowering plants and includes the
onion, garlic, chives, scallion, shallot,
and the leek.
• In many of these species of plants,
chromosome doubling has created a
large number of different phenotypes.
• This results is a number of
reproductively isolated but similar
populations.
Examples: of this are seen in 7 natural
populations Allium grayi. They showed
• tetraploid (2n=32)
• pentaploid (2n=40)
• hexaploid (2n=48) http://i.dailymail.co.uk/i/pix/2008/09/12/article-1054890-
029CF17900000578-854_233x364.jpg

31. 10.3 A.2 Speciation in the genus Alliumby polyploidy.
Allium grayi tetraploid (2n=32)
tetraploid (2n=32)

32. 10.3 A.2 Speciation in the genus Alliumby polyploidy.
pentaploid (2n=40)

33. 10.3 A.2 Speciation in the genus Alliumby polyploidy.
http://upload.wikimedia.org/wikipedia/commons/7/79/Allium_tulipifolium_(inflorescence).jpg
hexaploid (2n=48)