Evolution will _______ happen if all
five of the following happen!
1.
2.
3.
4.
5.
Population size is large
Random mating is occurring
No mutations
No genes are introduced or lost
No selection occurs
– Means: all genotypes can survive and reproduce equally
well
• This is referred to as the Hardy-Weinberg Principle
_________________
• rates are generally so low they have little effect on
proportions of common alleles
• are randomly occurring changes in the DNA
sequence of a cell within an organism
___________________
•Frequencies of particular alleles may
change by chance alone
Important in small
populations (e.g.;
grizzly bears may be
experiencing
genetic drift due to
isolated
populations)
Genetic drift
A) __________________: few individuals found new
population (small allelic pool)
Genetic drift
B) ___________________: drastic population
decrease, and gene pool size
Genetic Drift - Bottleneck Effect
15
Genetic drift
What type is this?
a)
Founder
or
b) Bottleneck?
Which graph shows the allele frequency of a
larger population? How are they different?
• ___________
total of all
alleles within
a population
• _________________ frequency: the
proportion of gene pairs (homozygous
dominant, homozygous recessive, or
heterozygous) in a population.
Example: TT:Tt:tt
• _________________ frequency: the
proportion of gene copies in a population
of a given allele.
• Example T:t
Heterozygote Advantage
•
Heterozygote advantage will favor
heterozygotes, and maintain both alleles
instead of removing less successful alleles
from a population.
– Sickle cell anemia
 Homozygotes exhibit severe anemia,
have abnormal blood cells, and usually
die before reproductive age.
 Heterozygotes are less susceptible to
malaria.
21
Sickle Cell and Malaria
22
The term “____________”, when
applied to DNA sequences…
• Refers to a gene that has only a single type
of allele.
____________
the movement of alleles
from one population to
another
_____________________
• Individuals with
certain
characteristics
sometimes mate
with each other
more frequently
than by chance
• (e.g., dark and light
wolves rarely
reproduce in a pack).
____________________________
• When natural selection acts to remove
the characteristics that differ from the
most common
•
•
•
•
all zebra have similar stripes
Birth weight in humans (and other species)
Number of eggs laid by a bird
Body diameter of a snake species
__________________________
When natural selection acts to remove the characteristics that
differ at one extreme from the most common
• average height and taller
giraffes are selected for, but
short giraffes are selected
against
• Large brains and reduced body
hair in humans
• Long noses and large size in
elephants
• Reduced size of salmon due to
overfishing
___________________________
When natural selection acts to
remove the most common
characteristics, leaving the
extremes as separate species.
• during an extended drought in the Galapagos Islands, the
ground finches with medium-sized beaks almost became
extinct, leaving separate populations with large beaks and
small beaks.
• Bill size in African seedeaters, Darwin’s finches, British
Columbia stickleback fishes
Forms of Selection
•
•
•
Disruptive selection
– Selection eliminates intermediate types.
Directional selection
– Selection eliminates one extreme from a
phenotypic array.
Stabilizing selection
– Selection acts to eliminate both extremes
from an array of phenotypes.
23
_________________ isolating mechanisms
• These mechanisms prevent fertilization directly or
indirectly by preventing mating
• Preventing mating can occur by:
– Ecological (Geographical) isolation
– Temporal isolation
– Behavioral isolation
• Preventing fertilization can occur by:
– Mechanical isolation
– Gametic isolation (DNA incompatibilities)
_____________
____________
Live in separate habitat or niche
___________________________
• Species that may be compatible
may remain isolated because they
mate or flower at different times.
• For example, the geographic
ranges of the western spotted
skunk and the eastern spotted
skunk overlap but they do not
interbreed because the former
mates in late summer and the
latter in late winter.
__________________________
• Distinctive mating
rituals in one
species will not be
recognized by
another. The signals
for attracting a mate
are very specific.
• Eg. Frigates
_____________________
• Structural differences in
reproductive organs prevent
copulation. (The parts don’t fit!)
• Bush babies, a group of small
arboreal primates, are divided into
several species based on
mechanical isolation. Each species
has distinctly shaped genitalia that,
like locks and keys, only fit with the
genitalia of its own species.
_____________________
• Prevents fertilization at the
molecular level.
• In the event that sperm from
another species reaches the egg,
it fails to fuse.
• Often occurs because the female
immune system recognizes
sperm as foreign and attacks it.
• In other cases, the sperm or
pollen may not be adapted to
the environment of the female
reproductive track.
________________________Isolating
Mechanisms
• In some cases, mating and fertilization between
different, but related, species can occur.
• However, if the offspring are not fertile or viable,
then the two species continue to be considered as
different.
• Mechanisms:
– Zygotic mortality
– Hybrid inviability
– Hybrid infertility
________________________
• In this case, the
fertilized zygotes or
embryos die before
birth.
• Usually, the
chromosomes are not
compatible.
________________________
• The embryo develops and is born, but the
hybrid is weak, and experiences reduced
survival.
• It will not survive to reproduce
_____________________
• A mule is the result of mating a horse and a donkey.
The mule has characteristics of both species but is
sterile. Since it cannot breed, it is not considered a
separate species but rather a hybrid.
Postzygotic mechanisms