Download Chapter 16: Evolution of Populations

Chapter 16:
Evolution
of
Populations
1. When Darwin developed his theory of
evolution, he did not understand:
• how heredity worked.
This left him unable to explain two
things:
a. source of variation
b. how inheritable traits pass
from one generation to the
next
In the 1940’s, Mendel’s work on genetics
was “rediscovered” and scientists began
to combine the ideas of many branches of
biology to develop a modern theory of
evolution. When studying evolution
today, biologists often focus on a
particular population. This evolution of
populations is called microevolution.
2. Vocabulary:
population: group of individuals of the
same species living in the same area that
breed with each other.
2. gene pool: combined genetic info.
for all members of a population
2. allele: one form of a gene
2. relative frequency of an allele: #
times an allele occurs in the gene pool
compared to other alleles (percent)
Example
Relative Frequency:
70% Allele B
30% Allele b
3. Sources of Variation:
a. mutations: any change in DNA sequence
♦ Can occur because of:
♦mistakes in replication
♦ environmental chemicals
♦ May or may not affect an organism’s
phenotype
3. Sources of Variation
b. Gene Shuffling: recombination of genes
that occurs during production of gametes
♦ Cause most inheritable differences between
relatives
♦ Occurs during meiosis
♦ As a result, sexual reproduction is a major
source of variation in organisms.
♦ Despite gene shuffling, the frequency of alleles
does not change in a population. Explain why
this is true.
Similar to a deck of cards – no matter how
many times you shuffle, same cards (alleles)
are always there.
4. Gene Traits:
A) Single gene trait: controlled by single
gene with two alleles
♦ Examples: widow’s peak, hitchhiker’s thumb,
tongue rolling
(4. Gene Traits:)
B) Polygenic trait: controlled by 2 or more
genes, each with 2 or more alleles
♦ Examples: height, hair color, skin color, eye color
Most human traits are polygenic.
Do the following graphs show the distribution of
phenotypes for single-gene or polygenic traits? Explain.
type: single gene
type: polygenic
why? Only two
phenotypes possible
why? Multiple (many)
phenotypes possible
Example: tongue roller
or non-tongue roller
Example: height range 4feet
to 9 feet all
5. Natural selection acts on phenotypes, not
genotypes.
Example: in a forest covered in brown leaves,
dirt and rocks which mouse will survive better
brown or white?
Brown, more hidden.
5. If brown is dominant can the a predator tell
the difference between:
BB
Bb
?
Mouse with highest fitness will have the most
alleles passed on to the next generation.
White mouse will have low fitness
5. Which mouse will have the lowest fitness?
White, bb (recessive)
BB
Bb
?
Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness
advantage of being brown
6. Three ways in which
natural selection affects
polygenic traits.
a. Directional Selection: individuals at
one end of the curve have higher fitness
so evolution causes increase in
individuals with that trait
Key
Food becomes scarce.
Low
mortalit
High
y,
high
mortalit
fitness
y, low
fitness
♦ Individuals with highest fitness: those at
one end of the curve
♦ Example: Galapagos finches – beak size
Directional Selection (page 398)
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
b. Stabilizing Selection: individuals at the
center of the curve have highest fitness;
evolution keeps center in the same position
but narrows the curve
Stabilizing Selection
Key
Individuals with
highest fitness: near
the center of the
curve (average
phenotype)
Example: human
birth weight
Low
mortality,
High
high fitness
mortality, low
fitness
Birth Weight
Selection
against both
extremes
keep curve
narrow and
in same
place.
Stabilizing Selection
Stabilizing Selection
Key
Low mortality, high
fitness
High mortality, low
fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
c. Disruptive Selection: individuals at
both ends of the curve survive better
than the middle of the curve.
Disruptive Selection
Largest and smallest seeds become more common.
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
♦ Individuals with highest fitness: both
ends of curve
Key
Beak Size
♦ Example: birds where seeds are either
large or small
Disruptive Selection (pg 399)
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Quiz Monday!!
• Evolution review ½ sheet. (yes some
questions are missing)
• Thursday and Friday’s concepts will be
on the quiz:
– Directional, Stabilizing and Disruptive
selection.
– Geographic, Behavioral, Temporal
Isolation
– Small populations caused by bottleneck
and founder effect
The Process of Speciation
• The formation of new biological
species, usually by the division of a
single species into two or more
genetically distinct one.
Three Isolating Mechanisms:
Isolate species forming subspecies
and perhaps causing speciation.
1. Geographic Isolation
2. Behavioral Isolation
3. Temporal Isolation
1. Geographic Isolation
• Two populations separated
by a geographic barrier;
river, lake, canyon, mountain
range.
Example: 10,000 years ago the
Colorado River separated two
squirrel populations.
• Kaibab Squirrel
Abert Squirrel
• Kaibab Squirrel
Abert Squirrel
This resulted in a subspecies, but
did not result in speciation
because the two can still mate
if brought together
2. Behavioral Isolation
• Two populations are capable of
interbreeding but do not interbreed
because they have different
‘courtship rituals’ or other lifestyle
habits that differ.
Example: Eastern and Western
Meadowlark
• Eastern and Western Meadowlark
populations overlap
middle of the US
in the
Example: Eastern and Western
Meadowlark
• Male birds sing a matting song
that females like, East and West
have different songs. Females only
respond to their subspecies song.
3. Temporal Isolation
Populations reproduce at
different times
January
7
1 2
3 4 5 6
8 9
10 11 12 13
Example: Northern Leopard Frog
& North American Bullfrog
• Mates in:
Mates in:
April
July
Conclusion:
• Geographic, Behavioral and
Temporal Isolation are all
believed to lead to speciation.
However:
–No examples ever observed in
animals
–A couple examples that may
demonstrate speciation exist
in plants and some insects.
Genetic Drift
• random change in allele
frequency that occurs in small
populations
The results of genetic crosses can
usually be predicted using the laws of
probability. In small
populations, however, these
predictions are not always accurate.
a. Founder effect: allele frequencies change due to
migration of a small subgroup of a population
♦ Example: fruit flies on Hawaiian islands
Two phenomena that result
in
small populations and cause
genetic drift
1. Founder Effect
2. Bottleneck Effect
Founder effect
allele frequencies change due
to migration of a small
subgroup of a population
Founder Effect: : Fruit Flies
on Hawaiian islands
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
2. Bottleneck effect
major change in allele
frequencies when population
decreases dramatically due to
catastrophe
♦ Example: northern elephant seals
decreased to 20 individuals in 1800’s, now 30,000
no genetic variation in 24 genes
Bottleneck Effect: Northern
Elephant Seal Population
♦ Hunted to near extintion
♦ Population decreased to
20 individuals in 1800’s,
those 20 repopulated so
today’s population is
~30,000
♦ No genetic variation in 24
genes
Bottleneck Effect
Original
population
Bottleneck Effect
Catastrophe
Original
population
Bottleneck Effect
Catastrophe
Original
population
Surviving
population
Another picture to illustrate
bottleneck effect