Transcript population - Damien Rutkoski

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
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
Individuals with
highest fitness: near
the center of the
curve (average
phenotype)
Example: human
birth weight
Stabilizing Selection
Key
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
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
Individuals with highest fitness: both
ends of curve
Example: birds where seeds are either
large or small
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
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 formation of new
biological species, usually by
the division of a single species
into two or more genetically
distinct one.
1.
2.
3.
Geographic Isolation
Behavioral Isolation
Temporal Isolation
 Two
populations
separated by a geographic
barrier; river, lake,
canyon, mountain range.

Kaibab Squirrel Abert
Squirrel

Kaibab Squirrel Abert
Squirrel
 Two
populations are capable
of interbreeding but do not
interbreed because they have
different ‘courtship rituals’ or
other lifestyle habits that differ.
 Eastern
and Western
Meadowlark populations
overlap
the US
in the middle of
birds sing a matting
song that females like, East
and West have different songs.
Females only respond to their
subspecies song.
 Male
Populations reproduce at
different times
January
7
1 2
3 4 5 6
8 9
10 11 12 13

Mates in:
Mates in:
April
July
 Geographic,
Behavioral and
Temporal Isolation are all
believed to lead to
speciation.
 No
examples ever observed
in animals
 A couple examples that
may demonstrate
speciation exist in plants
and some insects.

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
1.
2.
Founder Effect
Bottleneck 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
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
♦
♦
♦
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
Original
population
Catastrophe
Original
population
Catastrophe
Original
population
Surviving
population