Transcript Chapter 16: Population Genetics &Speciation

Chapter 16:
Population Genetics & Speciation
• Ch 16 will link the
understanding of the
theories of natural
selection & evolution
with principles of
genetics.
http://sps.k12.ar.us/massengale/population_genetics_notesbi.htm
I. Genetic Equilibrium
A. Traits vary within a population
1. Population Genetics –
the study of evolution from a
genetic point of view.
Population biologists –study
many different traits in
populations -such as size and color.
Charles Darwin's first sketch of an evolutionary tree from his First
Notebook on Transmutation of Species (1837) http://en.wikipedia.org/wiki/Speciation
2. Microevolution
• a small- scale change in the collective
genetic material (alleles) of a population.
• Microevolution can be contrasted with
macroevolution; which is the occurrence of
large-scale changes in gene frequencies, in a
population, over a geological time period
(i.e. consisting of lots of microevolution).
** remember:
- alleles -are the variations in genes that
code for traits)
- population - is a group of individuals of
the same species that routinely interbreed.
A population is the smallest group in which
evolution is observed.
- Individuals do not evolve, populations do.
3. Standard Bell Curve
- Traits vary among individuals & can be
mapped
- Shows that most individuals have
average traits
- a few individuals have extreme traits.
4. Causes of Variation of Traits
Variations in genotype arise by
-mutation (random change in a gene)
-recombination (reshuffling of genes in
an individual- remember meiosis- crossing
over, Independent assortment)
-random pairing of gametes (many
gametes, chance union)
B. The Gene Pool
• The total genetic information available in a
population.
• Imagine a “pool” with all the possible genes
for the next generation in it. - then make a
set of rules to predict expected genotypes
• So a gene pool is the sum of all
the individual genes in a given
population.
1. Allele frequency
The frequency of an allele is the number of
occurrences of that allele in that population
-Within a gene pool, every allele or gene variant
has a particular ratio or frequency.
-is determined by dividing the total number of a
certain allele by the total number of alleles of all
types in the population.
2. Review of phenotype & genotype
• Genotype- is the actual genetic information
(the combo of alleles for traits.)
• Three different genotypes:
BB (homozygous dominant)
Bb (heterozygous)
bb (homozygous recessive)
• Phenotype- what is seen.
• 3 genotypes , in complete dominance- show 2 phenotypes:
long bristles (BB and Bb),
3. Predicting Phenotype
a. Phenotype frequency
•
Predicting Phenotype
–
Phenotype frequency is equal to the number of
individuals with a particular phenotype divided
by the total number of individuals in the
population.
b. Counting & calculating
1. Count the alleles of each type in each generation.
Example- 12 R, 4r total 16 alleles in 8 individual
in 1st generation
2. Divide the type of each allele by the total number
of alleles.
Example- 12/16 = R = 0.75 & 4/16 = r = 0.25
Phenotype Frequency
The four o’clock flower illustrates how phenotype changes from
generation to generation. Compare 1st & 2nd generations.
Note that although the phenotypes change the allele frequencies
remain the same.
Gene pool
EXAMPLE:
15 individuals in the population (each organism has 2
alleles per trait), thus = 30 alleles for trait - if 6 alleles
in this population are of the b variety, & 24 are of the
B variety,
then frequencies of alleles are:
* 6/30 of the genes in the gene pool are b - a frequency of 0.2
* 6/24 of the gene in the gene pool are B - a frequency of 0.8.
Together, 0.2 + 0.8 = 1.0 (all the genes, 100%)
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_GenePool.html
Law of probability
The chances of 1 gamete having an allele &
meeting with any other allele is expressed:
frequency of R X frequency of R = frequency of RR pair
(example: 0.75 X 0.75 = 0.5625)
Frequency of r X frequency of r = frequency of rr pair
(example: 0.25 X 0.25 = 0.0625)
So that the frequency of Rr can be figured out by
subtracting the sum of RR + rr from 1.0.
(example 1.0 – (0.5625 + 0.0625) = 0.375 (Rr pairing)
C. Hardy-Weinberg Genetic
Equilibrium
• Englishman HARDY & German WEINBERG
• Showed frequency of alleles in a population stays constant
for generations if certain conditions are fulfilled.
• In other words-Allele frequencies in the gene pool do not
change unless acted upon by certain forces.
• Hardy-Weinberg genetic equilibrium is a theoretical
model of a population in which no evolution occurs &
the gene pool of the population is stable.
Five conditions for hypothetical
H-W population
1. No net mutations occur (# alleles remain the same)
2. No Individuals enter or leave the population
(Immigration or Emigration)
3. The population is LARGE
4. Individuals must mate randomly
5. Natural selection does not occur.
**Genetic Equilibrium is a theoretical state. Real
populations probably do not meet all these
conditions. Use equation to see causes of
DISRUPTION of genetic equilibrium
II. Disruption of Genetic
Equilibrium
A. Mutation
•
Evolution may take place when
populations are subject to genetic
mutations, gene flow, genetic drift,
nonrandom mating, or natural selection.
•
Mutations are changes in the DNA.
B. Gene Flow
1. Immigration – movement of individuals into
the group
2. Emigration-movement of individuals out of
the group
• Emigration and immigration cause gene flow
between populations and can thus affect gene
frequencies.
• Example- males of baboon troops- fight for dominance of
group of females. Females tend to stay in troop born into.
Less dominant or younger males move to a different troop.
This ensures gene flow.
Genetic Drift
• Genetic Drift- the phenomenon by which allele
frequencies in a population change as a result of
random events or chance.
• Genetic drift refers to the expected population
dynamics of neutral alleles (those defined as having
no positive or negative impact on fitness)
(Natural selection describes the tendency of beneficial
alleles to become more common over time (and
detrimental ones less common), genetic drift refers to
the tendency of any allele to vary randomly in
frequency over time due to statistical variation alone.)
C. Large populations
• Large populations tend to stabilize allele
frequencies.
• Genetic drift is more pronounced in small
populations where failure of even a single
individual to reproduce can change allele
frequencies in the next generation.
• See graph page 322
D. Non- random mating
(Sexual selection)
• Mating is nonrandom whenever individuals
may choose partners.
• Sexual Selection
– Sexual selection occurs when certain traits
increase an individual’s success at mating.
– Sexual selection explains the development of
traits that improve reproductive success but that
may harm the individual.
*Females are the limiting sex
- invest more in offspring than males
-many females are unavailable for fertilization
(because they are carrying for young or developing young)
-males tend to be in excess
*Sexual selection arises in response to either:
1. Female Choice: Intersexual selection, in which
females choose males based upon elaborate
ornamentation or male behaviors, or
2. Male Competition: Intrasexual selection, in which
males compete for territory or access to females, or
areas on mating grounds where displays take place.
Male-male competition can lead to intense battles for
access to females where males use elaborate
armaments (e.g., horns of many ungulates).
http://bio.research.ucsc.edu/~barrylab/classes/animal_behavior/SELECT
.HTM
E. Natural selection
•
•
One of the most powerful agents of genetic change
can influence evolution in one of three general
patterns:
1. Stabilizing selection- favors the formation of
average traits.
2. Disruptive selection -favors extreme traits rather
than average traits.
3. Directional selection -favors the formation of
more-extreme traits.
Stabilizing Selection
• Reduces variation
• Favors individuals with
an average phenotype
over the extremes.
• Example:
– very large fish cannot hide
under rocks
– very small fish move too slowly
– predators eat both of these
extremes
– average sizes survive best
Next 3 diagrams: http://bio.research.ucsc.edu/~barrylab/classes/animal_behavior/SELECT.HTM#anchor269237
Disruptive Selection
• Selects for phenotypes at
both extremes
• can creative two distinct
distributions from a single
distribution.
• Example
– large & small seeds available
to eat
– Birds with very large and very
small beaks survive best
– Average sizes not best suited
for survival.
Directional Selection
-A response to a change
in the environment
can select for traits
above or below
average
- we see a shift in the
mean for the trait
(either up or down)
III. Formation of Species
A. Definition of species
1. Morphological- a species is a populations of
organisms that look alike (same structures &
appearance)
2. Biological -a species is a population of
organisms that can successfully interbreed but cannot
breed with other groups.
Combined definition- a species is a group of
organisms that look alike & can successfully
interbreed to create fertile offspring.
B. Isolation & Speciation
1. Geographical Isolation & Allopatric
speciation
• Results from the separation of population
subgroups by geographic barriers.
• Geographical Isolation may lead to allopatric
speciation (Happens when species arise as a
result of geographical isolation)
2. Reproductive Isolation
– results from the separation of population subgroups by
barriers to successful breeding.
a. Prezygotic isolation – occurs before fertilization.
examples- different sizes-body structure prevents
mating, different mating ritual or behavior, different
breeding time, not recognizing songs or calls.
b. Postzygotic isolation – occurs after fertilization.
examples- embryo does not develop or creates a
hybrid organism that is infertile or weaker
Sympatric speciation
– Reproductive isolation within the same
geographic area is known as sympatric
speciation.
– May occur to give adaptive advantage
to organisms that use slightly different
niches.
C. Rate of Speciation
1. Gradualism
-The gradual model of speciation
-species undergo small changes at a constant
rate.
2. Punctuated Equilibrium
- new species arise abruptly
- differ greatly from their ancestors, and then
change little over long periods.
The illustration below shows two contrasting
models for rates of speciation.
Which model of speciation rates is illustrated
by model A in the graph?
F. gradualism
G. sexual selection
Gradualism
Punctuated
equilibrium
Questions:
1. Which type of selection
is modeled in the illustration?
What might cause this ?
2. What is the term for the total
genetic information in a population?
3. Saint Bernards and Chihuahuas (two breeds of
domestic dogs) cannot normally mate because they
differ so much in size. Thus, they are reproductively
isolated to some extent. What type of isolating
mechanism is operating in this case?
Directional, change in the environment.
Gene pool
prezygotic
Hardy Weinberg Equation
•
•
•
The gene frequency of a population in HardyWeinberg Equilibrium is written as: pp : 2pq : qq
where p = the frequency of the dominant allele, and q
= the frequency of the recessive allele. It follows that
p + q = 100% of all the genes in the gene pool.
When you have allele frequencies, you can then
calculate genotype frequencies using the H-W
equation, (AA) = p2, (Aa) = 2pq, and (aa) = q2.
Example:
• 16 pigs with 4 of them black (recessive aa).
• 16 pigs are 100%
• 4 pigs are 25% (aa 25%)
therefore: q2=0.25 -------> q=0.5
p + q = 1 ---------->p + 0.5 = 1 -----> p = 0.5
• AA (homozygous) are p2---->0.5X0.5= 0.25 = 25%
• 2Aa (heterozygotus) are 2pq----> 2 x (0.5) x (0.5) =
0.5 = 50%
so the equation is: AA + 2Aa + aa = 1
25% + 50% +25% = 1