Population Genetics & Speciation
Population Genetics –the study
of evolution from a genetic point of
view (microevolution)
Population biologists –study many
different traits in populations –such
as size and color.
Microevolution
▪ a small- scale change in the
collective genetic material
(*alleles) of a population.
▪ macroevolution: largescale 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
- a group of individuals of the same species that routinely
interbreed and live in the same area at the same time
- the smallest group in which evolution is observed
- Individuals do not evolve, populations do
Standard Bell Curve
- Traits vary among individuals & can be mapped
- Shows that most individuals have average traits
- a few individuals have extreme traits.
Causes
of
Variation
of
Traits
-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)
The Gene Pool
▪ the sum of all the individual
genes in a given population.
▪ within a gene pool, every allele
or gene variant has a particular
ratio or frequency.
▪ determined by dividing the total
number of a certain allele by the
total number of alleles of all
types in the population.
Predicting Phenotype
a. Phenotype frequency – predicting phenotypes
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.
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%)
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)
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)
Hardy-Weinberg Genetic Equilibrium
▪ Godfrey Hardy & Wilhelm Weinberg
▪ Showed 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
p = dominant allele
q = recessive allele
p+q=1
Video - Bozeman
Disruption of Genetic Equilibrium
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. Malemale 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
different mating ritual or behavior, different
not recognizing songs or calls.
mating,
breeding time,
b. Postzygotic isolation – occurs after fertilization.
examples- embryo does not develop or creates a
organism that is infertile or weaker
hybrid
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