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. Traits vary within a population
the study ofevolution 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
- alleles-are the variations in genes that code for traits)
A population is the smallest group in which evolution is observed.
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)
the individual genes in a given
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 dividingthe total number of a certain allele by the total number of alleles of all types in the population.
BB (homozygous dominant)
bb (homozygous recessive)
long bristles (BB and Bb),
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
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.
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 Bvariety, 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%)
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)
the gene pool of the population is stable.
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
1. Immigration – movement of individuals into the group
2. Emigration-movement of individuals out of the group
(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.)
*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).
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.
Next 3 diagrams: http://bio.research.ucsc.edu/~barrylab/classes/animal_behavior/SELECT.HTM#anchor269237
-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)
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.
1. Geographical Isolation & Allopatric speciation
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
-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.
Which model of speciation rates is illustrated by model A in the graph?
G. sexual selection
Gradualism Punctuated equilibrium
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 poolprezygotic