general ecology adaptation and evolution cont: population genetics
TRANSCRIPT
![Page 1: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/1.jpg)
General Ecology
Adaptation and Evolution cont: Population Genetics
![Page 2: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/2.jpg)
Evolution in Populations
Evolution is often a process of adaptation.
Adaptation is not possible w/o genetic variation.
Organisms present a phenotype to the environment (selective regime). The phenotype is an expression of the genotype.
![Page 3: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/3.jpg)
Evolution in Populations
The phenotypic variation we see can be a consequence of a number of things:– Genotypic or genetic variation.– Environmental variation.– Error
![Page 4: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/4.jpg)
Basic Population Genetics
We have defined evolution as a change in allele frequencies over time.
The sum of all genes in a population is the gene pool.
We characterize the gene pool be measuring allele frequencies.
![Page 5: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/5.jpg)
Popuation Genetics
In pea plants, there are red flowers and white flowers. Flower color is controlled by a single gene with 2 alleles.
R is dominant and r is recessive. RR and Rr individuals produce red
flowers while rr individuals produce white flowers.
![Page 6: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/6.jpg)
Population Genetics ‘RR’ is homozygous dominant ‘Rr’ is heterozygous ‘rr’ is homozygous recessive. If there are 100 individuals in a
population, there are 200 flower color alleles.
The number of those alleles that are ‘R’ is the allele frequency of ‘R’, and 1 minus this is the allele frequency of ‘r’.
![Page 7: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/7.jpg)
Population Genetics
If we note that the frequency of R has changed from .20 to .30 in 1 generation, then evolution has occurred.
Imagine we have 30 RR individuals, 20 Rr individuals, and 50 rr individuals.
Let p be the frequency of R and 1-p, or q, be the frequency of r. Note p + q = 1.
![Page 8: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/8.jpg)
Population Genetics
For our example,
p 2 30 20200 0 4( ) .
![Page 9: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/9.jpg)
Population Genetics
Also,
q 2 50 20200 0 6( ) .
![Page 10: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/10.jpg)
Population Genetics
Note that p+q=1. Now, imagine that the individuals in this
population mate panmictically. What is the probability that a R allele
will combine with a R allele? This is simply p2. This is also the expected frequency of
RR individuals in the next generation.
![Page 11: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/11.jpg)
Population Genetics
The probability of rr will be q2, and the probability of Rr is 2pq.
Note: there are 2 ways of getting Rr. You can get Rr or rR.
Note also that p2+2pq+q2 = 1 Finally, note that this is (p+q)2
![Page 12: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/12.jpg)
Population Genetics
What happens? If we repeat this for many generations, we find that the system reaches equilibrium: a point at which allele frequencies no longer change. This is called Hardy-Weinberg equilibrium. In other words, allele frequencies will not change unless something happens.
![Page 13: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/13.jpg)
Population Genetics
The system is dependent on a number of assumptions:– Panmictic breeding– Equal survival and reproduction of
individuals.– The population is closed.– No mutation.
Neither of these is likely to be true. Thus, evolution is inevitable.
![Page 14: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/14.jpg)
Mechanisms of Evolution
What can disrupt HW equilibrium?– Forms of selection already discussed.– Genetic drift.
• Here, changes in allele frequencies can occur by chance alone. This is a small population size phenomenon. Effective population size is critical.
– Gene flow• This can wash out changes in allele
frequencies, introduce new alleles, or promote change.
![Page 15: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/15.jpg)
Mechnisms of Evolution
Modeling natural selection Imagine the homozygous recessive
genotype does not survive as well as the heterozygote or homozygous dominant. We can denote this with a selction coefficient ‘s’.
If the fitness of the homozygous dominant and heterozygote are 1, then the fitness of the homozygous recessive is 1-s.
![Page 16: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/16.jpg)
Mechanisms of Evolution
We can use algebra to model changes in allele frequency. Let p = change in frequency of p from original generation, to generation after selection has occurred. So, p = p – p’
We want to compute the new frequency of p, p’. We do this just as before, but know that some alleles were removed by selection.
![Page 17: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/17.jpg)
Mechanisms of Evolution
The portion removed by selection is q2 * their selective disadvantage ‘s’. Thus, the number of alleles in the next generation is 1-sq2,
since (p2+2pq+q2)-sq2. Also, the total number of alleles is
2(1-sq2)
![Page 18: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/18.jpg)
Mechanism of Evolution
pp pq
sq
p
sq
pp
sq
p p pp
sqp
'( )
( )
( )
'
'
2 2
2 1
1
1
1
1
2
2 2
2
2
![Page 19: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/19.jpg)
Mechanism of Evolution
Note that the rate of change in allele frequency is a consequence of the strength of selection (s) and the initial allele frequency (p and q).
![Page 20: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/20.jpg)
Finally
We often characterize selection in a number of ways:
Density independent
Density dependent
Frequency independent
Hard Seln Density Depn
Frequency dependent
Freq. Depn
Soft Seln
![Page 21: General Ecology Adaptation and Evolution cont: Population Genetics](https://reader036.vdocument.in/reader036/viewer/2022070403/56649f2a5503460f94c43f60/html5/thumbnails/21.jpg)
Finally
Convergent evolution ESS: Evolutionary stable strategy. Wrights adaptive landscape.