Week 3 - Population Genetics (Haploid) Flashcards
Biological definition of evolution
“the origin and alteration over generations of: the frequencies of genotypes within populations, the proportion of differentiated populations within species, the proportion of species with different traits within a lineage” (Futuyma and Kirkpatrick)
Could also add ideas within society
natural selection definiton
the process whereby some individuals contribute more offspring to the next generation as a consequence of their carrying a trait or traits favorable to survival or reproduction
evo. by natural selection occurs whenever: (3 things)
- VARIANCE - individuals vary in some trait
- SELECTION - individuals with some trait value are more likely to live and/or reproduce
- HERITABILITY - parents have offspring with similar trait values
is it easy to say a change in genotype is because of natural selection?
NO - easy to document a change but not to say bc of selection
Haploid model of evolution
Variant alleles: A, a
Haploid population of size N, N = # of individuals in a population
Selection definition
differential survival or reproduction of different entities
p[t]
frequency of allele A at time t, from 0 to 1
q[t]
frequency of allele a at time t, from 0 to 1
p[t] + q[t] =
1
Fitness definition
Avg. contribution per parent to the next generation, including survival and reproduction
WA
fitness of A
Wa
fitness of a
equation for p[t+1]
= (WAp[t]) / {(WAp[t]) + Wa*q[t]}
equation for evolutionary change across one generation
(WA - Wa)p[t]q[t] / {WA*p[t] + Waq[t]}
If WA > Wa, when will change in p be greatest
in middle of change, when allele frequences are relatively similar (steepest part of the S curve)
Relative vs absolute fitness
evolution by natural selection depends only on relative fitness not absolute values
evolution after time t (p[t]
= (WA^t)p[t] / {((WA^t)p[t]) + (Wa^t)*q[t]}
When would there be low genetic variance
When one allele is nearly lost, or nearly fixed
if p[t]* q[t] = variance, highest variance is when around 0.5, 0.5
Haploid genetic variance
p[t] * q[t]
What part of change over generation equation reps: 1. variance, 2. selection, 3. heritability
- p[t]*q[t]
- WA - Wa
- p[t] (as A bearing parents pass A allele to offspring)
if WA = Wa
p[t] remains at p[0] - neutral
if WA> Wa
p[t] -> 1 - directional selection favouring A
if WA < Wa
p[t] -> 0 - directional selection favouring a
s
SELECTION COEFFICIENT - divide Wa and WA by Wa, typically done relative to the wildtype
so Wa = 1, and WA = 1 + s
if s is 10 times smaller…
takes 10 times longer to observe same amount of frequency change
W with a line over it [t]
mean fitness
WAp[t] + Waq[t]
mean fitness change across a generation
change in W bar = Wbar [t + 1] - Wbar [t]
why hasn’t everything evolved to be the same?
Fitness can change overtime
environmental change
the world isn’t static
organisms would never stay on evolutionary peak anyway even if our environment was static, because of mutation
evolutionary factors other than natural selection
Mutations
Chance
Sex and recombination
Alleles favoured for effects on some traits may affect others - PLEIOTROPY
Neighbouring alleles in genome can be dragged with selected alleles
