Emily Hager.

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Transcript Emily Hager.

Evolutionary Fitness
Absolute Fitness
• Reproductive success: change in proportion of
offspring in the next generation
• Can be determined for genotypes or phenotypes
• W=No/Np, the number of offspring over the
number of parents with the trait
• W=P*N, where P=probability of offspring
survival, N=average number of offspring
W/W(mean), where W(mean) is the mean fitness of
the population as a whole
Competitive Index
• Knight and Roberston, 1957
• CI=p/(1-p), where p is the proportion of the
offspring with the trait
• The relative fitness of two strains is given
Alternative Competitive Index
• Jungen and Hartl, 1979
• Just use p, not p/(1-p), as a measure of
fitness (ie p=N*/N(total))
• Relative fitness, W=p/q
Long Term/Equilibrium Tests
• Sved, 1971
• W=(r-h)/r(1-h) where r is the viability of
offspring of the strain, h is the equilibrium
frequency of the strain (average of the last 9
generations of tests)
• In test by Hartl, 1981, the longer term value
gave different results than the singlegeneration test
What can these tell us?
• These definitions can be useful, as long as we are
not trying to test the theory of natural selection
itself (ie, natural selection acts to increase W for a
• For example, used to figure out what
characteristics make an organism more fit in a
certain environment, or to track changes in the
characteristics of a population due to changes in
the selective pressures
Problem: Circularity
• These definitions are based on the theory of
natural selection. We can’t use them to test the
theory that natural selection acts to increase the
fitness of a population.
A phenotype is
“more fit” if
more of its
offspring survive
Natural selection
acts to increase
the fitness of
Other Measures
• Viability alone (measure survival rates of
offspring) - not tightly linked to fitness as defined
• Reproductive power: the rate at which organisms
transform energy to reproduction/offspring (fitness
~ energy used for reproduction)
• Model proposed by Brown, Marquet and Taper,
1993, widely discredited now, but work on other
Adaptive Landscapes
• Allele frequency in a population vs fitness
(multidimensional map)
• Looks at fitness of a population, not of a specific
• The landscape can change (changes in
environment, for example, or in competition,
available resources, etc) --> shifting surface,
different peaks --> no “end point”
• Local vs global maxima
• Natural selection acts to increase the fitness of the
population as a whole
Phenotype vs. Environment
Fitness vs. Allele Frequency
• The initial condition of the population can affect
the direction of its selection (extreme example: an
environment favoring bigger wings could cause a
population to head for more cells/wing or bigger
wing cells, or a combination of the two; less
extreme example: could get favored AA or BB
genotype if favor red flowers)
• Importance: local, not global, maxima in fitness
Random Drift on Adaptive Landscapes
http://www.jstor.org/view/00318248/ap010196/01a00100/5?frame=noframe&[email protected]
Life: The Science of Biology
Introduction to Genetic Analysis