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 offspringW/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 by

W=CI1/CI2=[p/(1-p)]/[q/(1-q)]

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 single-generation 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 population)

• 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 proportionally more of its offspring survive

Natural selection acts to increase the fitness of populations

?

Other Measures

• Viability alone (measure survival rates of offspring) - not tightly linked to fitness as defined previously

• 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 models

Adaptive Landscapes• Allele frequency in a population vs fitness

(multidimensional map)• Looks at fitness of a population, not of a specific

genotype• 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

Adaptive

Landscape

Random Drift on Adaptive Landscapes

Sources:

• http://links.jstor.org/sici?sici=0003-0147(199310)142%3A4%3C573%3AEOBSCO%3E2.0.CO%3B2-C

• http://links.jstor.org/sici?sici=0003-0147(199606)147%3A6%3C1087%3AEDOFYB%3E2.0.CO%3B2-O

• http://links.jstor.org/sici?sici=0269-8463(200110)15%3A5%3C696%3AEOBSLO%3E2.0.CO%3B2-G

• http://links.jstor.org/sici?sici=0031-8248(198206)49%3A2%3C268%3AOTPDOF%3E2.0.CO%3B2-T

• http://www.genetics.org/cgi/reprint/102/3/455

• http://www.genetics.org/cgi/reprint/137/2/509.pdf

• http://www.jstor.org/view/00318248/ap010196/01a00100/5?frame=noframe&userID=823ac019@swarthmore.edu/01c0a8346d00501c18850&dpi=3&config=jstor

• http://www.springerlink.com/content/x81q31h1m6052651/fulltext.pdf

• Life: The Science of Biology

• Introduction to Genetic Analysis

• http://images.google.com/imgres?imgurl=http://www.uwyo.edu/benkman/landscape.jpg&imgrefurl=http://www.uwyo.edu/benkman/rightsideresearch.html&h=992&w=1302&sz=151&hl=en&start=27&um=1&tbnid=aBv4T9zEahoniM:&tbnh=114&tbnw=150&prev=/images%3Fq%3Dadaptive%2Blandscape%2Bevolution%26start%3D20%26ndsp%3D20%26um%3D1%26hl%3Den%26client%3Dsafari%26rls%3Den%26sa%3DN