Measuring Fitness Differences of different Genotypes under variable Survivorship

                L
      Ro (lx)(mx replacement rate = # offspring in a lifetime
               x=1

        where
               lx = probability of survival from birth to age x  (cumulative)
                        [cf. survivorship = probability of survival to age x+1 from age x ]
               mx = fecundity (# offspring) at age x
                L
= maximum age (measured in appropriate units)

      Ro er at low population density (otherwise limited by logistic equation)

Suppose for some species:
     Maximum life span 4 seasons (days, months, years, etc).
     Reproductive Strategy a genetically determined phenotype:
        
Iteroparity: reproduction spread out over several seasons (iterated)
         semelparity
: reproduction limited to single season
     
         fecundity for each phenotype determined by genotype
                         
at a single locus with co-dominant alleles I & S
 
     Survivorship patterns are the same for all phenotypes at any one time:
            these are determined by the environment
                 25%, 50%, & 75%
in poor, average, and good environments
                 [Phenotype is perfectly genetic, AND perfectly environmental]


     Experimental data: Table records # offspring (male & female) born at each age to each female
            Ro = sum over all age classes

           Ro = 2 in population with stable size: each male / female parent pair produces two offspring

            [Blank worksheet to repeat calculations]



Iteroparous strategy in II mother: reproduction distributed over several years

Iteroparous
        strategy


Semelparous
strategy in SS mother
: reproduction limited to single year

Semelparous
            strategy

Additive fecundity in IS heterozygous mother: average contribution of both alleles
Additive
          Fecundity

Conclusion

Fitness differences are measurable as variable Ro for genetically-determined reproductive strategies under environmentally-variable survivorship regimes.


25% survivorship
50% survivorship
75% survivorship
Iteroparous (II)
0.719
2.000
4.031
Additive (IS) 0.859
2.000
3.516
Semelparous (SS)
1.000
2.000
3.000

Homework: for the summary table at bottom, answer the following questions & explain your answers by numerical examples
    1. Can phenotypes be ranked as consistently more or less "fit" over different environments?
    2. Is a population in which one phenotype is "most fit" necessarily increasing in population numbers?
    3. Is the "least fit" phenotype necessarily declining in population numbers?
    4. Does a genetically-determined trait have a constant phenotype in all environments ?


Figures and Text material © 2024 by Steven M. Carr