Life History, Evolution of Derek Roff, McGill University, Montreal, QC, Canada r 2001 Elsevier Inc All rights reserved This article is reproduced from the previous edition, volume 3, pp 715–728, r 2001, Elsevier Inc Glossary Genotype The genetic constitution of an organism Heterozygosity The presence of alternate alleles at a given locus in a diploid organism (e.g., A1A2) Homozygosity In a diploid organism, the presence of the same alleles at a given locus (e.g., A1A1) Introduction reproductive habit would be exactly equivalent to adding one individual to the average litter size (Cole, 1954, p118, Cole’s italics) Plants and animals show profound variation in all aspects of their life histories, which include age at maturity, age-specific fecundity, survival rate, size at birth, and so on This variation is evident at both the inter- and intraspecific levels For example, at the interspecific level, species of flatfish range in size from cm-long tropical species that reproduce within their first year of life to behemoths such as the Pacific Halibut (Hippoglossus hippoglossus), which exceed 200 cm and take over 10 years to mature Though the range in variation within a species is not as dramatic as between species, it is still impressive, as illustrated by variation in the flatfish, Hippoglossoides hippoglossoides In this species maturation occurs at age years at a length of 20 cm in populations off the coast of Scotland while the same species requires 15 years to reach maturity at a length of 40 cm on the Grand Banks of Newfoundland Longevity and maximum size are equally different in the two populations, Scottish fish reaching a maximum length of 25 cm and an age of years, compared to 60 cm and 20 ỵ years on the Grand Banks Similar observations on variation in life history characteristics could be made in most taxa But though the diversity of life histories is readily apparent, attempts to understand its origin and maintenance are still in their infancy Because fitness varies quantitatively among different life histories, a necessary tool for analysis is mathematical modeling An influential factor encouraging the use of mathematical investigation into life history variation was Lamont Cole’s 1954 paper, ‘‘The Population Consequences of Life History Phenomena,’’ which set out the basic mathematical framework by which the consequences of variation in life history traits can by analyzed Cole’s paper ushered in an era of research predicated on the integration of mathematics and biology in the study of the evolution of life history patterns In his review, Cole analyzed how changes in demographic attributes, such as the age at first reproduction, influenced the rate of increase of a population Cole’s paper gained widespread notice because of an apparent paradox with respect to the value of semelparity versus iteroparity: For an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial Encyclopedia of Biodiversity, Volume Iteroparity Repeat breeding (see semelparity) Semelparity Breeding once and dying; sometimes called ‘‘big bang’’ reproduction Thus, an annual species with a clutch size of 101 would increase in numbers as fast as a perennial that produces 100 young every year forever There is obviously something amiss with this result, for perennials are common While there is good evidence that survival and reproduction are negatively correlated it seems highly unlikely that perennial species are committing so much energy into reproduction that they cannot produce one more offspring Cole’s paradox derives from a failure to consider the consequences of juvenile and adult survival rates; specifically Cole implicitly assumed that the survival rate of juveniles is the same as that of the adults In general, juvenile survival is lower (often by a great amount) than that of the adults and this means that the reproductive output of the annual must be greater than one for the annual to have the same gain in rate of increase as a perennial, the precise amount depending on the mathematical details of the model Cole’s paradox illustrates the need to formulate an analysis of life history variation within a mathematical framework where the biological assumptions are explicitly stated Since Lamont Cole, the theoretical and experimental analysis of life history evolution has made enormous strides using two different analytical perspectives Two Frameworks for Analysis The evolution of life history variation has been approached via two different modes of analysis, here termed phenotypic models and genetic models Phenotypic Models In this approach no attempt is made to model the genetic basis of traits: it is simply assumed that there exists sufficient genetic variation that evolution is not constrained by genetic architecture Is this a reasonable assumption? Most life history traitsFsuch as age at first reproduction, fecundity, and survivalFare not determined by simple Mendelian mechanisms such as single locus, two allele systems More generally they http://dx.doi.org/10.1016/B978-0-12-384719-5.00087-3 631