Medium-term responses to and changes in fitness with selection for parameters of the allometric-autoregressive model

Abstract

The allometric-autoregressive model describes growth accurately and is useful in the characterization of growth responses. Hence, the potential of the model for selection was investigated. Rats were used in a selection
experiment, where selection was practised for slope (b) and intercept (ln a) of the allometric function: w = ln a + bv [where w = ln (body mass) and v = ln (cumulative feed intake)], and for ρ, the autoregression slope of ln (cumulative feed intake). Facilities for five selection groups of 40 rats each were available. Both upward and downward within-family selections were practised for ln a and b, and only downward selection for ρ. In the
medium-term, the observed direct responses to selection for parameters of the allometric-autoregressive model were irregular, and there was a marked discrepancy between the observed and expected responses. It was postulated that poor fit of the model in later generations, a lack of genetic variance, mutation, genetic drift and / or natural selection may be causing these discrepancies. The selection responses were accompanied by losses of fitness during certain generations, while in others, the responses disappeared altogether. In certain generations, however, fitness was regained and this was followed by renewed appearance of responses. This pattern of response points to the presence of natural selection, which was corroborated by the differences between the effective and expected selection intensities. It was concluded that the slope of the allometric
function seems to be the only parameter worth considering as a selection criterion.

Die allometriese-outoregressiemodel beskryf groei akkuraat en is bruikbaar vir die karakterisering van groeiresponsies. Gevolglik is die potensiaal van die model vir seleksiedoeleindes ondersoek. Die rot is gebruik in 'n seleksie-eksperiment waarin daar geselekteer is vir helling (b) en afsnit (ln a) van die allometriese funksie: w = ln a + bv [waar w = ln (liggaamsmassa) en v = ln (kumulatiewe voerinname)], sowel as vir ρ, wat die outoregressiehelling van ln (kumulatiewe voerinname) is. Fasiliteite vir vyf seleksiegroepe van 40 rotte elk was beskikbaar. Beide opwaartse en afwaartse binne-familieseleksie is vir ln a en b uitgevoer, terwyl daar slegs vir 'n lae ρ geselekteer is. In die mediumtermyn was die waargenome direkte responsies met seleksie vir parameters van die allometrieseoutoregressiemodel onreëlmatig, terwyl 'n aansienlike verskil tussen die waargenome en verwagte responsies gevind is. Daar is gepostuleer dat swakker passing van die model in later generasies, 'n tekort aan genetiese variasie, mutasie, genetiese drywing en / of natuurlike seleksie hiervoor verantwoordelik is. Die seleksievordering gedurende sekere generasies het gepaard gegaan met 'n verlies aan fiksheid, en soms het die vordering verdwyn. Telkens het fiksheid egter herstel waarna hernieude voorkoms van responsies ondervind is. Hierdie responsiepatroon dui op die teenwoordigheid van natuurlike seleksie, wat bevestig word deur die verskille wat tussen die effektiewe en verwagte seleksie-intensiteite voorgekom het. Die gevolgtrekking is gemaak dat die helling van die allometriese funksie die enigste parameter is wat as moontlike seleksiekriterium oorweeg kan word.

Keywords: Allometric-autoregressive model, fitness, genetic drift, natural selection, selection.