Evolution of threshold traits: the balance between directional selection, drift and mutation

Abstract

Threshold traits are characterized by showing discrete phenotypes (typically two) but by being controlled by many loci of small additive effect, the expression of the phenotype being a consequence of a threshold of sensitivity. In the case of a dimorphic threshold trait, individuals above the threshold display one morph and individuals below the threshold display the alternate. Many threshold traits, such as sex ratio, cyclomorphosis, paedomorphosis and wing dimorphism, are closely connected to fitness but have high heritabilities. The present study investigates the hypothesis that these large heritabilities can be maintained even in the face of directional selection by the countervailing force of mutation. This hypothesis is based on the observation that as selection proceeds to shift the frequency of one morph towards fixation, the selection intensity necessarily declines permitting mutation to restore genetic variation. The hypothesis is tested using a simulation model and a theoretical analysis, the latter assuming no genetic drift. It is shown that over 80 per cent of the original genetic variance can be maintained at equilibrium provided the population (N) and number of loci (n) are reasonably large (N 5000, n = 50). However, unless the selection coefficient is very small ( 0.001) the equilibrium frequency of the phenotypes ( 2 per cent) is considerably below that generally observed. I conclude that mutation could play a significant role in the maintenance of genetic variation in threshold traits but that some form of selection, such as frequency-dependent selection, is required to maintain the phenotypic variation.

Female Choosiness Leads to the Evolution of Individually Distinctive Males

“…For signaling and neutral region mutations, there is a 1:1000 chance that one allele of 12 will mutate to one of the three different states. This represents a 1:6000 mutation rate per locus, which is of the order used in other simulation models (Roff 1998). Our mutation rate of 1:500 per locus for attractiveness is substantially higher because we assume that attractiveness is the product of numerous alleles across the genome (which we represent as a single number); the mutational target is therefore likely to be relatively large, as it is for quality (Hunt et al 2004).…”

Evolution of pre-copulatory and post-copulatory strategies of inbreeding avoidance and associated polyandry

“…In contrast, individuals' phenotypic values for tendency for polyandry (P p ) cannot map directly onto their genotypic values (P g ) because P g can evolve to be negative, but females cannot mate with a negative number of additional males (e.g., Shuker et al, 2007;Evan & Gasparini, 2013). Rather, we considered polyandry "threshold trait", whereby continuous genotypic variation translates into expression of discrete phenotypic value(s) at some threshold (Lynch & Walsh, 1998;Roff, 1996Roff, , 1998. Accordingly, we allow individuals' phenotypic values for tendency for polyandry to equal genotypic values (P p P g ) if P g ¥ 0, but set P p 0 if P g 0.…”

“…Polyandry is therefore influenced by continuous genetic variation but only expressed when P g ¡ 0. One important general property of such threshold traits is that deleterious traits are occasionally expressed despite sustained negative selection because recombination among deleterious alleles can cause the underlying genotypic value to exceed the threshold for expression (Roff, 1996(Roff, , 1998.…”

How migratory populations become resident

“…In birds, migratory activity can be quantified in captivity by measuring its proxy, migratory restlessness [6][7][8]19]. This semi-continuous pattern of expression of migratory behaviour differs from the one expected for 'typical' threshold traits, which are dichotomous [20][21][22]. We may expect selection responses in semi-continuous threshold traits to be stronger than in dichotomous traits, since selection may act on both the categorical (migrant or resident) and the continuous (amount of migratory activity) aspect of the trait at the same time [8,15,19].…”