The decorated Easter egg, for example, is an evolution of the egg as an ancient symbol of renewed life, and the fat, laughing figure of the Japanese Hotei (god of luck) is both a deity and a ubiquitous folk charm.
Therefore, the evolution of the egg envelope digestion system, from a single enzyme to a two-enzymes system, has nearly overlapped with a change in the expression sites of zp genes.
Thus, the evolution of perfect egg mimicry by the cuckoo, resulting in an immaculate pale blue egg type with little variation is likely to have occurred relatively rapidly, giving a huge fitness advantage to the cuckoo.
Instead, the more homogenous markings seen in the CSA lineage might be viewed as an intermediate character state in the evolution of the species-specific egg-spots as found in the Astatoreochromis-lineage and in the modern haplochromines.
In either case, our results suggest that the evolution of mean egg size can be relatively decoupled from the evolution of egg size flexibility, due to constraints (e.g. egg size flexibility hit its physiological limit, as discussed below) or because the determination of mean egg size and the regulation of egg size flexibility involve different genes or genetic pathways [41], [42].
Egg rejection experiments showed that selection on parasites for countering the evolution of host egg types is evidently strong because hosts generally have evolved good abilities for rejecting even partly mimetic eggs.
Furthermore, selection on parasites for countering the evolution of host egg types appears to be strong, since ashy-throated parrotbills have evolved rejection abilities for even partially mimetic eggs.
We have presented evidence suggesting that population density and homogeneity of geographic distribution of hosts explain the evolution of specialized egg-morphs of the European cuckoo.
We have presented evidence suggesting that population density and homogeneity of geographic distribution of hosts explain, at least partly, the evolution of specialized egg-morphs of the European cuckoo.
Consequently, the evolution of different cuckoo egg morphs that mimic eggs of their hosts reflects specialization occurring in some, but not all suitable host species [ 13, 14].
Thus, the growing understanding of the evolution of egg size plasticity provides a useful background for interpreting and discussing our results.
