Also, the selfish genetic interests of interacting organisms tend to be aligned only insofar as those individuals are related (Hamilton, 1964; Mock & Parker, 1997), and pairs of individuals in a nuclear family differ dramatically

in their coefficients of genetic relatedness (r): a mother and her offspring normally share half their genes (r = 0.5) as do full sibs in a multi-birth litter; but half-sib progeny share only one-quarter of their genes (r = 0.25), and a sire and dam typically are unrelated (r = 0.0). For these and other reasons, each nuclear family is not simply a serene setting for harmonious interactions, but rather it can be an evolutionary minefield of oft-competing genetic fitness interests, both inter- and intragenerational (Trivers, 1972, 1974; Hausfater & Hrdy, 1984; Parmigiani Selisistat cell line and Vom Saal, 1994; Hudson & Trillmich, 2008). Furthermore, many of these conflicts play out forcefully within the mammalian womb. Thus, pregnancy becomes an evolutionary theatre for intergenerational conflict over parental resources – each offspring is under selection to seek as many maternal resources as possible (limited

only by any negative effects on its inclusive fitness that such demands impose on copies of its genes carried by its kin), whereas a dam can be expected to resist excessive demands by the fetus. The net result of each such evolutionary ‘tug-of-war’ (Moore & Haig, 1991) between mother and child is some ontogenetic balance in which each offspring must settle for fewer maternal resources than it ideally might wish and a mother surrenders more resources than she otherwise might prefer. But by evolutionary this website reckoning, any such maternal–fetal compromise during or after a pregnancy is less the result of a harmonious mutualism than it is an outcome of conflict mediation (Haig, 1993, 1999, 2010; Nesse & Williams, 1994). Of course, maternal–offspring relations entail elements of cooperation as well as conflict;

these two categories of interaction need not always be interpreted as mutually exclusive (Strassmann et al., 2011). Selective pressures that pregnancy promotes sometimes have led to outcomes that catch researchers totally off-guard. One such phenomenon is genetic imprinting: a situation in which a gene is expressed in progeny when inherited from one parent but MCE not from the other (Solter, 1988). In such cases, a gene can have very different effects on offspring (and therefore on the course of a pregnancy) depending on whether it was transmitted via the dam (egg) or sire (sperm). Genetic imprinting in animals appears to be confined mostly to viviparous mammals, but the phenomenon also is common in plants (Feil & Berger, 2007). In recent years, scientists have discovered imprinted genes in many marsupial and placental mammals, including Homo sapiens, where imprinting has been documented at approximately 100 loci to date.