Is this soap opera or science? It can be both, but in evolutionary biology it refers to the subtly different ways natural selection acts on genes expressed in parents and their offspring. To give a concrete example, consider a parent with two offspring: other things being equal, the parent is likely to be under natural selection to give equal amounts of food to each of its young. But natural selection is also likely to favour a gene expressed in one of the young that causes its bearer to grab more than its fair share of food.But your sibling is also likely to carry copies of your genes so won’t the ‘selfish’ gene be harming its own spread? Up to a point, but consider the case of a rare selfish mutant allele spreading through the population. If it is rare, only one parent is likely to carry the mutant allele, and if you, one of the offspring, carry the gene the probability your brother/sister also has it is only 1/2. So every time the gene is expressed it potentially benefits its bearer but only half the time harms a sibling carrier. Putting it crudely and anthropomorphically, the gene values its bearer twice as much as the bearer’s sibling as a vehicle for propagating itself. These arguments explain why the ‘selfish’ behaviour spreads and can ultimately become fixed.Isn’t there a logical inconsistency here: if I’m selfish as an offspring won’t I suffer as a parent because my offspring compete? No: why should suffering as a parent be more significant than benefiting as an offspring. The problem is that you are viewing evolution in terms of the fitness of individuals rather than genes. Most of the time it makes no difference but this is one of the situations where an explicit gene’s-eye view is essential. The selfish gene spreads because it does better than alternative ‘non-selfish’ alleles at the same locus (a type of frequency-dependent selection).I accept all this, but has it any relevance to the real world? Won’t the parent always be in a position to impose its will; won’t the resolution of the battle always be in the parent’s favour? Consider a moth that lays a clutch of eggs on an isolated host plant. If the young are not selfish, and use the resource optimally, she can lay a large clutch of eggs. But selection will favour selfish and wasteful behaviour by the young and this in turn will lead to selection on the parent to reduce clutch size. Clearly the resolution is not at the parental optimum.I’m not interested in moths, I want to understand King Lear! Surely my argument stands for birds and mammals where parents and young interact over an extended period of time? Have you never wondered why young birds in the nest beg so vigorously, or why babies cry so loudly and energetically that they appear to harm themselves?Y’r what? There is evidence that begging behaviour is energetically expensive, and can risk attracting predators. Baby birds and mammals beg or bleat when hungry, but why can’t they signal their need in a way that does not incur fitness costs?Any chance of reaching a point sometime soon? My point is that simple communication of need cannot evolve in the presence of the parent-offspring conflict battleground. Cost-free signalling is normally evolutionarily unstable because it is vulnerable to cheating. Instead, the only type of signalling system that can evolve and be stable is one in which signals are costly to produce and hence you are only selected to send them when your need is great. Parent–offspring conflict is thus relevant to the real world: it offers a (potential) explanation for the paradox of costly begging.I notice a parenthetical ‘potential’ in the last sentence... It is only fair to point out that there are other possible explanations for noisy begging. There is also a close parallel with signalling during courtship: a system in which males use cost-free signals to say how great they are is unstable, again because it is vulnerable to cheating; the peacock’s tail evolves because it is costly and it is only economical for high quality males to bear the costs. There are even parallels with the theory of advertising in economics.Any other squabbles in the family? Wherever natural selection operates differently on genes expressed in different individuals there is the potential for conflict. And it doesn’t stop there. Suppose garnering more food from your mother reduces her future survival: then a gene expressed on a maternally-derived chromosome will have a different optimum from one on a paternally-derived chromosome (molecular imprinting allows this type of conditional expression). Artificial embryos with two maternally-derived chromosomes make smaller than average placentas than those with two paternally-derived chromosomes. Again this is an example of where consideration of potential conflicts can explain otherwise very curious biological facts.