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A close biological association on which at least one partner depends for persistence or reproduction under the specified conditions. Obligacy does not itself imply mutual benefit, reciprocity or coevolution.

Both conflicts arise because family members value the same resource differently, but they concern different decisions. Sibling rivalry concerns allocation among offspring, whereas parent-offspring conflict concerns a parent's allocation relative to what an offspring would favour. Their consequences interact: offspring compete with one another inside a parental provisioning system that their own demands can also alter.

The basic mechanism is unequal genetic relatedness to the beneficiaries of an action. An offspring values its own additional survival or reproduction more strongly than an equal gain to a sibling. A parent, under a simple symmetric pedigree, is equally related to each of its offspring and can favour an allocation that maximizes their combined contribution. Neither statement implies that an offspring should demand everything: diverting resources from relatives can reduce its inclusive fitness. These differing interests underlie the original theory of parent-offspring conflict.^[ref]

Consider an explicit illustrative model. Let investment $x$ produce a direct offspring benefit $B(x)$ and an opportunity cost $C(x)$ measured in lost equivalent fitness contributions of other offspring. Assume a single parent, equal value of current and future offspring, no inbreeding, and relatedness $r$ between the focal offspring and those losing resources. After dropping a common parental relatedness factor, the parent's objective is $B-C$, whereas the offspring's is $B-rC$. Interior optima therefore satisfyWith diminishing marginal benefits and $0<r<1$, the offspring typically favours more investment. In particular, the range $r{C}^{\prime }<B^{\prime }<C^{\prime }$ contains increments favoured by the offspring but opposed by the parent. For full siblings under the stated pedigree, $r=1/2$; for maternal half-siblings, $r=1/4$. These values are not universal constants for all family structures, and parent survival, reproductive value and density dependence can alter the effective costs.

Sibling rivalry selects on relative access; parent-offspring conflict can also change the total supply. If one nestling obtains a larger fraction of a fixed food delivery, its sibling loses directly. Selection can favour size advantages, advantageous positions, earlier hatching or more effective solicitation. Severe asymmetries can permit siblicide or brood reduction. By contrast, a stronger demand directed at a parent can increase food delivery to the brood, potentially benefiting siblings too. The same begging behaviour can therefore redistribute a fixed supply and alter parental effort, with different consequences in each case.

A parent's preferred distribution need not be equal. A marginal food item may produce more surviving descendants when given to a smaller hungry chick, or to a larger chick that alone can survive a severe shortage. Parental allocation can consequently mitigate sibling asymmetry in one environment and reinforce it in another. Asynchronous hatching and brood-size adjustment are plausible parental means of structuring that contest; whether a particular asymmetry is an adaptation, a constraint or an offspring-driven outcome needs evidence. A parent's tolerance of brood reduction is not proof that every aggressive act serves parental interests.

Over evolutionary time, sibling rivalry can affect competitive morphology, growth schedules, solicitation, nest position and the survival distribution within broods. Parent-offspring conflict can affect the amount and duration of care, offspring demand, parental responsiveness, weaning schedules and the trade-off between current and future reproduction. Neither conflict predicts unlimited escalation. Energetic expenditure, injury, predation risk and losses to relatives can make competitive effort costly even when it obtains a larger resource share.

Information can produce compromise as well as waste. An offspring usually has private information about its hunger, while the parent integrates brood condition with its own prospects. In a costly-signalling model, offspring in greater need gain more from additional resources, so differing incentives can support informative solicitation at equilibrium. Such a result is conditional on the cost and benefit functions; it does not mean all begging is perfectly honest or that energetic expenditure alone establishes honesty.^[ref]

Signals can also be aimed at siblings. Experiments on barn owls found that nestlings reduced vocalization when a rival was hungrier, increased it after that rival was fed, and altered calling with experimental brood size. These calls occurred when parents were absent. They support sibling negotiation: assessing relative willingness to contest a future prey item can avert some unnecessary escalation. The experiment therefore cautions against treating every nestling call as a demand directed at the parent.^[ref]

A further consequence is coevolution among parental and offspring response rules. A demand phenotype expressed in young becomes part of the selective environment of provisioning adults, while parental allocation changes the returns to offspring competition. The same individual can experience both roles at different life stages, and genetic correlations between demand and care can constrain independent optimization. Thus observed care can be a compromise maintained by interacting strategies rather than the optimum of either party in isolation.

The comparison is consequently about overlapping arenas of selection. Sibling rivalry chiefly determines who obtains care, while parent-offspring conflict also determines how much care is supplied and when it ends. Both can generate competitive adaptations and costs, but both can also generate assessment, contingent responses and restraint. A strong empirical test must separate changes in total provisioning from redistribution, measure lifetime consequences rather than only immediate feeding success, and manipulate relatedness or information without simultaneously changing need.

Dependence evolves when a partner becomes a sufficiently reliable part of the environment that specialization on its services outcompetes maintaining an autonomous alternative. Reciprocal evolutionary change requires additional evidence. Ecological interdependence, obligate symbiosis and coevolution describe different properties: present-day benefit, inability to persist without assistance, and reciprocal heritable responses to selection, respectively.

An association can begin with incidental benefits. One species releases metabolic products, modifies habitat or consumes a toxin; another uses the altered environment. Initially neither partner need have evolved a service for the other. Repeated association can favour increased access to that benefit, spatial attachment or mechanisms that exchange complementary resources. Metabolic cross-feeding may arise from waste uptake before either producer becomes dependent on a particular recipient. This route requires no foresight and does not imply that the interaction started as costly altruism.

Once a service is sufficiently reliable, retaining an expensive redundant function may yield little advantage. Adaptive gene loss can then make the recipient dependent. Neutral deterioration under relaxed selection is another possible route, particularly when small populations and transmission bottlenecks weaken purifying selection. These explanations have different predictions about costs and selection and should not be conflated: a missing gene is not evidence, by itself, that losing it was adaptive. Moreover, dependence is conditional on the environment. A lineage unable to grow alone in one natural setting may grow without its usual partner when the missing resource or protection is experimentally supplied.

Experiments with Prochlorococcus illustrate this distinction. The cyanobacterium is vulnerable to extracellular hydrogen peroxide, and co-culture with peroxide-removing Alteromonas protected cells and enabled growth at low densities. Removing the surrounding microbial community also altered peroxide accumulation in seawater assays. These results establish an ecological dependence on a diffusible protective service in the tested conditions. They do not demonstrate that one specific helper species must be present, that helpers require Prochlorococcus in return, or that gene loss and reciprocal adaptation were observed evolving during the experiment.^[ref]

Reciprocal specialization can make dependence bilateral. If each partner increasingly relies on a different product or function supplied by the other, selection can favour complementary investment and reduced redundancy. Inherited association strengthens that feedback: successful host reproduction also transmits symbionts, so investment in host function can improve symbiont transmission. Vertical transmission and physical compartmentalization can therefore align some fitness consequences. They do not make interests identical, because symbionts can still compete within hosts and hosts can favour reduced costs of support.

Aphids and Buchnera aphidicola provide a strong example of a highly integrated association. The sequenced Buchnera aphidicola genome has extensive losses but retains pathways associated with essential amino-acid provision, consistent with nutritional complementation of the aphid's diet. The bacterium depends on the host-associated setting, while the host uses its biosynthetic contributions. This pattern supports derived specialization and loss of autonomy; the genome alone does not establish every metabolite exchanged or directly reconstruct the order of historical evolutionary changes.^[ref]

Dependence can also arise through repeated behavioural or ecological specialization rather than physical integration. A pollinator can specialize on a floral resource, or a predator on a particular prey. The affected partner may then evolve recognition, defence or counter-exploitation. Such interactions may remain facultative, asymmetric or antagonistic. Conversely, obligate association can arise through one lineage's losses while the other changes little. Thus the intensity of current dependence does not measure the amount of reciprocal evolution.

Coevolution requires each lineage to alter selection on the other, with a heritable response in both. A functional match between two traits is insufficient: both could have adapted to the same abiotic environment, one could have tracked the other, or different partners could produce the same match. An association might also result from ecological sorting of already compatible types. Immediate behavioural adjustment or altered gene expression is phenotypic plasticity unless an inherited change in the response itself is demonstrated.

Experimental host-parasite evolution gives a clearer test. In replicated populations of bacteria and bacteriophages, temporal comparisons of bacterial resistance and phage infectivity demonstrated reciprocal escalation, with divergence among replicate populations. Here changes in the antagonist's traits change the selective environment confronting the other lineage. This is evidence for an antagonistic arms race rather than merely a close ecological association. It also shows why coevolution is not restricted to beneficial partnerships.^[ref]

For a mutualism, reciprocal-transplant experiments can cross evolved hosts with ancestral and evolved symbionts, measuring both parties' performance in a common environment. Parallel evolution without the partner and controls for adaptation to the culture medium help isolate partner-mediated selection. In natural systems, time-shift assays, reciprocal genotype combinations and evidence that relevant traits affect partner fitness strengthen an inference; simple correlations and matching phylogenies remain vulnerable to shared environmental or transmission histories. Diffuse coevolution involving several interchangeable partners is possible and need not produce a one-to-one species pair.

The result may be complementary specialization and increasingly integrated reproduction, but stability is not guaranteed. Cheaters, changing environments, loss of partners and altered transmission can destabilize even an obligate association. The transition from incidental benefit to dependence is therefore explained by the relative costs of autonomy and association; the transition to coevolution is established only when reciprocal selection and heritable responses are demonstrated.