predation.

brood parasitism.

brood competition.

commensalism.

The behaviour does not reduce a donor’s fitness relative to a selfish alternative.

There must be some form of anti-cheating mechanism.

The helper and receiver must be related.

The probability of helper and recipient meeting again must be high.

Klebsiella bacteria that lives asymptomatically in the human gut, but gains antibiotic resistance via genes from ingested bacteria.

Mycobacterium tuberculosis which lives in the human lungs, multiplying when the immune system is compromised.

Rhizobium bacteria that invade the roots of legume plants, form nodules and fix atmospheric nitrogen in exchange for carbohydrates.

Bacteria which live on the human skin and gain nutrients from secretions, but do not harm the host.

A species of sponge producing chemicals that kill the symbiotic algae of a coral growing next to the sponge.

A bee taking nectar from a flower and, in the process, pollinates the flower.

A cuckoo laying her egg in the nest of another species of bird, the reed warbler. The reed warbler is then left to raise the cuckoo chick.

A Rafflesia plant using a specialised absorptive organ, known as a haustorium, to draw nutrients from other plants.

A moss gametophyte is observed to produce both male and female gametes.

The date palm (Phoenix dactylifera) has plants that produce flowers with only male structures or produce flowers with only female structures.

A pine tree is observed to produce male cones that shed pollen while the female cones, on the same tree, are still closed.

A plant has flowers containing male and female structures but the stamens are significantly shorter than the style.

The anther is shorter than the style.

The flower is most likely wind pollinated.

The flower most likely self pollinates.

Rhododendrons flower in response to short day length.

Plants alternate between seed-producing and spore-producing stages without any change in ploidy.

Plants alternate between sexual and asexual reproduction in each generation.

Plants alternate between flowering and non-flowering stages.

Plants switch between diploid and haploid stages, each forming a distinct multicellular organism.