predation.

brood parasitism.

brood competition.

commensalism.

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.

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.

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.

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.

gibberellic acid.

ethylene.

cytokinin.

auxin.

The large tree we observe is the gametophyte stage while the sporophyte stage is much smaller and contained in flowers.

The large tree we observe is the sporophyte stage while the gametophyte stage is much smaller and contained in flowers.

The large tree we observe is the gametophyte stage while the sporophyte stage is much smaller and contained in cones.

The large tree we observe is the sporophyte stage while the gametophyte stage is much smaller and contained in cones.