Symbiosis as a co-evolved intimate relationship between members of two species, including parasitism and mutualism, and social behaviour including social hierarchy, co-operative hunting, social defence, altruism, kin selection and the social organisation of insects and primates.

What this key area is asking

The SQA wants you to define symbiosis and distinguish parasitism from mutualism, and to describe social behaviour, including social hierarchy, co-operative hunting, social defence, altruism, kin selection and the social organisation of insects and primates.

Symbiosis: parasitism and mutualism

The two main forms are:

• Parasitism - the parasite benefits in terms of energy or nutrients while the host is harmed. Parasites are often highly specialised and may have a limited ability to survive away from a host, which is why many have complex life cycles involving more than one host.
• Mutualism - both species benefit from the relationship, so each partner gains, for example, food, protection or transport.

Both forms have co-evolved, meaning each species has evolved in response to the other over a long time, producing the close fit seen between parasite and host or between mutualistic partners.

Social behaviour

• A social hierarchy is a rank order among the members of a group; ritualised threat displays settle disputes without serious injury and reduce conflict, so energy and individuals are not wasted in fighting.
• Co-operative hunting lets a group catch larger or more prey, so each member can gain more energy than by hunting alone, even after the kill is shared.
• Social defence lets a group protect itself, for example by posting sentinels that warn of predators or by grouping the vulnerable young in the centre of the herd.

Altruism and kin selection

When the recipient is a relative (kin), the donor's apparent loss can be outweighed by the survival of shared genes in the relative, so the altruistic behaviour can be favoured by natural selection. The closer the relative, the more genes are shared, and the more likely altruism is to be favoured.

Social insects and primates

• Social insects (such as bees and ants) show a clear division of labour: only the queen reproduces, while sterile workers raise the young, gather food and defend the colony. Because the workers are closely related to the queen and her offspring, helping them is a close example of kin selection.
• Primates rely on complex social behaviours, such as grooming, alliances and appeasement gestures, to maintain bonds and reduce conflict within long-lived social groups, which improves the survival of group members.

Examples in context

Example 1. Cleaner fish and mutualism. Cleaner wrasse set up cleaning stations on coral reefs where larger fish come to have parasites and dead skin removed. The cleaner gains a meal, and the larger fish is rid of parasites, so both species benefit, making this a clear example of mutualism. The relationship is co-evolved: the large fish even suppress their normal predatory behaviour while being cleaned, and cleaners advertise their service with a distinctive dance.

Example 2. Honeybee colonies and kin selection. In a honeybee colony, only the queen reproduces, while thousands of sterile female workers forage, build comb, care for larvae and defend the hive, even dying when they sting. Because the workers are very closely related to one another and to the queen's offspring, helping the colony passes on shared genes. This division of labour and self-sacrifice is one of the strongest real examples of altruism explained by kin selection.

Try this

Q1. State the difference between parasitism and mutualism. [2 marks]

• Cue. In parasitism the parasite benefits and the host is harmed; in mutualism both species benefit.

Q2. Explain how kin selection can favour altruistic behaviour. [2 marks]

• Cue. Helping a relative survive passes on shared genes, so the altruistic behaviour can be favoured by natural selection.