Progressive change in structure and anatomy of organisms.

  1. Structural adaptation: Adaptive colouration
  2. Structural adaptation: For obtaining food.
  3. Different castes of termites.
  4. Different castes of bees.



Definition of Evolution

Organic evolution is the sum total of adaptive changes that have taken place over a long period of time in the life history of an organism. This process usually leads to the development of new species from pre- existing ones.

Organic evolution suggests that all living things have arisen from pre-existing ones by a gradual process of change over a long period of time.

Evolution proposes that life started in water; from marine water to fresh water to land and then to air. Progressive changes in the structure and anatomy of organisms have been going on as they inhabited these habitats at one time or the other. These changes enabled the organisms to be well adapted to the various habitats.

Organisms that live in water environment have streamlined body shape, soft and moist skin, water dependent reproductive system, efficient osmoregulator and other structural and anatomical adaptations.

The fore limbs of aquatic animals are also modified for swimming and orientation in water. These fore limbs in the course of progressive evolution changes are modified into crawling and hopping as in amphibians; running in reptiles; flight for birds and bat; climbing and swinging as in monkeys and walking and grasping as in humans and chimpanzees respectively.

All these animals have the same anatomical pattern of forelimbs but diversified structures for various adaptations in their respective habitats.

Similar structures that perform similar functions are called homologous structures. Dissimilar structures that perform similar functions are said to be analogous.

In the process of their evolution from water to land, Organisms progressively changed by possessing;

  1. complex body posture to withstand gravity,
  2. dry skin to prevent desiccation (drying up),
  3. internal fertilization mechanism to ensure reproduction on land and
  4. a modified osmo-regulator to conserve water on land.





  1. Describe how organisms evolved from one habitat to another.
  2. State the changes that occurred in limb structure as organisms progressively evolved from one habitat to another.

Galapagos Finches




To survive in their environments, organisms show different adaptations in their form and function. The following are some of the special adaptations shown by organisms for survival:

  1. Adaptive Colouration: Many organisms posses bright, attractive colour with special markings on their bodies to help them obtain food, escape or hide from enemies and to secure mates. Some flowering plants have brightly coloured flowers which attract animal pollinators.
  2. Warning Colouration: Many vulnerable preys develop bright colours and are bitter and unpalatable to their vertebrate predators. Distasteful insects are brightly coloured and conspicuous with contrasting pattern of different colour bands. Examples are lady bird beetles, wasps and butterflies. Even insects with nice taste but with colours like that of the unpalatable ones also survive.
  3. Mimicry: This is the close resemblance of an animal called a mimic to another different object referred to as a model in order to increase its chances of survival. A harmless organism is usually protected from its predator by mimicking a dangerous species. A mimic and a model are naturally unrelated. For good success a mimic must also behave like the model in some ways.
  4. Camouflage: Many animals are protected from their predators by the close matching of their body appearance with their surrounding background. Many grasshoppers and praying mantis have green pigments in their cuticles making them look like green vegetation. The rattlesnakes easily fit into the colour of the green environment. Another example is the chameleon.
  5. Colouration: Some animals possess bright body colouration which helps during mating. The bright colouration usually attracts the opposite sex for mating. Birds like peacock, turkey and cock exhibit colouration.
  6. Counter Shading: In some animals, the upper part of the body has a different colour from the lower part of the body. Such animals are said to be counter shaded. Most fishes have dark dorsal colour which tend to blend with the dark coloured water while their ventral sides are light in colour which blend with the sky above.




  1. List and discuss four structural adaptations for survival.





Some animals have special structural adaptations that enable them obtain food. For instance the beaks of birds are adapted to their individual diets.


Structure of beak

Food Eaten

Eagle, Falcon, Hawk, Kite

Hooked, sharp beak used to


kill and tear off flesh.


Long, strong pointed beak


Humming bird

Long beak and tubular-tipped




Short beak held wide open

Insects in flight

Weaver bird

Short, cone-shaped beak


Wood pecker

Long, narrow, pointed beak

Insect in tree bark

The mouth parts of some insects are also adapted to their feeding habits.

Flies and mosquitoes have proboscis as feeding organs with these they can freely feed on fluids by sucking (e.g. housefly) or piercing and sucking (e.g. Tse-tse fly and mosquitoes). Grasshoppers, termites and caterpillars of butterflies and moths have sharp mouth parts (mandible and Maxillae) for biting and chewing.

Structural Adaptations for Protection and Defence

Many organisms have exoskeletons by which they protect themselves from predators and other unfavourable environmental conditions.

Crabs, centipedes, snails, tortoise and turtle all have hard coverings or bony plates for protection. Others include the sharp thorns on some plants like acacia or spines on plants like Asparagus. Mammals like cows, deer and buffalo use horns to defend and protect themselves. Other animals defend and protect themselves with poisonous and toxic secretions when attacked. Some snakes spray poisonous venom on their predators.

For attack and defence, many animals bite their prey, predators or intruders with their teeth or mouthparts. Examples are insects (ants and termites), mammals (cat, dog, lion) and reptiles (snakes).

The swimming of fish and the flying of insects and birds aid them in moving away from predators.

Structural Adaptation for Regulating Body Temperature

The skin of mammals is well adapted to regulate body temperature.

The feathers of birds, the scales of some fish and reptiles and the shells of snails and crustaceans all help in regulating their body temperature.

Lizards are poikilothermic; their body temperature is determined by the temperature of their

surrounding environment. They bask in the sun to raise their body temperature and rest in the shade when their temperature rises above a critical level.


  1. Name and describe the structures used by four birds to obtain their food.
  2. Mention two insects and describe the structure they use to obtain food.
  3. List five animals and discuss their structural adaptation for protection and defence.
  4. State two differences between homiotherms and poikilotherms.


Write short notes on the following:

  1. hibernation,
  2. aestivation and
  3. migration



Termites are social animals. They live in highly organized groups. They usually live in underground tunnels. Termites feed on cellulose thereby damaging wooden materials, crops and young trees. They help in maintaining soil fertility by breaking down dead materials into humus.

Termites in a colony

A colony of termites is made up of the following castes:

  1. Queen and King: These are the reproductive members of the castes with the responsibility of mating and laying eggs. There is always only one king and one queen at any point in time though there could be other fertile potential kings and queens. A fully grown queen is about 9cm long.
  2. Workers: There are sterile females with no eyes, soft and pale exoskeleton and well developed mouthparts. Workers are responsible for building the termitarium, searching for and bringing in food for other members in colony. They also tend fungal gardens, collect eggs and care for the nymphs structural adaptations for regulating body temperature. After eating they regurgitate the partially digested food to feed the other members of the colony.
  3. Soldiers: These are sterile, blind and wingless males with large heads, thick exoskeletons and huge mandibles. They are responsible for protecting the colony from invaders and also protect the workers as they gather food for the colony.


  1. What is a caste?
  2. Name, describe and state the functions of the individuals that make up a termite caste.
  3. What is/are the economic importance of termites?
  4. How do termites regulate their body temperature?
  5. What are the key differences between ants and termites?


  1. Research and discuss the different types of termites, their roles in the colony, and their economic importance. Also address how they regulate their body temperature and any



DIFFERENT CASTES OF BEES AND THEIR FUNCTIONS A honey bee colony lives in a hive.

It is composed of:

  1. a single queen,
  2. a few hundred drones and
  3. several thousand workers.

Bees undergo complete metamorphosis unlike termites which undergo incomplete metamorphosis.

Drones hatch from unfertilized eggs. The drones are responsible for fertilizing eggs.

The queen and workers hatch from fertilized eggs. Larvae feeding on pollen grains and honey become workers. If fed on royal jelly which contains vitamins, a larva develops into a queen. The queen then secretes a queen substance that suppresses the development of ovaries in the workers.

The queen does the service of reproduction to replenish the castes.

The workers labour for the colony. They build and maintain the nest, collect food, feed other members of the hive, clean the nest and rear young bees.


  1. What is a honey bee colony?
  2. Name the different individuals that make up a honey bee colony, and briefly describe their roles.
  3. Compare and contrast bees and termites in terms of their developmental stages and caste systems.
  4. How do bees regulate the temperature of their nests?
  5. What are the economic and ecological impacts of bees on humans and the environment?

Bees in a colony


  1. Mention the castes in a bee colony.
  2. What are the functions of the castes mentioned above?
  3. What determines the caste a larva will develop into?

4. What is/are the economic importance of bees?


  1. The yellow and black stripes on the body of wasps is an example of ____ (a) counter shading (b) cryptic colouration (c) mimicry (d) warning colouration
  2. The main force that regularly produces evolutionary change is ___ (a) gene flow (b) genetic variation (c) mutation (d) natural selection
  3. Which is the odd one out? ___(a) bee (b) butterfly (c) termites (d) wasp
  4. The tendency for two or more genes to segregate together in a cross is known as ___ (a) co-dominance (b) incomplete dominance (c) sex-linkage (d) polygenic inheritance
  5. Organic evolution proposes that life started from ___ (a) air (b) fresh water (c) Land (d) water


  1. 1. (c) mimicry
  2. 2. (d) natural selection
  3. 3. (a) bee
  4. 4. (b) incomplete dominance
  5. 5. (d) water


  1. Explain the statement “individuals do not evolve; only populations evolve”
  2. What advantages do animals get by forming social groups?
  3. What are the differences between specialized insect and vertebrate societies?
  4. Why is there a rise in the number of pests and their resistance to pesticides?
  5. What are some possible solutions to this problem?