Evolution - Form 4 Biology | Secondary School

- All eukaryotic cells have organelles such as mitochondria, membrane-bound nuclei, ribosomes, Golgi bodies. Thus indicating that different organisms have a common ancestor.
- The presence of chloroplasts and cellulose cell walls indicates that green plants have a common ancestor.
- Blood pigments are conjugated proteins with a metal group. Similar pigments are found in different animal groups.

Mechanism of Evolution

- The mechanism of evolution can be described as a process of natural selection acting on the heritable variations that occur among the members of a population.
- A population consists of a group of individuals of the same species.
- Each individual has a set of hereditary factors(genes).
- All the genes in a population constitute a gene pool.
- When reproduction takes place, genes pair with one another randomly.
- Genes which occur in great numbers in the gene pool, will occur in greater numbers in the next generation.
- Several theories have been proposed over the years to explain how evolution took place.

Lamark's theory
- Lamark had observed that if a part of the body of an organism was used extensively, it became enlarged and more efficient;
- If a part of the body was not fully used, it would degenerate.
- By use and disuse of various body parts, the organism would change and acquire certain characteristics.
- He suggested that these characteristics would them be passed on to the offspring(next generation).
- In 1809, lamark published his book 'Theory Of Evolution'.
- He proposed that new life forms arise from use and disuse of parts of existing organisms and through the inheritance of acquired characteristics.
- Lamark's theory has been disapproved in that although use and disuse of parts does lead to acquired characteristics, such characteristics are not inheritable since they are effects produced by the environment and not by genes.

Evolution by natural selection
- In 1859, charles Darwin published his theory of evolution' in a book called origin of species by means of natural selection'.
- Darwin's theory was based on the following evidence;the population of a given species remains constant over a long period of time.
- The number of young ones is more than the number of adults.
- More offsprings are produced than can possibly survive.
Darwin made the following conclusions;
- There is a struggle for existence among individuals in a given population. Individuals who are not suitably adapted (e.i. who have unfavourable variations)are less able to pass their characteristics to the next generation.
- Natural selection operates on the population, selecting those individuals with favourable variations.
- They win competition e.g. for food and survive.i.e. 'survival of the fittest'.
- They attain sexual maturity and pass on the characteristics to their offspring.

Natural selection
- Peppered moth (Industrial melanism)
- The peppered moth, Biston betularia, exists in two distinct forms: A speckled white form(the normal form) and the melanic, dark form.
- The moths normally rest on the tree trunks and branches where they are camouflaged against predators.
- The first melanic moths were observed in 1848 around Manchester in Britain.
- Since that time, their numbers has increased tremendously, out-numbering the speckled white form.
- The increase in the population of the melanic form is correlated with environmental changes brought about by industrialization and pollution.
- Smoke and soot from factories have darkened the tree trunks over the years.
- This has resulted in the preservation of the mutation in Biston betularia leading to the evolution of the melanic form.
- This form is almost invisible against the dark background of the tree trunks and is less subject to predation than the speckled form.
- The peppered form is more abundant in areas away from the soot and smoke of factories.
- This is because it is well camouflaged by the lichen-covered tree trunks against which it rests and is therefore not easily detected by predators.
- The existence of two or more distinct forms within a species (as exemplified by Biston betularia) is called polymorphism.

Resistance to Drugs
- Certain strains of organisms have developed resistance to drugs and antibiotics.
- Following continued use of such drugs and antibiotics, some of the individuals in a population of bacteria or other microorganisms survive and are able to pass their characteristics to the next generation.
- When a patient fails to take full dosage of the antibiotics prescribed the pathogen develops resistance to the drugs hence become difficult to control.
- Some mosquitoes have developed resistance to certain pesticides.

- Comparison of Vertebrate Limbs - Limbs of various vertebrates are provided: e.g. fish- Tilapia, amphibian-frog reptiles, lizard; bird - domestic fowl (chicken), mammal- rabbit.
- Their anatomy can be studied.
- The following can be noted:
- That all limbs have five sets of bones;

• A single upper bone- the femur in hind limb and the humerus in fore limb.
• Two lower limb bones -i.e. the tibia & fibula in the hind limb & ulna & radius in the forelimb.
• Small bones - i.e. ankle (tarsals) and wrist bones (carpals).
• The bones making the foot and hand are metatarsals and metacarpals respectively.
• The bones of toes and of fingers i.e. phalanges.

- Observe the various modifications of these bones in the various animals. - Limbs of different mammals e.g. rabbit, cow, donkey reveal that the anatomy is adapted to mode or type of movement .
e.g. the horse has a single digit.
- An outdoor activity to observe various styles of movement in different mammals can be studied.
- It is noted that some move on tips of toes (donkey) others on the whole leg (rabbit).
- Comparison of Wings of bird-and insect.
- Wings of birds and insects (grasshopper, butterfly or moth) are obtained.
- A hand lens or a dissecting microscope is used to observe the specimens.
- The differences in their anatomy is noted.
- Insect wings are membranous while those of birds are made up of.