Variation
Variation among organisms

This refers to differences among organisms of the same species due to the differences in the genes they inherit and the environment they survive in.

Type of Variation:

There are two main types of variation namely, Environmental variation and genetic variation.

Environmental variation

This refers to differences amongst organisms of the same species due to the different factors of the environment they are exposed to. Eg , exposure of organism to different temperatures, light, humidity, nutrients, loss of body parts via accidents, dehorning of cattle by man, lightening of the skin using cosmetics.

Such variations, because they are not genetically acquired but enviromentary acquired or influenced characters, can not be inherited from parents to off their springs.

Genetic variation:

This refers to differences amongst organisms of the same species due to the differences in the genes they inherit from their parents. Eg some individuals are tall and others are short. This is because they inherited different genes from their parents. Such variations can be inherited because they are genetically determined.

Types of genetic variation:

There are two type of genetic variation and these are,

continuous variation
Discontinuous variation.
Continuous variation:

This is the type of variation of a given character/trait where by differences among organisms of the same species are slight and grade into each other.

These characters can be measured and mean, mode and median can be obtained. eg Height, weight, intelligence, waist line, length and width of structures, skin colour, yield of milk, fertility, number of grains on a maize cob.

When the above are measured for any group of organisms, the biggest percentage of the measurement are intermediates a few are of low grade and a few are of long grades

It give a normal distribution curve/bell shaped curve,/Gaussian curve when  we compare many organisms referring to one continuous character.

NB: No remarkable differences in value but continuous transmission from low to intermediate to high as shown by the normal distribution graph below.

Though genetically determined can be influenced by the environment. Such features can be used in the dichotomous key.

Discontinuous variation:

This the type of variation which shows clear-cut and sharp differences amongst organisms over a given trait.

In discontinuous variation differences do not merge into each other and there fore there are no intermediate grades. Features can not be measured but can be observed and there fore we can not obtain a normal distribution curve.

Features persist through out the life time of an organism. They show distinct differences. Eg sex, finger print, tongue rollers and non tongue roller, colour blindness, taster and non tasters to PTC (phenyl thio carbamide) blood groups, sickle cell anaemia, haemophilia, skin pigmentation (normal skin colour/albinism) eye colour etc. They are not affected by environmental condition.

These traits can be used when constructing a dichotomous key.

Causes of variation include;

1. Environmental factors such as a) diet, b) altitude, c) light intensity, d) temperature, e) pathogens and diseases, f) social function and g) age.

2. Genetic factors such as a) crossing over between homologous chromosomes during prophase I of meiosis, b) mutations (which bring about mainly discontinuous variation in a population)