Variety within a species

Natural selection

Within a population of animals, plants or any living organisms, there will be inherited variations. Within each species the individuals with the variations best suited to the environment will survive better than the others. More of them will survive to reproduce than the others. When they do, they pass on the genetic information for these variations to their offspring.

Species gradually evolve in this way. This process is called natural selection.

Over time a population can change so much it may even become a new species, unable to reproduce successfully with individuals of the original species.

One good example is the peppered moth:

When newly industrialised parts of Britain became polluted in the nineteenth century, smoke killed lichens growing on trees and blackened their bark. 

• Pale coloured moths which had been well camouflaged before when they rested on tree trunks became very conspicuous and were eaten by birds. 
• Rare dark moths, which had been conspicuous before, were now well camouflaged in the black background. 
• As birds switched from eating mainly dark moths to mainly pale moths, the most common moth colour changed from pale to dark. 
• Natural selection had caused a change in the British moth population. The moths had evolved.

Another example is Natural Selection in Darwin's Finches:

• These birds, although nearly identical in all other ways to mainland finches, had different beaks. 
• Their beaks had adapted to the type of food they ate in order to fill different niches on the Galapagos Islands. 
• Their isolation on the islands over long periods of time made them undergo a genetic change as well as the physical change.

Artificial selection

Artificial selection is when people use selective breeding to produce new varieties of a species. A variety is a type of a particular species that is different in some clear way from other varieties of that species.

For example, pedigree dogs come in lots of different varieties, called breeds of dog. They may be different colours and sizes, but they are all still dogs. They are all still the same species. Different varieties of dog have been produced by selective breeding.

Selective breeding of cows

Suppose you wanted a variety of cow that produced a lot of milk. This is what you could do:

• choose or select the cows in your herd that produce the most milk
• let only these cows reproduce
• select the offspring that produce the most milk
• let only these offspring reproduce
• keep repeating the process of selection and breeding until you achieve your goal.

Our own Jamaican Scientists Dr. T.P. Lecky carried out such and experiment to create tow new breeds or variety of Jamaican cows. The Jamaica Hope bread as milk producers for our tropical climate  and the Jamaica red which is our major beef producers.

Other examples of selective breeding

The key here is to identify the feature you want, and only breed from the individuals that have that feature. Here are some examples of what selective breeding can produce:

• hens that lay big eggs of a particular colour
• cattle that produce lots of meat
• tomato plants that produce lots of tomatoes
• crops that are resistant to certain plant diseases.

Genetic Variation - continuous & discontinuous

Continuous & discontinuous

Some of the features of the different organisms in a species show continuous variation, and some features show discontinuous variation.

Continuous variation

Human height is an example of continuous variation. Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values. So it is continuous variation.

For any species a characteristic that changes gradually over a range of values shows continuous variation. Individuals cannot be grouped into distinct & discrete phenotypes. Character is easily influenced by environment and can be measured or graded. 

Examples of such characteristics are:

• height
• weight
• foot length.

If you record the heights of a group of people and draw a graph of your results, it usually looks something like this:

The more people you measure, and the smaller the categories you use, the closer the results will be to the curved line. This shape of graph is typical of a feature with continuous variation. Weight and foot length would give graphs similar in shape to this.

Discontinuous variation

Human blood group is an example of discontinuous variation. There are only 4types of blood group. There are no other possibilities and there are no values in between. So this is discontinuous variation.

A characteristic of any species with only a limited number of possible values shows discontinuous variation. Individuals can be grouped into distinct & discrete phenotypes. Character cannot be measured or graded and is not influenced by environment. Here are some examples:

• gender (male or female)
• blood group (A, B, AB or O)
• eye colour.
• tongue rolling 

Inherited & environmental

Some variation within a species is inherited, and some variation is due to the environment.

Inherited causes of variation

Variation in a characteristic that is a result of genetic inheritance from the parents is called inherited variation.

Children usually look a little like their father, and a little like their mother, but they will not be identical to either of their parents. This is because they get half of their inherited features from each parent.

Each egg cell and each sperm cell contains half of the genetic information needed for an individual. When these join at fertilisation a new cell is formed with all the genetic information needed for an individual.

Here are some examples of inherited variation in humans:

• eye colour
• hair colour
• skin colour
• lobed or lobeless ears.

Gender is inherited variation too, because whether you are male or female is a result of the genes you inherited from your parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will will grow taller as it tries to reach more light.

Variation caused by the surroundings is called environmental variation. Here are some other examples of features that show environmental variation:

• your language and religion
• flower colour in hydrangeas - these plants produce blue flowers in acidic soil and pink flowers in alkaline soil.

Both types together

Some features vary because of a mixture of inherited causes and environmental causes. For example, identical twins inherit exactly the same features from their parents. But if you take a pair of twins, and twin 'A' is given more to eat than twin 'B', twin 'A' is likely to end up heavier.

Representing the data

Continuous Variation is a quantitative trait that can be measure so it can are represented using a histogram with can group and range the data in order for it to be properly analysed.

Discontinuous Variation is a qualitative trait that cannot be measure so a bar chart is more suited to represent this type of data.