variation and inheritance

What is variation?

All people are human. They belong to the same species. Your friends and classmates may have different eye colour and hair colour. Some will be boys and some will be girls. Some will be tall and some will be shorter. The presence of differences between living things of the same species is called variation.

Variation between different species is always greater than the variation within a species.

Variation and classification - 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. 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 4 types 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. Here are some examples:

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

Variation and classification - 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 informationneeded 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.

Natural selection

Within a population of animals, plants or any living organisms, there will beinherited 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.

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.

Different breeds of dogs

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.

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.

  Variation and classification - Genetic engineering
  Variation in living organisms can also be created by genetic engineering, also called genetic modification, or just 'GM'.
  Using laboratory techniques scientists can alter the genetic code within the DNA of a living organism. They can add genes from a different species to an organism's DNA
  For example, GM can be used to alter the DNA in a bacterium so that it produces insulin. This is a human hormone and valuable to people with diabetes. Bacteria reproduce quickly, so a lot of insulin can be made quickly and used to help people suffering from diabetes.

  Ethical questions

  Some people are concerned that there may be negative aspects to GM.
  For example, there are people that believe GM is unethical and should be banned.
  Other people are concerned that there may be unexpected long term problems of using GM, producing changes that could not be reversed. Such changes could create new pathogens or "superweeds" by the GM organisms breeding with other species. This could cause the natural balance of living organisms in the environment to be upset.
  Scientists, society and individuals need to think about the advantages and disadvantages of genetic engineering.

  Variation and classification - Classifying organisms
  There are millions of species on our planet. It would be difficult if we just tried to describe and name each one individually. Although species can be very different from each other, many of them have similar features that allow us to put them into groups.

  Modern classification system

  In the eighteenth century Carl Linnaeus started the modern system of putting species of organism into certain groups and giving them scientific names. Each species is given a name using Latin words, so that the same name can be used all over the world.
  For example, the scientific name for human beings is 'homo sapiens'. Putting different species into different groups according to their features is calledclassification.