These are the fundamental techniques used to develop plants useful to us. They have been the basis of plant breeding efforts from thousands of years ago to today.

1) Artificial selection

Plant breeding in its most basic form is very simple. The farmer selects which plants get to have offspring. As the offspring will resemble (on average) the plants they came from this determines what the next crop will be like.

This process is called Artificial Selection. The term comes from the phrase 'natural selection' coined by Darwin to describe the process by which the best adapted animals survive to pass on their genes. The same process is occurring here but is being artificially manipulated by humans to suit our needs.

Charles Darwin. Taken soon after he had published his famous book On the Origin of Species by Means of Natural Selection.

Photographer: Unknown. Copyright free

Example

Let's look at an example in a little more detail. Imagine you are a farmer with a field full of tomato plants. All these tomato plants are subtly different; some might have bigger leaves, some might have redder fruit etc.. The important part is these differences are heritable, they can be passed on to the plants offspring.

The reason these differences in the shape and colour can be passed down from parent to offspring is that they are due to differences in the genes (DNA) each plant contains. Genes from the parent plants are passed into their seed, from which their offspring will grow.

Now back to this imaginary tomato field. You decide that you are going to grow another field of tomatoes but you would like these to be bigger. You select the tomato plants with the biggest tomatoes and use the seed from these tomatoes to plant your new field. The plants in the new field will resemble the plants they came from and will, on average, have bigger tomatoes than the plants in the original field.

Big tomatoes

Photographer: David. Copyright: GFDL

Congratulations! You artificially selected for plants with bigger tomatoes and have improved your crop.

2) Cross breeding

Cross breeding plants means breeding two plants with each other. This is sometimes also called crossing or hybridisation.

This shows some-one using a cocktail stick to transfer pollen from one flower to another. This 'hand pollination' may be a necessary step in breeding two plants together.

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Cross breeding is a little more sophisticated than artificial selection. In cross breeding you are determining which plants breed together rather than just which plants to use seed from after they have been allowed to breed at random.

There are two main reasons to do this:

A) By determining who both the parents are, the breeder has more control over what the offspring will be like.

B) It allows qualities found in different plants to be combined into one. For example one variety of tomato might have big but bland tasting fruit while another might have small but sweet tasting fruit. By breeding plants of these two varieties together you might get some plants which have fruit which are both large and sweet tasting.

After cross breeding two plants it may still be desirable to select specific offspring from the cross to breed from in future.

The two processes above are the basis of all plant breeding. All further techniques are really just improvements on how we do these things.

3) Back crossing

Back crossing is a specific type of cross breeding. Back crossing is the breeding of a plant with one of its parents. It is usually performed when a plant breeder is trying to breed a particular trait from a wild species (or old cultivar) into a modern crop plant.

After breeding the crop plant and wild plant together a cyclic process of back crossing and artificial selection allows the plant breeder to obtain a plant which resembles the crop plant in most aspects but contains the trait of interest from the wild plant.

This is useful as while the new trait from the wild plant may be desirable modern crop plants have many advantageous traits bred in over time which the plant breeder would not want to lose.

The process in detail:

1) In the back crossing process a breeder will first make an initial cross between the wild plant and the cultivated plant. All of these offspring from this cross will contain half their genes from the wild plant and half their genes from the crop plant.

2) They then select out those offspring that have the trait of interest from the wild plant (say resistance to a certain disease).

3) These selected plants are then bred with the original crop plant.

4) From the offspring of this cross plants with the trait of interest are selected. These plants have the desired trait from the wild plant but now have three quarters of their genes from the crop plant and only a quarter from the wild plant.

By continuing with this process the breeder will get a plant with the large majority of its genes and hence attributes from the crop plant but at the same time ensure that it retains the trait in question from the wild plant.