(a) Introduction - why natural genetic variation matters

Any population of an organism which lacks variety in its gene pool is particularly susceptible to various factors.

This is the case

e.g. climate change - change in weather patterns

attack by a pathogen - bacteria, fungus or virus

If any of one these factors affect a population, in a varied gene pool, not all of the population will die.

Those individuals with the right set of genes will survive.

The survivors will be able to reproduce and pass on the advantageous genes.

These days new crop varieties in agriculture are bred for disease resistance - lost or poor crop yields costs money!

Increased yields of better quality food crops is a worthy economic goal.

Horticulturalists - plant breeders, are also interested in quality flowers, fruit or vegetables for garden cultivation.

The 'ideal aims' for crops i.e. beneficial characteristics of the variety are:

Plants that grow and mature quickly with good crop yields.

Crops that have a long shelf-life and store well (e.g. flowers), or can be frozen (e.g. vegetables.

Vegetables that have a distinctive taste, looks and texture.

Flowers that are attractive to look at e.g. in colour,  have a pleasing aroma-scent, perhaps larger and bigger than their wild origins!

However, its a good idea to preserve wild versions of plants we cultivate and use their wide-ranging gene pool for future breeding programmes.

In order to achieve these aims you need to:

Selectively breed a variety in large numbers of identical plants to give the greatest possibility of being able to select the plants with desired characteristics i.e. those listed above.

BUT, there is the danger of having crops that are genetically too uniform - as already discussed (at the start), a narrow gene pool is susceptible various things.

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(b) Traditional plant breeding methodology

In traditional plant breeding, a plant with desirable characteristics is crossed with another plant with the same or another desirable characteristic.

The plants are chosen from a mixed population or existing varieties.

Pollen is transferred from the flowers of one plant to the flowers of another to fertilise it - you can just dust off the pollen from the anther of one flower with a fine paint brush and dab the pollen onto the stigma of another flower.

To ensure the desired breeding direction - that is to concentrate the gene pool of desired traits, you must stop the plants from self-pollinating each other and also avoid cross-pollinating with a different plant variety.

From the new plant seeds the process is repeated over several generations until an acceptable new variety is created.

For more see reproduction in flowering plants

Need some examples - pictures

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(c) What it takes to produce a new variety by traditional breeding methods

The basic sequence to develop a new variety of a plant species is outlined below 

(its a sort of 'algorithm!)

Sequence	Stage in breeding programme	Comments on each step
1	A male flower is fertilised with pollen	You must prevent self-fertilising or cross-pollination with another plant
2	Seeds collected and the plants grown	You need thousands of individual plants to provide as wider gene pool as possible
3a	The plants are exposed to whatever you want the plant to be tolerant of e.g. to have disease resistance	The plants are tested for exposure to a disease induced by a bacteria, fungus or a virus
3b	OR maybe measuring the size and colour of a flower, or it might be the yield of a fruit	Plants examined for quality of flower or yield of fruit
4a	Select plants from stage 3a, looking closely at the characteristics e.g. signs of disease resistance etc.	You are the picking the plants that seem to show most disease resistance
4b	Select plants from stage 3b, monitoring the colour of the flower or the yield of fruit	You are the picking the plants that seem the 'best' colour' or the 'best' yield
5	Repeat breeding from the stage 4 selection - picking those plants closest to the desired traits	You keep on repeating the sequence and hope it works out to provide a healthy breeding line
6	Finally, you have a breeding line of plants with the desired characteristics - a new variety is born!	BUT, you are dealing with a narrowed gene pool.