الأسبوع 12 · الدرس الثاني «الانتخاب الطبيعي والانتخاب الصناعي»: الجماعات الحيوية ونموها والسعة التحملية والعوامل البيئية والوراثية والانتخاب الطبيعي والصناعي وتقنياته · 3 دروس
3
Artificial selection and keeping good traits
Science Prep 3 Term 1, Unit 3: pp. 75–83
GoalCompare natural and artificial selection, describe traditional and modern breeding methods, and explain the risk of low diversity.
New words
artificial selection · people choose which individuals breed, to get the traits they wanttissue culture · growing many new plants from small pieces of a plant in a clean, nutrient-rich mediumgene bank · a store that keeps seeds or cells of many varieties to protect genetic diversity
Explanation
In natural selection the environment decides who survives. In artificial selection people decide: farmers choose the best plants or animals to breed, for example the cows that give the most milk or wheat with the biggest grains. Over many generations the wanted trait becomes more common. Traditional methods are selection and cross-breeding of two varieties. Modern methods include tissue culture, which makes many identical plants from a small piece, and gene banks, which keep seeds and cells of many varieties. Artificial selection can also have a cost: if all the plants in a field are copies of one variety, a single disease may harm them all, so genetic diversity is a safety net. This is why farmers and scientists keep old varieties in gene banks. Whether to use new techniques is a matter for scientists, farmers and communities to discuss, with evidence.
Worked examples
A farmer picks, each year, the 10 tomato plants with the biggest fruit out of 100 and uses only their seeds. After many years the average fruit is bigger. This is artificial selection: the farmer, not the weather, chose the parents.
Tissue culture: from one small piece of a banana plant, hundreds of new plants can be grown in clean jars with nutrients. They are all copies of the parent plant, so there is no new variation.
A field has 1000 plants of one variety. A disease harms 35 % of that variety: 1000 × 0.35 = 350 plants lost. If the field had five varieties and the disease harmed only one of them, the loss would be much smaller. Invented numbers.
Class activity
Choose-the-parents game. Draw 10 flowers on paper, each with a different petal count (3 to 8) and colour. Your aim is flowers with many petals. Choose the 3 best as parents, draw 10 children that mostly have more petals, and repeat for 3 rounds. Write how the average petal count changed and one risk of keeping only one variety.
Practice
1
What is the difference between natural and artificial selection?
In natural selection the environment decides; in artificial selection people choose the parents.
2
Give one traditional and one modern method.
Traditional: selection or cross-breeding. Modern: tissue culture or gene banks.
3
Why are gene banks useful?
They store seeds and cells of many varieties to protect genetic diversity.
4
Why is a field of one single variety risky?
One disease can harm all the plants.
5
A disease harms 20 % of 500 plants. How many are lost?
