Variation and Selection
Senior 3 · Biology
Variation and Selection
Lesson 4 of 5
25 min
On 24 June 2021, at its Rwerere station in Burera District, the Rwanda Agriculture and Animal Resources Development Board (RAB) launched 19 new bean varieties: 13 climbing beans and 6 bush beans. Eight of them are high in iron and zinc; the others are high-yielding, with characteristics that farmers and markets prefer. Beans are eaten almost every day in many Rwandan homes, so beans with more iron can help fight iron deficiency.
New varieties like these are not found ready-made. They are produced by plant breeding: people choose parent plants with the characteristics they want and breed from them. This is selection, like natural selection, but the people, not the environment, decide which individuals reproduce.
Definition
Artificial selection (selective breeding): the choosing, by people, of plants or animals with wanted characteristics as parents for the next generation, repeated over many generations.
Inbreeding: breeding between closely related individuals, or self-pollination of a plant.
Outbreeding (crossbreeding): breeding between unrelated individuals, or between different varieties or breeds of the same species.
Hybrid vigour (heterosis): the greater size, yield, growth or health often shown by the offspring of a cross between two different, inbred varieties.
Inbreeding depression: the loss of fertility, vigour and health that follows repeated inbreeding.
Over long periods artificial selection can change a species greatly. Figure 1 shows six familiar vegetables that were all bred from a single wild plant.
Figure 1. Six vegetables bred from one wild species, each by selecting for a different part of the plant.
| Natural selection | Artificial selection | |
|---|---|---|
| Who or what selects | the environment (predators, climate, disease) | people |
| Characteristics favoured | those that help survival and reproduction | those useful to people, even if they harm survival in the wild |
| Speed | usually slow | faster: only the chosen individuals breed |
| Example | dark peppered moths on sooty trees | iron-rich beans; high-yielding dairy cows |
Breeders use inbreeding to fix a wanted characteristic. Mating close relatives, or self-pollinating a plant generation after generation, makes the offspring more and more homozygous, so they become pure-breeding and very uniform: every plant of an inbred line ripens at the same time and is the same height.
The danger is that relatives often carry the same rare recessive alleles, inherited from a shared ancestor. Inbreeding brings these together, so harmful recessive alleles become homozygous and show. Repeated inbreeding leads to inbreeding depression: lower fertility, smaller size, weaker growth and more inherited disorders.
Worked example
Worked example 1: a bull and his daughters. A bull carries a rare recessive allele, d, that causes a serious disorder in dd calves; he is Dd. He is first mated with unrelated cows, all DD. Later, a farmer mates him with his own daughters. What is the chance that a calf from a father–daughter mating is affected?
3. A daughter chosen at random is a carrier with chance , so the chance of an affected calf is .
Mated with unrelated DD cows, the bull's calves had no chance of being dd. Mating him with his daughters raises it to 12.5%. That is why breeders, and laws on human marriage in many countries, avoid close relatives.
Crossing two different inbred varieties brings together different alleles, so the offspring are heterozygous at many genes. Harmful recessive alleles from one parent are masked by dominant alleles from the other, and the F1 hybrids are often larger, healthier and higher-yielding than either parent: hybrid vigour. Hybrid maize is the best-known example.
Outbreeding can also combine good characteristics from two breeds. For example, a farmer might cross a hardy local breed of cattle, adapted to local heat and diseases, with a high-yielding dairy breed, hoping for calves that give more milk than the local breed and cope better with local conditions than the dairy breed.
Worked example
Worked example 2: why buy new hybrid seed? A farmer grows F1 hybrid maize and gets an excellent, even crop. She keeps some of the grain and plants it the next season. The new crop is uneven: some plants are tall and strong, many are weaker, and the yield falls. Explain.
Conclusion: to keep the benefit of hybrid vigour, new F1 seed must be bought each season, which costs the farmer money.
Warning
Breeding for one characteristic can harm others. Selecting only for the highest yield can reduce disease resistance or hardiness, and growing a few very similar varieties everywhere reduces genetic diversity, so one new disease can destroy a whole crop. Good breeders keep a wide range of varieties, including local ones, as a store of useful alleles.
Selective Breeding, from FuseSchool. Watch for the steps of selective breeding, and for its problems, such as inbreeding and loss of variety.
Tip
In = same, out = different. Inbreeding (relatives) makes organisms more homozygous: uniform, but recessive disorders appear. Outbreeding (unrelated) makes them more heterozygous: vigorous and varied.
Practice: artificial selection
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the Punnett square, the table, the worked examples and the practice questions.
