Variation and Selection

Lessons

Senior 6 · Biology

Variation and Selection

5 lessons

Lesson 4 of 5

Artificial selection

30 min

A grass that became a staple

Teosinte, a wild grass of southern Mexico, has a thin ear of a dozen or so grains, each locked inside a stone-hard case. It hardly looks like food. Yet genetic and archaeological evidence shows that it is the ancestor of maize, one of the world's most important crops. Over thousands of years, farmers kept seed from the plants they liked best and sowed it the next season. Each generation they chose again. Without knowing anything about genes, they changed the plant's form more than millions of years of natural selection had. This lesson looks at how such artificial selection works, how much a population can change in one generation, and what it costs.

Definition

Artificial selection (selective breeding): humans choosing which individuals breed, so that alleles for the characteristics they want become more common in later generations.

Inbreeding: breeding between closely related individuals, or self-fertilisation, which increases homozygosity.

Inbreeding depression: the reduced vigour, fertility and survival that often follow inbreeding, as harmful recessive alleles become homozygous.

Outbreeding (hybridisation): crossing unrelated individuals or different varieties. Hybrid vigour (heterosis): the greater size, yield or vigour of such hybrids compared with their parents.

Progeny testing: judging an animal's genetic value from the performance of its offspring.

Heritability (h²): the proportion, from 0 to 1, of the variation in a characteristic in a population that is due to genes that parents pass on.

Natural and artificial selection compared

Feature	Natural selection	Artificial selection
Selecting agent	The environment (selection pressures)	Humans
What is favoured	Characteristics that increase survival and reproduction	Characteristics useful or pleasing to humans, which may reduce the organism's fitness in the wild
Speed	Usually slow	Can be rapid, because only chosen individuals breed
Effect on variation	Can maintain or reduce it	Usually reduces genetic diversity strongly
Similarity	Both act on existing heritable variation and change allele frequencies over generations; neither creates new alleles

One wild plant, many vegetables

Selection for different parts of one species can produce very different crops. Cabbage, kale, Brussels sprouts, broccoli, cauliflower and kohlrabi all belong to the same species, Brassica oleracea, and can still interbreed:

Methods used by breeders

Mass selection: choosing the best individuals in each generation to breed, as early farmers did with maize.
Progeny testing: a bull cannot give milk, so his value as a sire of dairy cows is judged from the milk yields of his daughters; the best bulls are then used widely.
Artificial insemination and embryo transfer: semen from one tested bull can be used to inseminate thousands of cows, and embryos from an elite cow can be carried by others, spreading chosen alleles quickly.
Inbreeding to make uniform, true-breeding lines, and outbreeding to combine the characteristics of two lines and gain hybrid vigour.
Polyploidy: doubling chromosome sets, for example with the chemical colchicine, can give larger plants and fruits; bread wheat is a natural polyploid with six sets of chromosomes.
Marker-assisted selection: DNA markers linked to a wanted allele (Unit 10) let breeders identify the right seedlings or calves early, without waiting for them to grow up.

How much change in one generation?

Selection only works on the genetic part of the variation. If parents are chosen because they are tall, but some of their height came from good soil, that part is not passed on. Breeders predict the change, the response to selection $R$ , from the selection differential $S$ , the difference between the mean of the chosen parents and the mean of the whole population, and the heritability $h^{2}$ :

R = h^{2} \times S

Worked example 1: predicting the response

Illustrative values. (a) Use Figure 2: the population mean ear length is 15 cm, the chosen parents' mean is 19 cm and $h^{2} = 0.5$ . Predict the offspring mean. (b) In a dairy herd, the mean yield is 5 000 litres per lactation; cows chosen to breed average 6 500 litres, and $h^{2} = 0.3$ . Predict the daughters' mean yield.

(a) $S = 19 - 15 = 4$ cm. $R = 0.5 \times 4 = 2$ cm. Offspring mean $= 15 + 2 = 17$ cm.
(b) $S = 6\,500 - 5\,000 = 1\,500$ litres. $R = 0.3 \times 1\,500 = 450$ litres. Daughters' mean $= 5\,000 + 450 = 5\,450$ litres.

Interpretation. Only part of the parents' advantage is inherited, because some of it was environmental. Characteristics with low heritability, such as fertility in many livestock, respond slowly, so breeders improve the environment (feeding, health care) at the same time. Repeated over many generations, even small responses add up, as the history of maize shows.

Inbreeding and outbreeding

Breeding close relatives, or self-fertilising plants, makes offspring more and more homozygous. This produces pure lines that breed true and are very uniform. But every population carries rare, harmful recessive alleles, usually hidden in heterozygotes. Inbreeding makes them homozygous, so they show their effects: lower yield, poorer fertility, weaker resistance to disease. This is inbreeding depression.

Crossing two different inbred lines gives an F1 generation that is heterozygous at many loci and often shows hybrid vigour, outperforming both parents. Because every F1 plant has the same genotype, the crop is also uniform. Hybrid maize is grown this way, and new hybrid seed must be produced from the parent lines for each season, because the F1 plants' own seed segregates.

Worked example 2: how fast does selfing make a pure line?

A plant heterozygous at one gene (Aa) is self-fertilised, and so are its offspring, generation after generation. What proportion of the plants are heterozygous after (a) 1, (b) 5 generations?

Selfing Aa gives $\frac{1}{4}$ AA, $\frac{1}{2}$ Aa, $\frac{1}{4}$ aa. Homozygotes selfed give only homozygotes, so each generation halves the heterozygotes.
(a) After 1 generation: $\frac{1}{2}$ , or 50%.
(b) After 5 generations: $\left( \frac{1}{2} \right)^{5} = \frac{1}{32} \approx 3.1\%$ . About 97% are homozygous.

Consequence. The same halving happens at every heterozygous gene, so a few generations of selfing produce nearly pure lines, and any harmful recessive allele present has a good chance of becoming homozygous in some of them. Breeders discard those lines and keep the healthy ones, then cross different lines to restore heterozygosity.

Warning

Problems and ethics of artificial selection

Loss of genetic diversity. Strong selection and the use of a few elite sires or varieties remove many alleles, including some that may be needed later for disease resistance or a changing climate. Gene banks and the conservation of traditional varieties and wild relatives protect this diversity.
Animal welfare. Selection for one characteristic can harm others: some fast-growing meat chickens develop leg problems, and dog breeds selected for very short faces often have difficulty breathing.
Common mistake. "Selective breeding creates new genes." It only changes the frequency of alleles already present (new alleles must come from mutation).

HHMI BioInteractive: "Popped Secret: The Mysterious Origin of Corn". Watch how genetic and archaeological evidence show that maize was selected from teosinte, and how few genes made the biggest differences.

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Practice: artificial selection

Give two similarities and two differences between natural and artificial selection.
Explain how broccoli and Brussels sprouts can both be bred from wild cabbage.
Explain why progeny testing is needed to select bulls for milk production.
Population mean: 40 eggs per hen per season; selected hens' mean: 52; $h^{2} = 0.25$ . Predict the mean of their daughters.
Explain why characteristics with a low heritability respond slowly to selection.
Calculate the proportion of heterozygotes after 3 generations of selfing, starting from Aa.
Explain the causes of (a) inbreeding depression, (b) hybrid vigour.
A country replaces its many local varieties of a crop with one high-yielding variety. Evaluate this decision.

Show answersHide answers

Similar: both act on existing heritable variation; both change allele frequencies over generations. Different: humans versus the environment select; artificial selection can be faster and may favour features that reduce fitness in the wild.
Both belong to one species with variation in many parts. Breeders selected plants with the largest flower stalks and buds over many generations to get broccoli, and plants with the largest lateral buds to get Brussels sprouts.
Bulls do not produce milk, so their genes for milk yield can only be judged from the yields of their daughters.
$S = 52 - 40 = 12$ ; $R = 0.25 \times 12 = 3$ ; daughters' mean $= 43$ eggs.
Most of the variation is environmental, so the chosen parents' advantage is largely not inherited; $R = h^{2}S$ is small.
$\left( \frac{1}{2} \right)^{3} = \frac{1}{8} = 12.5\%$ .
(a) Inbreeding increases homozygosity, so rare harmful recessive alleles become homozygous and are expressed. (b) Crossing unrelated lines makes offspring heterozygous at many loci, masking harmful recessives and combining favourable alleles from both parents.
For: higher, uniform yields and easier marketing. Against: loss of genetic diversity; one disease or pest could destroy the whole crop; local varieties may suit local conditions or tastes and carry useful alleles. Conclusion: adopt it with care, keeping local varieties in gene banks and in use.

Key points

Artificial selection: humans choose which individuals breed; it changes allele frequencies, usually quickly, and reduces diversity.
Selecting different parts of Brassica oleracea gave kale, cabbage, Brussels sprouts, kohlrabi, broccoli and cauliflower; maize was selected from teosinte.
Methods: mass selection, progeny testing, AI and embryo transfer, inbreeding, outbreeding, polyploidy, marker-assisted selection.
Response to selection: $R = h^{2} \times S$ .
Inbreeding raises homozygosity and can cause inbreeding depression; crossing inbred lines gives uniform, vigorous F1 hybrids.

Sources and credits

18.1 Understanding Evolution, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the comparison of natural and artificial selection. Adapted by Ganzaa.
Photo: Maize-teosinte, by John Doebley, via Wikimedia Commons, licensed under CC BY 3.0.
Video: "Popped Secret: The Mysterious Origin of Corn", HHMI BioInteractive, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative values) and the practice questions were written by Ganzaa.

1A grass that became a staple

2Natural and artificial selection compared

3One wild plant, many vegetables

4Methods used by breeders

5How much change in one generation?

6Inbreeding and outbreeding