Artificial Selection and Genetic Modification

1Overview

Broccoli, cabbage, kale, cauliflower, Brussels sprouts and kohlrabi look like six different vegetables. They are descended from a single wild plant. No laboratory was involved: farmers over many generations kept seeds from plants with the features they liked. In the last fifty years a second route to change opened up. Scientists can now move a single gene from one organism into another, or deliver a working gene to a patient's cells. These two routes, artificial selection and genetic modification, are the subject of this lesson, along with the ethical questions they raise.

2Artificial selection: humans as the selective force

In artificial selection (selective breeding), humans choose which individuals will breed, based on desirable characteristics. The steps are:

1. Select parents that show the desired characteristics and breed them.
2. Select offspring that show the desired characteristics and breed them.
3. Repeat over many generations until the new breed reliably shows the selected characteristics in all offspring.

The mechanism is the same as in natural selection from the previous lesson: heritable variation, differential reproduction, and a change in allele frequencies. The difference is that humans, and not the environment, decide which individuals reproduce. Charles Darwin used this fact as an argument: if breeders can change populations so much, nature could do the same over longer periods.

Examples

• Crop plants: disease resistance, higher yield, drought tolerance, better-tasting fruit, and large or unusual flowers.
• Dogs: breeders choose which dogs mate to raise the chance of puppies with traits such as coat colour. Many breeds exist today, all of one species and descended from a single ancestor.
• Livestock and other domesticated animals, selected for products or behaviour useful to people.

Artificial versus natural selection

Why artificial selection matters for understanding evolution

Artificial selection shows in miniature what natural selection can do. Within human history, a single species of wild plant has been reshaped into forms as different as a leafy head and a swollen stem, and breeds of one species of dog differ greatly in size, coat and behaviour. The speed of change comes from the strength of the selection: only chosen individuals reproduce. The same species can be shaped by different pressures into different forms, which is also a core idea in the next lesson on speciation and divergence.

3Genetic modification: changing DNA directly

Biotechnology uses biological agents for technological purposes, with major applications in medicine, agriculture and industry. Genetic modification goes beyond selecting among existing variants: a specific piece of DNA is added to or changed in an organism.

Recombinant DNA: the basic technique

Foreign DNA is cut with restriction enzymes (restriction endonucleases) and joined into a plasmid vector by DNA ligase, creating a recombinant DNA molecule. The plasmid is placed into a host such as a bacterium, which then makes the protein encoded by the foreign gene. Proteins made this way are called recombinant proteins.

GMOs and transgenic organisms

A genetically modified organism (GMO) has received recombinant DNA. If the foreign DNA comes from another species, the organism is transgenic. Examples:

• Bt crops: plants engineered to produce a toxin from the bacterium Bacillus thuringiensis that harms many insect pests.
• Transgenic animals such as sheep and goats that make human proteins in their milk.
• Transgenic plants with disease resistance, herbicide resistance, better nutrition or longer shelf life.
• Insulin made by bacteria, as above.
• The Flavr Savr tomato, the first GM crop on the market (1994), which used antisense RNA to slow spoilage and did not stay on the market because of maintenance and shipping problems.

4Gene therapy

Gene therapy treats or prevents disease by correcting the underlying genetic problem instead of relying only on drugs or surgery. Two main approaches are described:

• Gene addition (gene transfer): the earliest method. A working copy of a gene is introduced to take the place of a faulty one, or a new gene is added to help fight disease.
• Genome editing: a newer method, such as CRISPR-Cas9, which changes the cell's existing DNA instead of adding new genetic material. Studied uses include fixing a faulty gene, turning a helpful gene on, turning a harmful gene off, and removing DNA that disrupts gene function.

In many gene-addition treatments the healthy gene is delivered to diseased cells by a modified virus that has had its disease-causing genes removed. An example is the treatment of severe combined immunodeficiency (SCID). Gene therapies are used to treat a small number of diseases, including Leber congenital amaurosis (an eye disorder) and spinal muscular atrophy (a muscle disorder), while many more are in research. See the article on CRISPR.

5Evaluating genetic modification: benefits, risks and ethics

The curriculum asks for ethical considerations, which means weighing benefits and risks and asking who decides. Reputable bodies describe the following.

Each GM food is assessed case by case, so general statements about the safety of all GM foods cannot be made. Assessments cover toxicity, allergenicity, specific components, gene stability, nutritional effects and unintended effects. GM foods on the international market have passed national safety assessments, and no health effects have been shown in the general population where they are approved. Countries regulate GM foods differently, and labelling is debated, with no worldwide consensus.

Ethical questions for gene therapy and genome editing

For gene therapy, ethical questions include: who should have access if treatments are expensive; how the risks of an early-stage treatment should be weighed against the severity of the disease; whether changes should be limited to the treated person's body cells or could ever involve eggs and sperm, which would pass changes to later generations; and who should decide. These are questions of values as well as science, which is why Curricular Competencies ask students to examine the social, ethical and environmental effects of artificial selection and genetic modification.

6Cloning and the limits of biotechnology

Cloning produces an organism that is genetically almost identical to its donor. Dolly the sheep, the first cloned mammal, lived seven years and died of respiratory complications; the donor cell's age may have affected her lifespan. Cloned animals often show facial, limb and cardiac abnormalities, and therapeutic cloning has faced bioethical objections. Cloning shows a general pattern of new technologies: success in one respect does not guarantee safety or ethical acceptance in others, and risk must be assessed case by case.

7Weighing a technology: a decision framework

When asked to evaluate a use of artificial selection or genetic modification, a structured approach avoids opinion alone:

1. Define the change: which gene or trait, and by which method (selection, transgenic modification, gene therapy, genome editing).
2. List benefits and who receives them: farmers, patients, consumers, ecosystems.
3. List risks and who bears them: health, environmental, economic, cultural.
4. Identify the evidence: what has been tested, how, and what remains unknown.
5. Consider alternatives: conventional breeding, other treatments, or no intervention.
6. Decide how to monitor: post-market monitoring is recommended for GM foods, for example.

The same framework can be applied to fish farms, an example the Curricular Competencies mention for evaluation: farmed fish can supply food, but questions about the ecosystem effects and about wild fisheries, including First Peoples fisheries, require evidence and consultation.

8Summary

• Artificial selection is humans choosing which individuals breed; it works with existing variation and can be fast. The cabbage family came from one wild plant.
• Genetic modification changes the DNA of an organism directly; recombinant DNA uses restriction enzymes and ligase to move genes, even between species.
• GMOs include Bt crops and bacteria making insulin; they are assessed case by case.
• Gene therapy adds a working gene (often via a modified virus) or edits existing DNA (genome editing such as CRISPR-Cas9).
• Ethical evaluation weighs benefits, risks, access and who decides.

?Quick check

Try each question first, then reveal the answer.