Natural selection
Evolution is the gradual change in the inherited features of a species over many generations. The process that drives it is natural selection, first explained by Charles Darwin. It follows the same four steps every time:
| Step | What happens |
|---|---|
| 1. Variation | Individuals in a species differ because of differences in their genes (from mutation and reproduction). |
| 2. Competition | More offspring are born than can survive, so they compete for limited food, space and mates. |
| 3. Survival | Individuals with features best suited to the environment are more likely to survive and reproduce ("survival of the fittest"). |
| 4. Inheritance | Survivors pass on the alleles for those helpful features, so they become more common in the next generation. |
Repeat this over thousands of generations and a species can change so much that a new species forms.
- Natural selection
- The process by which organisms better suited to their environment tend to survive, reproduce and pass on their alleles, so advantageous features become more common over time.
Antibiotic resistance: natural selection in action
The rise of "superbugs" is natural selection you can watch happen in years rather than millennia, which makes it a perfect example.
Explain how a population of bacteria becomes resistant to an antibiotic.
A common misconception
Bacteria do not "decide" to become resistant, and the antibiotic does not make them resistant. The resistant ones were already there by chance; the antibiotic just removes their competition. Avoid saying an organism changes because it "needs to".
Selective breeding
Selective breeding (also called artificial selection) is when humans, not nature, choose which organisms reproduce. Over generations it produces plants and animals with features we find useful.
The method is straightforward and repeats each generation:
- Choose the parents with the most desirable feature (for example cows that give the most milk).
- Breed them together.
- From the offspring, choose those that show the feature most strongly.
- Breed those together, and repeat over many generations.
It has given us high-yield crops, dairy cattle and friendly dogs, but there is a cost: breeding from a small group reduces variation in the gene pool (inbreeding), which can leave a breed vulnerable to disease or inherited defects.
Genetic engineering
Genetic engineering is faster and more direct than breeding: a gene is taken from one organism and put into another, so the second organism gains a new feature in a single generation. The classic example is putting the human insulin gene into bacteria so they produce insulin for people with diabetes.
The process in outline:
- The useful gene (for example the human insulin gene) is cut out of its chromosome using enzymes called restriction enzymes.
- A small ring of bacterial DNA, called a plasmid, is cut open with the same enzyme.
- The gene is joined into the plasmid using another enzyme (ligase). The plasmid is now the vector carrying the gene.
- The plasmid is put back into a bacterium.
- The bacterium reproduces, and all the copies make the useful protein, here insulin.
- Genetic engineering
- Changing the genes of an organism by transferring a gene from another organism into it, giving it a new characteristic. Organisms changed this way are called genetically modified (GM).
A balanced look at the ethics
Genetic engineering is powerful, and powerful tools raise hard questions. A good MYP answer weighs both sides rather than just picking one.
| Potential benefits | Potential concerns |
|---|---|
| Cheap, reliable medicines such as insulin and vaccines. | Long-term effects on health and the environment are not fully known. |
| Crops that resist pests or disease, or contain more vitamins, could reduce hunger. | GM genes might spread to wild plants and affect ecosystems. |
| Possible future treatments for inherited diseases. | Worries about "designer" organisms and where to draw the line. |
| Less need for chemical pesticides on resistant crops. | A few large companies could control the seed supply, affecting farmers. |
Where this is assessed
Explaining natural selection is Criterion A (knowing and understanding). Weighing the benefits and risks of genetic engineering, and how it affects people and the environment, is exactly Criterion D (reflecting on the impacts of science). Give reasons on both sides, then a considered conclusion.
Check yourself
1. State the four steps of natural selection. +
Variation (individuals differ genetically), competition (more offspring than can survive), survival (the best-suited survive and reproduce), and inheritance (survivors pass on their alleles, so helpful features become more common).
2. Explain why using antibiotics can lead to resistant bacteria. +
A few bacteria are already resistant by random mutation. The antibiotic kills the non-resistant ones but the resistant ones survive and reproduce, passing on the resistance allele until the whole population is resistant. The antibiotic selects, it does not create, resistance.
3. Give one difference between selective breeding and genetic engineering. +
Selective breeding chooses which existing organisms mate and takes many generations, only combining genes already in the species. Genetic engineering transfers a gene directly from one species into another and works in a single generation.
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