IB Biology HLNatural SelectionPaper 1 & 2~11 min read
Artificial Selection
Cabbage, broccoli, kale and cauliflower are the same species. Every dog from a chihuahua to a great dane came from wolves. Humans have been running natural selection by hand for thousands of years, and doing it much faster than the environment ever could.
📚 What you need to know
Artificial selection is the process by which humans choose organisms with desirable traits and selectively breed them to enhance those traits over many generations.
It is also called selective breeding.
Individuals are chosen by their phenotypes, not their genotypes, so no knowledge of alleles is needed.
Breeders can accidentally enhance other traits that are genetically linked to the desirable one, sometimes harming the organism’s health.
Animals are bred for milk and meat yield, egg size, temperament, wool quality and speed.
Plants are bred for disease resistance, crop yield, hardiness, taste and flower size.
All domestic dogs descend from wolves; cabbage, cauliflower, broccoli, Brussels sprouts, kale and kohlrabi all descend from wild brassica.
Artificial selection involves a deliberate choice of traits; natural selection does not.
What artificial selection is
Definition
Artificial selection is the process by which humans choose organisms with desirable traits and selectively breed them together, to enhance the expression of those traits over many generations
Humans have been doing this for thousands of years, long before anyone understood the genetics behind it. That is possible because you do not need to know anything about alleles to do it — individuals are selected by their phenotypes, the characteristics you can actually see, measure or taste.
There is a cost to working blind. Because the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable one, and those other traits can sometimes negatively affect the organism’s health.
The process
🧩 How selective breeding is carried out
The population shows phenotypic variation — there are individuals with different characteristics.
A breeder selects an individual with the desired phenotype.
A second individual with the desired phenotype is selected. The two should not be closely related to each other.
The two selected individuals are bred together.
The offspring reach maturity and are tested for the desirable trait. Those showing it to the greatest degree are selected for further breeding.
The process continues for many generations, until all the offspring display the desirable trait.
Compare this with the five-step chain of natural selection. Only step one is different: a human decides who breeds, instead of the environment.
Selective breeding in animals
Animals are selectively bred for a long list of characteristics, including:
cows, goats and sheep that produce a higher yield of milk or meat;
chickens that lay large eggs;
domestic dogs with a gentle nature;
sheep with good quality wool;
horses with fine features and a very fast pace.
The domestic dog is the clearest example of how far this can go. Every breed, whatever its size, shape or temperament, is descended from wolves. Humans selected different traits in different places, and a few thousand years produced more visible variety than the wolf lineage managed in a very long time on its own.
Where it goes wrong. Breeding hard for one visible trait can drag linked alleles along with it. Dog breeds selected for a very short face often have breathing difficulties, and breeds selected for a particular body shape often suffer joint problems. The trait the breeder wanted was achieved. The trait that came with it was never chosen by anyone.
Selective breeding in plants
Selective breeding of plants works in exactly the same way. Plants are bred for:
disease resistance in food crops;
increased crop yield;
hardiness to weather conditions, such as drought tolerance;
better tasting fruits;
large or unusual flowers.
The showpiece example is wild brassica. Breeders selected a different part of the same plant each time, and each choice produced a vegetable you would recognise from a supermarket shelf.
This is artificial selection at its most visible. Nothing was invented — existing variation was pushed to an extreme, six different times.
Natural and artificial selection compared
The key difference is intention. Artificial selection involves a deliberate choice of traits by humans. Natural selection does not choose anything.
The example that catches people out is antibiotic resistance. Resistant bacteria evolved because of the overuse of antibiotics in humans, so it looks like our fault — but it is natural selection. Nobody used antibiotics in order to produce resistant bacteria. It was an unintended consequence, and the selecting was done by the drug, not by a breeder.
Feature
Natural selection
Artificial selection
Who selects
The environment
Humans
Deliberate?
No
Yes — a specific trait is chosen
Trait selected for
Whatever improves survival and reproduction
Whatever humans find useful or attractive
Speed
Usually slow, over many generations
Much faster, because selection is intense
Result
Organisms better adapted to their environment
Organisms that may be poorly suited to their environment
Genetic diversity
Variation is maintained by mutation and recombination
Often reduced, because few individuals are bred from
Both processes are identical underneath. Variation exists, some individuals reproduce more than others, allele frequencies change. The only difference is what decides who reproduces. Say that sentence in an exam and you have understood the topic.
Worked examples
WE 1
Describe the process
Describe how a farmer could use selective breeding to increase the milk yield of a herd of cows. (4 marks)
Step 1: variation
There is phenotypic variation in milk yield within the herd.
Step 2: selection
The farmer selects the cows with the highest yields, and a bull from a high-yielding family. The two should not be closely related.
Step 3: breeding and testing
They are bred together, the offspring are raised to maturity, and their milk yields are measured.
Step 4: repetition
The highest-yielding offspring are selected for further breeding, and the process is repeated over many generations until yield is consistently high.
Select, breed, test, repeat — over many generations“over many generations” is a mark on its own and is the phrase most often left out
WE 2
Explain an unwanted outcome
Some pedigree dog breeds suffer from inherited health problems. Explain why selective breeding can cause this. (3 marks)
Point 1: selection is by phenotype
Breeders select for a visible characteristic and have no knowledge of the genotype, so they cannot see what else they are selecting.
Point 2: linked alleles
Alleles that are genetically linked to the desired trait are passed on with it, and these can negatively affect health.
Point 3: reduced diversity
Only a small number of individuals are bred from, so genetic diversity falls and harmful recessive alleles appear more often in the homozygous state.
You get the trait you chose, plus whatever travels with itthis is why the process says the two selected individuals should not be closely related
WE 3
Natural or artificial?
A student claims that antibiotic resistance in bacteria is an example of artificial selection because humans made the antibiotics. Evaluate this claim. (2 marks)
Judgement
The claim is incorrect. This is natural selection.
Reason
Artificial selection requires humans to deliberately choose which individuals breed in order to enhance a trait. Nobody used antibiotics in order to produce resistant bacteria — resistance was an unintended consequence, and the antibiotic acted as an ordinary selection pressure.
Human-caused is not the same as human-chosen“evaluate” wants a clear judgement first, then the reasoning that supports it
💡 Exam tips
Use the phrase desirable trait or desired phenotype when describing the selection step.
Always finish a description with over many generations — it is the mark people forget.
Mention that individuals are chosen by phenotype, not genotype.
Learn one animal example and one plant example properly. Dogs from wolves and brassica cover almost everything.
The key difference from natural selection is deliberate choice. Lead with that word.
If asked for a disadvantage, give linked harmful traits and reduced genetic diversity.
⚠ Common mistakes
Calling antibiotic resistance artificial selection. It is natural selection with a human-made selection pressure.
Saying breeders change the organism’s genes. They choose from the variation that already exists; that is not genetic engineering.
Describing one round of breeding. Selective breeding is a repeated cycle.
Assuming artificial selection produces better-adapted organisms. Many bred organisms would not survive in the wild.
Forgetting variation. Without phenotypic variation the breeder has nothing to select from.
Confusing selective breeding with cloning. Selective breeding needs two parents and sexual reproduction.
That completes Natural Selection. Look back at what the eight pages actually said: variation appears at random, something decides who breeds, and allele frequencies shift. Change the thing doing the deciding — a fox, a female guppy, a drought, a farmer — and you have explained every example in the topic.
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