🧬 Variation and competition
1. Individuals in a population show variation in their phenotypes.
2. Not all of them will survive. They have to cope with the conditions around them, such as predators, disease and a shortage of food.
Variation and Evolution » Evolution by Natural Selection
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.2.2
- What evolution is, and how natural selection brings it about
- How variants with the best-suited phenotypes survive and pass on their genes
- How a new species can form
- The idea that all living things evolved from simple life forms
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Unlock This CourseWhat is evolution?
In the last lesson you saw that every population shows variation, and that new variants arise from mutations. This lesson shows what happens to those variants over many generations.
The living things on Earth today are not the same as the ones that lived long ago. Species change. That change is called evolution.
Key terms:
- Evolution: a change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species.
- Natural selection: the process in which individuals with phenotypes best suited to their environment are more likely to survive and reproduce, passing on their genes.
Notice the word inherited. A broken tooth or a tattoo is not inherited, so it cannot make a population evolve. Only characteristics that are passed on in the genes count.
The theory of evolution by natural selection
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
That is a very long time. Over billions of years, small changes in populations have added up. The huge variety of plants, animals, fungi and microorganisms alive today all came from those first simple life forms.
How natural selection works
Natural selection happens in a few clear steps. Follow them in order.
🏆 Survival and inheritance
3. Individuals with phenotypes best suited to the environment are more likely to survive and reproduce.
4. They pass on the genes for those phenotypes to their offspring, so these phenotypes become more common over generations.
The environment does the selecting. It does not make new variants appear. The variants are already there, thanks to mutations. The environment simply decides which variants do well.
A worked example: rabbits in the snow
Imagine a population of rabbits living in a cold, snowy place. Foxes hunt them. The rabbits show variation in fur colour. Some have white fur and some have brown fur. This is a genetic difference.
Worked example
Step 1: The rabbits vary. Some are white, some are brown.
Step 2: Brown rabbits stand out against the snow, so foxes catch more of them.
Step 3: White rabbits are better suited to this environment. More of them survive to breed.
Step 4: White rabbits pass on the allele for white fur to their offspring.
Step 5: Over many generations, a greater proportion of the population has white fur.
The inherited characteristics of the population have changed. That is evolution by natural selection.
Notice that no individual rabbit changed its colour. The population changed, because the variants suited to the environment left more offspring.
Natural selection when the environment changes
Natural selection also acts when the environment changes. A phenotype that was once only average might become the best suited one.
Suppose the snow in the rabbits' habitat stopped falling and the ground stayed bare brown earth. Now white rabbits would stand out. Brown rabbits would be better suited and more of them would survive. Over time the population would move back towards brown fur.
So "best suited" always depends on the environment. There is no variant that is best everywhere.
How a new species forms
A species can change so much that it becomes something new. The spec gives a clear rule:
New species
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.
Key terms:
- Interbreed: mate with each other.
- Fertile: able to reproduce.
Both parts of the rule matter. The two populations must be unable to interbreed and produce fertile offspring. A horse and a donkey can mate and have a baby, called a mule. But a mule is sterile, so it cannot have offspring of its own. Horses and donkeys are therefore two different species.
Common mistakes
1. Saying that individuals "change to suit the environment". Individuals do not change their genes. The variants that are already suited survive and breed more.
2. Saying that animals "try" or "want" to adapt. Natural selection has no goal.
3. Forgetting to say that the useful phenotype is passed on to offspring. Survival alone is not enough. The genes must be inherited.
4. Using "survival of the fittest" to mean the strongest. It means the phenotype best suited to its environment.
Exam-style question
A population of beetles lives on dark soil. Some beetles are dark green and some are light green. Birds eat the beetles. After many generations most of the beetles in the population are dark green.
(a) Explain how natural selection led to most of the beetles being dark green. [4 marks]
(b) A scientist says that two populations of these beetles have become two new species. Describe what must be true of the two populations. [2 marks]
Model answer
(a) There is variation in colour, with some dark green and some light green (1). Dark green beetles are harder for birds to see on the dark soil, so they are more likely to survive (1) and reproduce (1). They pass on the genes for dark green colour to their offspring, so over generations more of the population is dark green (1).
(b) The two populations are so different in phenotype that they can no longer interbreed (1) to produce fertile offspring (1).
Exam tip
In a natural selection answer, always finish the chain: survive, reproduce, pass on genes. Many students lose the last mark by stopping at "more of them survive".