Variation and evolution (AQA GCSE Biology Combined Science): Revision Notes
Variation and evolution
What is evolution?
Evolution is when the inherited characteristics of a population change over time. This happens because of natural selection. Sometimes evolution can create completely new species.
A population means all the individuals of one species that live in the same place at the same time.
Evolution is a gradual process that occurs over many generations. It's important to understand that individual organisms don't evolve - populations do!
Causes of variation
Variation means the differences you see between individuals in a population. For example, people have different heights, eye colours, and abilities.
Most variation happens because of two main causes:
Genetic causes
- These are differences that come from the genes you inherit from your parents
- Examples: eye colour, blood type, natural hair colour
- You cannot change these characteristics
Environmental causes
- These are differences that come from the conditions around you as you grow and develop
- Examples: scars from injuries, language you speak, muscle development from exercise
- These can change during your lifetime
Most variation is caused by a combination of both genes and environment working together. For example, your height depends on genes from your parents AND how well you eat as you grow.
Evolution by natural selection
The theory of evolution by natural selection explains how all living things developed from simple life forms over three billion years ago.
Here's how it works:
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Individuals in a population show variation - they are all slightly different from each other
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Some individuals are better suited to their environment - they have characteristics that help them survive
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These individuals are more likely to survive and reproduce - they pass their helpful characteristics to their offspring
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Their offspring are also more likely to survive and reproduce - because they inherit the helpful characteristics
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Over many generations, helpful characteristics become more common in the population
Natural selection doesn't create new characteristics - it acts on the variation that already exists in a population. The individuals with the most advantageous traits are simply more likely to pass on their genes.
Variation and mutation
There is lots of genetic variation within any population of a species. All this genetic variation comes from mutations.
What are mutations?
Mutations are random changes in genes that happen all the time. Most mutations:
- Have no effect on the phenotype (how an organism looks and behaves)
- Some influence the phenotype slightly
- Very few determine the phenotype completely
How do mutations cause evolution?
- Mutations happen continuously in all living things
- Very rarely, a mutation creates a new phenotype that is better suited to the environment
- If the environment changes, this new phenotype might help the organism survive better
- This can lead to relatively rapid change in a species
Common Misconception: Mutations don't happen "because" an organism needs them. They occur randomly, and natural selection determines whether they become more common in the population.
When populations become separate species
Sometimes two populations of the same species become so different that they cannot breed together anymore to produce fertile offspring. When this happens, they have become two separate species.
Worked Example: African Elephant Tusks
The situation: Poachers illegally hunt African elephants for their tusks. Some elephants are born without tusks because of a mutation.
The data: In Uganda in 1930, only 1% of elephants were born tuskless. By 2010, 15% of female elephants and 9% of male elephants were born without tusks.
The explanation: Elephants with tusks were more likely to be poached and killed. They could not survive to pass on their genes. Tuskless elephants were more likely to survive and reproduce. So the allele (gene version) for no tusks became more common in the population over time.
This shows how human activity can act as a selective pressure, causing rapid evolutionary change in just 80 years!
Key Points to Remember:
- Evolution is the change in inherited characteristics of a population over time due to natural selection
- Variation comes from both genetic and environmental causes working together
- Natural selection means individuals better suited to their environment are more likely to survive and reproduce
- Mutations create genetic variation - most have no effect, but some can be helpful
- Evolution can lead to the formation of new species when populations become too different to breed together