Variation and evolution (AQA GCSE Biology): Revision Notes
Variation and evolution
What is evolution?
Evolution happens when the inherited features of a population change over time. This occurs because of natural selection. Sometimes evolution can lead to the formation of completely new species.
A population is all the individuals of one species that live in the same place at the same time. For example, all the robins living in your local park.
Causes of variation
Variation means the differences between individuals in a population. No two individuals are exactly the same (except identical twins).
There are three main causes of variation:
Genetic causes
- These are differences you inherit from your parents
- They come from the genes passed down to you
- Examples: eye colour, blood type, natural hair colour
Environmental causes
- These are differences caused by your surroundings and experiences
- They develop during your lifetime
- Examples: scars, language you speak, muscle development from exercise
Combination of both
- Most variation is caused by both genes and environment working together
- Examples: your height, body mass, intelligence
Key fact: Most variation comes from a combination of genetic and environmental factors.
Evolution by natural selection
The theory of evolution by natural selection explains how all living things developed. It says that all species evolved from simple life forms that first appeared over three billion years ago.
Here's how natural selection works:
- Individuals show variation - Members of a population have different characteristics
- Some are better suited - Individuals with certain features are better adapted to their environment
- They survive and reproduce more - Well-adapted individuals are more likely to survive and have offspring
- Alleles get passed on - These individuals pass their advantageous alleles to their children
- Population changes - Over time, more individuals in the population will have the helpful characteristics
Natural selection is the process where individuals with favourable traits are more likely to survive and reproduce, passing these traits to future generations.
Variation and mutation
Genetic variation
Most populations have lots of genetic variation. All genetic variants come from mutations - changes in DNA.
Types of mutations
There are different types of mutations:
- Most have no effect on the phenotype (what you look like)
- Some influence the phenotype slightly
- Very few determine the phenotype completely
How mutations lead to evolution
- Mutations happen all the time in populations
- Usually a mutation doesn't lead to a new phenotype
- Sometimes a mutation creates a new phenotype that helps an organism survive
- If the environment changes, this new phenotype might give the organism an advantage
- This can lead to rapid changes in the species
Formation of New Species
Sometimes two populations of the same species become so different that they can no longer breed together to produce fertile offspring. When this happens, they have become two separate species.
Real-World Example: Elephant Evolution in Uganda
In Uganda, poachers hunt elephants for their tusks. This created pressure for natural selection:
- Elephants with tusks are more likely to be hunted and killed
- Elephants without tusks are more likely to survive
- Tuskless elephants can reproduce and pass on the "no tusks" allele
- Over time, more elephants in the population become tuskless
The Evidence: The data shows this change: in 1930, only 1% of elephants were tuskless. By 2010, 15% of female elephants and 9% of male elephants were born without tusks.
Key Points to Remember:
- Evolution is change in inherited characteristics of populations over time
- Variation has three causes: genetic, environmental, and combination of both
- Natural selection means well-adapted individuals survive and reproduce more
- Mutations create genetic variation - most have no effect but some can be advantageous
- New species form when populations become too different to breed together