Evolution

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What is Evolution?

Evolution is the process by which new species (different types of organisms) are formed from species that already existed. This happens gradually over a very long time—often thousands or millions of years.

The key idea is this: populations of organisms change over time because their gene pools change from one generation to the next.

What is a gene pool?
A gene pool is the complete set of all the genes (and all the different versions of each gene, called alleles) in a population at any given time. Think of it as the "genetic library" available to that group of organisms.

How does the gene pool change?
Several processes can change which alleles are common or rare in a population:

• Natural selection — organisms with helpful traits survive and reproduce more, so their alleles become more common
• Genetic drift — random chance events that change how common different alleles are (explained in detail below)
• Mutations — new alleles appear when DNA changes by accident
• Gene flow — organisms moving between populations bring new alleles with them

Over many generations, these changes add up. Eventually, the population may become so different from the original that it can no longer breed with the original species. At this point, a new species has formed.

Important Differences: Natural Selection vs Genetic Drift

Similarities: Both processes involve a change in allele frequency, and both can lead to evolutionary changes over time.

The Founder Effect

The founder effect happens when a small group of organisms splits off from a larger population to start a new colony somewhere else (for example, on a distant island).

Because this founding group is small, it's likely to have a different mix of alleles compared to the original large population — just by chance, they might carry certain alleles more or less often.

Example:
Imagine a large population where 60% of individuals have allele A and 40% have allele B. If only 10 individuals leave to start a new colony, they might by chance be 80% allele A and 20% allele B. The new population will have different allele frequencies right from the start.

As this small founding population reproduces, the new population's gene pool will reflect those starting frequencies, not the original population's frequencies. This is genetic drift caused by the founder effect.

The Bottleneck Effect

The bottleneck effect occurs when a population suffers a drastic reduction in size due to a natural disaster (like a flood, fire, earthquake, or disease outbreak).

Most individuals die, and only a small number survive. The survivors carry only a fraction of the genetic diversity (variety of alleles) that the original population had.

When the population recovers and grows again, it will have much less genetic variation than before. Again, this is genetic drift — but this time caused by a disaster, not by a group splitting off.

Example:
Think of a population like beads in a bottle. If you pour them through a narrow bottleneck and only a few make it through, the next generation (refilled bottle) will only have the colours that happened to survive. The genetic diversity is reduced.

Comparing Founder Effect and Bottleneck Effect

Both are examples of genetic drift and both reduce genetic diversity in the affected population.