Evolution may lead to speciation

Introduction: The Story of Biodiversity

Hi there! Welcome to one of the most exciting parts of your Biology A Level. Have you ever wondered why there are so many millions of different species on Earth? From tiny bacteria to giant blue whales, the variety is staggering. This chapter is all about the "How" and "Why." We are going to explore how tiny changes in the DNA of a population can, over thousands of years, lead to the birth of entirely new species. Evolution isn't just a slow crawl; it's a dynamic process driven by survival, luck, and isolation. Don't worry if this seems a bit "big picture" at first—we'll break it down into small, manageable steps!

1. Where Does Variation Come From?

Before evolution can happen, there must be differences between individuals. If every organism were a perfect clone, nothing would ever change! We call these differences phenotypic variation.

Why do we look different?
1. Genetic Factors: This is the "blueprint" variation. The primary source is mutation (random changes in DNA). During sexual reproduction, meiosis and the random fertilisation of gametes mix these genes up like a deck of cards, creating unique combinations.
2. Environmental Factors: Things like diet, temperature, or light can change how an organism grows.

Quick Review: Evolution only acts on the variation that can be passed down to the next generation (the genetic stuff!).

2. Natural Selection: Survival of the "Fittest"

In nature, life is tough. There are "selection pressures" like predation, disease, and competition for food or mates.

Step-by-Step: How Natural Selection Works
• Step 1: A population shows variation in phenotypes due to different alleles.
• Step 2: Certain individuals have phenotypes that give them a selective advantage (e.g., they are faster or better camouflaged).
• Step 3: These individuals are more likely to survive and successfully reproduce. This is called differential reproductive success.
• Step 4: They pass on their favourable alleles to the next generation.
• Step 5: Over many generations, the allele frequency of the favourable allele increases in the gene pool.

The Definition of Evolution: In your exam, remember that evolution is defined as a change in the allele frequencies within a population over time.

3. Three Types of Selection

Selection doesn't always work the same way. Think of a graph showing the height of people. Most people are in the middle (average), with a few very short or very tall people at the edges.

Stabilising Selection: The environment stays stable. The "average" individuals are best adapted. The extremes (very tall or very short) are selected against. Example: Human birth weight—babies that are too small or too large have lower survival rates.

Directional Selection: The environment changes. One extreme phenotype becomes an advantage. The "average" shifts in that direction. Example: Antibiotic resistance in bacteria.

Disruptive Selection: This is the rarest but most important for speciation! Both extremes are better than the average. Over time, the population may split into two distinct groups. Example: Small birds with small beaks for seeds and large beaks for nuts; middle-sized beaks are bad at both.

Key Takeaway: Disruptive selection is the most likely type to lead to the formation of a new species.

4. Speciation: The Birth of a New Species

How do we know when a new species has formed? According to the biological species concept, two organisms belong to different species if they can no longer interbreed to produce fertile offspring. This happens through reproductive separation.

Allopatric Speciation (The "Physical Wall")
This happens when a population is split by a geographical barrier (like a mountain range, a river, or an ocean).
1. The two populations are geographically isolated.
2. There is no gene flow between them.
3. Both populations experience different selection pressures (different food, climate, or predators).
4. Mutations occur independently in each group.
5. Over a long time, their gene pools change so much that they become reproductively isolated. Even if the barrier is removed, they can't breed anymore.

Sympatric Speciation (The "Invisible Wall")
This happens in the same area, without a physical barrier. The isolation is usually biological or behavioural.
• Example: Some insects might start feeding or mating on a different type of fruit in the same forest. Eventually, they only mate with others who prefer that fruit, leading to isolation.

Did you know? Even a change in "courtship song" or the time of year an animal is active can be enough to trigger sympatric speciation!

5. Genetic Drift: The Power of Luck

Natural selection isn't the only way allele frequencies change. Sometimes, it's just down to chance. We call this Genetic Drift.

Why is it important only in small populations?
Imagine a giant city (a large population). If one person with a rare eye colour leaves, the overall percentage of that eye colour barely changes. But if you are in a tiny village of 10 people (a small population) and the only person with blue eyes leaves, that allele is lost forever from that village.

In small populations, chance events have a huge impact on the gene pool, which can lead to rapid evolutionary change.

6. Summary & Common Mistakes to Avoid

Common Mistakes:
• "Individuals evolve." No! Individuals stay the same; populations evolve over generations.
• "Evolution is a choice." No! Organisms don't "try" to adapt. It is a passive process of those with the best genes surviving by chance and necessity.
• Confusing Allopatric and Sympatric. Remember: Allopatric = Apart (geographic barrier). Sympatric = Same place.

Key Takeaways:
• Evolution is the change in allele frequencies.
• Isolation is the key to speciation.
• Mutation is the ultimate source of all new variation.
• Long periods of evolutionary change have resulted in the great diversity of species we see today.

Don't worry if this feels like a lot to take in! Just remember the core story: Variation + Selection + Isolation = New Species. You've got this!