Biology (9610) | Evolution may lead to speciation

Introduction: The Birth of Biodiversity

Welcome to one of the most exciting topics in Biology: how new life forms arise! In the last chapter, we looked at how genes change within a population (evolution). Now, we are taking the final step to understand how that evolutionary change leads to the creation of entirely new species—a process called speciation.

Don't worry if this sounds complex! We will break down this journey from tiny genetic shifts to massive biodiversity using clear steps and relatable examples.

Section 1: The Foundation of Change—Evolution and Selection

1.1 Evolution: A Shift in Alleles

Before we can talk about new species, we must remember the core definition of evolution (3.3.8.1):

Evolution is simply defined as a change in the allele frequencies in a population over successive generations.

This process relies on two key things:

• Phenotypic Variation: Individuals in a population show a wide range of characteristics (phenotypes). This variation is usually due to mutations, meiosis, and sexual reproduction.
• Differential Survival: Not all individuals survive equally. Factors like predation, disease, and competition for resources mean some individuals are more "fit" for the current environment.

The result is that organisms with phenotypes providing selective advantages are more likely to survive, reproduce, and pass their favourable alleles to the next generation. This changes the allele frequency in the gene pool, leading to evolution.

1.2 The Effects of Selection (3.3.8.2)

Natural selection acts differently depending on which phenotypes are being favoured. These different pressures result in three main types of selection:

Stabilising Selection

• What it does: Favours the 'average' phenotype and selects against the extreme variations.
• Effect on population: Reduces the range of variation, making the population more uniform. The mean phenotype stays the same.
• Example: Human birth weight. Babies that are average weight have the highest survival rate, while very small or very large babies have lower survival rates.

Analogy: Think of a target shooter aiming for the bullseye. Stabilising selection punishes those who shoot too far left or too far right, concentrating all shots right in the middle.

Directional Selection

• What it does: Favours one extreme phenotype and selects against the mean and the other extreme.
• Effect on population: The mean phenotype shifts towards the favoured extreme over time. This often happens when the environment changes.
• Example: Bacteria exposed to an antibiotic. Only the most resistant individuals survive and reproduce, shifting the entire population toward higher resistance.

Disruptive Selection

• What it does: Favours both extreme phenotypes and selects against the mean (intermediate) phenotype.
• Effect on population: Increases variation and can eventually split the population into two distinct groups.
• Example: A population of birds where very small beaks are good for eating tiny seeds, and very large beaks are good for cracking hard nuts, but medium-sized beaks are inefficient for both food sources.

Key Takeaway for Section 1: Evolution changes allele frequencies, and natural selection can drive these changes in three distinct ways (stabilising, directional, or disruptive).

Section 2: Speciation—The Ultimate Separation

2.1 Defining Speciation

Speciation is the evolutionary process by which new species arise (3.3.8.2).

Remember the definition of a species (3.1.10.1): a group of organisms capable of interbreeding and producing fertile offspring.

Therefore, speciation occurs when two populations of the same species accumulate so many genetic differences that they can no longer interbreed to produce viable, fertile offspring.

2.2 The Role of Reproductive Separation (3.3.8.2)

The crucial first step towards speciation is reproductive separation (or isolation). This prevents gene flow between two groups of the same species.

Once separated, three things happen:

1. Different Selection Pressures: The isolated populations face different biotic (e.g., new predators) and abiotic (e.g., different climate) conditions.
2. Differential Evolution: Natural selection acts on each gene pool independently, favouring different alleles in each location.
3. Genetic Divergence: Over long periods, genetic differences accumulate due to selection, mutation, and genetic drift.

Eventually, these genetic differences lead to reproductive isolation, meaning even if the populations meet again, they cannot successfully interbreed. A new species has arisen.

Did You Know? Genetic drift (3.3.7.2), which is change due to chance, has a much larger effect in small, reproductively separated populations, speeding up the divergence process.

Section 3: How Speciation Happens—Allopatric vs. Sympatric (3.3.8.2)

There are two main ways that reproductive separation can occur, which gives us the two types of speciation:

3.1 Allopatric Speciation

The term 'Allopatric' literally means 'other country'.

• Mechanism: Occurs when populations are separated by a geographical barrier.
• Barriers include: Mountains, rivers, oceans, or even human developments like major roads.
• Process:
  1. A population is split by a physical barrier (e.g., a massive glacier forms).
  2. Gene flow stops completely between the two resulting populations.
  3. Each isolated population faces different selective pressures and experiences independent mutations and genetic drift.
  4. Genetic differences accumulate until, if the barrier were removed, members of the two populations could no longer interbreed.

Analogy: Imagine two groups of friends who share a specific slang term. If one group moves to a completely different city, their language will evolve differently, and when they meet years later, their slang will no longer match.

3.2 Sympatric Speciation

The term 'Sympatric' literally means 'same country'.

Don't worry if this seems tricky at first—it's less intuitive than allopatric speciation because there is no geographical barrier involved.

• Mechanism: Occurs when populations diverge while inhabiting the same geographical area.
• Separation Requirements: Reproductive separation is achieved by non-geographical factors, known as Reproductive Isolating Mechanisms (RIMs).
• Examples of RIMs (non-syllabus detail, but helpful for understanding):
  • Temporal Isolation: Individuals breed at different times of day or different seasons.
  • Behavioural Isolation: Changes in mating rituals or courtship displays prevent recognition between groups.
  • Ecological Isolation: Populations inhabit different niches within the same environment (e.g., one group feeds on one type of plant, another on a different type).

Example: A species of insects feeds and mates on one type of host plant. A mutation causes a small subgroup to prefer a different host plant nearby. They only mate on their preferred plant, even though they share the same physical habitat. Over generations, they become genetically distinct.

3.3 The Final Test of Speciation

Regardless of whether the isolation was allopatric or sympatric, the speciation process is only complete when the genetic differences prevent the separated populations from successfully interbreeding and producing fertile offspring. If they were to meet again, they would be two distinct species.

Summary and Key Takeaways

The chapter "Evolution may lead to speciation" shows how sustained changes in allele frequencies (evolution) can eventually create permanent genetic boundaries, resulting in new species:

• Evolution is the change in allele frequency, driven by selection.
• Natural selection can be stabilising, directional, or disruptive.
• Speciation requires reproductive separation to stop gene flow.
• New species are formed when accumulated genetic differences prevent successful interbreeding and the production of fertile offspring.
• Allopatric Speciation involves a geographical barrier separating populations.
• Sympatric Speciation involves non-geographical barriers in the same area.