7.3 Evolution may lead to speciation

Why do individuals in a population show a wide range of phenotypic variation?

• Genetic factors:

  • Mutations – primary source of new alleles.

  • Crossing over during meiosis.

  • Independent assortment of homologous chromosomes.

  • Random fertilisation of gametes.

• Environmental factors:

  • Differences in nutrition, light, temperature, etc. can influence phenotype.

What is evolution?

• A change in allele frequency in a population over many generations, driven by natural selection.

What drives natural selection?

• Selection pressures such as:

  • Predation

  • Disease

  • Competition for resources (food, water, mates, space)

Describe the process of natural selection in 5 steps.

1. Mutation – random mutation produces a new allele.

2. Advantage – new allele provides a selective advantage under a given selection pressure.

3. Survival & Reproduction – individuals with the advantageous allele are more likely to survive and reproduce.

4. Inheritance – advantageous allele is passed to offspring.

5. Allele Frequency Change – over generations, the frequency of the advantageous allele increases in the gene pool.

What is stabilising selection?

• Selection that favours intermediate phenotypes and reduces extreme variations.

Give an example of stabilising selection.

• Human birth weight – babies of average weight have higher survival rates than very small or very large babies.

What is the effect of stabilising selection on allele frequencies?

• Increases frequency of alleles coding for average traits.

• Decreases frequency of alleles coding for extreme traits.

What is the effect of stabilising selection on the population’s range/variation?

• Reduces the range / standard deviation – population becomes more uniform.

What is directional selection?

• Selection that favours one extreme phenotype, causing the population’s average trait to shift in one direction over time.

What is disruptive selection?

• Selection that favours both extreme phenotypes over intermediate ones, potentially leading to speciation.

How does natural selection lead to evolution?

• By differentially affecting survival and reproduction, it changes allele frequencies in the gene pool over time.

What is the role of mutations in evolution?

• They are the primary source of genetic variation upon which natural selection acts.

What is directional selection?

• Natural selection that favours one extreme phenotype in a population.

What is the effect of directional selection on allele frequencies?

• Increases frequency of alleles coding for the favoured extreme.

• Decreases frequency of alleles coding for the other extreme.

Give an example of directional selection.

• Bacteria evolving antibiotic resistance – bacteria with higher resistance survive and reproduce.

What is disruptive selection?

• Natural selection that favours both extreme phenotypes over intermediate ones.

What is the effect of disruptive selection on allele frequencies?

• Increases frequency of alleles coding for both extremes.

• Decreases frequency of alleles coding for average traits.

What can disruptive selection lead to over time?

• Speciation – formation of two distinct species.

What is speciation?

• The formation of a new species from an existing one.

What must happen for speciation to occur?

• Reproductive isolation between populations, leading to genetic divergence until they can no longer interbreed to produce fertile offspring.

What is allopatric speciation?

• Speciation that occurs due to geographical isolation of populations.

Describe the steps of allopatric speciation.

1. Geographical isolation – a physical barrier splits a population.

2. Reproductive isolation – gene flow stops between populations.

3. Different mutations occur in each population.

4. Different selection pressures act on each population.

5. Allele frequencies change independently over generations.

6. Genetic differences accumulate until the populations become reproductively isolated.

What is reproductive isolation?

• When two populations can no longer interbreed to produce fertile, viable offspring.

Name two types of reproductive isolation barriers.

• Prezygotic – before fertilisation (e.g., temporal, behavioural, mechanical).

• Postzygotic – after fertilisation (e.g., hybrid inviability, hybrid sterility).

What role do mutations play in speciation?

• Provide the genetic variation upon which natural selection acts, leading to divergence.

What is the ultimate result of speciation?

• Two or more distinct species that cannot produce fertile offspring together.

What is sympatric speciation?

• Speciation that occurs without geographical isolation; populations live in the same area but become reproductively isolated by other mechanisms.

What causes reproductive isolation in sympatric speciation?

• Genetic mutations leading to:

  • Gamete incompatibility

  • Different breeding seasons (temporal isolation)

  • Different courtship behaviours

  • Mechanical changes preventing mating

  • Polyploidy in plants

How does sympatric speciation proceed?

1. Reproductive isolation arises within a population.

2. Different selection pressures act on each sub-population.

3. Different advantageous alleles are selected.

4. Allele frequencies change independently over generations.

5. Populations can no longer interbreed to produce fertile offspring.

What is genetic drift?

• A mechanism of evolution where allele frequencies change due to random chance, not natural selection.

When is genetic drift most significant?

• In small populations where chance has a greater effect.

Why does genetic drift have a stronger effect in small populations?

• The gene pool is smaller, so random events can disproportionately affect allele frequencies.

What is the bottleneck effect?

• A sharp reduction in population size due to a catastrophic event, leading to loss of genetic diversity.

What is the founder effect?

• When a small group breaks away from a larger population to form a new colony, carrying only a subset of the original gene pool.

What are the consequences of genetic drift?

• Reduced genetic diversity

• Some alleles become fixed (frequency = 1)

• Some alleles become lost (frequency = 0)

How does genetic drift differ from natural selection?

• Genetic drift: changes due to chance; not adaptive.

• Natural selection: changes due to selective advantage; adaptive.

Can genetic drift lead to speciation?

• Yes, especially in small, isolated populations where drift can cause significant genetic divergence.

What is an example of the founder effect?

• Amish populations with higher frequencies of certain genetic disorders due to a small founding population.

What is an example of the bottleneck effect?

• Northern elephant seals: hunted nearly to extinction, now have very low genetic diversity.