# Speciation and the Origin of Diversity

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/speciation-and-the-origin-of-diversity

## Drawing a Line Around a Species

The **biological species concept** defines a species as a group of populations that actually or potentially interbreed and are reproductively isolated from other such groups. It is the standard definition and it has real limits: it cannot be applied to asexual organisms, to fossils, or cleanly to populations that hybridise occasionally. Other concepts — morphological, ecological, phylogenetic — are used where it fails, and none works everywhere.

**Speciation** is the splitting of one lineage into two reproductively isolated ones, and the models differ by geography:

| Model | Geography | Mechanism |
|---|---|---|
| Allopatric | Populations physically separated | Divergence without gene flow |
| Sympatric | Same area | Ecological or chromosomal isolation |
| Parapatric | Adjacent, touching ranges | Divergence despite some gene flow |

**Allopatric** is the commonest and the easiest to understand: a barrier stops gene flow, the two populations diverge under drift and different selection, and by the time contact resumes they no longer interbreed.

Isolating mechanisms are conventionally split by timing. **Prezygotic** barriers act before fertilisation, different habitats, different breeding seasons, incompatible courtship, mechanical mismatch, gamete incompatibility. **Postzygotic** ones act after, inviable hybrids, or sterile ones such as the mule. Prezygotic barriers are generally favoured by selection, because a wasted mating costs less than a wasted offspring.

Population genetics gives a quantitative handle. Under **Hardy-Weinberg** equilibrium, allele frequencies $p$ and $q$ satisfy $p + q = 1$ and genotype frequencies are

$$p^2 + 2pq + q^2 = 1$$

The equilibrium holds only under strict conditions — no selection, no mutation, no migration, random mating, infinite population — so its practical value is as a **null model**: a population deviating from it is telling you that one of those conditions is being violated.

The fossil record shows diversity rising over time, punctuated by **five mass extinctions**. The largest, at the end of the Permian, removed an estimated 90 % of marine species. Recovery took millions of years and produced radically different faunas, extinction is not merely subtraction, but a reshuffling of which lineages get to radiate.

> **Common pitfall:** treating Hardy-Weinberg as a prediction about real populations. It is a null model describing what happens when nothing is happening. Its usefulness lies entirely in the deviations, which is where the evolution shows up.

## Practice questions

6 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why is Hardy-Weinberg equilibrium useful even though real populations rarely satisfy its conditions?

A. It accurately predicts allele frequency trajectories across most wild populations
B. It is a null model, so deviations from it reveal which evolutionary forces are acting
C. It defines the stable genetic endpoint that every evolving population tends towards
D. It allows researchers to calculate real genotype frequencies without empirical sampling

**Answer:** B. It is a null model, so deviations from it reveal which evolutionary forces are acting

**Why:** The equilibrium describes a population in which no evolutionary force acts. Its value lies in the *deviations*: a population out of equilibrium is telling you that selection, drift, migration or non-random mating is at work.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/why-is-hardy-weinberg-equilibrium-useful-even-though-real-populations

### 2. In a population at Hardy-Weinberg equilibrium the dominant allele has frequency $p = 0.7$. What percentage of the population are heterozygotes?

**Answer:** 42 (within ±1)

**Why:** $q = 0.3$, so $2pq = 2 \times 0.7 \times 0.3 = 0.42$, or **42 %**. Heterozygotes peak at 50 % when both alleles are equally common, and fall away as either allele becomes rare.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/in-a-population-at-hardy-weinberg-equilibrium-the-dominant-allele-has

### 3. Sort each isolating mechanism by when it acts.

**Answer:**

- Prezygotic, before fertilisation: The two species breed in different seasons, Courtship displays are not recognised across species, Gametes are chemically incompatible
- Postzygotic, after fertilisation: Hybrid offspring die before maturity, Hybrid offspring are sterile, like the mule

**Why:** Prezygotic barriers stop a zygote forming; postzygotic ones act on the hybrid. Selection generally favours prezygotic barriers, because a wasted mating is cheaper than a wasted offspring.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/sort-each-isolating-mechanism-by-when-it-acts

### 4. Natural selection systematically favours prezygotic over postzygotic reproductive barriers during divergence. What drives this evolutionary asymmetry, and what does it mean for hybridising taxa?

A. Prezygotic barriers arise first purely because behavioural changes evolve faster
B. Postzygotic barriers fail because sterile hybrids can still compete for resources
C. Prezygotic isolation avoids the energetic waste of producing non-viable offspring
D. Postzygotic mechanisms cannot evolve since natural selection cannot act on hybrids

**Answer:** C. Prezygotic isolation avoids the energetic waste of producing non-viable offspring

**Why:** Postzygotic barriers still consume parental investment before reproductive failure occurs, so selection actively rewards traits preventing wasted gametes. Hybrids are subject to selection through parent fitness, and behavioural divergence is not inherently faster than genetic incompatibility.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/natural-selection-systematically-favours-prezygotic-over-postzygotic

### 5. Arrange the stages of allopatric speciation in order.

**Answer:**

1. A single interbreeding population occupies a continuous range
2. A geographic barrier divides the range in two
3. The separated populations diverge under drift and different selection
4. Reproductive isolation becomes established
5. The two populations no longer interbreed even if contact resumes

**Why:** Separation stops gene flow, divergence accumulates independently, and reproductive isolation emerges as a *by-product* rather than a goal. The final test is whether renewed contact produces interbreeding, if it does not, speciation is complete.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/arrange-the-stages-of-allopatric-speciation-in-order

### 6. The end-Permian mass extinction removed an estimated 90 % of marine species. Set the percentage that survived.

**Answer:** 10 (within ±3)

**Why:** Only about **10 %** of marine species survived, the most severe extinction in the record. Recovery took millions of years and produced a radically different fauna, which is why extinctions reshape rather than merely reduce diversity.

Page: https://tryals.app/practice/biology-i/speciation-and-the-origin-of-diversity/the-end-permian-mass-extinction-removed-an-estimated-90-of-marine