# The Origin and Evolution of Sex

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

## An Expensive Habit That Persists

Reproduction comes in two forms with very different economics.

**Asexual** reproduction is fast, needs no partner, and passes on 100 % of the parent's genes. Every offspring is a clone, so a well-adapted genotype is preserved intact. In a stable environment it is hard to beat.

**Sexual** reproduction is expensive. It needs a partner, costs time and risk in finding one, and each parent passes on only 50 % of their genes to each offspring. That last figure is the **twofold cost of sex**: an asexual mutant in a sexual population should, all else equal, double its representation each generation. Yet sex is nearly universal among eukaryotes, so something must be paying for it.

The answer is **variation**. Sex generates it three ways: **independent assortment** of chromosomes in meiosis, **crossing over** between homologues, and **random fertilisation** combining two gametes. Independent assortment alone gives $2^n$ distinct gametes for $n$ chromosome pairs, which for humans ($n = 23$) is over 8 million before crossing over is counted at all.

Variation pays in a **changing** environment, and especially against parasites, the **Red Queen hypothesis**, where host and parasite must keep evolving simply to hold their position. A clonal host presents the same target every generation; a sexual one does not.

The prediction is testable and holds: asexual reproduction dominates in stable environments and among colonising species, while sexual reproduction dominates where conditions vary and parasites are abundant. Many organisms do both, switching to sex under stress.

**Fisher's principle** explains why sex ratios sit near 1:1. If males became rare, each male would have more offspring on average, so parents producing sons would do better, until the advantage disappeared at parity. The ratio is an equilibrium maintained by frequency-dependent selection, not a fixed constant of biology.

> **Common pitfall:** describing sex as "good for the species". Selection acts on individuals and genes, not on species. Sex persists because sexual *individuals* leave more surviving descendants under the right conditions, an explanation that has to work at the individual level or it does not work at all.

## Practice questions

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

### 1. What best explains why sexual reproduction persists despite its twofold cost?

A. It preserves successful, well-adapted genotypes intact across successive generations
B. It generates variation, which pays off in changing environments and against parasites
C. It requires significantly less energy and courtship time than asexual reproduction
D. It produces a higher number of surviving offspring per parent than asexual methods

**Answer:** B. It generates variation, which pays off in changing environments and against parasites

**Why:** Sex trades reproductive efficiency for variation, and variation is what lets a lineage track a moving target, most sharply a coevolving parasite. Preserving genotypes intact is the *asexual* advantage, and sex produces fewer offspring per parent, not more.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/what-best-explains-why-sexual-reproduction-persists-despite-its

### 2. Sort each feature by the mode of reproduction it belongs to.

**Answer:**

- Asexual reproduction: Offspring are genetically identical to the parent, No partner is required, The parent passes on all of its genes
- Sexual reproduction: Offspring differ genetically from both parents, Each parent contributes half of the offspring genome, Meiosis and fertilisation are both involved

**Why:** Asexual reproduction is efficient and conservative; sexual reproduction is costly and generative. The 100 % versus 50 % gene transmission is precisely the twofold cost that any explanation of sex has to overcome.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/sort-each-feature-by-the-mode-of-reproduction-it-belongs-to

### 3. An organism has 6 pairs of chromosomes. How many genetically distinct gametes can independent assortment alone produce?

**Answer:** 64

**Why:** $2^6 = 64$ distinct gametes from independent assortment alone. Crossing over then recombines within each chromosome, so the true figure is far larger, effectively no two gametes are alike.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/an-organism-has-6-pairs-of-chromosomes-how-many-genetically-distinct

### 4. Fisher's principle explains the 1:1 sex ratio as an evolutionary equilibrium rather than a fixed physiological constant. What follows from this distinction if a sudden disease halves the male population?

A. The population adapts by permanently raising male birth rates above unity
B. Females adapt by producing identical clonal daughters in equal numbers
C. Parents producing sons gain higher relative fitness until parity returns
D. Selective pressure acts immediately to preserve the species from collapse

**Answer:** C. Parents producing sons gain higher relative fitness until parity returns

**Why:** Equilibria rely on individual differential reproductive success rather than species-level rescue or permanent mutations. When males are scarce, individual parents yielding sons enjoy higher genetic representation, driving the ratio back to parity via negative frequency-dependent selection.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/fishers-principle-explains-the-1-1-sex-ratio-as-an-evolutionary

### 5. Which conditions favour sexual over asexual reproduction?

A. Strong competition among close relatives
B. Heavy pressure from coevolving parasites
C. A rapidly changing environment
D. A completely stable environment with no parasites

**Answer:** A. Strong competition among close relatives; B. Heavy pressure from coevolving parasites; C. A rapidly changing environment

**Why:** Change, parasites and sibling competition all reward variation. In a genuinely stable, parasite-free environment the asexual clone wins, which is exactly what is observed in stable habitats and among colonising species.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/which-conditions-favour-sexual-over-asexual-reproduction

### 6. Sexual reproduction persists because it benefits the species as a whole.

**Answer:** False

**Why:** False, selection acts on individuals and genes, not species. Any account of sex must explain why sexual *individuals* leave more surviving descendants; group-level reasoning here is a classic misstep that gets the mechanism wrong.

Page: https://tryals.app/practice/biology-i/the-origin-and-evolution-of-sex/sexual-reproduction-persists-because-it-benefits-the-species-as-a