Comparing Sexual and Asexual Reproduction: Variation and Survival

1Two strategies for passing on genes

Strawberry plants can reproduce in two quite different ways. They can send out runners that form genetically identical daughter plants, or they can flower, be pollinated and make seeds, each seed a genetic mixture of two parents. Why would the same organism use two methods? And why is sexual reproduction so common among animals, when it takes two individuals, costs time and energy, and halves the number of parents that can have babies?

The BC curriculum frames this puzzle as an inquiry question: What are the advantages and disadvantages of sexual and asexual reproduction? This lesson answers that question by pulling together what the previous lessons established about mitosis, asexual reproduction, meiosis and fertilization.

2A side-by-side comparison

3The case for asexual reproduction

• Offspring are copies of a parent that already survives well. In a habitat that does not change, copying a successful design makes sense.
• Rapid reproduction. Organisms can multiply quickly when conditions are good, using budding, fragmentation, spores or fission.
• No partner is needed. This helps solitary organisms and can make it easier for a species to colonize new habitats, since a single individual can start a population.
• Every individual can reproduce. In a sexual population, males do not produce offspring directly, so a population of asexual individuals could in theory grow about twice as fast.

The costs of asexual reproduction

• The only source of new variation is mutation, which is rare.
• In an unpredictable environment, the lack of variation can leave the whole population vulnerable at once.
• Multicellular organisms that rely only on asexual reproduction are very rare.

4The case for sexual reproduction

The benefit of sexual reproduction is variety. Offspring differ from one another and from their parents. On average, a sexual population leaves more descendants than a similar asexual one over the long term, because variation helps populations survive and reproduce when conditions change.

The costs of sexual reproduction

• A mate must be found.
• Only part of the population produces offspring directly, so growth is slower.
• Gametes must be made by meiosis and brought together, which takes time and energy.

Biologists still study why sexual reproduction is so widespread, and the question is not fully resolved. One influential idea, the Red Queen hypothesis (proposed by Leigh Van Valen in 1973), points out that species are in constant competition with others, such as predators with prey and parasites with hosts. Whenever one species gains an advantage, pressure increases on its competitors, which must also improve. Variation from sexual reproduction lets a species change quickly, so that none gets far ahead. The name comes from the Red Queen in Lewis Carroll's story, who must keep running just to stay in the same place.

5Where variation comes from

The previous two lessons described how variation arises in sexual reproduction. It is useful to put the sources side by side and compare them with asexual reproduction.

1. Mutation. Rare changes in DNA. In asexual organisms, this is the only source. Mutations in cells that make gametes can be passed to offspring in sexual reproduction as well.
2. Crossing over. In prophase I, matching segments are exchanged between homologous chromosomes, creating new combinations of alleles on a single chromosome.
3. Independent assortment. At metaphase I, each pair lines up independently, so a human can make over eight million different combinations of chromosomes even without crossing over.
4. Random fertilization. Any sperm could meet any egg. Two parents can therefore make a very large number of different offspring.

6Many organisms use both strategies

The two strategies are not rivals in every species. Many organisms switch between them or combine them.

• Plants. A strawberry plant spreads by runners (cloning) and also flowers and makes seeds. Many plants alternate between a multicellular haploid stage and a multicellular diploid stage, with meiosis making spores.
• Fungi. Fungi produce huge numbers of asexual spores for rapid spread, and they can also produce sexual spores after two mating types fuse.
• Animals. Some invertebrates (such as aphids and water fleas) use parthenogenesis, in which an unfertilized egg develops into an individual, while others reproduce sexually at other times. Bees use both: unfertilized eggs become male drones and fertilized eggs become females.
• Hermaphrodites. Some animals, such as earthworms and snails, have both male and female reproductive parts. They may mate with another individual or, in some, fertilize themselves; in animals with limited mobility, such as barnacles and clams, self-fertilization is common.

Evaluating the strategies: a reasoning example

The curricular competencies ask students to use evidence to support a claim, and to evaluate it. A useful pattern is claim, evidence, reasoning.

Questions to practise the same pattern: Why might a species of fungus produce both asexual and sexual spores? Why might an isolated animal that can reproduce by parthenogenesis be at an advantage in a new habitat, and what is its main weakness?

Analyzing a data set

Claims about advantages and disadvantages are stronger when supported by data. Imagine an experiment in which a student compares two groups of single-celled organisms in a dish after a harmful chemical is added. (The numbers below are invented for practice.)

Sex determination: another difference among species

Species that reproduce sexually also differ in how the sex of an offspring is decided. In mammals, the Y chromosome drives male development, and individuals with two X chromosomes are female. Birds, some fish, insects and reptiles use a different system (Z and W chromosomes), in which the W chromosome is essential for female development. In some reptiles, such as turtles and crocodiles, the temperature during incubation determines the sex. Some species even change sex during their lives: oysters, for example, begin as males and later become females.

These examples show that “sexual reproduction” is not a single pattern but a family of strategies built on the same core idea: halving the chromosome number by meiosis and combining two gametes at fertilization.

Two misconceptions about the comparison

The inquiry question revisited

A balanced answer to “What are the advantages and disadvantages of sexual and asexual reproduction?” can now be given in a few sentences. Asexual reproduction is fast, simple, needs one parent, and preserves successful combinations of genes, but it produces little variation, so populations are vulnerable to change. Sexual reproduction is slower and needs a mate, but it continually produces new combinations of genes, which gives populations a better chance of surviving changes such as disease, new predators or shifting climate. Because the costs and benefits depend on the circumstances, living things have evolved many variations on both strategies, and some use both.

7Summary

• Asexual reproduction: one parent, mitosis, identical offspring. It is fast and needs no mate, but gives little variation.
• Sexual reproduction: two parents, meiosis and fertilization, unique offspring. It needs a mate and is slower, but variation helps populations to survive change.
• Variation arises from mutation, crossing over, independent assortment and random fertilization; asexual reproduction relies on mutation alone.
• The advantage of each strategy depends on the environment, and many organisms use both.

The next lessons widen the view from the cell to the whole planet: how the energy of the Sun and the cycling of matter connect all living things.

?Quick check

Try each question first, then reveal the answer.