The existence of sexual reproduction has baffled biologist for centuries. How can sexual reproduction have evolved when it greatly diminishes the speed of reproduction? How can it continue to be widespread in nature when asexual species produce much more offspring?
The twofold cost of sexual reproduction
Sexual reproduction has a twofold cost: The cost of males and the cost of passing only half of the genes over to an offspring.
The cost of males: I don’t know if you have noticed, but males do not produce offspring. Not directly. The amount of new individuals that can be created in a population only depends on the number of females (given that there is at least one male). If a female would produce only two daughters, and these would also only produce two daughters, in four generations there would be 8 offspring. If a female produces one daughter and one son, and the daughter would produce again one daughter and one son, in four generations there would be only 2 offspring present in the population.
The cost of passing only half of the genes over to an offspring: In sexual reproduction half of the genes in an offspring came from the father and half from the mother. The genetic relatedness between parent and offspring is therefore much lower than when the offspring is a direct genetic copy of the parent. Alleles have only a 50% chance of being passed on to the next generation every time an offspring is created. From the viewpoint of genes this is costly; twice as many offspring need to be created to have the same allele frequency in the next generation compared to asexual reproduction.
Asexual competitive advantage
The twofold cost of sexual reproduction causes asexual individuals of a species to spread their genes faster into the population. Over time, one would expect that the genes that cause asexual reproduction will out compete genes for sexual reproduction. In controlled laboratory conditions with species that have both asexual and sexual individuals (e.g. some stick insect species) this holds true. Over some generations the asexual variants out compete the sexual variants, causing the genes for sexual reproduction to disappear from the population. In nature sexual reproduction is widespread and in most species asexual reproduction does not occur. Whats the catch to asexual reproduction?
Genetic diversity and risk of extinction
Sexual reproduction as one big advantage over asexual reproduction: it increases genetic diversity. When a mutation arises in one individual it can spread to individuals with a different genetic background through sexual reproduction. In asexual reproduction a mutation is always ‘locked’ inside the genetic background it first occurred in. With sexual reproduction mutations can be combined with other mutations, they can be removed or they can survive in the population until they have a big enough impact on the fitness of the individual to be selected for or against. Sexual reproduction thus allows for a bigger genetic diversity and this allows species to adapt to changes in environment or to changes in other species it interacts with.
Theories that go more into this effect are Muller’s ratchet and the Red Queen Hypothesis.
The benefits outweigh the costs
Sexual reproduction evolved and has stayed for billions of years. This proves that the benefits of sexual reproduction outweigh the costs for most species. Not for all species, as micro-organisms can testify. There are examples of species that have been asexual for millions of years, the most famous being Bdelloid rotifers. In general species with a short generation time are much more likely to reproduce asexually than species with a long generation time.