Some animals have males and females, some are hermaphrodite, and some have no sex at all.
Sex
We normally think in terms of two sexes, but some invertebrates are hermaphrodite. Slugs and Snails, for example, are both male and female at the same time. These differences do not matter when we look at the advantages of sexual reproduction (over the many forms of asexual reproduction seen in many animal and plant species. Sexual reproduction allows the genes of a species to be mixed in novel combinations each generation. This leads to the variation necessary for evolution to proceed and allows species to adapt to long-term environmental changes. The fact that most (vertebrate and invertebrate) species use sexual reproduction, at least some of the time, shows how important sex is.
No Males
Some Stick Insects are ‘parthenogenetic’, in other words males are not necessary for reproduction. At first glance this would seem to cast doubt on the benefits of sexual reproduction, but a more detailed study reveals that there is still the mixing of genes with each generation – this is not quite the same as asexual reproduction. It appears that the ‘eggs’ of parthenogenetic stick insects develop in an unusual way. These stick insect ‘eggs’ begin to develop by making a few cells by a process which halves the number of genes (meiosis) - in the same way that male animals produce sperm and females make eggs. These cells then fuse together in the same way that sperm fertilise eggs. In this way there is a regular ‘re-shuffling’ of the genes housed in the females. This is not as ‘good’ as re-shuffling the genes found in the whole population, so it is not surprising to find that males occasionally appear.
No Sex
There are many animals (such as some Rotifers) that never indulge in sexual reproduction. Offspring are genetically identical to the parent and any variation here must rely on mutation not the re-shuffling of genes. Asexual reproduction is far more energy-efficient than sexual reproduction, so there must be some advantage to the sexual process for it to have arisen by evolution and survived in so many cases. Exactly what this advantage is is still a mystery to evolutionary biologists. Maybe the advantage has something to do with the speed with which organisms can adapt to change?
Information Carried by Genes
Monarch Butterflies show how much information can be passed on from one generation to the next within the genes. Adult butterflies who hatched and grew up as far north as Canada are able to navigate back to the same trees in Mexico or California that were used by their ancestors in the previous winter. (It is ‘ancestors’ not ‘parents’ because there are usually several generations bred in a single summer.) This shows that very complex navigational information can be genetically encoded (see ‘Migration’). Maybe this illustrates a case where the regular mixing of genetic information through sexual reproduction allows a species to adapt rapidly (to the need for changes in migratory behaviour in this case).
(See also ‘Evolution, genetics and ecology’).
Other articles by John Blatchford
