Chapter 17: Sex, evolution, and the biological species concept
Chapter 17: Sex, evolution, and the biological species concept
Sex, Evolution, and Species
Although for many organisms sex is crucial to the ability to reproduce, sex is an important concept to biology for more reasons than this. Sex is critical to two other highly significant biological topics: the process of evolution and the concept of species, two things that are in fact interconnected. Evolution describes changes through time. What is it that changes? It is not an individual organism, whose changes are described as ‘development’. What evolves are groups of organisms, i.e., populations (Fig. 1). Populations are usually described as a group of organisms of the same species. T he criterion that puts individuals together into a group called a species is the ability to interbreed, i.e., species are groups of organisms united by the fact that they can potentially interbreed, and the only way to interbreed is through sex. Sex is the process that connects individuals together in to something called a species, a group of interbreeding organisms. Species, i.e. interbreeding groups, are thought to be the biological entity that evolves. Additionally, sex is significant to the process of Darwinian evolution because:
- sex can be an important process that generates variation within a population and variation is significant to the evolution ary process, providing the ‘raw material’ upon which the process of natural selection can act.
- sex provides a mechanism to spread particular features within a population, allowing favorable characteristics to spread.
It should be pointed out that sex is certainly not required for the process of evolution. There are a number of groups (e.g., bacteria) that evolve readily and have no sexual process (although bacteria DO have processes that allow characteristics to spread within a population, it is not sex).
Moreover, groups that do not have sex cannot be categorized as having species (although oftentimes they are!) because sex is what delineates an ‘interbreeding group’, which is the criterion usually used to define species. Thus, although sex and species are significant to evolution, they are not essential to evolution. As noted earlier, several of the groups considered here (e.g., dinoflagellates) do not have sex, but the groups’ diversity clearly point to the fact that evolution has been, and presumably still is, operating.
Even within groups that are considered to be sexual, e.g., flowering plants, there are groups that do not participate in sex. Dandelions produce seeds but do so in a manner that does not involve sex. Such groups might be considered evolutionary ‘dead-ends,’ yet, as noted above, sex is not essential to the process of evolution.
The process of sex is significant to the grouping and naming practices discussed in Chapter 2. To deal with diversity, biologists group organisms that are phylogenetically connected (reflected in the fact that organisms within the group ‘look alike’). There are different lev