Species Level Natural Selection
Natural selection at species level relies on there being a geographical separation between groups within a species so that they can follow their own independent evolutionary path. Eventually, the genomes of two groups will become so different that they have difficulty interbreeding. For example, a male donkey and a female horse will produce a sterile mule. Ultimately, they will become separate “child” species and incapable of interbreeding. This process is known as speciation.
Population pressure among successful “child” species can cause them to migrate and come into contact with “sibling” species. There can only be one species in each ecological niche. If there are more, then competition for the niche will result in the fittest species, normally the migratory one, prospering and the least fit one becoming extinct. It is theoretically possible for this process to take place but, because millions of years would be required and there is, therefore, relatively little evidence of it, not all evolutionary biologists believe that it does. It may, however, have occurred among hominins.
Hominins are human-like species that evolved after our predecessors and those of the chimpanzees speciated between 12 and 5 million years ago. Since then, there are believed to have been 15 to 20 species of hominins, all of which, apart from our own, have become extinct. The migration of homo sapiens from Africa, where we originated, into Asia may have resulted in the demise of Homo Erectus, and our migration into Europe in the demise of the Neanderthals. Neanderthals were a sub-species, and some are known to have been subsumed by modern humans through interbreeding. This is confirmed by the existence of part of the Neanderthal genome in non-African branches of our species. However, most were probably outcompeted by modern humans. It is unclear whether Homo Erectus was an entirely separate species and became extinct or whether it too was subsumed in a similar way.
Presently, it is difficult to identify any behavioural traits which may have evolved in modern humans as a result of species level selection as this would require a comparison with other, now extinct, hominin species.
Ecosystem Level Natural Selection
The final level in the organisation of life comprises the world’s ecosystems. These are the final, and largest, Russian dolls on which individual organisms depend for their survival and ability to procreate.
A natural ecosystem comprises all the non-living ingredients for life, e.g., a source of energy, water, minerals, atmospheric gases and so on. It also comprises numerous species, each of which has its own niche or role to play, and each of which interacts with other species to form a complex system. Each ecosystem is adapted to its own habitat, and these can be highly variable to include, for example, freshwater, marine, tropical, mountainous, and desert habitats.
The roles played by species are classified using the food chain. Generally, there are only up to 4 or 5 levels, which typically comprise:
- Producers: organisms that produce food for all other species in the ecosystem, e.g., green plants which convert inorganic substances into organic material through photosynthesis.
- Primary consumers or herbivores: animals that consume plants, e.g., sheep and goats.
- Secondary consumers or carnivores: animals that feed on others, e.g., the big cats and sharks.
- Tertiary Consumers. These are also carnivores but ones that consume other carnivores, e.g., polar bears and crocodiles.
- Decomposers: organisms which feed on dead organic material and help in the recycling of nutrients, e.g., fungi and earthworms.
The flow of energy in a natural ecosystem is largely unidirectional. Plants, which take their energy from sunlight, were the first to evolve and altered the environment, thereby permitting the evolution of herbivores, which take their energy from plants, followed by carnivores, which take their energy from herbivores.
Some species do not fit neatly into these classes. For example, humans are omnivorous, consuming both animals and plants. There are also parasites which feed on a living host. Nevertheless, the above classification is a helpful guide.
All levels of natural selection exist within an ecosystem: individual, kin, group, and species. However, for ecosystem level selection to be possible, there must be more than one ecosystem competing to control the same habitat. This is not apparent in the natural world. Rather, it appears to have been introduced by mankind, as will be discussed in the next post.