166 EVOLUTION also be facilitated by the appearance of major innovations, such as the evolution of hard parts by a variety of different taxa close to the Precambrian–Cambrian boundary The two are not necessarily coupled For example, eukaryotes, the complicated internally divided cells with which we are most familiar, evolved more than Ga ago (possibly much more), but they did not become widespread or common until much later at around Ga ago They could not radiate until there was adequate oxygen present in the Earth’s atmosphere and oceans, as they depended on this molecule for respiration It seems likely that evolutionary selection can work at the species level as well as at the level of an individual within a population Species-level selection favours species that have lower extinction rates and higher origination rates than their ‘competitors’ In the long-term, these species become more abundant at the expense of their less-successful competitors Distinguishing between the levels of selection is extremely difficult in practice, but the theory helps to demonstrate the emergent properties of species Some types of extinction, or reductions in diversity, may also be explained by macroevolution and, in turn, throw light on the mechanisms of evolution It is clear that the evolution of a new species will increase competition for resources and may force another species to become extinct if it is unable to compete successfully The pattern of species extinction would be expected to be one of increasing chance of extinction with species age, but in some cases this does not seem to be so Instead, species age does not appear to correlate with the likelihood of extinction Van Valen has used this observation (which is itself somewhat contentious) to suggest a novel hypothesis for macroevolutionary patterns He suggested that competition for resources produces a dynamic equilibrium between species, in which each will continue to evolve in order to survive This is the core of the Red Queen hypothesis, which suggests that organisms evolve to keep their biological place or, to paraphrase the quotation from Alice in Alice Through the Looking Glass, they ‘run to keep still’ The characters that help organisms to survive at times of low extinction rate may be different from those that make survival of mass extinctions more likely In other words, the criteria by which species are selected may vary with extinction rate Specialist species tend to have greater survival potential at times when extinction rates are low and reduced survival potential when extinction rates are high In addition, it has been suggested that small species have a greater chance of surviving mass extinctions than larger species, though the overall trend in evolution is clearly not towards smaller species The level of understanding of genetics is now so great that it is possible to explore macroevolution in this way In traditional views of macroevolution, a set of ways in which different forms could be produced with small changes in the genome was known as heterochrony The idea was that different parts of the body grew at different rates In some examples, this might be a difference in the rate at which sexual maturity was reached relative to the rate at which the rest of the organism (the somatic portion) developed If sexual reproduction became possible at an earlier stage in body development, this was known as pedomorphosis The classic living example of this is the axolotl This resembles a juvenile salamander, complete with external gills, but reproduces at this stage of development If it is injected with extract from the thyroid gland, an axolotl will develop into an adult salamander A genetic view of this kind of evolution is that there has been a change in the regulatory genes that switch on and off the protein-coding gene sequences within cells If these genes start to operate at new rates, then the phenotype will change shape, in some cases dramatically It is now known that some genes, especially a group known as Hox genes, control development by instructing the different parts of the growing embryo on which part of the body should be built It is known that these genes are more common in vertebrates than in other groups of animals and that there was a single period when these genes duplicated (or rather, duplicated twice), so that vertebrates carry four times as many of these genes as invertebrates This multiplication occurred between the evolution of the cephalochordates and proper vertebrates, probably during the Cambrian period It is tempting to assume that this evolutionary event facilitated the increase in complexity needed to produce vertebrates and may have made them more ‘evolvable’ since Whether or not cause and effect can be proved in this example, it points to a growing understanding of the relationship between genes and macroevolution See Also Biodiversity Biological Radiations and Speciation Famous Geologists: Darwin; Lyell Fossil Invertebrates: Trilobites Origin of Life Palaeozoic: Cambrian Precambrian: Eukaryote Fossils Further Reading Darwin C (1859) On the Origin of Species Penguin Books (edited by J W Burrow) Eldredge N and Gould SJ (1972) Punctuated equilibria: an alternative to phyletic gradualism In: Schopf TJ (ed.)