This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we examine how symbiotic relationships can result in parallel speciation events.
In the last post in this series, we introduced the concept of coevolution: reciprocal interactions between two species that act as major evolutionary influences on each other. In some cases, two species act not only as the primary influence on the other, but do so in a close, long-term relationship. Such relationships are examples of a symbiosis (literally, “living together”). As one might expect, symbiotic relationships are excellent places to explore coevolution. In fact, the closeness of a symbiotic relationship can lead to what is known as cospeciation – the simultaneous speciation of two species in tandem as a consequence of their close association.
Parasitism and cospeciation
One (rather unpleasant) type of symbiotic relationship is the one found between parasites and their hosts. Some parasites are obligate parasites, meaning that they are restricted to only one host species and depend on it for their continued survival and reproduction. In this case, the host species effectively constitutes the environment for the parasite species. Given the close association of the two species, it is not surprising that in many cases we can find evidence that parasites cospeciate with their host species. As the host species undergoes speciation (starting with reproductive isolation between two populations, as we have discussed previously) an obligate parasite will also be divided into two genetically isolated populations. If, over time, the host species divides into fully separate species (making the genetic isolation permanent), then it is likely that the parasite populations will also diverge from one another enough to be recognized as distinct species. In cases of cospeciation, the phylogenies of the host species and the parasite species match each other, with identical divergence times:
One example of evidence for host-parasite cospeciation comes from studies of primate lice. Lice (singular, louse) are obligate parasites that feed on the blood of their hosts, and many primate species have a unique species of body lice (including humans). For humans, chimpanzees and gorillas, the phylogeny of their lice matches the primate phylogeny, with the closest louse relatives found on the closest primate relatives:
Beyond the matching pattern of speciation, the divergence times for human and chimpanzee body lice agrees with the speciation times for their hosts. Human and chimpanzee body lice separated approximately 5.6 million years ago, which falls within the range of times estimated for the human / chimpanzee divergence (4.5 – 6.0 million years ago), though this estimate is based on a very limited sample of gene sequences from the louse species.
In contrast, the split between the common ancestral population of the two Pendiculus species and Pthirus (~11.5 million years ago) is not as well matched to the corresponding primate speciation event (the divergence of the gorilla lineage from the lineage leading to the common ancestral population of humans and chimpanzees at 6.0 – 8.0 million years ago). The evidence thus suggests a straightforward cospeciation event for Pendiculus humanus and Pendiculus schaeffi, but a more complex history for Pthirus. One possibility (aside from the observation that this date is also based on a limited data set) is that the louse and primate divergence times are correct, with the louse event occurring prior to the primate event. One way for this to happen is for the parasite to maintain separate populations within the host population for a long period, perhaps facilitated by switching between closely related hosts. Interestingly enough, there is evidence for such an effect within present-day populations of Pendiculus humanus – there are populations of the parasite that appear to have been separate for longer than our species has existed. An ancient lineage of Pendiculus humanus found only in the New World (i.e. North and South America) diverged from Old World Pendiculus humanus about 1 million years ago and arrived with humans migrating over the Bering land bridge from Asia. This ancient split between Pendiculus humanuslineages predates the arrival of our own species in the fossil record at about 200,000 years ago. One hypothesis for how it is that we carry such an ancient lineage of an obligate parasite is that some human populations acquired it from a related hominid lineage in the Old World prior to migrating to North America – an example of host switching. This hypothesis recently received support from genomics evidence indicating interbreedingbetween some human populations and other hominid groups (Neanderthals and Denisovans), which would provide the necessary close contact for a host switch, especially if the Neanderthal and/or Denisovan lineages were previously in contact with even older groups such as Homo erectus. It will be interesting to see if this hypothesis continues to be supported as more genetic data from human louse populations is gathered and analyzed.
Mutualism and cospeciation
In contrast to parasitism, mutually beneficial symbiotic relationships can arise between species. One example of this effect (known as mutualism) is the striking relationship between some plants and their pollinating insects. Some plants and insects have a reciprocally obligate relationship: the insect is the sole pollinator for the plants, and the plant is the sole food source for the developing insect larvae. In this case the relationship is beneficial to both species – the plant receives the benefit of having a highly efficient pollinator, and the insect receives a food source tailored to its larvae.
One widespread group of plants that uses obligate insect pollinators are the numerous species of fig trees. Almost every fig tree species (of the hundreds worldwide) hosts a separate species of fig wasp that acts as its obligate pollinator and in turn is restricted to its fig tree host for its larval development. Fig wasps need to develop inside a developing fig fruit, and fig trees need their wasp species to pollinate them. Not surprisingly, this mutualistic, symbiotic relationship is also a recipe for cospeciation – recent analysis of over 200 fig / wasp pairs (of the over 750 species pairs known worldwide) strongly supports cospeciation for the vast majority, with only limited host switching between closely related groups. For these wasps, the widespread diversification of their host trees has been a major force in their evolution.
In the next post in this series, we’ll explore an even deeper level of symbiosis –endosymbiosis, where one species lives within the other – and examine the evidence that modern-day mitochondria and chloroplasts are descended from free-living prokaryotes.