Part Three: Mass Extinctions and the Geological Record
To continue with the theme I have set up amassed from my exam revision notes from university (something I would like to stress – the quality of writing is not very high in these posts!), I would like to ask the question: are mass extinctions vital to the evolution of complex life?
How do we view complex evolution: 1) evolution of the animals and plants that we see today or 2) the development of a multi-celled organism to suit its habitat to the perfect extent?
If it is the former, it is taking the stance that everything has happened to come to some sort of culmination in present day fauna. It could also imply that then yes, there has been more and more complex evolution even thought there have been mass extinctions. Therefore, yes, they do contribute to the evolution of complex life. However, I would like to think of evolution in the latter sense, and would like to discuss here the implications of mass extinctions and evolution of complexity with regards to an organism best suiting its habitat.
In recovery from mass extinctions there are several variables that decide how life is affected, especially post the evolution of complex organisms in the last 550Myr. There are various kinds of mass extinctions i.e. pulsed and final extinctions (climatic and dramatic – meteor, large igneous province eruption). These give dramatically different results to the kinds of evolutionary filtering seen.
In the case of pulsed climatic extinction (such as Ordovician and Cretaceous) extinction event filtered out those that were immediately not able to adapt, however with periods of 500,000 years in between pulse events it is possible that some will be able to adapt in this time and create complex ecosystems. Therefore there is possibly increased evolution of complexity within a short amount of time. However, it does not mean to say that the organisms who died off were not capable of possible increased complexity if the mass extinction never occurred. Therefore it comes down to the fact that although mass extinctions could help certain organisms evolve, it is probably not a pre-requisite for the evolutionary process.
In saying that, maybe in this kind of event it will select the organisms that were genetically prone to adapt quicker to certain environments and they could progress quicker, becoming specialised. Those that would not have had the capabilities in the first place would be wiped out selectively. In this case, other kinds of dramatic mass extinctions (say, a massive eruption or an impact event) would still reset the clock and previous selected organisms might die and leave once again ‘generalised’ organisms. Generalised organisms that are not fully evolved to their own specific niche (that have not evolved specific and complex habits to use in this environment) would probably have less of a chance at surviving a mass extinction.
At the end of a mass extinction there is usually an opportunistic species that pops up out of nowhere and tries to take over niches, such as in the Triassic there was a burst of stromatolites. During the diversification and recovery phase entirely new species can pop up that are extremely environment specific, and then decline. At the end of the Ordovician global ice age the melting of the glaciers fed anoxic waters into the oceans and this killed off highly speciated organisms that had adapted to the current environment, hence adding a second pulse of extinction to the first.
In the Silurian there was limited species numbers but high abundances, and rising sea levels expanded the shallow areas of continental shelf available for colonisation. The taxa followed different evolutionary trajectories, depending on their evolutionary and ecological characteristics. Conodonts lived in the seas during the glaciation, and then migrated onto land with sea level fall where they were able to diversify. Despite the magnitude of the extinction, the ecological structure of the Ordovician was relatively unchanged in the Silurian with little biological innovations present. This demonstrates that the event of the mass extinction does not mean massive new innovations and increases in complexity of life. One thing that is puzzling is the massive increase after the Permo-Triassic extinction event, maybe because it was such a large event wiping out over 90% of all marine life it left so many gaps that life could almost start again and so many more species evolved. But it has been seen that many, as much as 30% of organisms, came back to life such as gastropods. There was a very long time before these Lazarus taxa came back after the event, which may have made many niches available to other new species within that time, and then the recurrence of the large percent of Lazarus taxa increased this exponentially.
Extinction terminates lineages and removes unique genetic variations and adaptations. But on the scale of evolutionary timescales it can totally reshape the evolutionary landscape in creative ways. The differential survivorship of lineages and evolutionary opportunities given by mass extinctions can also be a major stepping stone in genetic speciation. Mass extinctions produce non-constructive selectivity, selecting any organism whether it be locally abundant, diverse or well adapted. This means that in general, organisms which are geographically widespread have the advantage in the case of mass extinctions and these do not include ones which may be particularly opportunistic or highly generalised species. Mass extinctions cannot contribute to complicated ecosystems, even if they contribute to complex life, because in wiping out the more selectively adapted organisms that rely on maybe one specific other organism for life then this can destroy the ecosystem. For example, certain insects that can only pollinate one flower that can only grow on one tree with one animal that can disperse the seeds etc.
Although extinction does not by itself provide any creative contribution to evolution of complex structures i.e. wings or limbs it is definitely decisive in the sustaining or eliminating of these structures. Of course, more ecosystems will evolve and possibly new and creative ones that may eventually be more complex that previous ones, but the next mass extinction will likely destroy them. It is most likely, that even if certain complex life is spared in an extinction, the ecosystems that it relied upon and relied upon it are gone. The exception to this is humanity, as we are complex but do not complete a chain of complete reliability, but we have yet to live through a mass extinction to prove this, and who knows, we might bring it upon ourselves.
Sometimes, there is constructive evolution, such as the mass extinction that would have occurred at the end Proterozoic, where cyanobacteria started producing oxygen. With eventual release of free oxygen in the atmosphere and the harsh decline of organic molecules to feed on for the originally anaerobic bacteria there would have been one of the most profound extinctions. This however is most definitely one that caused significant development of the evolution of complexity, as larger and more energy-efficient organisms were then able to evolve. But, even if there was creation of new complexity, there was still destruction of complex ecosystems that may have already evolved. For other extinctions there may be similar benefits, but I think it is far too early to know.
Life has found a way to become increasingly complex, regardless of the fact that mass extinctions happen. However one thing I think that can safely be said is that mass extinctions were not crucial to this happening. If anything they have inhibited what could have been in the past, but it is not for better or for worse. It has changed the Earth completely from what it could have been and is vital in understanding Earth and life history as we know it, but not necessarily vital in creating in the first place.
Dawkins R, (2010), The Ancestor’s Tale, Hachette UK, Available online: http://books.google.co.uk/books?id=Scxg1Pu3dOsC&source=gbs_navlinks_s
York, R and Clark B, (2011), Stephen Jay Gould’s Critique of Progress, Monthly Review, 62(9), Available online: http://monthlyreview.org/2011/02/01/stephen-jay-goulds-critique-of-progress