Table of Contents

Question

After a mass extinction, how can the life that survives reach the same level of biological diversity as it was before?

Initial Model

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Claim

Even if there is a mass extinction, life will eventually build back up to as complex of a diversity as before. The new species will arise from the survivors of the extinction as mutations and natural selection lead to niche-filling and evolution over time.

Evidence


Several mass extinction events have occured on the earth over the course of 4.5 billion years...five to be exact. And each time, life rebounds to the same level of diversity,
though not necessarily the same kind of life. But how does this rebounding process occur? Consider the largest wipeout in Earth's history, the Permian-Triassic extinction event. 90% of all life was wiped out (1) but life eventually came back. The 10% of the surviving organisms now had unlimited space to reproduce and live. These "disaster taxa" species (1) were able to fill their niche (and the empty niches) without much competition or predators and therefore could flourish and reproduce at an extensive rate. But with gene replication for reproduction comes mutations...and mutations are the basis of variety and evolution. Through the process of evolution over time, other vacant niches can be filled. Species descened from mutants of a common ancestor can fill the niches they are best suited for. When all the niches are filled again, life is back to it's previous level of diversity, even though most of the genes and mutations of the Earth's previous life will never be seen again (2).

Here's a hypothetical example concerning niche-filling: Let's say there's a small population of rodent-resembling mammals (after the extinction of the dinosaurs). They have short tails, short legs, and are best suited for scavenging and eating carrion (like I said, this is hypothetical). Over time, a gene destined for a gamate is mutated during gene replication. This gamate gets fertilized and the offspring survives because the mutation isn't deadly - in fact, it's beneficial. Not only can the mutant eat carrion, it can hunt for other organisms (like insects) because of it's longer limbs and tail. Because the mutant has a wider variety of food it can eat, it survives better than the original rodent-resembling mammals. It can fill the niche of "insect-hunter" - a totally different niche than "scavenger." Say this mutant is incredibly successful: it reproduces and fills the new niche and becomes a new species. Eventually, with the processes of mutation, reproduction, natural selection, and evolution repeated, new species will stem from the new rodent and fill other niches.

After the Permian-Triassic event, life rebounded rather quickly, even if it was only for a short time (3). A real life example of niche-filling and diversifying comes in with the rise of the Lystrosaurus. With little to no competition, these oraganisms reproduced at an astounding rate and filled their own niche, as well as others. Through the same processes as the hypothetical rodent-like mammal, new species arose from the Lystrosaurus and filled other vacant niches (3). And after the extinction of the dinosaurs (the Cretaceos-Tertiary mass extinction event) niches further opened up for not only mammals but for insects and small reptiles. Though the extinction of the dinosaurs isn't completely attributed to the rise of the mammals (4), their disappearance left niches wide open. Nature abhors a vacuum.

Consider two more recent, much smaller extinctions: the beginning and ending of the Pleostocine Ice Age. This era is generally associated with the rise and fall of the woolly mammoth. The woolly mammoth, descending from the steppe mammoths (who in turn came from an ancestral mammoth who had split from a species of African-dwelling elephant-like animals (5)), came into full existence as their mutant traits were much better suited for the harsh, cold weather of the Ice Age north than the other mammoths unused to such conditions. After the previous mammoths died out of the north, woolly mammoths filled a niche as top herbivore. They spread
throughout the northern hemisphere and began to diversify as mutations and evolution occured. One of the best examples of woolly mammoth diversification is the dwarf mammoths (such as the ones on Wrangel Island, Russia) (6, 7). Their size mutations allowed them to better fit small islands and isolated places.
When the Ice Age ended, however, all the advantages of the woolly mammoth became useless. Though the absolute causes of the woolly mammoth's extinction are still unsure, it is clear that a warmer environment was detrimental to the woolly mammoths. Natural selection worked against them, and a niche was then left open.

Lystrosaurus_BW.jpg early_mammals.jpg woolly_mammoth.jpg
Lystrosaurus, Early Mammals, and Woolly Mammoth

Sources:
(1) http://www.eurekalert.org/pub_releases/2008-01/uob-rfa011808.php
(2) http://www.physics.ohio-state.edu/~wilkins/energy/Companion/E16.8.pdf.xpdf
(3) http://rspb.royalsocietypublishing.org/content/275/1636/759.full
(4) http://www.livescience.com/7230-rise-modern-mammals-occurred-long-dinosaur-demise.html
(5) http://news.nationalgeographic.com/news/2001/11/1101_WoolyMammoth_2.html
(6) http://www.crystalinks.com/woollymammoth.html
(7) http://www.donsmaps.com/bcmammoth.html
(8) google images...

Final Model


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Final Claim

Based on my evidence, even if there is a mass extinction, life will eventually build back up to as complex of a diversity as before. The new species will arise from the survivors of the extinction as mutations and natural selection lead to niche-filling and evolution over time.

Reasoning


Life today is incredibly diverse (even though species are becoming extinct every day), and only 65 million years ago, a mass extinction took place (the "dinosaur wipeout," in layman's terms). When a bunch of species are killed, niches are left open in the ecosytem. Even though the environment has changed (one of the reasons for extinction), the surviving organisms must have already had adaptations (resulting in previous mutations) that allowed them to succeed in the new environment. With
almost no competition or predators and a wide space to spread out, the survivors can quickly reproduce and fill their niche (along with others). And like any other species, organisms with a beneficial mutated trait in their genome will be born, given plenty of time. Advantageous mutations give the mutant, well, an advantage over the others of its species. But because there are so many niches to fill after an extinction, the new species (if there are enough mutants over time and if natural selection
is favorable to them) doesn't neccessarily have to replace the old one but can fill a new niche. In this manner, through mutations, natural selection, niche-filling, and evolution over time, life can rebound back to as complex of a level of diversity as it was before the mass extinction.