Charles Darwin (1809-1882) assumed that species have gone extinct in a steady and progressive way through the history of life; that is, over time. The diversity of life is reflected in the large number of present-day species, but a much larger number has existed over time, owing to the fact that extinction is the ultimate fate of all species; 99% of all species that have ever lived on Earth are estimated to have become extinct since life originated, about 3.5 billion years ago. A species’ duration on Earth is very short on the geologic timescale; normally, a species becomes extinct within 10 million years after its origin. Thus, it is now widely accepted that species become extinct, but this fact was not known 200 years ago. It was previously thought that species were fixed in form, until Jean-Baptiste de Lamarck established that species change over time into new species. However, Lamarck assumed that lineages of species continue indefinitely without branching, so they did not go extinct.
Georges Cuvier (1769-1832) is accredited as the first to establish the extinction of species. He studied the fossils from large terrestrial mammals that had never been observed alive and realized that such fossils must not be overlooked. He considered that each species had had a different origin, and then it stayed stable in its form. Successive catastrophic events then drove these species extinct.
The principle of uniformitarianism, developed by the geologist Charles Lyell in his Principles of Geology (1830-1833), was essential for Darwin. It is simply that the basic physical processes involved in the present-day structure of the Earth are assumed to have been the same everywhere and at all times. In the pivotal work On the Origin of Species (1859), Darwin considered that the modern variety of species was the result over time of the process of descent with modification. Thus, the successive species are linked to each other by a branching genealogy, the tree of life. This tree-like evolution, where species disappear gradually rather than as a result of catastrophic events, allows for extinction. Darwin assumed that due to competition with other species, the causes of the extinction of species were mostly biological. However, there were a series of peak times when extinction rates appeared to be exceptionally high, now called mass extinctions, but Darwin attributed them to gaps in the fossil record rather than real catastrophic events. Today, the absolute dating of rocks by radioisotope methods has ruled that possibility out. Thus, Darwin held that extinction is coupled with the process of natural selection and is, therefore, a main element of organic evolution.
The Fossil Record: Evidence for Extinction
The fossil record is the main source of information on the history of life and it provides strong evidence for evolution and, therefore, for extinction. However, the fossil record is neither complete nor perfect. It contains merely a small part of all the species that have ever existed and not in a representative way. The proportion of species known from the fossil record is about 0.02%; in other words, about 250,000 fossil species. The fossil record is biased in favor of mineralized hard-skeleton invertebrate species, long life span species, and geographically and ecologically widespread distributed species. Foraminifera, single-celled eukaryote marine microorganisms, are one of the best groups to evidence organic evolution, as they show a widespread and almost continuous record with intermediate forms between species. But even with the best-recorded species, the exact time of extinction of a species is impossible to determine. The last appearance of a species in the fossil record usually happens before the time of extinction as a consequence of the lack of completeness in the fossil record. The misinterpretation of the fossil record may lead to what is called pseudo-extinctions. It means that a species (or higher taxon) apparently becomes extinct but it reappears in younger rocks. It may be due to taxonomic artifacts or to the incompleteness of the fossil record.
The divergence of species is usually based on physical expressed characteristics, that is, morphological and behavioral characteristics, but the latter are generally not observed in fossil species. Thus, two species that recently evolved from a common species may blur into each other, as would be expected in an evolutionary process. Furthermore, the extreme forms of an evolving lineage may seem sufficiently different that they are identified as different species, even more if due to the incompleteness of the fossil record when there are only few specimens recorded from the extremes so the evolving lineage cannot be detected. A second kind of pseudo-extinction due to taxonomic artifacts can occur in situations involving higher taxa above the species level. A paraphyletic group (an artificial group of taxa comprising a common ancestor but not all of its descendants) could become extinct although some descendants of that group continue to exist. For instance, the birds are descendants of one dinosaur group, which became extinct, but birds survived that extinction.
The so-called Lazarus effect is a kind of pseudo-extinction due to the incompleteness of the fossil record. A Lazarus species is one that disappears temporarily from the fossil record, but it can be inferred to have existed during that time by its reappearance in younger rock strata. However, its disappearance would be a pseudoextinction if its later occurrence is overlooked. A mass extinction can seem artificially more abrupt, gradual, or stepped than it actually was because of the Signor-Lipps effect. Sampling gaps in the fossil record, through an interval of time before a mass extinction, may lead to the scattering of the last occurrence of taxa prior to their actual extinction.
The fossil record has been used to estimate the change of the diversity of life and therefore of the extinction rate through the history of life, compiling the time distributions of taxa in the fossil record. In the 19th century, the geologist John Philips, after plotting diversity against time, was able to recognize the great faunal transitions that mark the boundary between the Paleozoic, Mesozoic, and Cenozoic eras, that is, the major extinction events now called the end-Permian and Cretaceous-Tertiary (K-T) mass extinctions. But the most widely used compilation is that developed by Jack Sepkoski and published in a series of papers beginning in 1981. Three main conclusions are inferred from it: The average extinction rate seems to decrease from the Cambrian period (about 500 million years ago) to the present; there are five time periods of particularly high extinction rate, which have become known as the Big Five mass extinctions; and a periodic pattern of peaks in extinction rate occurs every 26.2 million years.
Extinction Over Time: Background Extinctions and Mass Extinctions
Extinctions have occurred throughout the history of life as a consequence of the branching structure of evolution. These extinctions are called background extinctions. The decline of the average extinction rate from the Cambrian period to the present is a controversial subject, because it may be an artifact caused by the way the extinction rate is measured. Diversity in the Cambrian is known to be lower than in the Mesozoic and Cenozoic eras. Consequently, for a given extinction rate, fewer taxa needed to go extinct. As a result of the branching structure of evolution, the numbers of species per genus and per family (higher taxa) have increased over time. Thus, fewer genera and families will go extinct through time for a given number of species going extinct. But the decrease in extinction rate may be real. Taxa may have become more resistant to extinction, perhaps suggesting progressive adaptive advances. Species may have initially occupied central niches (the position of a species within its environment and community, i.e., the habitats it occupies and the resources it consumes) that are subject to more intense competition than are marginal niches. Thus, a higher turnover of occupying species occurs. Over time, more marginal niches are occupied where the species may last longer and, consequently, have lower extinction rates.
The five major mass extinctions occurred at or near the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. The Permian mass extinction is the biggest in the history of life, with about 95% of the species going extinct. In the K-T mass extinction, where dinosaurs and ammonites (coiled-chambered shell cephalopods similar to the pearly nautilus) were driven finally extinct, at least half (and perhaps 75%) of species suffered extinction. Even though it is clear that the extinction rate was extremely high through mass extinctions, it is not clear whether mass extinctions are truly different from background extinctions. Thus, mass extinctions could just be the intensification of background extinctions, fully random or selective. In the last case, the most plausible idea, they would follow different rules than would background extinctions, which are responsible for the extinction of the vast majority of species.
It has been estimated that there was a periodic pattern of peaks in extinction rate every 26.2 million years during the Mesozoic and Cenozoic eras. However, not all predicted peaks coincide with known extinction events in the fossil record, and there was a lack of those peaks in the Paleozoic. Recently, another periodic pattern has been calculated every 27 million years, which clearly matches with the 26.2-million-year periodicity.
The combination of Darwin’s theory of evolution by natural selection and Mendel’s genetic theories constitute the modern synthesis, or neo-Darwin- ism. Species usually have excess fecundity so they generate many more offspring than can survive. This implies a competition within every species to survive, as the genes in the population of offspring are a random sample of the genes present in the parental population. The better-adapted forms to the environmental conditions, and thus the variants that improve survival or reproductive success, will increase in frequency and, eventually, a new species will be originated. Darwin considered that competition between species is the main cause for extinction, and thus all extinctions are selective. It is obvious that extinction does not affect all species in the same way, but more factors than only competition between species are implied. Species with large body size, ecological specialism traits, low reproductive rate, and slow growth are more likely to suffer extinction. Species with large body size are usually characterized by small populations and low reproductive rates; that is, they are rare. Species with broad geographic ranges are much less prone to extinction than species with smaller ranges, but not when extinction is so severe as during mass extinctions. In this case, survival depends on the geographic distribution of the entire group but not of the individual species. Tropical taxa usually suffer much more extinction than high-latitude taxa, but there is also no difference during mass extinctions. Species-rich taxa have lower extinction rates. Not all taxonomic groups show the same mean duration of their species, so those with shorter duration have higher extinction rates.
Taxonomic selectivity is not common, but occasionally it is prominent. Such is the case of the extinction of the trilobites (arthropods gone extinct at the Permian mass extinction), dinosaurs, and ammonites at the Mesozoic mass extinction. This evidence—that taxonomic selectivity may operate at higher levels than the species level—makes it possible that diverse higher taxa have become extinct. Therefore, mass extinctions can drive taxa extinct not because they are poorly adapted to normal environmental conditions, but because they happen to lack adaptations to the new conditions or characteristics, such as lacking extensive geographic ranges that favor survival during the extinction. These contrasts in selectivity between mass extinctions and background extinctions evidence the evolutionary importance of mass extinctions. Even though they account for less than 5% of the total number of species that have become extinct through the history of life, they can drive to extinction well- adapted and successful taxa. This makes sense in terms of evolution, as natural selection cannot ensure that its taxa will survive sudden abrupt environmental shifts. The species selection pattern can even shed light onto the nature of the mass extinction event.
Understanding the causes of the mass extinctions has become an important and controversial issue since the proposal of a giant meteorite impact as the main or triggering cause for the K-T boundary mass extinction. Other common abiotic causes of major extinctions are climate change, volcanism, sea-level changes, anoxia (low-oxygen conditions), and shifting continental position. However, the evolutionary consequences of mass extinctions are probably more important than the events themselves.
Evolutionary Consequences of Mass Extinctions
The evolutionary process following mass extinctions is a key to the postextinction role of the diversity of life as new evolutionary opportunities are created by the demise of dominant groups. After the dinosaurs became extinct at the K-T mass extinction, the mammals radiated rapidly and occupied the ecological space vacated by dinosaurs. Mass extinctions triggered the diversification of survivors, not only in terms of numbers of species but also in terms of morphological or ecological variety. However, the fossil record shows that not all survivors radiate after mass extinctions, and therefore survival alone does not guarantee evolutionary success. The diversity of survivor groups can follow different ways over time after a mass extinction. Some groups accelerate diversification following the extinction event, as is the case with mammals after the K-T mass extinction. Others groups suffer a delay just after the extinction event. Some of them rapidly recover and continued their diversification, but others never really recover from the mass extinction. The latter usually disappear several million years after the mass extinction, although some of them manage to survive until the present day, and they are often called living fossils (e.g., the pearly nautilus). Immediately after the mass extinction event, severe environmental conditions persist, and they can even prompt more extinctions and delays in species recovery. Mass extinctions, which are especially severe among rare and restricted distributed groups, tend to homogenize the postextinction fauna. Low-diversity assemblages dominated by generalist species characterize these intervals. Generalist species are often small in size, and under normal conditions they are rare, with very few populations. Even though mass extinctions have provided major evolutionary opportunities, they did not trigger such a big diversification of life as the one at the beginning of the Paleozoic era, the Cambrian explosion, when most of the major groups of animals appeared for the first time, as evidenced in the fossil record.
The recovery process after a mass extinction can show considerable variety all over the world. Furthermore, recovery processes are different for each mass extinction as different taxa and different ecological, climatic, and oceanographic settings are involved. Thus, the speed of recovery may differ between mass extinctions, but the fossil record shows that it always takes a long time to recover, as much as 10 million years.
Today, the extraordinary proliferation of just one species, our own, is triggering what is considered to be the sixth mass extinction. It may be viewed as not natural, but the fossil record provides strong evidence for mass extinctions throughout the history of life. Moreover, the fossil record provides evidence of recovery after all of them. Apparently, we should not be worried about a sixth mass extinction of all life on Earth due to the misguided actions of our own species. However, the fossil record also shows us that the evolutionary recovery of biodiversity is extremely slow compared to a human lifetime and a long time compared to the total life span of our species so far. Impoverishment is the legacy of extinction on human timescales, but extinction also creates new evolutionary opportunities. Even so, survivors of mass extinction are not always the best adapted, just the luckiest. Of course, not all survivors are going to be successful in terms of ongoing evolution.
See also Dinosaurs; Extinction; Extinction and Evolution; Extinctions, Mass; K-T Boundary
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Raup, D. M. (1995). The role of extinction in evolution. In W. M. Fitch & F. J. Ayala (Eds.), Tempo and mode in evolution: Genetics and paleontology 50 years after Simpson. Washington, DC: National Academies of Sciences.
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