Evolution and Extinction

Evolution and Extinction 

Darwin’s Grand Idea 

As a young man in England in the early 19th century, Charles Darwin had been unable to settle on a career but had developed a strong interest in natural history. Therefore, he jumped at the opportunity to serve as a naturalist aboard HMS Beagle on an around-the-world surveying cruise. During the five years of the cruise, from 1831 to 1836, Darwin made detailed observations of plants, animals, and geology in the field and amassed an immense specimen collection from South America, Australia, and Africa. Just before Darwin departed on the voyage, a friend gave him a copy of Charles Lyell’s 1830 textbook, Principles of Geology, which argued in favour of James Hutton’s proposal that the Earth had a long history and that geologic time extended much further into the past than did human civilization.

A visit to the Galápagos Islands, off the coast of Ecuador,  led to a turning point in Darwin’s thinking during the voyage. The naturalist was most impressed with the variability of Galápagos finches. He marveled not only at the fact that different varieties of the bird occurred on different islands, but at how each variety had adapted to  utilize a particular food supply. With Lyell’s writings in mind, Darwin developed a hypothesis that the finches had begun as a single species but had branched into several different species when populations of the birds became isolated on different islands. This can happen because offspring can differ from their parents and new traits can be transferred to succeeding generations. If enough change accumulates over many generations, the living population ends up being so different from its distant ancestors that the population can be classified as a new species. During the course of evolution, old species vanish and new species appear. The accumulation of changes may eventually yield a population that is so different from its ancestors that taxonomists consider it to be a new genus greater differences may result in a new class, an order, or even a new phylum. Such change in a population over a succession of generations, due to the transfer of inheritable characteristics, is the process of evolution.

Darwin and his contemporary, Alfred Russel Wallace, not only proposed that evolution took place, but also came up with an explanation for why it occurs. The crux of the explanation is simply this: populations of organisms cannot increase in numbers forever, because they are limited by competition for scarce resources in the environment. In nature, only organisms capable of survival can pass on their characteristics to the next generation. In each new generation, some individuals have characteristics that make them more fit, whereas some have characteristics that make them less fit. The fitter organisms are more likely to survive and produce offspring. Thus, beneficial characteristics that they possess get passed on to the next generation. Darwin called this process natural selection, because it occurs on its own in nature. According to Darwin, when natural selection takes place over long periods of time geologic time it eventually yields a population of new organisms that differ so significantly from their distant ancestors that the new organisms can be considered to constitute a new species. If environmental conditions change, or if competitors enter the environment, species that do not evolve and become better adapted to survive the new conditions eventually die off. 

Darwin’s view of evolution has been successfully supported by many observations, and so far has not been definitively disproved by any observation or experiment. Also, it can be used to make testable predictions. Thus, scientists now refer to Darwin’s idea as the theory of evolution by natural selection. The theory of evolution provides a conceptual framework in which to understand paleontology. By studying fossils in sequences of strata, paleontologists are able to observe progressive changes in a species and in an assemblage of species through time, and can determine when some species die out and other species appear. But because of the incompleteness of the fossil record, many questions remain as to the rates at which evolution takes place during the course of geologic time. Originally, it was assumed that evolution happened at a constant, slow rate this concept is called gradualism. More recently, researchers have suggested that evolution takes place in fits and starts: it occurs very slowly for quite a while (the species are in equilibrium), and then, during a relatively short period, it takes place very rapidly. This concept is called punctuated equilibrium. Factors that could cause sudden pulses of evolution include:

1. a geologic catastrophe that causes many species to go extinct, leaving ecological niches open for new species to colonize; 
2. a sudden change in the Earth’s climate that puts stress on organisms, organisms that evolve to survive the new stress survive, whereas others become extinct; 
3. the sudden formation of new environments, as may happen when rifting splits apart a continent and generates a new ocean with new coastlines; and 
4. the isolation of a breeding population. 

Regardless of the process of evolution, the survivability of different kinds of organisms are not all the same. Some populations are very durable, in that they survive as an identifiable genus or even species for long intervals of geological time  (tens to over 100 million years). But others appear and then disappear within a relatively short interval of geologic time (less than a few million years).

Extinction: When Species Vanish 

Extinction occurs when the last members of a species die, so there are no parents to pass on their genetic traits to offspring. Some species become extinct as a population evolves into new species, whereas other species just vanish, leaving no hereditary offspring. These days, we take for granted that species become extinct, because we've seen a great number vanish during human history. Before the 1770s, however, few geologists thought that extinction occurred; they thought instead that fossils that didn't resemble known species must have living relatives somewhere on this planet. Considering that large parts of the Earth remained unexplored, this idea wasn't so far fetched. But by the end of the 18th century, it became clear that numerous fossil organisms did not have modern-day counterparts anywhere. The bones of mastodons and woolly mammoths, for example, were too different from those of elephants to be of the same species, but the animals were too big to hide. 20th-century studies led to the conclusion that many different phenomena can contribute to extinction.

Some of the geologic factors that cause extinction include:

• Global climate change: At times, the Earth’s mean temperature has been significantly colder than today’s, whereas at other times it has been much warmer. Because of a change in climate, an individual species may lose its habitat, and if it cannot adapt to the new habitat or migrate to stay with its old one, the species will disappear.
• Tectonic activity: Tectonic activity causes vertical movement of the crust over broad regions, changes in sea-floor spreading rates, and changes in the amount of volcanism. These phenomena can modify the distribution and area of habitats. Species that cannot adapt die off. 
• Asteroid or comet impact: Many geologists have concluded that impacts of large meteorites with the Earth have been catastrophic for life. A large impact would send dust and debris into the atmosphere that could blot out the Sun and plunge the Earth into darkness and cold. Such a change, though relatively short lived, could interrupt the food chain. 
• Voluminous volcanic eruption: Several times during Earth history, incredible quantities of lava have spilled out on the surface and/or incredible volumes of ash and gas have spewed into the air. These eruptions, perhaps due to the rise of superplumes in the mantle, were accompanied by the release of enough gas into the atmosphere to alter the climate. 
• The appearance of a new predator or competitor: Some extinctions may happen simply because a new predator appears on the scene. Researchers suggest that this phenomenon explains the mass extinction that occurred during the past 20,000 years, when a vast number of large mammal species vanished from North America. The timing of these extinctions appears to coincide with the appearance of the first humans (fierce predators) on the continent. If a more efficient competitor appears, the competitor steals an ecological niche from the weaker species, whose members can’t obtain enough food and thus die. 

Some extinctions happen over long time intervals, when the replacement rate of a population simply becomes lower than the mortality rate, but others happen suddenly, when a cataclysmic event leads to the rapid extermination of many organisms. For example, in 1870 the global population of passenger pigeons in North America exceeded 3 billion. Due to widespread hunting by people, the population dropped rapidly during the next two decades and the last member of the species died in 1914.

This graph shows how the diversity of life has changed with time. Sudden drops indicate periods when mass extinctions occurred.

Of note, paleontologists have found that the number of different genera of fossils, a representation of biodiversity (the overall variation of life), changes over time and has abruptly decreased at specific times during Earth history. A worldwide abrupt decrease in the number of fossil genera is called a mass extinction event. At least five major mass extinction events have happened during the past half-billion years (figure above). These events define the boundaries between some of the major intervals into which geologists divide time. For example, a major extinction event marks the end of the Cretaceous Period, 65 million years ago. During this event, all dinosaur species (with the exception of their modified descendants, the birds) vanished, along with most marine invertebrate species. A huge extinction event also brought the Permian to a close. Some researchers have suggested that extinction events are periodic, but this idea remains controversial.

Credits: Stephen Marshak (Essentials of Geology)

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