The Ecology Book Page 3
Darwin’s field work on the islands of the Galapagos archipelago off South America in the fall of 1835 provided especially strong evidence for his later theory of evolution by natural selection. Here, he observed that the shape of the carapaces (shells) of giant tortoises varied slightly from island to island. Darwin was also intrigued to find that there were four broadly similar, yet clearly distinct, varieties of mockingbirds, but that no single island had more than one species of the bird. He saw small birds, too, that looked alike but had a range of beak sizes and shapes. Darwin deduced that each group possessed a common ancestor but had developed diverse traits in different environments.
CHARLES DARWIN
Born in Shropshire, UK, in 1809, Darwin was fascinated by natural history from a young age. While at Cambridge University, he became friendly with several influential naturalists, including John Stevens Henslow. As a result, Darwin was invited to join the HMS Beagle expedition around the world. Henslow helped Darwin catalog and publicize his finds.
Darwin’s research brought him fame and recognition—the Royal Society’s Royal Medal in 1853, and fellowship of the Linnean Society in 1854. In 1859, his book On the Origin of Species sold out instantly. Despite continuing ill-health, Darwin fathered 10 children and never stopped studying and developing new theories. He died in 1882.
Key works
1839 Zoology of the Voyage of HMS Beagle
1859 On the Origin of Species by Means of Natural Selection
1868 The Variation of Animals and Plants under Domestication
1872 The Expression of Emotions in Man and Animals
Darwin’s conclusions
On Darwin’s return to England, the differing beaks of the small birds he had found on the Galapagos, usually called “finches” although they are not in the true finch family, set him thinking. He knew that a bird’s beak is its key tool for feeding, so its length and shape offer clues to its diet. Later research revealed that there are 14 different finch species on the Galapagos islands. The differences in their beaks are marked and significant. For example, cactus finches have long, pointed beaks that are ideal for picking seeds out of cactus fruits, while ground finches have shorter, stouter beaks that are better suited for eating large seeds on the ground. Warbler finches have slender, sharp beaks, which are ideal for catching flying insects.
Darwin speculated that the finches were descended from a common ancestral finch that had reached the archipelago from the mainland of South America. He concluded that a variety of finch populations had evolved in different Galapagos habitats, each group adapted for a more or less specialist diet by a process that he would later call “natural selection.” Over time, the finch populations had become distinct species.
In the early 21st century, researchers at Harvard University uncovered new evidence of how this happens at a genetic level. Their findings, published in 2006, showed that a molecule called calmodulin regulates the genes involved in shaping birds’ beaks, and is found at higher levels in longer-beaked cactus finches than in shorter-beaked ground finches.
Refining the theory
Darwin was influenced by Thomas Malthus’s An Essay on the Principle of Population (1798), in which Malthus predicted that population growth would eventually outstrip food production. This idea matched the evidence Darwin had observed of ongoing competition between individual animals and species for resources. This competitive aspect formed the backbone of Darwin’s coalescing theory of evolution.
By 1839, Darwin had developed an idea of evolution by natural selection. He was, though, reluctant to publish because he understood that the theory would unleash a storm of controversy from those who would view it as an attack on religion and the Church. When, in 1857, he began receiving communications from fellow British naturalist Alfred Russel Wallace, who had independently arrived at very similar conclusions, Darwin realized he had to publish his ideas. Papers by Darwin and Wallace were jointly presented at a meeting of the Linnean Society of London in July 1858, under the title “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.”
The following year, Darwin published the theory in On the Origin of Species by Means of Natural Selection. It offended some scientists because it differed from Lamarck’s ideas of transmutation, and also upset creationists who argued that it undermined a literal interpretation of the Bible. Others felt that the theory did not account for the huge range of characteristics in species and called it “unguided” and “nonprogressive.”
Darwin was confident. He knew that all individual organisms in a species show a degree of natural variation; some have longer whiskers, or shorter legs, or brighter colors, for instance. Because members of all species compete for limited resources, he deduced that those whose traits are best suited to their environment are more likely to survive and reproduce. He also argued that characteristics that helped an individual organism live longer and reproduce more successfully would be passed on to more offspring, while those that made the organism less successful would be lost. Darwin called this “natural selection”—a process that, over generations, enabled a population of any given species to adapt better and thrive in its chosen habitat.
“I see no good reasons why the views given in this volume should shock the religious views of anyone.”
Charles Darwin
Sexual selection
Darwin also developed a theory of sexual selection. First outlined in On the Origin of Species, this was developed further in The Descent of Man, and Selection in Relation to Sex (1871). This theory was distinct from natural selection, as Darwin recognized that animals select mates based on characteristics that do not simply favor survival. For example, when Darwin considered the spectacular but cumbersome tails of male peafowl (peacocks), he could not imagine the tail playing any role in helping the individual bird to survive. He concluded that they were designed to boost an individual’s chance of reproductive success. Peahens choose males with the brightest tails, so the genetic material of these showy males is passed to the next generation. Bright tail feathers indicate that the bird is healthy, so choosing a mate with a bright tail is a good strategy for the peahen. However, Darwin’s idea that females choose a mate came under fire; 19th-century society could accept that males competed to reproduce (intrasexual selection), but intersexual selection, where one sex (usually the female) makes the choice, was ridiculed.
Reproductive success is clearly essential for the future of a species. Natural selection is often described as “survival of the fittest,” but longevity alone is not particularly helpful. If individual A lives 10 times as long as individual B, but the latter produces twice as many offspring that then also breed, B will pass on more genes to the next generation than the longer-lived A.
The peacock with the most splendid tail will attract the most peahens. Its bright tail will be passed on to its male offspring, which will find it similarly easy to attract mates.
Building on the theory
Many of Darwin’s and Wallace’s ideas have proved remarkably accurate, despite the fact that the workings of genetics were not understood at the time. Although Darwin himself had used the term “genetic” as an adjective to describe the as-yet-unknown mechanism of inheritance, it was British biologist William Bateson, in the early 20th century, who first used the term “genetics” in a description of the scientific process. In 1930, British geneticist Ronald Fisher wrote The Genetical Theory of Natural Selection, which combined Darwin’s theory of natural selection with the ideas of heredity that the 19th-century Austrian scientist Gregor Mendel had developed. In 1937, Ukrainian–American geneticist Theodosius Dobzhansky put forward the idea that regularly occurring genetic mutations are sufficient to provide the genetic diversity—and therefore different traits—that makes natural selection possible. He wrote that evolution was a change in the frequency of an “allele” in the gene pool, an allele being one of the alternative forms of a gene that arise by mutation.<
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A mutation is a permanent alteration in the sequence of deoxyribonucleic acid (DNA), the molecule that makes up a gene in one individual, resulting in a sequence that differs from that of other members of the species. Mutations may occur as the result of the miscopying of DNA during cell division, or they may be caused by environmental factors, such as damage resulting from the sun’s ultraviolet radiation. One mutation might affect only the individual organism carrying it, whereas another might affect all its offspring and future generations.
Inherited mutations may or may not alter an individual’s phenotype – its physical traits and behavior. If mutations do affect the phenotype, they may be to its advantage or disadvantage, helping or hindering an organism’s ability to survive and reproduce successfully. If they hinder, they are likely to disappear from the population; if they help an organism adapt better to its environment, they become more common over the course of generations. Over time, they may produce large enough divergences from the parent population for a new species to evolve—a process called speciation.
Mutation rates are usually very low, but the process is ever-present. The changes may be beneficial, neutral, or harmful. They do not occur in response to an organism’s needs, and are, in that respect, random. However, some types of mutations occur more frequently than others. Scientists now know, for example, that evolution can take place very rapidly in bacteria because of their frequent mutations.
Albinism, as in this albino leopard gecko, is a mutation causing a lack of pigment. This mutation hinders the gecko’s chances of survival, making it lighter colored and sensitive to light.
“Why do some die and some live?… the answer was clearly, that on the whole the best fitted live.”
Alfred Russel Wallace
Kin selection
In honeybee colonies, female worker bees look after the queen bee. They build the honeycomb, gather nectar and pollen, and feed larvae, but they do not breed.
The term “kin selection” was first used by British biologist John Maynard Smith in 1964. It is the evolutionary strategy that favors the reproductive success of an organism’s relatives, prioritizing them above the individual’s own survival and reproduction. It occurs when an organism engages in self-sacrificial behavior that benefits its relatives. Charles Darwin was the first to discuss the concept when he wrote about the apparent paradox represented by altruistic nonbreeding social insects, such as worker honeybees, which leave reproduction to their mothers. British evolutionary biologist William Donald Hamilton proposed that bees, for example, behave in an altruistic manner—assisting others in reproduction—when the genetic closeness of the two bees and the benefit to the recipient outweigh the cost of altruism to the giver. This is called Hamilton’s Rule.
“The vast majority of large mutations are deleterious; small mutations are both far more frequent and more likely to be useful.”
Ronald Fisher
Different rates of evolution
The ancestors of all life on Earth were very simple organisms. Recent scientific research suggest that the earliest “biogenic” rocks—derived from early life forms—date back nearly four billion years. In that time, highly complex life forms have evolved, and later fossils of species that look more similar to those of today reveal what has occurred. For example, a fossil record stretches back 60 million years for ancestors of the horse. The earliest of these were dog-sized forest-dwelling animals with several toes on each foot. Evolution produced much larger horses with just a single hoof on each foot, adapted for life on open grasslands where they would often have had to outrun predators.
Peppered moths (biston betularia) reveal change over a shorter period. The moth is usually pale, providing camouflage against the bark of birch trees, but a mutation produces some black moths. Before the 19th century, most peppered moths were pale. During the Industrial Revolution (1760–1840), however, smoky air left deposits of soot on trees and buildings in British cities, and the black form became much commoner. By 1895, 95 percent of peppered moths in Britain’s cities were black, as paler moths were eaten by birds because their coloring provided no camouflage. This phenomenon continues to act as an example of Darwin’s theory in action today, as the pale moth becomes common once more due to the declining soot concentrations in Britain’s cities.
Two peppered moths exhibit evolution at work, the lower one an example of industrial melanism. The dark variety began to appear in British cities in the early 1800s.
“Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science.”
Theodosius Dobzhansky
Evolution in real time
Escherichia (E.) coli bacteria can cause serious gut and other infections that will be increasingly difficult to treat as drug-resistant strains of E. coli multiply.
Richard Lenski, a professor at Michigan State University, established the Long-term Experimental Evolution project in 1988. For more than 25 years, he studied 59,000 generations of the E. col bacterium. During this time, he observed that the species used the glucose solution it lived in more efficiently, increasing in size but also growing faster. Also, a new species had evolved that was able to use a compound in the solution called citrate, which the parent bacterium could not. Evolving bacteria can pose a potential threat to humans. Increasing antibiotic use destroys many disease-causing bacteria, but not those with mutations that make them resistant to the drugs. As the non-resistant bacteria are killed off, the resistant strains become more dominant, multiplying and passing on their mutations to future generations. That is natural selection at work.
See also: Early theories of evolution • The rules of heredity • The role of DNA • The selfish gene • The food chain • Mass extinctions • Population viability analysis
IN CONTEXT
KEY ECOLOGIST
Gregor Mendel (1822–84)
BEFORE
1802 French biologist Jean-Baptiste Lamarck suggests that traits acquired during the lifetime of an organism are transmitted to its offspring.
1859 Charles Darwin proposes his theory of evolution and natural selection in his book On the Origin of Species by Means of Natural Selection.
AFTER
1869 Swiss chemist Friedrich Miescher identifies DNA, which he terms “nuclein.”
1953 Molecular biologists—including Briton Francis Crick and American James Watson—discover the structure of DNA.
2000s Researchers in the field of epigenetics describe inheritance by mechanisms other than through the DNA sequence of genes.
Long before scientists cracked the genetic code, in 1866 an Austrian monk named Gregor Mendel was the first to show how traits are transferred through the generations. By means of much painstaking research, Mendel accurately predicted the basic laws of inheritance.
When Mendel began his experiments, scientists believed that the various traits seen in plants and animals were handed down through a “blending” process. However, Mendel noticed that this was not the case when he was working in his monastery garden. When he crossed a plant that always produced green peas with one that always produced yellow peas, the result was not yellowish-green peas—instead, all the peas were yellow.
Mendel’s experiment with growing peas proved that the gene carrying the yellow coloration was dominant while the gene for green was recessive.
Mendel’s labors
During the course of his research (1856–63), Mendel grew nearly 30,000 pea plants over several generations and carefully recorded the results. He focused on traits (phenotypes) that had only two distinct forms—for example, white or purple flowers. When examining the trait of yellow or green peas, Mendel took green pea plants and cross-pollinated them with yellow pea plants. The peas produced from this parent generation were all yellow and Mendel named them the F1 generation. He then cross-pollinated pea plants from the F1 generation with each other to produce the F2 generation. He found that some peas produced were yellow and some were green. The F1 gener
ation showed only one trait (yellow), which Mendel called “dominant.” However, in the F2 generation 75 percent had the dominant yellow trait and 25 percent displayed the nondominant—or “recessive”—green trait.
“Heredity provides for the modification of its own machinery.”
James Mark Baldwin
American psychologist
Laws of inheritance
Mendel theorized that every pea plant has two factors controlling each trait. When plants are cross-pollinated, one factor is inherited from each plant. A factor can be dominant or recessive. When both inherited factors are dominant, the resulting plant will show the dominant trait. With a pair of recessive factors, the plant will show the recessive trait. However, if one dominant and one recessive factor are present, the plant will show the dominant trait.
Pea plants provided the raw data that Mendel used to develop his theories explaining the transmission of traits from one generation to the next.
Pioneering geneticist
Mendel published his paper in 1866, but no one took much notice until 1900, when the botanists Hugo de Vries, Carl Erich Correns, and Erich Tschermak von Seysenegg discovered his work. Scientists then began proving Mendel’s theories more widely.
Within just ten years, scientists named the pairs of factors “genes” and showed that they are linked on chromosomes. It is now known that inheritance is far more complex than Mendel recognized, but his meticulous research continues to form the basis for modern studies.