Evolution

Evolution is the process of genetic change by which species adapt to their environment or develop new ways of coping with environmental stress. The theory of evolution is the most fundamental organizing principle in biology. It can be invoked to explain the origin of almost any feature of living things. By the early nineteenth century enough geological and fossil evidence had accumulated to challenge the prevailing idea that Earth and the life on it were unchanging. Jean-Baptiste Lamarck then proposed the ultimately rejected theory that species evolved by passing on new characteristics that were acquired by changes during their lifetime. For example, it was suggested incorrectly that early giraffes stretched their necks by reaching for leaves from high branches and that their offspring retained this change.

Two English naturalists then developed a better explanation for evolution. From 1831 to 1836, Charles Darwin served on the round-the-world voyage of HMS Beagle. His observations in South America, especially in the Galapagos Islands off Ecuador, led him to form the modern theory of evolution. Darwin wrote a summary of the theory in 1842 but did not publish it immediately. Independently, and several years later, Alfred Russell Wallace developed the same ideas, even using the same term, natural selection. After Wallace published several papers he sent one to Darwin, asking him to forward it to the Linnean Society for publication, which Darwin did. Darwin then rushed his own life's work into print in 1859 (giving proper credit to Wallace) in the form of the book The Origin of Species.

Our modern understanding of evolution is based on genetic theory. However, its original development preceded that knowledge. Here a detailed discussion of heredity and genetics is deferred to Chapter 6. Evolution can be described based on the understanding that individuals in a population vary in numerous traits (observable characteristics) that can be inherited by their offspring. The traits may be of various types, such as morphological (related to structure, such as height or shape of the eyes), biochemical (such as the ability to produce their own vitamin C), or even behavioral (e.g., aggressiveness in dogs).

The theory of evolution can be reduced to two mechanisms that act in combination:

1. Random variation. The sources of the variation in traits within a population are random mutations in the genetic code, and sorting and recombination of genetic material that occurs during meiosis, a type of cell reproduction. Only a minority of genetic changes may confer an advantage on an individual. In fact, most changes are probably fatal and are not passed on to future generations. Mutations are caused by errors in the biochemical processes of reproduction in which the genetic material is copied for progeny, or by damage from chemical or physical agents such as ionizing radiation.

2. Natural selection. Because organisms have an inherent reproductive growth rate that would cause the population to exceed the ability of the environment to support it, not all individuals survive to reproduce. Individuals with heritable traits that do confer an advantage tend to leave more offspring than those without such traits. Consequently, those traits become more common in succeeding generations.

These cause the traits held by a population to tend to change with time either because novel traits are developed randomly that confer an advantage in the current environment, or because different traits are selected for when the environment changes, such as by climate change, introduction of new competing species to the area, or various forms of human intervention. Favorable traits, which increase the fitness of a population to an environment, are called adaptations. The Galapagos Islands, which have become a field laboratory for evolution, furnish an example. In 1977 a drought wiped out 85% of a species of finch. Studies showed that the survivors were mostly birds with larger beaks. It was found that this was because during the drought there were fewer herbs and grasses that produced small seeds. The birds with small beaks were unable to eat the larger seeds that remained, and they did not survive to pass on their characteristics.

Prior to our modern understanding of genetic theory and molecular biology, the theory of evolution could be supported by three types of evidence: the fossil record, comparisons between the structure and function of different species, and by an analysis of the geographic distribution of existing species. The fossil record shows that (1) different organisms lived at different times, (2) different organisms lived in the past than are in existence today, (3) fossils found in adjacent sedimentary layers (and therefore relatively close to each other on a geological time scale) are similar, (4) intermediate forms of species are sometimes found, and (5) older rocks tend to have simpler forms.

Comparison of species falls into three categories: comparative anatomy, comparative embryology, and comparative biochemistry. Comparative anatomy shows that similar organisms have similar structures, but structures that serve different functions. For example, the same bones that a human has in the forearm are found in the flipper of a whale and the wing of a bat. It was easier for nature to modify existing structures of these mammals than to develop completely new, specialized structures. Sometimes a structure loses its function altogether, forming a vestigial organ. For example, whales and snakes retain the pelvis (hipbone) and femur (thighbone). Comparative embryology finds that similar organisms have similar embryos (the earliest multicellular form of an individual). For example, all vertebrate embryos, including humans, have gill slits, even if the adult does not. Evolution accounts for this by explaining that those features are retained from ancestral forms. In an example of comparative biochemistry, techniques of molecular biology have shown that similar species have similar genetic material. It is possible to compare species based on the degree of similarity between their DNA (the chemical in the nuclei of cells that stores the hereditary information). This has shown definitively that species that are similar on an evolutionary scale (based on other evidence) are also similar genetically. Furthermore, the code that converts DNA into proteins is the same in all living things from bacteria to humans (see Table 6.2). There is no fundamental reason that this should be so unless all these organisms developed from a common ancestor.

The third line of evidence is from biogeography, the study of geographic distribution of living things. This type of evidence was particularly striking to Darwin. He observed many unique species in the Galapagos, off South America, and in the Cape Verde Islands, off Africa. Although the two island groups have similar geology and climate, their species are more similar, although not identical, to those on the nearby mainland than to each other. This suggested that the islands were colonized from the nearby mainland by organisms swimming, flying, or rafting on floating vegetation, and that evolution continued through their subsequent isolation. At the same time, unrelated organisms of the two island chains had similar characteristics, suggesting that evolution formed similar structures in response to similar requirements.

The study of evolution has helped to understand changes in populations other than the formation of new species. Random changes in traits can occur in a population, resulting in what is called genetic drift. This is seen when a population becomes divided by some circumstance, such as the formation of an island from a peninsula by rising water levels. If two populations are isolated from each other long enough, they can diverge to form distinct species, even if both are in similar environments. This is called divergent evolution. The differences between Galapagos species and the mainland species with which they share ancestry is an example.

Another form of genetic drift is called the bottleneck effect. This occurs if some catastrophe destroys a large portion of a population. As a result, only those traits carried by the survivors are found in future generations. Many of the less common traits will disappear, and some rare ones may become common (see Figure 2.1). The bottleneck effect is also seen when a small number of a species are introduced to a new ecosystem and flourish there. The descendents share a few recent ancestors and limited genotypes.

Environmental stresses can cause changes in the genetic makeup of a population by favoring organisms with certain alleles more than others. This is, in fact, the normal way that populations can adapt rapidly to changes in their environment without mutations being required to produce new adaptations. It is also the reason why populations with genetic diversity are more likely to survive in the face of change. However, there is another side of this phenomenon related to human impacts on populations. Toxins added to the environment exert selection pressure for individuals that are more tolerant of the toxins. One negative impact of this is that it can reduce the genetic diversity of a population, making it vulnerable to further stresses. Another problem occurs if the organism is a pest and the toxicant is an agent such as a pesticide or antibiotic. As a result of the selection pressure, the population seems to develop tolerance or resistance to the agent, which then becomes less effective.

Original population contains many rare alleles.

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Recovered population has lost many of the rare alleles, and a few rare alleles become common.

Figure 2.1 Bottleneck effect. (Based on Wallace et al., 1986.)

Sometimes very different species develop similar characteristics in response to similar environmental conditions. This is called convergent evolution. Because of this it is not always possible to consider species to be related evolutionarily because of superficial similarities. For example, most of Australia's original mammals are marsupials, which nurture their fetuses in pouches; whereas North America is dominated by placental mammals, whose fetuses grow internally in the uterus until ready to live independently. Despite their being very different groups, similar species have arisen on both continents, such as marsupial analogs to the wolf, mouse, and even the flying squirrel.

The Pace of Evolution The traditional view of evolution has been that it proceeds by the accumulation of small increments of change. This view is called gradualism. Some fossil evidence supports this. For example, fossils have shown a series of species leading from a small, dog-sized animal, to the modern horse. However, the fossil record is more compatible with the view that species remained stable for millions of years before suddenly disappearing and being replaced by new ones. An alternative view, called punctuated equilibrium, was proposed in 1972 by Niles Eldredge and Stephen Jay Gould. This predicts that evolutionary changes occur rapidly over short periods, forming new species in small populations. These stay relatively unchanged for millions of years until they become extinct. Gould suggests that the rapid changes could be caused by small, yet influential genetic modifications. For example, radical changes in the body plan of an organisms could be mediated by a small number of mutations.

The theory of punctuated equilibrium answers the criticisms directed at evolution theory for the absence of "missing links'' in the fossil record. However, gradualism also explains some of the features observed in the living world. The structure of the eye has been cited as being so complex as to defy explanation in terms of development from simpler forms. However, four different species of mollusk illustrate stages of a continuum in eye development (see Figure 2.2).

Extinction Extinction is the elimination of a species from Earth. The term is also used to describe elimination of a species from a particular area or ecosystem. Today, there is serious concern because human activities are causing the extinction of numerous species each year. Activities that destroy ecosystems or even just reduce their size cause loss of species. Some biologists estimate that human population pressure on natural ecosystems could eliminate 20% of Earth's species over the next 25 or 30 years. The term biodiversity describes the taxonomic variation on Earth or in an ecosystem. The loss of species due to human activities eliminates adaptive information created by nature over eons. Many hope to protect against loss of biodiversity. Besides the moral motivation, there is also the utilitarian concern that lost species might have been useful, such as for drug development or to help control biological pests.

Extinctions also occur naturally, of course. A species may become extinct because of competition or increased predation from another species. This could occur if the other species is one that is newly evolved or that has invaded the ecosystem from other locations. Such invasions are caused by geological and/or climatic changes such as land bridges that form between major islands and continents due to fluctuation sea levels, or elimination of a climatic barrier between, say, ecosystems separated by mountains or desert. Human importation of exotic species, intentional or not, produces a similar effect.

The fossil record shows that a number of mass extinctions have occurred in the past. At the end of the Permian Period, some 250 million years ago, 96% of species, such as

Figure 2.2 Development of the eye in mollusks: limpet, nautilus, scallop, octopus. (From Postlethwait and Hopson, 1995.)

the trilobite, disappeared from the fossil record. Some 65 million years ago, at the end of the Cretaceous Period, known as the K-T boundary, as much as 76% of species disappear, including the dinosaurs. Recently, geologic evidence has established that this mass extinction was caused by a large meteorite or comet striking Earth at the Yucatan Peninsula in what today is Mexico. Shock waves traveling through Earth's crust seem to have focused on the opposite point of the globe, southwest of India, causing massive volcanic outpourings. The impact plus the volcanism are thought to have sent dust and smoke into the atmosphere, darkening the sun, directly wiping out many ecosystems and reducing the primary productivity of the Earth (Chapter 14), starving many species into extinction.

Mass extinctions seem to be followed by a period of accelerated evolution of new species. The elimination of dinosaurs, for example, paved the way for the further development of mammals. Human activities are currently being blamed for causing the extinction of great numbers of species, mostly by encroaching on their habitats.

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