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Evolution

Biological evolution is a process that results in changes in physical, chemical and behavioral features of populations across more than one generation. These changes must be genetically inheritable. Acting over geological time, evolution accounts for the vast diversity of life on Earth, and the word evolution is used by many to refer to the history of change as well as the process. Charles Darwin called evolution “descent with modification”, although he did not know about the genetic mechanisms by which the modifications were passed from one generation to the next. Darwin did recognize natural selection as the most important mechanism by which evolution takes place. Today biologists realize that populations of individuals are variable in many traits, and some of this variability results from underlying genetic differences caused by mutation. Individuals with traits that allow them to produce more descendants are better represented in succeeding generations, as are the heritable traits that led to their reproductive success. Through this mechanism, natural selection leads to changes in gene frequencies and in the traits they code for.

Darwin’s original theory has been greatly modified over time. He did not, for example, have any knowledge of genetics as we currently understand it. Growth in our understanding of the cell and of the genetic machinery that controls cells and organisms has in turn greatly refined our understanding of how evolution works. Evolution is universally accepted among scientists as the best explanation for the relationships we observe between all living and fossil life forms. Scientists are still uncovering the specifics of how, when, and why evolution produced the life we see on Earth today. These discussions concern the mechanisms and timing of evolution, not whether descent with modification has occurred.

Evolution explains why groups of organisms share physical characteristics to different degrees. For example, all vertebrates are structurally supported by an internal skeleton with a vertebral column (backbone). This shared characteristic reflects their descent from a common ancestor in which these structures first evolved. A giraffe, for example, has seven vertebrae in its neck and so does a mole--in fact, nearly all mammals do. This is because all mammals evolved from an ancestor that had seven neck vertebrae. Evolution also explains why organisms are different. Mammal necks are of different lengths because of lengthening or shortening of each vertebra. These modifications allow different kinds of mammals to use their necks in different ways.

Most importantly, the process of descent with modification produces the pattern that scientists call the “Tree of Life.” This “tree” describes the relationships of organisms to one another. The branching pattern of relationships that results from the evolutionary process is called the phylogeny of life. Scientists have used these evolutionary relationships to develop a system of classification. Within this system, organisms have been identified, named, and classified in order to permit scientists to communicate easily about them.

The species is the “working unit” of evolution. A species is a collection of populations, all genetically related. These populations are composed of individual organisms that are capable of breeding with each other to produce fertile offspring, thus passing genetic information from one generation to the next. Through descent with modification (mutation and natural selection) a population will accumulate genetic changes until it is so different from other populations of the parent species that interbreeding is no longer possible. In this way, a new species has formed. Speciation is the term biologists and paleontologists use to describe such an event.

How does speciation occur? Evolution has two basic components: the origin of new variation, which occurs primarily through mutation of genetic material (DNA), and the sorting of new mutations by natural selection. Mutations are usually caused by energetic radiation or DNA-altering chemicals, and thus do not occur in response to “needs” on the part of the individuals in which they occur. Most mutations, in fact, are either harmful or neutral. Only rarely is a mutation beneficial to the offspring of the organism in which the mutation occurred. In any population, mutations occur that make some individuals different from others. Natural selection is the means by which helpful and harmful mutations are sorted out. Because organisms compete for resources, natural selection favors those individuals with beneficial mutations, permitting them to reproduce more than other organisms. Over time, the genes that confer these advantages become more common in the population. Natural selection works through such mechanisms as competition for resources, different abilities to tolerate physical conditions, different abilities to detect predators or resist parasites, and so forth, resulting in differential reproductive success. But competition is not always a physical battle. For example, an individual with a longer neck might be able to reach more food. In a year when resources are limited, this adaptation would allow the production of more offspring than other individuals in the population. Over time, the average length neck in the population would shift toward longer necks. If these selective pressures persisted for some time, the population would become significantly different from other populations in the species. Then, if this population became isolated from the other populations of the species, either through later physical isolation or through associated mutations that limited cross-breeding with other individuals, an entirely new species would appear.

The rate of evolution has been debated for decades. One of Darwin’s key points was that evolution worked very slowly, through the gradual, incremental acquisition of small differences. This view continues to be widely held, although in somewhat modified form. Dr. Ernst Mayr, a well-known 20th century biologist, proposed that new species tend to form allopatrically, in small populations that are geographically isolated from the main species. These isolated populations are more likely to harbor high proportions of unusual mutations, because each individual represents a larger percentage of the local population than individuals in larger populations at the center of the species’ range.

Under certain conditions, evolution might occur more rapidly than visualized in even the modified versions of Darwin’s theory. In the extreme case, for example, a population of just a few individuals, all sorts of unusual mutations could become fixed simply because the number of individuals was so small--each mutation has a much higher likelihood of survival because competition among mutant forms is lower. This process is called genetic drift and the fixation of a mutation in such a population is called the “founder effect.” Through this process a new species can arise in few generations. Consideration of such processes led paleontologists Stephen Jay Gould and Niles Eldredge in 1972 to pose a challenge to the prevailing evolutionary thought of the time. They argued for “punctuated equilibrium,” a concept that proposed explicitly that speciation was an “event”--in other words, species did not originate in a series of gradual steps, each resulting from a mutation with a small effect, slowly changing ancestor into descendant. Rather, the genetic changes that led to the formation of new species had large effects and happened over relatively few generations. While not denying that much of evolution is indeed slow and gradual, they suggested that slow change was not the underlying process that gave rise to most new species. Punctuated equilibrium helped to explain why many transitional forms apparently were missing from the fossil record. According to the hypothesis of punctuated equilibrium, transitional forms existed for brief periods of time, and so were unlikely to become fossils.

This view has not been universally accepted, by paleontologists or biologists working with modern species. Some, such as paleontologist Philip Gingerich, have argued that many mammal species indeed evolved gradually, without any apparent episodes of rapid change. Others have argued that small mutations could not produce the large changes in organismal structure needed to drive the process of punctuated equilibrium. However, the discovery of Hox genes--which code for entire structures, rather than single components--identified a potential mechanism for rapid evolutionary change.

This relationship between a structure (such as a long neck) and a function (such as feeding) is important to understanding its evolutionary origin. If, in the course of its evolution, a structure was very closely tied to a specific function, it is called an adaptation. Many structures are not closely linked to particular adaptations, however. They may have evolved for other purposes, and were later co-opted (or “exapted”) for a new function. Or they may simply be the by-products of evolutionary selection on another structure to which they were connected.

Structures in different species that were inherited from a common ancestor are described as homologous. Homologous structures may look similar (such as the legs of elephants and rhinos), or different (such as the front legs of elephants and flippers of whales), but they always share this quality of common ancestry. In contrast, analogous structures are always superficially similar (such as the fins of fish, whales, and ichthyosaurs) but evolved independently. Whales are descended from land mammals that lacked fins, so the must have evolved fins independently of fish.

Analogous structures are the product of convergence. Convergence occurs when similar structures evolve in species that are not closely related. For example, wings have evolved a number of times in the course of the history of life. In each instance of independent appearance, the structure is unique. Insects, reptiles, birds, and mammals have all evolved wings, but they are constructed of different materials. This convergence has occurred because genetic mutations combined with natural selection created structures that permitted flight in different organisms. The wings of these different forms are really only similar in superficial ways, but they have similar functions. In order to fly, an organism must obey the laws of aerodynamics. (Even though the organisms themselves don’t understand them or know that they exist, such physical laws control the flight of an airplane as surely as they control that of a dragonfly.) Physical factors thus constrain the shape of a successful wing.

Coevolution occurs when two unrelated species evolve in tandem. In effect, the species become linked through a period of sustained ecological interaction such as the need for food and the need for pollination that link certain insects and flowers. An evolutionary change in one of the two species will often favor the survival of mutations that cause a change in the other. In predator-prey interactions, the evolution of prey defenses in one species can result in the subsequent evolution of a means to foil these defenses in the other. This is often referred to as a coevolutionary “arms race.” Coevolution also occurs in mutualistic relationships, where two species have a mutually beneficial dependence on one another. For example, some species of acacia tree have evolved features in tandem with a particular species of ant. The ant-acacias provide shelter by producing hollow thorns in which the ants can live. The trees may also produce nectar or fatty food bodies that the ants consume. The dispersion of these rewards around the tree ensures that the ants will visit different parts of the tree. The ants in turn create a space around the tree by trimming encroaching vegetation that might shade the acacia or compete for soil nutients. The ants will also attack any organism that enters this space. It is to the advantage of the ants to protect the tree from which they derive food and shelter. It is to the advantage of the tree to attract the ants that will protect it.


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