What is evolutionary theory and what are causes of evolution and the evidence of evolution?
The term ‘evolution’ evokes varied connotations. In simple and direct terms it is a process of change or development over a long period of time. Defined so, evolution can refer to any phenomena– evolution of universe, evolution of human culture etc. — that change over a period of time. However, in common parlance, evolution refers to biologic or organic evolution, which is the formation and development of life on earth. Biological evolution is “an unpredictable and natural process of temporal descent with genetic modification that is affected by natural selection, chance, historical contingencies and changing environments.” (NABT, 1997) It is the change in genetics of a population over time.
Theory of Evolution:
The first scientific studies on evolution were conducted in the 1700s by two French biologists – Comte de Buffon and Baron Cuvier — by studying the fossils and their anatomy, concluding that life on earth has undergone a series of changes. Chevalier de Lamarck, the French naturalist, proposed the first theory of evolution in 1809 suggesting that an animal’s body part could change, depending on the extent to which it is used and such acquired traits became hereditary. Though his theory of the inheritance of acquired characteristics interested many scientists, it was only in 1858 when Charles R. Darwin, the English biologist presented his theory of evolution that the subject of evolution began gaining adequate scientific consideration. Since then, the scientific advancements and investigation has led to many refinements of the theory though the principal ideas of evolution remain more or less unchanged.
There is a tendency to reject evolutionary theory, considering it is as only a ‘theory’. However theory of evolution is supported by a range of evidence from varied scientific disciplines and an evidentially corroborated theory becomes accepted as a scientific fact. “Today, nearly all biologists acknowledge that evolution is a fact. The term theory is no longer appropriate except when referring to the various models that attempt to explain how life evolves… It is important to understand that the current questions about how life evolves in no way implies any disagreement over the fact of evolution.” (Campbell, 1990: p. 434) Today, the theory of evolution is considered the most important fundamental concept in the study of biological studies. (Dobzhansky, 1973)
The principal idea behind the theory of evolution is that all living things evolved from simple organisms and developed through the years to produce millions of species. This generally accepted scientific theory about the development of life since it origination has three major aspects.
The ancestral lineage between organisms– living and extinguished
The appearance of new traits in a lineage
The reasons and processes that cause some traits to endure while others give way (Winkipedia, 2003)
The evolutionary theory explains these aspects. Evolutionary theory proposes that all species probably evolved from a single form of life, which existed about three and half billion years ago. Over the ages, the original basic form of life is considered to have evolved into more species and these, in turn, into many other species. This ‘speciation’ process as it is called led to the development of over 10 million species that live on earth today. The idea of speciation leads to another essential idea underlying it – the idea of common ancestry. As all species evolved from one basic life form, it is implied that any two species once had a common ancestor. But how can the idea of common ancestry are explained in the light of widely varying species? It only means that closely related or seemingly identical species, like human beings and gorillas, have a more recent ancestor, where as dissimilar species, like human beings and reptiles, have a common ancestor, which inherited the earth in a very distant past. Evolutionary changes only occur over long spans of time, ranging from decades of years to millions of years it is considered that evolution continues to occur today at rates, which are comparable to those of the past.
Causes of Evolution:
The evolutionary theory and its versions essentially attribute the cause of evolutionary change to the interaction of three processes — mutation, natural selection and genetic drift. Mutation is a permanent change in the hereditary component of an organism. Mutation produces random or chance variation in the genetic or inherited features of an organism. To understand the process of mutation it is essential to understand how characteristics are inherited. The hereditary or genetic characteristics are carried by chromosomes, which in turn carry infinite number of genes, in the cells. Genes consist of DNA — deoxyribonucleic acid — that contains the coded information that establishes the hereditary characteristics. Mutation of a hereditary characteristic occurs with the alteration of DNA in the genes, which can either result from exposure to environmental factors including radiations and chemicals or by errors in copying of DNA during cell division. After a gene is altered, it duplicates itself in the altered form. The presence of such mutated or mutant genes in the egg or sperm cells of an organism, leads to the alteration of inherited characteristics or introduce new hereditary characteristics. Mutations are thus the building blocks of evolutionary development and explain the development of new species as well.
The process of natural selection differentiates the random changes brought about by mutation and selects those changes that enhance the individual’s reproduction and survival. The natural selection thus ensures that only those variations that make a species better adapted to its environment are passed on future generation. Not only that, natural selection also eliminates those changes that limit the ability of species to reproduce and survive. Natural selection involves such characteristics as appearance, physiology, body chemistry and also behavior. The process of natural selection is said to occur only when two biological conditions are met. The first biological requirement is that the individuals of a population must vary in their hereditary or genetic characteristics.
Variations in genetic characteristics include differences in physiological appearance such as height, weight, and color of skin, hair and other features. It also includes such intrinsic features as the thickness of bones, the characteristics of blood etc. The second biological requirement for natural selection process to occur is that certain different characteristics inherited need to affect chances for reproduction and survival. Only if this occurs, can the fittest individuals pass on more copies of their genes to future generations than the other individuals. Over a period of time, the species gathers genes that increase its ability to reproduce and survive in its environment. There are essentially three types or natural selection.
Directional selection: This process produces new characteristics or features that enable a species to adapt to its environment.
Stabilizing selection: The most common type of natural selection, stabilizing selection occurs when a species is well adapted to its environment.
Sexual selection: This selection occurs primarily in animals. Preferring sexual partners with certain specific characteristics result from sexual selection.
Genetic Drift is the third process that explains the cause of evolutionary change with reference to the evolution of population. This theory has not received as much acceptance as the natural selection theory. Genetic drift involves a random change in the occurrence of genes in populations. It is said to be caused by the random manner in which the sperm and egg cells receive certain chromosomes from each parent as they form. As these reproductive cells contain only half the respective sets of chromosomes, half of a parent’s genes are only present in an egg or sperm. If the parents produce a limited number of off springs, some of their genes may not be passed on to the off springs even. The process of genetic drift does not help species to adapt to their living environment as it causes only random changes in the occurrence or frequencies of characteristics.
However, over time genetic drift modifies the genetic make up of a population. In this regard it is to be noted that biologists and naturalists even today remain uncertain about the relative importance of genetic drift and natural selection in explaining the evolutionary change. Many species have characteristics that have no obvious or apparent benefit in adapting to its particular living environment. It is quite not possible to say that certain features like the change in color or shape of a genetic feature affects the organism’s reproduction and survival or they are just random variations caused by genetic drift. This limitation has brought about much criticism to Darwin’s theory of evolution. (Smith, 1989: p 180)
Population Dynamics and Evolution of Population
Population dynamics is the study of changes in the number and composition of individuals in a population, and the factors that influence those changes. Population dynamics involves five basic components of interest to which all changes in populations can be related: birth, death, sex ratio, age structure, and dispersal. Natural selection in combination with the genetic drift process explains the population dynamics, as they are essentially group processes. It causes the evolution of a population as a whole and not the evolution of an individual. The natural selection process is supposed to gradually shift the average characteristics of the group over time. However, why most populations demonstrate relatively stable dynamics is a question yet to be answered, and biologists are largely uncertain regarding the processes by which population stability may evolve through natural selection (Mueller et al., 2000: p 1284)
Evidence of Evolution
Evolutionary changes are obviously not observable directly as such changes involve decades and centuries. However, there are other evidential facts that explain and demonstrate the occurrence of evolution. The main sources that evidence evolution includes: fossil records, the geographic distribution of species, embryology, vestigial organs and artificial selection. The fossil record offers the most substantial evidence for evolution. Fossils are the remains of organisms turned into rocks after years of buried preservation under layers of mud. The fossil record illustrates a definite progression from the earliest and original species of single-celled organisms to the first multi-celled organism and from those orgasms to many simple and complex organisms inhabiting the earth today. However, fossil record presents many gaps, as it is probable that only a few species were preserved. Even so, palaeontologists have collected enough fossils to present a comprehensive record of the history and development of life on earth.
Geographic distribution of species, generally called biogeography, is the second main source of evidence supporting the occurrence of evolution. The limited and peculiar nature of species of organisms present in specific geographical areas suggests that organisms, which have migrated from other regions, have evolved into new species, adapting to the new environmental conditions. Embryology is the study of the way in which organisms develop during the early stages of life present another source of evidence for evolution. The embryonic development of many organisms involves very specific events that can be explained only by the evolution of the organism from other species. The repeated occurrence of embryonic features of evolutionary ancestors during the early development of an organism is called recapitulation.
The presence of vestigial organs is also suggestive of the occurrence of evolution. Vestigial organs are the remains of organs, which are generally purposeless to the current species, but were once useful to the evolutionary ancestor. The best example of vestigial organ is the human appendix, a narrow tube attached to the large intestine, which serves no purpose to human beings but are useful to apes, considered the ancestors of humans, for digesting plant material.
Artificial selection, which is essentially the process, used by breeders to produce new varieties, also evidence the occurrence of evolution. Breeders select only those organisms in a species that demonstrate the desired qualities. It is a very evident fact that such selections eventually lead to substantial changes in the species. Artificial selection differs from natural selection in only that in artificial selection it is human beings that determine the characteristics that give an advantage in reproduction, whereas in natural selection it is the environment that selects the characteristics. The occurrence of vast changes in the characteristics of species in a short time by artificial selection only strengthens the idea that natural selection can cause larger changes over the vast period of time.
Though the natural process of evolution is normally not visible or observable being very slow, in certain cases evolution occurs rapidly in living species, when it undergoes a genetic change in response to a disturbance of its environment by human beings. The rapid development of immunity in insects to chemicals is one such example that also suggests that organisms evolve while adapting to its environment.
Dobzhansky Theodosius “Nothing in Biology Makes Sense Except in the Light of Evolution,” American Biology Teacher vol. 35 March, 1973
Evolution,” Wikipedia The Free Encyclopedia available at www.wikipedia.org/wiki/Evolution- Wikipedia.htm
Mueller, L.D., Joshi, A. & Borash, D.J. “Does population stability evolve?” Ecology 81, 2000-1273-1285.
Neil A. Campbell, Biology 2nd ed., Benjamin/Cummings, 1990, p. 434
Smith J. M), “The limitations of evolutionary theory,” in Did Darwin Get It Right? J.M. Smith (Ed.), New York: Chapman and Hall, 1989 pp. 180-191.
Statement on teaching evolution, National Association of Biology Teachers, Available at http://www.nabt.org/Evolution.htm. accessed on 1.08.2003
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