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Introduction To Evolution What is Evolution? Evolution is the process by which all living things have developed from primitive organisms through changes occurring over billions of years, a process that includes all animals and plants. Exactly how evolution occurs is still a matter of debate, but there are many different theories and that it occurs is a scientific fact. Biologists agree that all living things come through a long history of changes shaped by physical and chemical processes that are still taking place.
It is possible that all organisms can be traced back to the origin of Life from one celled organisms. The most direct proof of evolution is the science of Paleontology, or the study of life in the past through fossil remains or impressions, usually in rock. Changes occur in living organisms that serve to increase their adaptability, for survival and reproduction, in changing environments. Evolution apparently has no built-in direction purpose. A given kind of organism may evolve only when it occurs in a variety of forms differing in hereditary traits, that are passed from parent to offspring.
By chance, some varieties prove to be ill adapted to their current environment and thus disappear, whereas others prove to be adaptive, and their numbers increase. The elimination of the unfit, or the survival of the fittest, is known as Natural Selection because it is nature that discards or favors a particular being. Evolution takes place only when natural selection operates on a population of organisms containing diverse inheritable forms. HISTORY Pierre Louis Moreau de Maupertuis (1698 - 1759) was the first to propose a general theory of evolution. He said that hereditary material, consisting of particles, was transmitted from parents to offspring. His opinion of the part played by natural selection had little influence on other naturalists.
Until the mid- 19 th century, naturalists believed that each species was created separately, either through a supreme being or through spontaneous generation the concept that organisms arose fully developed from soil or water. The work of the Swedish naturalist Carolus Linnaeus in advancing the classifying of biological organisms focused attention on the close similarity between certain species. Speculation began as to the existence of a sort of blood relationship between these species. These questions coupled with the emerging sciences of geology and paleontology gave rise to hypotheses that the life-forms of the day evolved from earlier forms through a process of change. Extremely important was the realization that different layers of rock represented different time periods and that each layer had a distinctive set of fossils of life-forms that had lived in the past. Lamarckism Jean Baptiste Lamarck was one of several theorists who proposed an evolutionary theory based on the use and disuse of organs.
Lamarck stated that an individual acquires traits during its lifetime and that such traits are in some way put into the hereditary material and passed to the next generation. This was an attempt to explain how a species could change gradually over time. According to Lamarck, giraffes, for example, have long necks because for many generations individual giraffes stretched to reach the uppermost leaves of trees, in each generation the giraffes added some length to their necks, and they passed this on to their offspring. New organs arise from new needs and develop in the extent that they are used, disuse of organs leads to their disappearance.
Later, the science of Genetics disproved Lamarck's theory, it was found that acquired traits cannot be inherited. Malthus Thomas Robert Malthus, an English clergyman, through his work An Essay on the Principle of Population, had a great influence in directing naturalists toward a theory of natural selection. Malthus proposed that environmental factors such as famine and disease limited population growth. Darwin After more than 20 years of observation and experiment, Charles Darwin proposed his theory of evolution through natural selection to the Linnaean Society of London in 1858.
He presented his discovery along with another English naturalist, Alfred Russel Wallace, who independently discovered natural selection at about the same time. The following year Darwin published his full theory, supported with enormous evidence, in On the Origin of Species. Genetics The contribution of genetics to the understanding of evolution has been the explanation of the inheritance in individuals of the same species. Gregor Mendel discovered the basic principles of inheritance in 1865, but his work was unknown to Darwin.
Mendel's work was rediscovered by other scientists around 1900. From that time to 1925 the science of genetics developed rapidly, and many of Darwin's ideas about the inheritance of variations were found to be incorrect. Only since 1925 has natural selection again been recognized as essential in evolution. The modern theory of evolution combines the findings of modern genetics with the basic framework supplied by Darwin and Wallace, creating the basic principle of Population Genetics.
Modern population genetics was developed largely during the 1930 s and 40 s by the mathematicians J. B. S. Haldane and R. A. Fisher and by the biologists Theodosius Dobzhansky, Julian Huxley, Ernst Mayr, George Gaylord SIMPSON, Sewall Wright, Bernard Reach, and G.
Ledyard Stebbins. According to the theory, variability among individuals in a population of sexually reproducing organisms is produced by mutation and genetic recombination. The resulting genetic variability is subject to natural selection in the environment. POPULATION GENETICS The word population is used in a special sense to describe evolution.
The study of single individuals provides few clues as to the possible outcomes of evolution because single individuals cannot evolve in their lifetime. An individual represents a store of genes that participates in evolution only when those genes are passed on to further generations, or populations. The gene is the basic unit in the cell for transmitting hereditary characteristics to offspring. Individuals are units upon which natural selection operates, but the trend of evolution can be traced through time only for groups of interbreeding individuals, populations can be analyzed statistically and their evolution predicted in terms of average numbers.
The Hardy-Weinberg law, which was discovered independently in 1908 by a British mathematician, Godfrey H. Hardy, and a German physician, Wilhelm Weinberg, provides a standard for quantitatively measuring the extent of evolutionary change in a population. The law states that the gene frequencies, or ratios of different genes in a population, will remain constant unless they are changed by outside forces, such as selective reproduction and mutation. This discovery reestablished natural selection as an evolutionary force. Comparing the actual gene frequencies observed in a population with the frequencies predicted, by the Hardy-Weinberg law gives a numerical measure of how far the population deviates from a non evolving state called the Hardy-Weinberg equilibrium. Given a large, randomly breeding population, the Hardy-Weinberg equilibrium will hold true, because it depends on the laws of probability.
Changes are produced in the gene pool through mutations, gene flow, genetic drift, and natural selection. Mutation A mutation is an inheritable change in the character of a gene. Mutations most often occur spontaneously, but they may be induced by some external stimulus, such as irradiation or certain chemicals. The rate of mutation in humans is extremely low; nevertheless, the number of genes in every sex cell, is so large that the probability is high for at least one gene to carry a mutation. Gene Flow New genes can be introduced into a population through new breeding organisms or gametes from another population, as in plant pollen. Gene flow can work against the processes of natural selection.
Genetic Drift A change in the gene pool due to chance is called genetic drift. The frequency of loss is greater the smaller the population. Thus, in small populations there is a tendency for less variation because mates are more similar genetically. Natural Selection Over a period of time natural selection will result in changes in the frequency of alleles in the gene pool, or greater deviation from the non evolving state, represented by the Hardy-Weinberg equilibrium. NEW SPECIES New species may evolve either by the change of one species to another or by the splitting of one species into two or more new species. Splitting, the predominant mode of species formation, results from the geographical isolation of populations of species.
Isolated populations undergo different mutations, and selection pressures and may evolve along different lines. If the isolation is sufficient to prevent interbreeding with other populations, these differences may become extensive enough to establish a new species. The evolutionary changes brought about by isolation include differences in the reproductive systems of the group. When a single group of organisms diversifies over time into several subgroups by expanding into the available niches of a new environment, it is said to undergo Adaptive Radiation.
Darwin's Finches, in the Galapagos Islands, west of Ecuador, illustrate adaptive radiation. They were probably the first land birds to reach the islands, and, in the absence of competition, they occupied several ecological habitats and diverged along several different lines. Such patterns of divergence are reflected in the biologists scheme of classification of organisms, which groups together animals that have common characteristics. An adaptive radiation followed the first conquest of land by vertebrates.
Natural selection can also lead populations of different species living in similar environments or having similar ways of life to evolve similar characteristics. This is called convergent evolution and reflects the similar selective pressure of similar environments. Examples of convergent evolution are the eye in cephalad mollusks, such as the octopus, and in vertebrates; wings in insects, extinct flying reptiles, birds, and bats; and the flipper like appendages of the sea turtle (reptile), penguin (bird), and walrus (mammal). MOLECULAR EVOLUTION An outpouring of new evidence supporting evolution has come in the 20 th century from molecular biology, an unknown field in Darwin's day.
The fundamental tenet of molecular biology is that genes are coded sequences of the DNA molecule in the chromosome and that a gene codes for a precise sequence of amino acids in a protein. Mutations alter DNA chemically, leading to modified or new proteins. Over evolutionary time, proteins have had histories that are as traceable as those of large-scale structures such as bones and teeth. The further in the past that some ancestral stock diverged into present-day species, the more evident are the changes in the amino-acid sequences of the proteins of the contemporary species. PLANT EVOLUTION Biologists believe that plants arose from the multicellular green algae (phylum Chlorophyta) that invaded the land about 1. 2 billion years ago. Evidence is based on modern green algae having in common with modern plants the same photosynthetic pigments, cell walls of cellulose, and multi cell forms having a life cycle characterized by Alternation Of Generations.
Photosynthesis almost certainly developed first in bacteria. The green algae may have been p readapted to land. The two major groups of plants are the bryophytes and the tracheophytes; the two groups most likely diverged from one common group of plants. The bryophytes, which lack complex conducting systems, are small and are found in moist areas. The tracheophytes are plants with efficient conducting systems; they dominate the landscape today. The seed is the major development in tracheophytes, and it is most important for survival on land.
Fossil evidence indicates that land plants first appeared during the Silurian Period of the Paleozoic Era (425 - 400 million years ago) and diversified in the Devonian Period. Near the end of the Carboniferous Period, fern like plants had seedling structures. At the close of the Permian Period, when the land became drier and colder, seed plants gained an evolutionary advantage and became the dominant plants. Plant leaves have a wide range of shapes and sizes, and some variations of leaves are adaptations to the environment; for example, small, leathery leaves found on plants in dry climates are able to conserve water and capture less light.
Also, early angiosperms adapted to seasonal water shortages by dropping their leaves during periods of drought. EVIDENCE FOR EVOLUTION The Fossil Record has important insights into the history of life. The order of fossils, starting at the bottom and rising upward in stratified rock, corresponds to their age, from oldest to youngest. Deep Cambrian rocks, up to 570 million years old, contain the remains of various marine invertebrate animals, sponges, jellyfish, worms, shellfish, starfish, and crustaceans.
These invertebrates were already so well developed that they must have become differentiated during the long period preceding the Cambrian. Some fossil-bearing rocks lying well below the oldest Cambrian strata contain imprints of jellyfish, tracks of worms, and traces of soft corals and other animals of uncertain nature. Paleozoic waters were dominated by arthropods called trilobites and large scorpion like forms called eurypterids. Common in all Paleozoic periods (570 - 230 million years ago) were the nautiloid, which are related to the modern nautilus, and the brachiopods, or lamp shells. The odd graptolite's, colonial animals whose carbonaceous remains resemble pencil marks, attained the peak of their development in the Ordovician Period (500 - 430 million years ago) and then abruptly declined. In the mid- 1980 s researchers found fossil animal burrows in rocks of the Ordovician Period; these trace fossils indicate that terrestrial ecosystems may have evolved sooner than was once thought.
Many of the Paleozoic marine invertebrate groups either became extinct or declined sharply in numbers before the Mesozoic Era (230 - 65 million years ago). During the Mesozoic, shelled ammon oids flourished in the seas, and insects and reptiles were the predominant land animals. At the close of the Mesozoic the once-successful marine ammon oids perished and the reptilian dynasty collapsed, giving way to birds and mammals. Insects have continued to thrive and have differentiated into a staggering number of species. During the course of evolution plant and animal groups have interacted to one another's advantage. For example, as flowering plants have become less dependent on wind for pollination, a great variety of insects have emerged as specialists in transporting pollen.
The colors and fragrances of flowers have evolved as adaptations to attract insects. Birds, which feed on seeds, fruits, and buds, have evolved rapidly in intimate association with the flowering plants. The emergence of herbivorous mammals has coincided with the widespread distribution of grasses, and the herbivorous mammals in turn have contributed to the evolution of carnivorous mammals. Fish and Amphibians During the Devonian Period (390 - 340 million years ago) the vast land areas of the Earth were largely populated by animal life, save for rare creatures like scorpions and millipedes. The seas, however, were crowded with a variety of invertebrate animals.
The fresh and salt waters also contained cartilaginous and bony Fish. From one of the many groups of fish inhabiting pools and swamps emerged the first land vertebrates, starting the vertebrates on their conquest of all available terrestrial habitats. Among the numerous Devonian aquatic forms were the Crossopterygii, lobe- finned fish that possessed the ability to gulp air when they rose to the surface. These ancient air- breathing fish represent the stock from which the first land vertebrates, the amphibians, were derived. Scientists continue to speculate about what led to venture onto land. The crossopterygians that migrated onto land were only crudely adapted for terrestrial existence, but because they did not encounter competitors, they survived.
Lobe-finned fish did, however, possess certain characteristics that served them well in their new environment, including primitive lungs and internal nostrils, both of which are essential for breathing out of the water. Such characteristics, called pre adaptations, did not develop because the others were preparing to migrate to the land; they were already present by accident and became selected traits only when they imparted an advantage to the fish on land. The early land-dwelling amphibians were slim-bodied with fishlike tails, but they had limbs capable of locomotion on land. These limbs probably developed from the lateral fins, which contained fleshy lobes that in turn contained bony elements.
The ancient amphibians never became completely adapted for existence on land, however. They spent much of their lives in the water, and their modern descendants, the salamanders, newts, frogs, and toads still must return to water to deposit their eggs. The elimination of a water-dwelling stage, which was achieved by the reptiles, represented a major evolutionary advance. The Reptilian Age Perhaps the most important factor contributing to the becoming of reptiles from the amphibians was the development of a shell- covered egg that could be laid on land. This development enabled the reptiles to spread throughout the Earths landmasses in one of the most spectacular adaptive radiations in biological history. Like the eggs of birds, which developed later, reptile eggs contain a complex series of membranes that protect and nourish the embryo and help it breathe.
The space between the embryo and the amnion is filled with an amniotic fluid that resembles seawater; a similar fluid is found in the fetuses of mammals, including humans. This fact has been interpreted as an indication that life originated in the sea and that the balance of salts in various body fluids did not change very much in evolution. The membranes found in the human embryo are essentially similar to those in reptile and bird eggs. The human yolk sac remains small and functionless, and the exhibits have no development in the human embryo. Nevertheless, the presence of a yolk sac and allantois in the human embryo is one of the strongest pieces of evidence documenting the evolutionary relationships among the widely differing kinds of vertebrates. This suggests that mammals, including humans, are descended from animals that reproduced by means of externally laid eggs that were rich in yolk.
The reptiles, and in particular the dinosaurs, were the dominant land animals of the Earth for well over 100 million years. The Mesozoic Era, during which the reptiles thrived, is often referred to as the Age of Reptiles. In terms of evolutionary success, the larger the animal, the greater the likelihood that the animal will maintain a constant Body Temperature independent of the environmental temperature. Birds and mammals, for example, produce and control their own body heat through internal metabolic activities (a state known as endotherm, or warm-blooded ness), whereas todays reptiles are thermally unstable (cold-blooded), regulating their body temperatures by behavioral activities (the phenomenon of ectotherm). Most scientists regard dinosaurs as lumbering, oversized, cold-blooded lizards, rather than large, lively, animals with fast metabolic rates; some biologists, however notably Robert T. Bakker of The Johns Hopkins University assert that a huge dinosaur could not possibly have warmed up every morning on a sunny rock and must have relied on internal heat production.
The reptilian dynasty collapsed before the close of the Mesozoic Era. Relatively few of the Mesozoic reptiles have survived to modern times; those remaining include the Crocodile, Lizard, snake, and turtle. The cause of the decline and death of the large array of reptiles is unknown, but their disappearance is usually attributed to some radical change in environmental conditions. Like the giant reptiles, most lineages of organisms have eventually become extinct, although some have not changed appreciably in millions of years. The opossum, for example, has survived almost unchanged since the late Cretaceous Period (more than 65 million years ago), and the Horseshoe Crab, Limulus, is not very different from fossils 500 million years old. We have no explanation for the unexpected stability of such organisms; perhaps they have achieved an almost perfect adjustment to a unchanging environment.
Such stable forms, however, are not at all dominant in the world today. The human species, one of the dominant modern life forms, has evolved rapidly in a very short time. The Rise of Mammals The decline of the reptiles provided evolutionary opportunities for birds and mammals. Small and inconspicuous during the Mesozoic Era, mammals rose to unquestionable dominance during the Cenozoic Era (beginning 65 million years ago). The mammals diversified into marine forms, such as the whale, dolphin, seal, and walrus; fossorial (adapted to digging) forms living underground, such as the mole; flying and gliding animals, such as the bat and flying squirrel; and cursorial animals (adapted for running), such as the horse. These various mammalian groups are well adapted to their different modes of life, especially by their appendages, which developed from common ancestors to become specialized for swimming, flight, and movement on land.
Although there is little superficial resemblance among the arm of a person, the flipper of a whale, and the wing of a bat, a closer comparison of their skeletal elements shows that, bone for bone, they are structurally similar. Biologists regard such structural similarities, or homologies, as evidence of evolutionary relationships. The homologous limb bones of all four-legged vertebrates, for example, are assumed to be derived from the limb bones of a common ancestor. Biologists are careful to distinguish such homologous features from what they call analogous features, which perform similar functions but are structurally different.
For example, the wing of a bird and the wing of a butterfly are analogous; both are used for flight, but they are entirely different structurally. Analogous structures do not indicate evolutionary relationships. Closely related fossils preserved in continuous successions of rock strata have allowed evolutionists to trace in detail the evolution of many species as it has occurred over several million years. The ancestry of the horse can be traced through thousands of fossil remains to a small terrier-sized animal with four toes on the front feet and three toes on the hind feet.
This ancestor lived in the Eocene Epoch, about 54 million years ago. From fossils in the higher layers of stratified rock, the horse is found to have gradually acquired its modern form by eventually evolving to a one-toed horse almost like modern horses and finally to the modern horse, which dates back about 1 million years. CONCLUSION TO EVOLUTION Although we are not totally certain that evolution is how we got the way we are now, it is a strong belief among many people today, and scientist are finding more and more evidence to back up the evolutionary theory. 339
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