Sunday 22 April 2007
EvolutionScience Investigator
Scientific evidence to Prove Theory of Evolution
In science, one of the definitions of theory is a logical explanation, a proven model of the manner of interaction of a set of natural phenomena, and thus it is capable of predicting future occurrences or observations of the same kind. Such theories can be falsified through empirical observations. The theory of evolution is such a theory and it is the central organizing principle of modern biology, and providing a unifying explanation for the diversity of life on Earth. The theory of Evolution has been scientifically proven by scientific evidence and thus is the underlying principle and basis on which many other theories were formed.
Scientific Evidence for Evolution
The reason why this theory has been widely accepted by scientists in the world, and also taught in textbooks is because it is proven by much scientific evidence as follows:
1. Evidence from palaeontology
The study of past life based on fossil records and their relations to different geological time periods.
Palaeontology is used to study macroevolution, or large evolutionary trends.
Remains of organisms, which are preserved and found embedded in rock deposits, are called fossils. These fossils are extremely important in the study of the evolutionary history of life on Earth. This concrete evidence of evolution in the ancestry of organisms show us a record of past life. Fossils have attests to the fact that there is a huge variety of living things, and that extinct species had traits which were transitional between major groups of organisms. These transitional fossil specimens also prove that organisms evolve over time.
The most common of fossils discovered are skeletal structures of organisms buried. After an animals is buried (by mud), mineral salts will infiltrate the bones and fill up the pores, thereby hardening the bones into fossils.
There are also “fossils” showing moulds or imprints of previous organisms. Examples include leaves and footprints, the fossils of which are made in layers that then harden.
It is possible to see how an animal has evolved by putting the
fossils found on it in a chronological sequence. This can be determined if the fossils were found in sedimentary rock, which is formed by a horizontal series of layers called strata. Each layer contains fossils typical of the specific time period they are made. Geologists can estimate the time periods in which these strata are formed (by radiometric dating), and thus predict the age of fossils found. it. This is thePrinciple of Superposition, which is fundamental to understanding the age of rocks; at any one place it indicates the relative ages of the rock layers and of the fossils they contain.
Radiometric dating can be done using minerals that contain naturally-occurring
radioactive elements and measuring the amount of change or decay in those elements to calculate approximately how many years ago the rock formed. Radioactive elements can serve as natural clocks, because the rate of
emission or decay is measurable and because it is not affected by external factors.
Rock strata can also give clues about the environments in which an animal or plant lived. The chemical make-up of these strata can tell us the balance of gases in ancient atmospheres. Major cataclysmic events such as eruptions and meteor strikes also leave there mark on the fossil record.
By studying the number and complexity of different fossils at different stratigraphic levels, the older fossil-bearing rocks contains fewer organisms or simpler structures while younger rocks contain fossils of a variety of organisms, often with increasing complex s
tructures. Many species occur at a certain stratigraphic and then disappear layer. These species are interpreted to have orignated in those time periods (derived form the age of rock) and then became extinct, by the Law of Fossil Succession.This could have been due to the constantly changing geographical regions and climatic conditions, where these extinct organisms are replaced by other organisms which have adapted to the new environment. This again provides evidence fot the mechanism of natural selection in the theory of evolution.
*Example* of Transitional Forms of Life – Lobed-finned Fishes
Tetrapods are vetebrate animals which had legs, and this makes them distinct from the other vetebraes which had fins (fishes). The lobe-finned fish is an example of a transitional form of life, and are known to be the first tetrapods.They have bony structures that are organized much like the bones in the forelimbs and hind limbs of tetrapods, but the finger-like bones look
like unjointed fin rays, rather than the truly jointed finger bones of tetrapods.
*Example* of Transitional Forms of Life – the Archaeopteryx
Fossils of the Archaeopteryx were discovered in southern Germany in 1861. It had bird characters, feathers and wings, but also had reptilian characters, the skeleton of a small theropod (flesh-eating) dinosaur, with a long bony tail, fingers with claws on the leading edge of the wing, and teeth in the jaws.
Later, new bird specimens were found in China and Spain, and were 30 to 40 million years younger. These bird specimens have short bony tails and reduced hand claws – they were becoming more bird-like. Thus a long series of fossils through the Jurassic and Cretaceous periods, a span of 140 million years, document the evolutionary transition from reptile to bird.
*Example* Evolution of Whales
In 1994,the fossils of Ambulocetus natans, whose name means “walking whale that swims,” wasdiscovered from middle the Eocene rocks of Pakistan. This species provides fossil evidence of
the origin of aquatic locomotion in whales. Ambulocetus preserves large forelimbs and hind
limbs with large hands and feet, and the toes have hooves as in mesonychians. Ambulocetus
is regarded as having webbing between the toes and it could walk on land as well as swim;
thus, it lived both in and out of the water.
From late Eocene time onward, evolution in whales shows reduction of the hind-limbs,
modification of the forelimbs and hands into flippers for steering and development of a
massive tail – all of which are modifications for the powerful swimming of the modern whale.
The skull of the modern beluga whale (below right) has its nostrils placed at the top of its skull. Past specimens of the whale had nostrils at the front of the skull. Transitory fossils of whales are shown to have nostrils in the middle of the skull.
“Missing Links”
There are, however, limitations on the information fossils can supply. Fossilization is an improbable event. Most often, bones and other materials are crushed or consumed before they can be fossilized. In addition, fossils can only form in areas where sedimentary rock is formed, such as ocean floors. Organisms that live in these environments are therefore more likely to be fossilized. Erosion of exposed rock faces or through the crushing action of geological movements can destroy fossils even after they are formed. All of these conditions lead to large and numerous gaps in the fossil record.
Therefore, there are gaps in the fossil record due to the incomplete data collection, but the more we learn about the evolution of specific species line, these gaps are filled by traditional fossil specimens, which are rarely bizarre or unexpected; they fit into the predictions of evolutionary trees. Dinosaurs with feathers and whales with legs are pretty startling discoveries, but biologists were convinced they existed from the predictions of their evolutionary trees.
2. Evidence from Comparative Anatomy
Body Structure
Many organisms share similar body structures as they inherited the characteristics from a common ancestor, an example being the vertebrates (animals with internal skeletons). For instance, the arms of humans, the forelegs of dogs and cats, the wings of birds, and the flippers of whales and seals all have the same types of bones (humerus, radius, and ulna) because they have retained these traits of their shared common ancient vertebrate ancestor.
The degree of resemblance would show the how closely related two organisms are in evolution. If structures are homologous (fundamentally similar), a common origin is suggested. If these structures serve different purposes in the adult of the organisms, their origin and embryonic development is traced. The studying of embryonic development is called embryology. During the course of development, the embryo passes through stages in which it resembles its ancestors. For example, human embryos have a tail at some stages, like those of our primate ancestors. Thus a similar developmental origin suggests that they are the same structures and thus are likely to be derived from a common ancestor. Some examples are shown below:
*Example* Pentadactyl limb
This pattern of limb bones are an example of homologous structures found in all classes of tetrapods (amphibians to mammals). It can be traced back to the fins of certain fossil fishes from which the first amphibians are thought to have evolved. Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life.
For instance:
In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming.
In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, and the hook-like first digit remains free for hanging from trees.In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.
Vestigial Organs
These organs are smaller and simpler compared to the ancestral species, as they are underdeveloped or degenrated. These changes could be due to environmental changes, or the modes of life of the species. The organs may have since become unnecessary or non-functional, however some have developed other purposes. Examples of vestigial organs are the hind limbs of whales, and the wings of flightless birds such as the ostrich (now used in mating rituals).
Much of the anatomic differences between these species are explained by relatively few genetic differences that have a large effect upon the organism.
3. Evidence from Biogeography
Historical Biogeography
This is concerned with the origins and evolutionary histories of species on a long time scale. It depends heavily on other evidence, such as fossil records which tell historical biogeographers the distribution and past interactions of organisms, and molecular biology provides them with metabolic molecules which track the relatedness of the species in relation to change in time.
Historical bioclassified in their relatedness, but the species names are replaced by the geographic location they were found in. geographers make use of an area cladogram, which consists of a taxonomic tree where the organisms are classified in their relatedness, but the species names are replaced by the geographic location they were found in.
The cladogram allows them to scientists to determine how the difference in environments has affected the evolutionary history of different species of common origin.
Ecological Biogeography
Ecological Biogeograhers study the ways in which species develop and interact in the presence of other species and different environments. They tend to study island communities as a type of experimental system to test hypotheses about species development.
In ecological biogeography, they are concerned with species
richness, which is the number of species an area supports. They had developed the species richness equilibrium model which is used to determine the changes in extinction and immigration rates which will tend toward the equilibrium point, which is different for every island, depending on resources and degree of separation from other areas.
4. Evidence from Geographical Distribution
Major isolated land areas and island groups form their own distinct group of plant and animal communities. For instance, Australia had more than 100 species of marsupials but none of the modern placental species, and the only living representatives of primitive egg-laying mammals (monotremes),such as the echidna and platypus, can be found only in Australia and are totally absent in the rest of the world. The isolated islands of Hawaii and New Zealand had unique species of insects, birds and plants, but land mammals were entirely absent. This evidence implies that these organisms in each continent show adaptive radiation and evolve along their own lines when isolated.Even in similar habitats in similar geographic areas, the species are entirely different. For example, there were short-tailed monkeys, elephants, lions and giraffes in Africa, but there were long-tailed monkeys, cougars, jaguars and llamas in South America.
Evidence for migration and isolation
The fossil record for the camel indicated that evolution of camels started in North America, from which they migrated across the Bering Strait into Asia and hence to Africa, and through the Isthmus of Panama into South America. Once isolated, they evolved along their own lines, giving the modern camel in Asia and Africa and llama in South America.
Continental drift
The same kinds of fossils are found from areas known to be adjacent to one another in the past but which, through the process of continental drift, are now in widely divergent geographic locations. For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica. Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates. This shows that these organisms have come from a common ancestor and then evolved in their own unique biotic environment after continental drift.
*Example* Marsupials
Marsupial mammals are found in the Americas as well as Australia and New Guinea.
Fossils of the marsupials were also found in Australia, South America and Antartica. Scientists later found out that these three areas were actually part of the same land mass called Gondwana, which split apart 160 to 90 millions of years ago. Thus the marsupials had actually rode with the continents to their present locations, and thus these animals which are similar had most likely come from a common ancestor.
5. Evidence from Chemical / Molecular Similarities
DNA Complex
Though there is a great diversity of life on Earth, we all share complex molecular machinery, such as the whole DNA/ RNA code of life and protein synthesis machinery and the ATP system of energy transfer. We have similar DNA coding, which contains the instructions to creating tremendous numbers of proteins in living organisms, and these are only made from 20 amino acids. All living organisms share the ability to create complex molecules form carbon, of which 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements. These plants and animals inherit particular combinations of genes from their parents. This molecular similarity implies that all living organisms come from a similar ancestry.
Also, the degree of difference between proteins in different species was proportional to the time that they split apart. For instance, the proteins found in humans would be more similar to that of chipanzees, than that of cows. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons.[1] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes.[ Thus the amount of difference is proportional to the time of divergence on the evolutionary tree.
We can make use of molecular Biology as molecular clocks, in the hypothesis of neutral evolution, where the variability in the structure of the molecule does not affect the functionality of the molecule.
Molecular clocks must be present in all the organisms being studied, and must be under strong functional constraint such that functional regions are highly conserved. An example is Cytochrome c.
The variance of the Cytochrome
c protein in living cells is measured by the number of differing amino acids it contains. Each differig amino acid is due to a base pair substitution (mutation). If the average time it takes for a base pair of the cytochrome c gene to mutate is N years, and the number of amino acids making up the cytochrome c protein in monkeys differ by one from that of humans, this leads to the conclusion that the two species diverged N years ago.
Depending on the rate of substitution, molecules may be used to determine ancient relationships or relatively recent ones. Ribosomal RNA has a very slow rate of substitution, so it is most commonly used in conjunction with fossil information to determine relationships between extremely ancient species.
Life processes
Life organisms all need energy for growth, repair, and reproduction of cells, and this energy is obtained from sunlight by photosynthesis (plants) or by consuming plants, and other organisms eating plants.
These major chemical and anatomical similarities can be explained by the theory of evolution, where all living organisms came into existence from a common ancestor or similar natural processes.
6. Evidence from Genetic Changes over Generation
Antibiotic and Pesticide Resistance
Bacteria have been found to become resistant to antibiotics, especially when they are in an environmental crisis. This happens when antibiotics cause most bacteria to die off, leaving the some immune bacteria left, which then thrives and forms a new generation of antibiotics-resistant bacteria.
This is the same for plants and insects as well, to become resistant to pesticides and insecticides. These organisms tend to mature and reproduce large numbers in a short amount of time and thus have a potential for very fast evolutionary changes.
*Examples*
· The appearance of vancomycin resistant Staphlococcus aureus, and the danger it poses to hospital patients is a direct result of evolution through natural selection.
· The appearance of DDT resistance in various forms of Anopheles mosqitoes.
· The appearance of myxomatosis resistance in breeding rabbit populations in Australia.
These examples are all evidence of the existence of evolution in situations of evolutionary selection pressure in species in which generations occur rapidly.
Artificial Selection
Artificial selection provides us with a model that helps us undersyand natural selection. It helps us to envision natural conditions acting selectively on populations and thus causing natural changes. Examples include the domesticated plants and animals that humans have produced through experiments that show drastic changes in organisms through selective breeding.
Ecology
The environment affects the evolution of living things. Populations evolve to meet the demands of their surroundings. Organisms either have genetic tools to make opportunies to live in an ecosystem, or they do not. In this way, the environment exerts “pressure” on the population to evolve, with raw material provided by persistant variation within a population.
*Example* Colobines
The colobines (a sub-family of Old-World Monekeys) are leaf eaters, unlike the Old World monkeys, which prefer eating fruits or insects. The donc langur, a colobine monkey from Asia) are found to possess a second enzyme (RNASE 1B), other than the one (RNASE 1) found in most monkeys. This RNASE 1B was produced during the duplication of RNASE, and the former enzyme works best in PH6.3 while the original one works best at PH 7.4. Interestingly, the colobine’s small intestines have an acidity of PH 6 to 7, unlike the PH levels of most monkeys and humans, which range from 7.4 to 8.
Scientists and researchers have also inferred that the duplication of the second enzyme happened after the colobines broke off from the Old World Monkeys. This evidence clearly implies evolution as these monkeys now have enzymes that work better at digesting dietary RNA.
References
Websites
Scientific Theories
http://en.wikipedia.org/wiki/Theory
The Theory of Evolution
http://en.wikipedia.org/wiki/Theory_of_evolution
Evidence of Evolution
http://en.wikipedia.org/wiki/Evidence_of_evolution
http://anthro.palomar.edu/evolve/evolve_3.htm
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_01
http://www.gate.net/~rwms/EvoEvidence.html
http://www.agiweb.org/news/evolution.pdf
http://evolution.berkeley.edu/evolibrary/search/topicbrowse2.php?topic_id=46
http://www.sparknotes.com/biology/evolution/evidence/
http://www.txtwriter.com/backgrounders/Evolution/EVcontents.html
http://bioweb.cs.earlham.edu/9-12/evolution/HTML/live.html
Books
Selection in natural populations by Jeffry B. Mitton.
Oxford ; New York : Oxford University Press, 1997.
Darwin and the theory of evolution by Robert Greenberger.
New York : Rosen Central Primary Source, 2005.
The theory of evolution: what it is, where it came from, and why it works by Cynthia Mills
Hoboken, N.J. : Wiley, c2004.
All the information in this webpage is done by Lew GuiQi (16) Class 309 for Biology PT on Evolution, in the role of the Science Investigator.