“It is not the strongest of the species that survives, nor ...



“It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most adaptable to change.” - Charles DarwinPrinciples of EvolutionPre-Darwinian TheoriesJames UssherAlthough Darwin deserves much of the credit for modern evolutionary theory, he was not the first person to come up with the idea. Evolution is the process of biological change by which descendants come to differ from their ancestors. This concept had been discussed for more than 100 years before Darwin proposed his theory of how evolution works. Evolution is the central theme in all fields of biology today.Charles LightfootDr. Charles Lightfoot of Cambridge University in England had the last word though.He proclaimed that the time of creation was 9:00 A.M. on October 23, 4004 B.C.Great Chain of BeingThis belief that the earth and life on it are only about 6000 years old fit neatly with the then prevalent theory of the "Great Chain of Being.”This idea was that God created an infinite and continuous series of life forms, from simplest to most complex, and that all organisms, including humans, were created in their present form and that they have remained unchanged since then.Given these strongly held beliefs, it is not surprising that 17th and 18th century European biology consisted mainly of the description of plants and animals as they are with virtually no attempt to explain how they got to be that way.John RayThe concept of genus and species was actually developed in the late 1600's by John Ray, an English naturalist and ordained minister.It was Carolus Linnaeus who used this system to name us Homo sapiens.Linnaeus also placed us in the order Primates along with all of the apes, monkeys, and prosimians.At the time this was radical since it implied that people were part of nature, along with other animals and plants.It also meant that we were biologically closer to the other primates than to all other animals.Carolus LinnaeusCarolus Linnaeus , (a.k.a. Karl von Linne) was the leading biological scientist of the mid-18th century.His 180 books are filled with precise descriptions of nature, but he did little analysis or interpretation.Linnaeus believed that he was just revealing the unchanging order of life created by God.He rejected the idea of biological evolution based on his belief in the “great chain of being”.Late in his life he was troubled by the fact that plant hybrids could be created by cross pollination.His most important contribution to science was his logical classification system for all living things.George LeclercLate in the 18th century, a small number of European scientists began to quietly suggest that life forms are not fixed.The French mathematician and naturalist, George Louis Leclerc actually said that living things do change through time.He speculated that this was somehow a result of influences from the environment or chance.He believed that the earth must be much older than 6000 years and in 1774 he speculated that the earth must be at least 75,000 years old.Leclerc was the first to suggest that humans and apes are related. Leclerc was careful to hide his radical views in a limited edition natural history book series called Histoire Naturelle.By doing this, he avoided broad public criticism.Leclerc was a quiet pioneer in asserting that species can change over generations, but he publicly rejected the idea that species could evolve into other species.One of his most significant contributions to the biological sciences was his insistence that natural phenomena must be explained by natural laws rather than theological doctrine.Erasmus DarwinAnother late 18th century ‘closet-evolutionist’ was Erasmus Darwin, the grandfather of Charles Darwin.Erasmus was an English country physician, poet, and amateur scientist.Darwin believed that evolution had occurred in living things including humans, but he had fuzzy ideas about what might be responsible for this change.He wrote of his ideas about evolution in poems and a relatively obscure book called Zoonomia.In this book he suggested that the earth and life on it must have been evolving for "millions of ages before the commencement of the history of mankind."Jean-Baptiste LamarkThe first evolutionist who confidently and very publicly stated his ideas about biological change was Jean-Baptiste Chevalier de Lamarck. Unfortunately, his theory about these processes was incorrect. Lamarck believed that microscopic organisms appear spontaneously from inanimate materials and then evolve, gradually and progressively into more complex forms through a constant striving for perfection.Use-Disuse TheoryHe also believed that evolution was mostly due to the inheritance of acquired characteristics as creatures adapted to their environments.For example, evolution occurs when an organism uses a body part in such a way that it is altered during its lifetime and this change is then inherited by its offspring.Lamarck’s “Use-Disuse Theory” said that giraffes evolved their long necks by each generation stretching further to get leaves in trees and that this change in body shape was then inherited.He also believed that birds, like herons and egrets, evolved their long legs by stretching them to remain dry.Lamarck believed that creatures could develop new organs or change the structure and function of old ones as a result of their use or disuse.Lamarck did not invent the idea of inheritance of acquired characteristics, but stated it clearly and publicly in an 1809 publication called Philosophie Zoologique.George CuvierDespite his criticism of Lamarck, Cuvier did not reject the idea that there had been earlier life forms.He was the first scientist to document extinctions of ancient animals and was an internationally respected expert on dinosaurs.But, he rejected the idea that their existence implied that evolution had occurred – he strongly maintained the idea of the "fixity" of species.CatastrophismCuvier advocated the theory of catastrophism, like most other leading scientists of his day.Casastrophism is the violent and sudden natural catastrophes such as great floods and the rapid formation of major mountain chains.According to Cuvier, plants and animals living in those parts of the world where such events occurred were often killed off.Then new life forms moved in from other areas.As a result, the fossil record for a region shows abrupt changes in species.Cuvier's explanation relied solely on scientific evidence rather than biblical interpretation.James HuttonIn the late 1700s, the Scottish geologist James Hutton proposed that the changes in landforms resulted from slow changes over a long period of time, a principle that became known as gradualism.He argued that the laying down of soil or the creation of canyons by rivers cutting through rock were not the result of large-scale events. They resulted from slow processes that had happened in the past. This idea has become so important to evolution that today the term gradualism is often used to mean the gradual change of a species through evolution.Charles LyellA careful examination of European geological deposits in the early 19th century led the English lawyer and geologist, Charles Lyell, to conclude that Cuvier's catastrophism theory was wrong.He believed that there have been slower, progressive changes.In his work, Principles of Geology, Lyell documented the fact that the earth must be very old and that it has been subject to the same sort of natural processes in the past that operate today in shaping the land.These forces include erosion, earthquakes, glacial movements, volcanoes, and even the decomposition of plants and animals.Lyell provided evidence for the theory of uniformitarianism, which was developed originally by the 18th century Scottish geologist, James Hutton.Uniformitarianism held that the natural forces now changing the shape of the earth's surface have been operating in the past much the same way.In other words, the present is the key to understanding the past.All of these events have potentially affected the rate and direction of biological evolution.While George Cuvier and Charles Lyell strongly disagreed about how the earth got to be the way it is today, they both rejected the idea of biological evolution.But, neither man accepted a traditional Biblical account of creation and a young earth.Cuvier did not live long enough to learn about Charles Darwin's proof of evolution, but Lyell did.Charles DarwinCharles Darwin was born into a moderately wealthy family in Shrewsbury, England in 1809.His father, Robert, had the largest medical practice outside of London at the time and his mother, Susannah Wedgwood, was from a family of wealthy pottery manufacturers.She died when Charles was only 8 years old.He was then raised mostly by his father and older sisters.Darwin grew up in comparative luxury in a large house with servants.Robert & Susannah DarwinThis was a socially very conservative time in England that set narrow limits on a young man's behavior and future possibilities.The constraints on women in Darwin's social class were even greater.Most were given only enough education to efficiently manage the homes of their future husbands and raise their children.Young men were expected to go to university in order to prepare themselves to become medical doctors, military officers, or clerics in the Church of England.Charles DarwinMost other occupations were considered somewhat undesirable.At his father's direction, Darwin started university at 16 in Edinburgh, Scotland as a medical student.He showed little interest in medicine and was revolted by the brutality of surgery and dropped out after two years of study in 1827.His father then sent him to Cambridge University in 1828 to study theology.It was there that his life's direction took a radical direction.Adam Sedgwick & John HenslowHe became very interested in the scientific ideas of the geologist Adam Sedgwick and the naturalist John Henslow with whom he spent considerable time collecting specimens from the countryside around the university.At this time in his life, Darwin rejected the concept of biological evolution, just as his mentors Sedgwick and Henslow did.Charles DarwinHowever, Darwin had been exposed to the ideas of Lamarck while he was a student in Edinburgh.Following graduation from Cambridge in 1831 with a degree in theology, Darwin was clearly more interested in biology and geology than he was in a religious career.The Voyage of the BeagleJohn Henslow was able to help Darwin get a job on a British naval mapping expedition that was going around the world on what would become a five-year voyage.Initially, Darwin's father refused to allow him to go but was eventually persuaded by Charles and even agreed to pay for his passage and for that of his servant on the journey.They sailed two days after Christmas in 1831 aboard the survey ship H.M.S. Beagle with Darwin acting as an unpaid naturalist and gentleman companion for the captain, Robert Fitzroy.Darwin’s VoyageDarwin was only 22 years old at the time.The H.M.S. Beagle was a compact 90 foot long ship with a crew of 74; there was little space, even for the captain.Darwin shared a cramped 10 x 11 foot cabin with two other men and their belongings.Because of the Beagle's small size, it was generally thought by naval men that it was not suited for the rough seas it would encounter, especially at the southern tip of South America.Darwin’s VoyageDarwin frequently suffered from sea sickness on the voyage.It was during the beginning of the voyage that Darwin read the early books of Charles Lyell and became convinced by his proof that uniformitarianism provided the correct understanding of the earth's geological history.This intellectual preparation along with his research on the voyage were critical in leading Darwin to accept evolution.Galapagos IslandsEspecially important to the development of this understanding was his 5 week-long visit to the Galapagos Islands in the Eastern Pacific Ocean.It was there that he began to comprehend what causes plants and animals to evolve, but he apparently did not clearly formulate his views on this until 1837.Galapagos IslandsThe Galapagos Islands have species found in no other part of the world, though similar ones exist on the west coast of South America.Darwin was struck by the fact that the birds were slightly different from one island to another. He realized that the key to why this difference existed was connected with the fact that the various species live in different kinds of environments.Darwin identified 13 species of finches in the Galapagos Islands.Galapagos IslandsThis was puzzling since he knew of only one species of this bird on the mainland of South America, nearly 600 miles to the east, where they had all presumably originated.He observed that the Galapagos species differed from each other in beak size and shape.He also noted that the beak varieties were associated with diets based on different foods.He concluded that when the original South American finches reached the islands, they dispersed to different environments where they had to adapt to different conditions.Galapagos IslandsOver many generations, they changed anatomically in ways that allowed them to get enough food and survive to reproduce.We use the term adaptive radiation to refer to this sort of branching evolution in which different populations of a species become reproductively isolated from each other by adapting to different ecological niches and eventually become separate species.Darwin’s ObservationsDarwin’s ObservationsDarwin was struck by the variation of traits among similar species that he observed in all his travels. Variation is the difference in the physical traits of an individual from those of other individuals in the group to which it belongs. Variation can occur either among members of different species (interspecific variation) or among individuals of the same species (intraspecific variation). Darwin noted that the species found on one island looked different from those on nearby islands and that many of the islands’ species looked different from those on the nearest mainland.The differences between species on different islands was especially noticeable in the Galpagos Islands. For example, saddle-backed tortoises, which have long necks and legs, lived in areas with a lot of tall plants. Domed tortoises, with their shorter necks and legs, lived in wet areas rich in mosses and short plants. Finches with strong, thick beaks lived in areas with a lot of large, hard-shelled nuts, while those species of finch with more delicate beaks were found where insects or fruits were widely available.These observations led Darwin to realize that species may somehow be able to adapt to their surroundings. An adaptation is a feature that allows an organism to better survive in its environment. Adaptations can lead to genetic change in a population over time.Fossil And Geologic EvidenceOn his voyage, Darwin found fossil evidence of species changing over time. In Argentina, he found fossils of huge animals, such as Glyptodon, a giant armadillo. The fact that these fossils looked like living species suggested that modern animals might have some relationship to fossil forms. These fossils suggested that in order for such changes to occur, Earth must be much more than 6000 years old.During his voyage, Darwin also found fossil shells of marine organisms high up in the Andes mountains. Darwin later experienced an earthquake during his voyage and saw firsthand the result: land that had been underwater was moved above sea level. This experience explained what he saw in the Andes. Darwin’s observations on his voyage supported Lyell’s theory that daily geologic processes can add up to great change over a long period of time.Thomas MalthusIn 1798, Thomas Malthus, an English clergyman and economist, published Essay on the Principles of Population.In it he said that human populations will double every 25 years unless they are kept in check by limits in food supply.In 1838, Darwin read Malthus' essay and came to realize that all plant and animal populations have this same potential to rapidly increase their numbers unless they are kept in check by predators, diseases, and limitations in food, water, and other resources.Darwin’s TheoryThis fact was key to his understanding of the process of natural selection.Darwin realized that the most fit individuals in a population are the ones that are least likely to die of starvation and are most likely to pass on their traits to the next generation.Darwin didn’t believe that evolution follows a predetermined direction or that it has an inevitable goal.His explanation that evolution occurs as a result of natural selection implied that chance plays a major role.He understood that it is a matter of luck whether any individuals in a population have variations that will allow them to survive and reproduce.If no such variations exist, the population rapidly goes extinct because it cannot adapt to a changing environment.Unlike Lamarck, Darwin did not believe that evolution inevitably produces more complex life forms and that the ultimate result of this process is humans.Darwin and Natural SelectionMost educated people in Europe and the Americas during the 19th century had their first full exposure to the concept of evolution through the writings of Charles Darwin.But he didn’t invent the idea; that happened long before he was born.Darwin carried out the necessary research to document that evolution had occurred and then made the idea acceptable for scientists and the general public.This wasn’t easy since the idea of evolution had been strongly associated with radical scientific and political views coming out of post-revolutionary France.These ideas were widely considered to be a threat to the established social and political order.Publishing the WorkDarwin didn’t rush his ideas about evolution and natural selection into print.He first concentrated his efforts on writing the account of his voyage on the Beagle and analyzing the many preserved animal and plant specimens and extensive notes that he brought back with him.Another major influence was the widespread Christian evangelical influence in England during the 1830's and 1840's.He could have been charged with sedition and blasphemy for widely publishing his unpopular theory.Publishing the WorkAfter returning from the voyage around the world, he settled down in England, married Emma Wedgwood (his wealthy first cousin), raised a large family, and quietly continued his research at his newly purchased country home 16 miles south of London.In 1842 he wrote a 35 page summary of his theory about evolution.This was expanded to a 230 page manuscript in 1844, but it was not published and apparently was only known to a few people in British scientific circles.Darwin busied himself over the next 20 years establishing his reputation as an important naturalist by growing and studying orchids, pigeons, earthworms, and other organisms at his home.He spent 8 of these years studying and writing about barnacles that people had sent him from around the world.It wasn’t until he was 50 years old, in 1859, that Darwin finally published his theory of evolution in full for his fellow scientists and for the public at large.Publishing the WorkHe did so in a 490 page book titled On the Origin of Species.From the very beginning it was very popular and controversial. The first edition came out on November 24, 1859 and sold out on that day.By 1872 it went through six editions.Alfred WallaceWhat finally convinced Darwin that he should publish his theory for the general public was the draft of an essay that he received in the summer of 1858 from a younger British naturalist named Alfred Wallace. Wallace was hard at work collecting biological specimens in Southeast Asia for sale to museums and private collectors.Darwin was surprised to read that Wallace had come upon essentially the same explanation for evolution. Being a fair man, Darwin insisted that Wallace also get credit for the natural selection theory during debates over its validity that occurred at a meeting of the British Association for the Advancement of Science at Oxford University in 1860.We now know that Darwin deserves most of the credit. In 1837, one year after he returned from the voyage on the Beagle, he made detailed notes on the idea of evolution by means of natural selection.At that time, Wallace was only 14 years old.On the Origin of SpeciesIt was Darwin's book, rather than Wallace's essay, that had the most impact on the Victorian public. Darwin not only described the process of natural selection in more detail, but he also gave numerous examples of it.It was his On the Origin of Species that convinced most scientists and people in the late 19th century that life forms do change through time.This prepared the public for the acceptance of earlier human species and of a world much older than 6000 yearsBut, both Darwin and Wallace failed to understand an important aspect of natural selection.They realized that plant and animal populations are composed of individuals that vary from each other in physical form.They also understood that nature selects from the existing varieties those traits that are most suited to their environment.If natural selection were the only process occurring, each generation should have less variation until all members of a population are essentially identical to each other.That does not happen. Each new generation has new variations.Darwin was aware of this fact, but he did not understand what caused the variation.The first person to begin to grasp why this happens was an obscure Central European monk named Gregor Mendel.Through plant breeding experiments carried out between 1856 and 1863, Mendel discovered that there is a recombination of parental traits in offspring.Sadly, Darwin and most other 19th century biologists never knew of Mendel and his research.Charles Darwin's convincing evidence that evolution occurs was very threatening to many Christians who believed that people were created specially by God and that they have not changed biologically since that creation.The idea that there could have been prehistoric humans who were anatomically different from us was rejected for these reasons.However, Charles Lyell's geological evidence that the earth must be much older than 6,000 years along with the rapidly accumulating fossil record of past evolution convinced people in the 1860's to think what had been unthinkable earlier.Archaeological confirmation of the existence of prehistoric Europeans had been accumulating since the 1830's.Until the late 1850's, it had been widely rejected or misinterpreted.Much of this fossil evidence had been collected by Jacques Boucher de Perthes, a customs officer in northern France during the early 1800's.His hobby was collecting ancient stone tools from deep down in the Somme River gravel deposits.Since he found these artifacts in association with the bones of extinct animals, he concluded that they must have been made at the time that those animals lived.Boucher de Perthes tried to publish his findings in 1838. They were rejected by all important scientists and scientific journals.By 1858, his claims were beginning to be accepted by some Western European scientists.Darwin's publication of On the Origin of Species the following year convinced even more people that Boucher de Perthes had been right.FootnoteCharles Darwin was an active collector of plant and animal specimens and a prodigious note taker on the voyage of H.M.S. Beagle.By the time the ship returned to England in 1836, he had accumulated 5,436 plant and animal specimens that had been dried or preserved in alcohol.He had 368 pages of notes on plants and animals as well as 1,383 pages of geological observations.In addition, he had a 770 page diary that was the basis for his later popular book of his narrative on the voyage.From the time that Charles Darwin published "On the Origin of Species" in 1859 on up to the present, the presumptions of many people led them to misread the title.They assumed that it was "On the Origin of the Species".The implication of adding "the" is that his book was about human evolution.In fact, that was not the case, though it had implications for human evolution.It focused on non-human animals and the mechanisms of evolution.He did not pointedly address the question of human evolution until the publication of his 1871 book "Descent of Man and Selection in Relation to Sex".Darwin died in 1882 at the age of 73.Throughout his life he struggled with his health spending a great deal of time bed-ridden and incapable of work.Charles and Emma had 10 children, 6 boys and 4 girls, and 8 grandchildren.The last direct descendant of Darwin, Charles Galton Darwin, died in 1962.Theory of Natural SelectionDarwin’s TheoryDarwin realized that the most fit individuals in a population are the ones that are least likely to die of starvation and are most likely to pass on their traits to the next generation.Darwin didn’t believe that evolution follows a predetermined direction or that it has an inevitable goal.His explanation of evolution occuring as a result of natural selection implied that chance plays a major role.Darwin understood that it is a matter of luck whether any individuals in a population have variations that will allow them to survive and reproduce.If no such variations exist, the population rapidly goes extinct because it cannot adapt to a changing environment.Unlike Lamarck, Darwin did not believe that evolution inevitably produces more complex life forms and that the ultimate result of this process is humans.Natural SelectionDarwin’s first major principle is the theory of natural selection.Natural selections occurs when nature selects the best adapted varieties to survive and to reproduce.There are four main principles to the theory of natural selection: variation, overproduction, adaptation, and descent with modification.Variation It’s the inherited traits, or variations, that exist in every population are the basis for natural selection. These differences among individuals result from differences in the genetic material of the organisms, whether inherited from a parent or resulting from a genetic mutation.OverproductionWhile having many offspring raises the chance that some will survive, it also results in competition between offspring for resources. Adaptation Sometimes a certain variation allows an individual to survive better than other individuals it competes against in its environment. More successful individuals are “naturally selected” to live longer and to produce more offspring that share those adaptations for their environment.Descent With ModificationNatural selection will result in species with adaptations that are well suited for survival and reproduction in an environment. More individuals will have the trait in every following generation, as long as the environmental conditions continue to remain beneficial for that trait.Darwin came to understand that any population consists of individuals that are all slightly different from one another.Those individuals having a variation that gives them an advantage in staying alive long enough to successfully reproduce are the ones that pass on their traits more frequently to the next generation.As a result, their traits become more common and the population evolves.Darwin called this "descent with modification."Survival of the FittestDarwin didn’t believe that the environment was producing the variation within the finch populations.He correctly thought that the variation already existed and that nature just selected for the most suitable beak shape and against less useful ones.By the late 1860's, Darwin came to describe this process as the "survival of the fittest."This is very different from Lamarck's incorrect idea that the environment altered the shape of individuals and that these acquired changes were then inherited.Evidence of EvolutionEvidence For EvolutionThe evidence for evolution comes from four indisputable sources:the fossil record of change in earlier speciesthe chemical and anatomical similarities of related life formsthe geographic distribution of related speciesthe recorded genetic changes in living organisms over many generationsThe Fossil RecordFossil remains of animals and plants found in sedimentary rock deposits give us an indisputable record of past changes through time.This fossil evidence proves the fact that there has been a tremendous variety of living things.Fossil existence confirms that species are not fixed but can evolve into other species over time.Fossil evidence also shows that what appear to be gaps in the fossil record are due to incomplete data collection.The more that we learn about the evolution of specific species lines, the more that these so-called gaps are filled with fossil specimens.One of the first of these gaps to be filled was between small two-legged dinosaurs and birds.Just two years after Darwin published his book, “On the Origin of Species”, a 150-145 million year-old fossil of Archaeopteryx was found in southern Germany.It had jaws with teeth and a long bony tail like dinosaurs, broad wings and feathers like birds, and skeletal features of both.This discovery verified the assumption that birds had reptilian ancestors.Since the discovery of Archaeopteryx, there have been many other crucial evolutionary gaps filled in the fossil record.Geographic Distribution of Related SpeciesAnother clue to patterns of past evolution is found in the natural geographic distribution of related species.It’s clear that major isolated land areas and island groups often evolved their own distinct plant and animal communities.Before humans arrived 60-40,000 years ago, Australia had more than 100 species of kangaroos, koalas, but none of the more advanced terrestrial mammals such as dogs, cats, bears, horses.Land mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand.Each of these places have a great number of plant, insect, and bird species that are found nowhere else in the world.The most likely explanation for the existence of these unique environments is that the life forms in these areas have been evolving in isolation from the rest of the world for millions of years.EmbryologyThe relationship between crabs, which can walk, and barnacles, which are fixed in one place as adults, fascinated Darwin. He collected barnacles for many years and noted that the larvae of the crabs and barnacles were similar. Barnacle and crab larvae both swim and look alike, but the adult animals look and behave very differently.Like larvae, embryos of vertebrates can be hard to tell apart. Fish, birds, reptiles, and mammals all have gill slits as embryos. The gill slits become gills in adult fish. In mammals, the gill slits develop into structures of ears and throats. These observations formed an important part of Darwin’s evidence for common descent. The similar features of embryos in very different organisms suggest evolution from a distant common ancestor.Chemical SimilaritiesAll living things on earth share the ability to create complex molecules out of carbon and a few other elements.99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements.Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things. Anatomical SimilaritiesSome of Darwin’s best evidence came from comparing the body parts of different species. Chief among this evidence are homologous structures. Homologous structures are features that are similar in structure but appear in different organisms and have different functions. Their appearance across different species offers strong evidence for common descent. It would be unlikely for many species to have such similar anatomy if each species evolved independently.Many groups of species share the same types of body structures because they inherited them from a common ancestor that had them.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 because they have retained these traits of their shared common ancient vertebrate ancestor.All of these major chemical and anatomical similarities between living things can be accounted for because they either share a common ancestry or they came into existence as a result of similar natural processes.These facts make it difficult for many biological scientists to accept a theory of special and independent creation of different species.Structural PatternsSome organisms have structures or organs that seem to lack any useful function, or at least are no longer used for their original purpose.Snakes have tiny pelvic bones and stump-like limbs, even though snakes don’t walk. Underdeveloped or unused features are called vestigial structures. Vestigial structures are remnants of organs or structures that had a function in an early ancestor. As vertebrates, snakes share a common ancestor with lizards and dogs. The tiny pelvic bones and hind limbs in many snakes are homologous to the pelvic bones of the lizard and dog.The wings of ostriches are another example of vestigial structures. Ostriches have wings that they use for balance but not to fly.In humans, the appendix is an example of a vestigial structure.Genetic Changes Over GenerationsThe earth's environments are constantly changing in subtle and complex ways.When the changes are so great as to go beyond what most members of a population of can tolerate, widespread death occurs.Darwin observed that not all individuals always perish.Natural populations have genetic diversity. Those individuals whose characteristics allow them to survive an environmental crisis likely will be the only ones able to reproduce.As a result, their traits will be more common in the next generation - evolution of the population occurs.Patterns of EvolutionPatterns of EvolutionBiologists often use the term macroevolution to refer to large-scale evolutionary patterns and processes that occur over long periods of time.There are six important topics in macroevolution: extinction, adaptive radiation, convergent evolution, co-evolution, punctuated equilibrium and changes in developmental genes.ExtinctionMore than 99 percent of all species that have ever lived are now extinct. Extinctions usually happen for the reasons that Darwin proposed.However, several times in Earth's history mass extinctions wiped out entire ecosystems. Food webs collapsed and this disrupted energy flow through the biosphere.Under these environmental pressures, extinction is not necessarily related to ordinary natural selection.Until recently, most researchers looked for a single, major cause for each mass extinction. One hypothesis suggests that at the end of the Cretaceous Period, about 65 million years ago, the impact of a huge asteroid wiped out the dinosaurs and many other organisms. Scientific evidence confirms that an asteroid did strike Earth at that time. The impact threw huge amounts of dust and water vapor into the atmosphere and probably caused global climate change. It would be reasonable to assume that this kind of event played a role in the end of the dinosaurs.However, many paleontologists think that most mass extinctions were caused by several factors. During some mass extinctions, many large volcanoes were erupting, continents were moving, and sea levels were changing. Adaptive RadiationStudies of fossils or of living organisms show that a single species, or a small group of species, has evolved into diverse forms that live in different ways. This process is known as adaptive radiation. In the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species.Adaptive radiation can also occur on a much larger scale. Dinosaurs were the products of a spectacular adaptive radiation among ancient reptiles.The first dinosaurs and the earliest mammals evolved at about the same time. Dinosaurs and other ancient reptiles underwent an adaptive radiation first and “ruled” Earth for about 150 million years. During that time, mammals remained small and relatively scarce.But the disappearance of the dinosaurs cleared the way for the great adaptive radiation of mammals.Convergent EvolutionAdaptive radiations can have an interesting evolutionary “side effect.” They can produce unrelated organisms that look remarkably similar to one another. Sometimes, groups of different organisms, such as mammals and dinosaurs, undergo adaptive radiation in different places or at different times but in ecologically similar environments.These organisms start out with different “raw material” for natural selection to work on.But they face similar environmental demands, such as moving through air, moving through water, or eating similar foods. In these situations, natural selection may mold different body structures, such as arms and legs, into modified forms, such as wings or flippers. The wings or flippers function in the same way and look very similar. This process, by which unrelated organisms come to resemble one another, is called convergent evolution.Convergent evolution has occurred time and time again in both animals and plants.Convergent evolution involving fishes, two different groups of aquatic mammals, and swimming birds has resulted in sharks, dolphins, seals, and penguins whose streamlined bodies and swimming appendages look a lot alike. CoevolutionSometimes organisms that are closely connected to one another by ecological interactions evolve together. Many flowering plants can reproduce only if the shape, color, and odor of their flowers attract a specific type of pollinator. These kinds of relationships can change over time. An evolutionary change in one organism may also be followed by a corresponding change in another organism. The process by which two species evolve in response to changes in each other over time is called coevolution.The pattern of coevolution involving flowers and insects is so common that biologists in the field often discover additional examples. Charles Darwin saw an orchid with a long structure called a spur. Inside the tip of that 40-centimeter spur is a supply of nectar, which serves as food for many insects. Darwin predicted the discovery of a pollinating insect with a 40-centimeter structure that could reach the orchid's nectar. About fifty years later, researchers discovered a moth that matched Darwin's prediction.Punctuated EquilibriumDarwin was enormously impressed by the way Hutton and Lyell discussed the slow and steady nature of geologic change.Darwin, in turn, felt that biological change also needed to be slow and steady, an idea known as gradualism. In many cases, the fossil record confirms that populations of organisms did change gradually over time.There is also evidence that this pattern does not always hold.Some species, such as horseshoe crabs, have changed little from the time they first appeared in the fossil record. Much of the time these species are in a state of equilibrium, which means they do not change very much. At several points in the fossil record, changes in animals and plants occurred over relatively short periods of time. Some biologists suggest that most new species are produced by periods of rapid change.Rapid evolution after long periods of equilibrium can occur for several reasons.It may occur when a small population becomes isolated from the main part of the population.This small population can then evolve more rapidly than the larger one because genetic changes can spread more quickly among fewer individuals.Or it may occur when a small group of organisms migrates to a new environment. That's what happened with the Galapagos finches. Organisms evolve rapidly to fill available niches. Mass extinctions can open many ecological niches and provide new opportunities to those organisms that survive.That’s why some groups of organisms have evolved rapidly following mass extinctions.Scientists use the term punctuated equilibrium to describe this pattern of long, stable periods interrupted by brief periods of more rapid change.Developmental Genes and Body PlansBiologists have long suspected that changes in the genes for growth and differentiation during embryological development could produce transformations in body shape and size.Scientists can now perform experiments with gene expression by turning genes on or off and examining the results.These studies shed new light on how genetic change can produce major evolutionary transformations.“Master control genes,” called hox genes, guide development of major body structures in animals. Some determine which parts of an embryo become front and rear, or top and bottom.Others control the size and shape of arms, legs, or wings.Homologous control genes serve similar functions in animals as different as insects and humans - even though these animals haven't shared a common ancestor in at least 700 million years.Small changes in the activity of control genes can affect many other genes to produce large changes in adult animals. If one gene, called “wingless,” is turned on in an insect body segment, that segment grows no wings.This is interesting because some ancient insects had wing-like structures on all body segments. Yet modern insects have wings on only one or two segments. Changes in the activation of this gene could have enabled many-winged ancestors of modern insects to evolve into four-winged and two-winged forms.Small changes in the timing of cell differentiation and gene expression can make the difference between long legs and short ones, between long, slender fingers or short, stubby toes.Recent studies suggest that differences in gene expression may cause many of the differences between chimpanzee brains and human brains. ................
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