Evolution—The Theory and Its Supporting Evidence Chapter 7 Evolution—The Theory and Its Supporting Evidence

Darwin and the Galápagos During Charles Darwin’s five-year voyage (1831-1836) on the HMS Beagle, he visited the Galápagos Islands where he made important observations that changed his ideas about the then popular concept called the fixity of species an idea holding that all present-day species had been created in their present form and had changed little or not at all Darwin fully accepted the Biblical account of creation before the voyage

Route of HMS Beagle Map showing the route (red line) followed by Charles Darwin when he was aboard HMS Beagle from 1831 to 1836 The Galápagos Islands are in the Pacific Ocean west of Ecuador

The Galápagos Islands The Galápagos Islands are specks of land composed of basalt in the eastern Pacific

Darwin Developed the Theory During the voyage Darwin observed that fossil mammals in South America are similar yet different from present-day llamas, sloths, and armadillos that the finches and giant tortoises living on the Galápagos Islands vary from island to island and still resemble ones from South America, even though they differ in subtle ways These observations convinced Darwin that organisms descended with modification from ancestors that lived during the past the central claim of the theory of evolution

Galápagos Finches Darwin’s finches from the Galápagos Islands arranged to show evolutionary relationships Notice that beak shape varies depending on diet Berry eater Insect eaters Seed eaters Cactus eaters

Why Study Evolution? Evolution involving inheritable changes in organisms through time is fundamental to biology and paleontology Paleontology is the study of life history as revealed by fossils Evolution is a unifying theory like plate tectonic theory that explains an otherwise encyclopedia collection of facts Evolution provides a framework for discussion of life history

Evolution: Historical Background Evolution, the idea that today’s organisms have descended with modification from ancestors that lived during the past, is usually attributed solely to Charles Darwin, but it was seriously considered long before he was born, even by some ancient Greeks and by philosophers and theologians during the Middle Ages Nevertheless, the prevailing belief in the 1700s was that Genesis and the works of Aristotle explained the origin of life and contrary views were heresy

Evolution: Historical Background During the 18th century, naturalists were discovering evidence that could not be reconciled with literal reading of the Bible In this changing intellectual atmosphere, scientists gradually accepted a number of ideas: the principle of uniformitarianism, Earth’s great age, that many types of plants and animals had become extinct, and that change from one species to another occurred What was lacking, though, was a theoretical framework to explain evolution

Lamarck Jean-Baptiste de Lamarck According to this theory, (1744-1829) is best remembered for his theory of inheritance of acquired characteristics, even though he greatly contributed to our understanding of the natural world According to this theory, new traits arise in organisms because of their needs and are somehow passed on to their descendants Lamarck’s theory seemed logical at the time

Lamarck’s Theory Lamark’s theory was not totally refuted until decades later with the discovery that genes units of heredity cannot be altered by any effort by an organism during its lifetime

Darwin In 1859, Charles Robert Darwin (1809-1882) in which he detailed published On the Origin of Species in which he detailed his ideas on evolution formulated 20 years earlier and proposed a mechanism for evolution

Natural Selection Plant and animal breeders practice artificial selection by selecting those traits they deem desirable and then breed plants and animals with those traits thereby bringing about a great amount of change Observing artificial selection gave Darwin the idea that a process of selection among variant types in nature could also bring about change Thomas Malthus’ essay on population suggested that competition for resources and high infant mortality limited population size

Artificial Selection Through artificial selection humans have giving rise to dozens of varieties of domestic dogs, pigeons, sheep, cereal crops, and vegetables Wild mustard was selectively bred to yield broccoli, cauliflower, kale, and cabbage

Darwin and Wallace Darwin and Alfred Russel Wallace (1823-1913) read Malthus’ book and came to the same conclusion, that a natural process was selecting only a few individuals for survival Darwin’s and Wallace’s idea called natural selection was presented simultaneously in 1859

Natural Selection—Main Points Organisms in all populations possess heritable variations such as size, speed, agility, visual acuity, digestive enzymes, color, and so forth Some variations are more favorable than others some have a competitive edge in acquiring resources and/or avoiding predators Those with favorable variations are more likely to survive and pass on their favorable variations

“Survival of the Fittest” In colloquial usage, natural selection is sometimes expressed as “survival of the fittest” This is misleading because natural selection is not simply a matter of survival but involves inheritable variations leading to reproductive success

Alternate Uses for Structures Critics claim that complex features such as eyes and bird’s wings could not have evolved by natural selection Anything less than the existing structure would be useless The “to complex to have evolved” argument fails because it assumes that structures have always been used exactly as they are now

Not only Biggest, Strongest, Fastest One misconception about natural selection is that among animals only the biggest, strongest, and fastest are likely to survive These characteristics might provide an advantage but natural selection may favor the smallest if resources are limited the most easily concealed those that adapt most readily to a new food source those having the ability to detoxify some substance and so on...

Limits of Natural Selection Natural selection works on existing variation in a population It could not account for the origin of variations Critics reasoned that should a variant trait arise, it would blend with other traits and would be lost The answer to these criticisms existed even then in the work of Gregor Mendel, but remained obscure until 1900

Mendel and the Birth of Genetics During the 1860s, Gregor Mendel, an Austrian monk, performed a series of controlled experiments with true-breeding strains of garden peas strains that when self-fertilized always display the same trait, such as flower color Traits are controlled by a pair of factors, now called genes Genes occur in alternate forms, called alleles One allele may be dominant over another Offspring receive one allele of each pair from each parent

Mendel’s Experiments The parental generation consisted of true-breeding strains, RR = red flowers, rr = white flowers Cross-fertilization yielded a second generation all with the Rr combination of alleles, in which the R (red) is dominant over r (white)

Mendel’s Experiments The second generation, when self-fertilized produced a third generation with a ratio of three red-flowered plants to one white-flowered plant

Importance of Mendel’s Work The factors (genes) controlling traits do not blend during inheritance Traits not expressed in each generation may not be lost Therefore, some variation in populations results from alternate expressions of genes (alleles) Variation can be maintained

Genes and Chromosomes Complex, double-stranded helical molecules of deoxyribonucleic acid (DNA) called chromosomes are found in cells of organisms Specific segments of DNA are the basic units of heredity (genes) The number of chromosomes varies from one species to another fruit flies 8; humans 46; horses 64

Sexually Reproducing Organisms In sexually reproducing organisms, the production of sex cells pollen and ovules in plants sperm and eggs in animals results when cells undergo a type of cell division known as meiosis This process yields cells with only one chromosome of each pair so all sex cells have only 1/2 the chromosome number of the parent cell

Meiosis During meiosis, Formation of sperm is shown here sex cells form that contain one member of each chromosome pair Formation of sperm is shown here Eggs form the same way, but only one of the four final eggs is functional

Fertilization The full number of chromosomes The egg (or ovule) then is restored when a sperm fertilizes an egg or when pollen fertilizes an ovule The egg (or ovule) then has a full set of chromosomes typical for that species As Mendel deduced, 1/2 the genetic makeup of fertilized egg comes from each parent The fertilized egg grows by mitosis

Mitosis Mitosis is cell division In this example, Mitosis takes place that results in the complete duplication of a cell In this example, a cell with four chromosomes (two pairs) produces two cells each with four chromosomes Mitosis takes place in all cells except sex cells

Mitosis Once an egg has been fertilized, the developing embryo grows by mitosis

Modern View of Evolution During the 1930s and 1940s, paleontologists, population biologists, geneticists, and others developed ideas that merged to form a modern synthesis or neo-Darwinian view of evolution They incorporated chromosome theory of inheritance into evolutionary thinking They saw changes in genes (mutations) as one source of variation

Modern View of Evolution They completely rejected Lamarck’s idea of inheritance of acquired characteristics They reaffirmed the importance of natural selection According to modern evolutionary theory, populations rather than individuals evolve. Individuals with favorable traits are more likely to survive and reproduce if their variations are favorable As a result descendant populations possess variations in greater frequency

What Brings about Variation? Evolution by natural selection works on variation in populations most of which is accounted for by the reshuffling of genes from generation to generation during sexual reproduction The potential for variation is enormous with thousands of genes each with several alleles, and with offspring receiving 1/2 of their genes from each parent New variations arise by mutations change in the chromosomes or genes

Mutations Mutations result in a change in hereditary information Mutations that take place in sex cells are inheritable, whether they are chromosomal mutations affecting a large segment of a chromosome or point mutations individual changes in particular genes Mutations are random with respect to fitness they may be beneficial, neutral, or harmful

Mutations If a species is well adapted to its environment, most mutations would not be particularly useful and perhaps would be harmful But what was a harmful mutation can become a useful one if the environment changes

Neutral Mutations Information in cells is carried on chromosomes which direct the formation of proteins by selecting the appropriate amino acids and arranging them into a specific sequence Neutral mutations may occur if the information carried on the chromosome does not change the amino acid or protein that is produced

What Causes Mutations? Some mutations are induced by mutagens agents that bring about higher mutations rates such as some chemicals ultraviolet radiation X-rays extreme temperature changes Some mutations are spontaneous occurring without any known mutagen

Species Species is a biological term for a population of similar individuals that in nature interbreed and produce fertile offspring Species are reproductively isolated from one another Goats and sheep do not interbreed in nature, so they are separate species Yet in captivity they can produce fertile offspring

Speciation Speciation is the phenomenon of a new species arising from an ancestral species It involves change in the genetic makeup of a population, which also may bring about changes in form and structure During allopatric speciation, species arise when a small part of a population becomes isolated from its parent population

Allopatric Speciation A few individuals of a species on the mainland reach isolated island 1 Speciation follows genetic divergence in a new habitat.

Allopatric Speciation Later in time, a few individuals of the new species colonize island 2 In this new habitat, speciation follows genetic divergence.

Allopatric Speciation Speciation may also follow colonization of islands 3 and 4 Invasion of island 1 by genetically different descendants of the ancestral species!

Speciation in Songbirds Two species of Eurasian songbirds appear to have evolved from an ancestral species Adjacent populations (A & B, F & G) can interbreed But E and H cannot interbreed

Rate of Speciation Although widespread agreement exists on allopatric speciation scientists disagree on how rapidly a new species might evolve Phyletic gradualism the gradual accumulation of minor changes eventually brings about the origin of new species Punctuated equilibrium holds that little or no change takes place in a species during most of its existence Then evolution occurs rapidly giving rise to a new species in as little as a few thousands of years

Styles of Evolution Divergent evolution occurs when an ancestral species gives rise to diverse descendants adapted to various aspects of the environment Divergent evolution leads to descendants that differ markedly from their ancestors Convergent evolution involves the development of similar characteristics in distantly related organisms Parallel evolution involves the development in closely related organisms

Styles of Evolution In both convergent and parallel evolution, similar characteristics developed independently in comparable environments

Divergent Evolution Divergent evolution of a variety of placental mammals from a common ancestor Divergence accounts for descendants that differ from their ancestors and from one another

Convergent Evolution Convergent evolution takes place when distantly related organisms give rise to species that resemble one another because they adapt in comparable ways

Parallel Evolution Parallel evolution involves the independent origin of similar features in related organisms

Microevolution and Macroevolution Microevolution is any change in the the genetic make-up of a species, and involves changes within a species Macroevolution involves changes such as the origin of a new species or changes at even higher levels For example, the origin of birds from reptiles The cumulative effects of microevolution are responsible for macroevolution

Cladistics and Cladograms Traditionally, scientists have depicted evolutionary relationships with phylogenetic trees in which the horizontal axis represents anatomical differences and the vertical axis denotes time In contrast, a cladogram shows the relationships among members of a clade a group of organisms including its most recent common ancestor Cladistics focus on derived characteristics sometimes called evolutionary novelties as opposed to primitive characteristics

Phylogenetic Tree A phylogenetic tree showing the relationships among various vertebrate animals

Cladogram A cladogram showing inferred relationships Some of the characteristics used to construct this cladogram are indicated

Evolutionary Novelties All land-dwelling vertebrate animals possess bone and paired limbs so these characteristics are primitive and of little use in establishing relationships among land vertebrates However, hair and three middle ear bones are derived characteristics because only one subclade, the mammals, has them

Evolutionary Novelties If considering only mammals, hair and middle ear bones are primitive characteristics, but live birth is a derived characteristic that serves to distinguish most mammals from the egg-laying mammals

Cladograms Three different interpretations of the relationships among bats, dogs and birds

Cladograms Bats and birds fly, Dogs and birds which might suggest a closer relationship than to dogs Dogs and birds appear closely related Hair and giving birth to live young indicate that bats and dogs are more closely related

Cladistics for Fossils Cladistics and cladograms work well for living organisms, but are trickier for fossils Care must be taken in determining what are primitive verses derived characteristics, especially in groups with poor fossil records Paleontologists must be especially careful of characteristics resulting from convergent evolution

Cladistics for Fossils Nevertheless, cladistics is a powerful tool that has more clearly elucidated the relationships among many fossil lineages, and is now used extensively by paleontologists

Evolutionary Trends Evolutionary changes do not involve all aspects of an organism simultaneously A key feature we associate with a descendant group might appear before other features typical of that group For example, the oldest known bird had feathers and the typical fused clavicles of birds, but it also retained many reptile characteristics Mosaic evolution is the concept that organisms possess recently evolved characteristics as well as some features of their ancestral group

Phylogeny Phylogeny is the evolutionary history of a group of organisms If sufficient fossil material is available, paleontologists determine the phylogeny and evolutionary trends for groups of organisms For example, one trend in ammonoids extinct relatives of squid and octopus was the evolution of an increasingly complex shell

Evolutionary Trends Abundant fossils show the evolutionary trends of the Eocene mammals, Titanotheres These extinct relative of horses and rhinoceroses evolved from small ancestors to giants standing 2.4 m at the shoulder developed large horns and the shape of their skull changed Only 4 of the 16 known genera are shown

Evolutionary Trends Size increase is However, trends are complex one of the most common evolutionary trends However, trends are complex they might reverse more than one can take place at the same time at different rates Trends in horses included generally larger size but size decreased in some now-extinct horses changes in teeth and skull lengthening legs reduction in number of toes These trends occurred at different rates

Adaptations Evolutionary trends are a series of adaptations to changing environment or in response to exploitation of new habitats Some organisms show little evolutionary change for long periods Lingula is a brachiopod whose shell has not changed significantly since the Ordovician

“Living Fossils” Several organisms have shown little or no change for long periods If these still exist as living organisms today they are sometimes called living fossils For example: horseshoe crabs Latrimaria (fish) Gingko trees Some of these are generalized and can live under a wide variety of environments

A Living Fossil Latimeria belongs to a group of fish once thought to have gone extinct at the end of the Mesozoic Era A specimen was caught off the coast of East Africa in 1938

A Second Living Fossil Ginkgos have changed very little for millions of years

Randomness in Natural Selection? But isn’t evolution by natural selection a random process? If so, how is it possible for a trend to continue long enough to account just by chance for such complex structures as eyes, wings, and hands?

Two Steps in Natural Selection Evolution by natural selection is a two-step process Only the first step involves chance Variation must be present or arise in a population Whether a mutation is favorable is a matter of chance The natural selection of favorable variations is not by chance

Extinctions Perhaps as many as 99% of all species that ever existed are now extinct Organisms do not always evolve toward some kind of higher order of perfection or greater complexity Vertebrates are more complex but not necessarily superior in some survival sense. Bacteria have persisted for at least 3.5 billion years! Natural selection yields organisms adapted to a specific set of circumstances at a particular time

Background and Mass Extinction The continual extinction of species is referred to as background extinction It is clearly different from mass extinction during which accelerated extinction rates sharply reduce Earth’s biotic diversity Extinction is a continual occurrence but so is the evolution of new species that usually quickly exploit the opportunities another species’ extinction creates Mammals began a remarkable diversification when they began occupying niches the extinction of dinosaurs and their relatives left vacant

Evidence in Support of Evolution Darwin cited supporting evidence for evolutionary theory such as classification embryology comparative anatomy geographic distribution fossil record, to a limited extent He had little knowledge of the mechanism of inheritance, and biochemistry and molecular biology were unknown at his time

Evidence in Support of Evolution Since Darwin’s time, studies from additional fields in biochemistry molecular biology more complete and better understood fossil record have convinced scientists that the theory is as well supported by evidence as any other major theory

Is the Theory of Evolution Scientific? An idea can only be a truly scientific theory if testable predictive statements can be made from it No theory in science is ever proven in the final sense, although substantial evidence may support it All theories are always open to question, revision and occasionally to replacement by a more comprehensive theory

Theories Must Be Predictive Not all predictions are about future events, and evolutionary theory cannot make predictions about the far distant future Nevertheless, we can make a number of predictions about the present-day biological world and about the fossil record that should be consistent with evolutionary theory if it is correct

Some Predictions from Evolution If evolution has taken place, closely related species such as wolves and coyotes should be similar in anatomy and biochemistry, genetics, and embryonic development

Testable Suppose that contrary to evolutionary prediction wolves and coyotes were not similar in terms of their biochemistry, genetics and embryonic development, then our prediction would fail and we would at least have to modify the theory If other predictions also failed, for example, if mammals appeared in the fossil record before fishes then we would have to abandon the theory and find a better explanation for our observations

Classification Classification uses a nested pattern of similarities Carolus Linneaus (1707-1778) proposed a classification scheme in which organisms receive a two-part name consisting of genus and species for example, the coyote is Canis latrans Linnaeus’s classification is an ordered list of categories that becomes more inclusive as one proceeds up the list

Linnaean Classification Most inclusive the coyote, Canis latrans Animalia Chordata Vertebrata Mammalia Carnivora Kingdom Phylum Subphylum Class Order Canidae Canis latrans Family Genus Species Least inclusive

Classification —shared Characteristics Subphylum vertebrata including fishes, amphibians, reptiles, birds and mammals, have a segmented vertebral column Only warm-blooded animals with hair/fur and mammary glands are mammals

Coyote, Canis latrans 18 orders of mammals exist including order Carnivora The Family Canidae are doglike carnivores and the genus Canis includes only closely related species Coyote, Canis latrans, stands alone as a species

Coyote and Wolf Coyote (Canis latrans) and wolf (Canis lupus) share numerous characteristics as members of the same genus They share some but fewer characteristics with the red fox (Volpes fulva) in the family Canidae All canids share some characteristics with cats, Bears, and weasels in the order Carnivora which is one of 18 living orders of the class Mammalia Shared characteristics are evidence for evolutionary relationships

Biological Evidence Supporting Evolution If all existing organisms descended with modification from ancestors that lived during the past, all life forms should have fundamental similarities: all living things consist mainly of carbon, nitrogen hydrogen and oxygen their chromosomes consist of DNA all cells synthesize proteins in essentially the same way

Evolutionary Relationships Biochemistry provides evidence for evolutionary relationships Blood proteins are similar among all mammals Humans’ blood chemistry is related most closely to the great apes then to Old World monkeys then New World monkeys then lower primates such as lemurs Biochemical tests support the idea that birds descended from reptiles a conclusion supported by evidence in the fossil record

Structures with Similarities Homologous structures are basically similar structures that have been modified for different functions They indicate derivation from a common ancestor. Analogous structures are structures with similarities unrelated to evolutionary relationships that serve the same function but are quite dissimilar in both structure and development

Homologous Structures Forelimbs of humans, whales, dogs, and birds are superficially dissimilar, yet all are made up of the same bones, have similar arrangement of muscles, nerves and blood vessels, are similarly arranged with respect to other structures, have similar pattern of embryonic development

Analogous Structures Wings of insects and birds serve the same function but differ considerably in structure and development

Vestigial Structures Vestigial structures are remnants of structures in organisms that were functional in their ancestors Why do dogs have tiny, functionless toes on their feet (dewclaws)? Ancestral dogs had five toes on each foot, all of which contacted the ground As they evolved they became toe-walkers with only four toes on the ground and the big toes and thumbs were lost or reduced to their present state

Remnants of Rear Limbs in Whales The Eocene-aged whale, Basilosaurus, had tiny vestigial back limbs but it did not use limbs to support its body weight.

Evolution in Living Organisms Small-scale evolution can be observed today. For example adaptations of some plants to contaminated soils insects and rodents developing resistance to new insecticides and pesticides development of antibiotic-resistant strains of bacteria Variations in these populations allowed some variant types to live and reproduce, bringing about a genetic change

What do We Learn from Fossils? The fossil record consists of first appearances of various organisms through time One-celled organisms appeared before multicelled ones plants appeared before animals invertebrates before vertebrates Fish appeared first followed in succession by amphibians, reptiles, mammals, and birds

Advent of Various Vertebrates Times when major groups of vertebrates appeared in the fossil record Thickness of spindles shows relative abundance

Fossils Are Common Fossils are much more common than many people realize However the origin and initial diversification of a group is generally the most poorly represented But fossils showing the diversification of horses, rhinoceroses, and tapirs from a common ancestor are known as are ones showing the origin of birds from reptiles and the evolution of whales from a land-dwelling ancestor

Horses and Their Relatives This cladogram shows the relationship among tapirs, rhinoceroses, and horses

Horses and Their Relatives These might seem an odd assortment of animals but fossils and studies of living animals indicate that they shared a common ancestor As we trace these animals back in the fossil record, differentiating one from the other becomes increasingly difficult The earliest members of each group are remarkably similar, differing mostly in size and details of their teeth As their diversification proceeded the differences became more apparent

Missing Links Missing links or fossils bridging gaps between presumed ancestor and descendant groups of organisms are claimed by some non-scientists to invalidate the theory of evolution There are many intermediate fossils The transition from fish to amphibians is well documented by fossils Some of the most advanced mammal-like reptiles and earliest mammals are very similar

The Evidence: Summary Scientists agree that the theory of evolution is as well-supported by evidence as any other theory Transitional fossils provide compelling evidence but fossils aren’t the only evidence Much evidence comes from comparative anatomy biogeography molecule biology genetics embryology and biochemistry

Summary The central claim of evolution is that all organisms have descended with modification from ancestors that lived during the past. Jean Baptiste de Lamarck proposed the first mechanism to account for evolution Inheritance of Acquired Characteristics Darwin’s observation of variation in populations and artificial selection and his reading of Malthus’ essay on population helped him formulate his idea of natural selection

Summary In 1859 Charles Darwin and Alfred Wallace published their ideas of natural selection which hold that in populations of organisms, some have favorable traits that make it more likely that they will survive and reproduce natural selection

Summary Gregor Mendel’s breeding experiments with garden peas provided some of the answers regarding how variation is maintained in populations Mendel’s work is the basis for present-day genetics Genes are the hereditary units in all organisms Only the genes in sex cells are inheritable

Summary Sexual reproduction and mutations account for most variation in populations Evolution by natural selection has two steps First, variation must exist or arise and be maintained in interbreeding populations, and second, favorable variants must be selected for survival

Summary Most species evolve by allopatric speciation which involves isolation of a small population from its parent population, that is then subjected to different selection pressures Divergent evolution involves A common ancestor stock giving rise to diverse species The development of similar adaptive types in different groups of organisms results from parallel and convergent evolution

Summary Microevolution involves changes within a species, while microevolution encompasses all changes above the species level. Scientists traditionally used phylogenetic trees to depict evolutionary relationships but now they more commonly use cladistic analyses and cladograms to show these relationships

Summary Background extinctions occur continually, but several mass extinctions have also taken place during which Earth’s biologic diversity has decreased markedly The theory of evolution is truly scientific because we can make observations that would falsify it That is, prove it wrong

Summary Much of the evidence supporting the theory of evolution comes from classification, comparative anatomy, embryology, genetics, biochemistry, molecular biology, and present-day examples of microevolution The fossil record also provides evidence for evolution in that it shows a sequence of different groups appearing through time, and some fossils show features we would expect in the ancestors of birds or mammals, horses, whales, and so on