Evolution—The Theory and Its Supporting Evidence

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

2. Evidence for Evolution • Some of the evidence for evolution • is provided by fossils • such as this Early Pleistocene mammoth • known as Archidiskodon meridonalis • on display in the Museum of Geology and Paleontology at the University of Florence in Italy

3. 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

4. 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

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

6. The Galápagos Islands

7. 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

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

9. 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 • encyclopedic collection of facts • Evolution provides a framework • for discussion of life history • in later parts of the term

10. Misconceptions about Evolution • Many people have a poor understanding • of the theory of evolution • and hold a number of misconceptions, • which include: • evolution proceeds strictly by chance • nothing less than fully developed structures • such as eyes are of any use • there are no transitional fossils • so-called missing links • connecting ancestors and descendants • humans evolved from monkeys • so monkeys should no longer exist

11. 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 • explained the origin of life • and contrary views were heresy

12. Evolution: Historical Background • During the 18th century, • naturalists were discovering evidence • that could not be reconciled • with literal reading of Scripture • 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

13. Lamarck • Jean-Baptiste de Lamarck • (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 • and was widely accepted

14. 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

15. Lamarck’s Giraffes • ancestral short-necked giraffes • stretched their necks • to reach leaves high on trees • their offspring were born • with longer necks • According to Lamarck’s theory of inheritance of acquired characteristics

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

17. 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’s essay on population • suggested that competition for resources • and high infant mortality limited population size

18. Darwin and Wallace • Darwin and Alfred Russel Wallace (1823-1913) • read Malthus’s 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

19. Natural Selection—Main Points • Organisms in all populations • posses 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 • Not all young survive to reproductive maturity • Those with favorable variations • are more likely to survive • and pass on their favorable variations

20. Naturally Selected Giraffes • According to the Darwin-Wallace theory • of natural selection, giraffe’s long neck evolved • because ancestors with longer necks • had an advantage • and reproduced more often

21. “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 differential rates • of survival and reproduction

22. 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...

23. 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

24. 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

25. 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)

26. 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

27. 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

28. Genes and Chromosomes • Complex, double-stranded helical molecules • of deoxyribonucleic acid (DNA) • called chromosomes • are found in cells of all organisms • except bacteria, • which have ribonucleic acid (RNA) • 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

29. 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

30. Meiosis • During meiosis, • 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

31. Fertilization • 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 • The full number of chromosomes • is restored when a sperm fertilizes an egg

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

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

34. 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

35. Modern View of Evolution • They completely rejected Lamarck’s idea • of inheritance of acquired characteristics • They reaffirmed the importance of natural selection • But since then, • some scientists have challenged the emphasis • in modern synthesis • that evolution is gradual

36. What Brings about Variation? • Evolution by natural selection • works on variation in populations • most of which is accounted for by the reshuffling • of alleles 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. Allopatric Speciation • Reduction of the area occupied by a species • may leave a small isolated population • Two peripheral isolates evolved into new species • Isolation might result from a marine transgression.

44. Allopatric Speciation • Geographic barriers may form across parts • of a central population’s range, • thereby isolating small populations that speciate

45. Allopatric Speciation • A few individuals may somehow reach • a remote area and no longer exchange genes • with the parent population • This out-migration can lead to the formation • of a peripheral isolate that gives rise to a new species • while the parent population persists without change

46. Finch Speciation • Darwin’s finches from the Galápagos Islands • underwent allopatric speciation • due to isolation of birds among the many islands

47. 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 • This view was held by Darwin and reaffirmed by modern synthesis

48. Rate of Speciation • holds that little or no change • takes place in a species • during most of its existence • Punctuated equilibrium • then evolution occurs rapidly • giving rise to a new species • in perhaps as little as a few thousand years

49. Misconceptions • Ideas about speciation • commonly involve misconceptions • One antievolution argument is • “If humans evolved from monkeys, • “why are there still monkeys?” • This involves two misconceptions • No scientist has ever claimed • that humans evolved from monkeys • Even if they had, that would not preclude • the possibility of monkeys still existing

50. Various Possibilities • In allopatric speciation • a small population may evolve • whereas the larger parent population may • remain unchanged, • evolve in some other direction, • or become extinct