Evolution —The Theory and Its Supporting Evidence

1. Evolution—The Theory and Its Supporting Evidence

2. Evidence for Evolution • Some of the evidence for evolution • comes from fossils such as this 2.3-meter-long Ceresiosaurus which belonged to a groupof Triassic marine reptiles known as nothosaurs. -On display at Glacier Garden, Lucerne, Switzerland

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 collection of facts Evolution provides a framework for discussion of life history

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 and the works of Aristotle • 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 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

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

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 during its lifetime

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 which 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’ 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’ 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 • 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 • 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 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

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) • produces 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 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

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

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

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

46. Honeycreeper Speciation • More than 20 species of Hawaiian honeycreepers have evolved • from a common ancestor as they adapted to diverse food sources on the 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. 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 • of similar characteristics • in closely related organisms

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