Evolution and Systematics

1. Evolution and Systematics Chapter 19

2. Diversity of Life • Relevant fields of study • Taxonomy • Process of sorting and naming life forms • Evolution • Process by which living species change and new species come into being • Systematics • Effort to find how modern life forms are related • Look for evolutionary steps that led from ancient to modern forms of life  phylogeny (“origin of groups”)

3. What is a Species? • A group of organisms that are more closely related to one another than to organisms of any other kind • May look more like one another • Interbreed more freely with one another than with organisms outside the group

4. What is a Species? • Characters • Traits of organisms ranging from shapes and colors of body parts to DNA • Used to define most currently known species • Phenetic species • Species that are defined by combinations of traits • Example: citrus trees • Characterized partly on distinctions between their fruits

5. What is a Species? • Type specimen • An organism placed in museum or botanical garden when species is first named • Used for comparison • Does not always reflect all members of that species

6. What is a Species? • Mating test • If organisms from two populations mate and produce fertile offspring under natural conditions, then the two populations belong to same species • Biological species • Species defined by mating test

7. What is a Species? • Problems associated with mating test • Does not apply to organisms that lack sexual reproduction • Many plant species can interbreed with closely related species and produce offspring that are weakly fertile

8. Taxonomy • Need formal system for assigning names for scientific communication • Hierarchy of levels within levels • Begun by Carolus Linnaeus • 1753 published book • Named about 6,000 species of plants • Assigned them to 1,000 groups called genera • Genus  group of species that are similar enough to be obviously related

9. Taxonomy • Wrote short description of each species • Gave every species an abbreviated two-word name  binomial • Every species has a binomial, or species name • First word is genus (always capitalized) • Second word is specific epithet (never capitalized) • Both words are written in italics • Example: Zea mays

10. Taxonomy Classification of common garden nasturtium (Tropaeolum majus)

11. Taxonomy • Extra levels may be needed to divide up multiple species • Examples • Superfamily  group of several families • Subfamily  smaller division of family • Subspecies, varieties (races, among animals), forms  divisions below species • Important in cultivated plants • Cultivar  equivalent to variety • Used to describe products of human selection within a species

12. Taxonomy • Taxon (plural, taxa) • Taxonomic group at any level • Examples: species, kingdom

13. Taxonomy • Original taxonomic plan • Two kingdoms • Plant • Animal • Examples of problems with this scheme • Some microscopic organisms have both plant-like and animal-like characteristics • Fungi have more in common with animals than plants

14. Taxonomy • Early 20th century biologists divided plant kingdom into four new kingdoms • Monera • Fungi • Plantae • Protista

15. Taxonomy • Mid 20th century • Electron microscope provided information showing bacteria have simpler cell structure than other organisms • No envelope around DNA • Prokaryotic • Cells of plants, animals, fungi, protists • Most of DNA enclosed in membranous envelope (true nucleus) • Eukaryotic

16. Taxonomy • Carl Woese • Found prokaryotes included two distinct groups of organisms • Probably evolved separately • Evidence came from analysis of ribosomal RNA called rDNA

17. Taxonomy • Needed higher level above kingdom to accommodate new system of classification • Domain  contains one or more kingdoms • Three domains • Bacteria • Archaea • Eukarya • “Kingdom” Protista • Questionable as to where many members belong • Many smaller groups do not fit into the three established kingdoms within Eukarya

18. Taxonomy • Two eukaryotic groups have been proposed for kingdom status • Alveolates • Heterokonts • Remains to be seen how domains Bacteria and Archaea will be divided into kingdoms

19. Taxonomy

20. Evolution • Fossils • Relics of life such as bones and leaves embedded in stone • Observation of how older fossils differ from more recent ones challenged view that species did not change • 300 million years ago, horsetails were tree-sized and exhibited secondary growth and wood • Modern horsetails are herbs

21. Evolution • Charles Darwin and Alfred Wallace • English naturalists • Came up with idea that hereditary characteristics of species could change, or evolve, over many generations • Darwin’s ideas took shape during trip around world • Stop at Galápagos Islands made strongest impression • Examined finches on island that differed in many ways from those he had seen in Ecuador

22. Evolution • Darwin kept thoughts to himself until he received letter from Wallace stating same ideas • Darwin • 1859 • Published The Origin of Species

23. Evolution • Darwin’s mechanism of evolution based on following assertions • Changes in heredity occur in the individuals of a population, leading to varied progeny. • Populations produce more progeny than the environment can support. This leads to competition among the progeny.

24. Evolution • The progeny that are best adapted to the *environment will reproduce most abundantly. • Repeated over many generations, the preceding three factors could lead to great changes in heredity, and, hence, great changes in the forms of life. * Natural selection – Darwin’s term for effect of environment

25. Evolution • Darwin’s ideas suggested • No ideal body form for each species • Forms can change as environment changes

26. Evolution • In order for changes to be passed from one generation to the next, changes must occur in DNA • Two main sources of change in DNA • Mutation • Recombination

27. Mutations • Mutations • Random changes in DNA • Primary source of new hereditary information • Base substitution • Type of mutation in which wrong base is inserted in DNA copying process • Body heat keeps molecules in motion causing collisions that sometimes cause this type of mutation

28. Mutations • Some mistakes are corrected • Others are missed • Errors occur at random locations • When error occurs in DNA of reproductive cells, altered gene can produce new hereditary characteristics in progeny

29. Mutations • Mutagens • Agents that cause mutations • Body heat • High-energy radiations  dental X-rays, ultraviolet light from sun, high-energy particles released from radioactive decay • Chemicals • Normal metabolism • Most mutations have little or no effect on evolution • Cause damage that leads to their elimination • Occasionally a mutation helps organism, spreads through population, contributes to evolution • Example: appearance of antibiotic resistance in bacteria that cause human disease

30. Mutations • If mutation occurs at critical point in gene for vital protein • Cell makes copies of protein • Leads to cell death • If mutation damages proteins that control cell division • Cells multiply without limit • Produce tumors and cancers (in animals)

31. Recombination • Process that creates new combinations of genes by joining parts of DNA molecules from separate organisms • Ways recombination occurs • Transduction • Viruses carry DNA of one host organism to another

32. Recombination • Transformation • Bacteria take up segments of DNA that are released from decaying organisms • Enzymes insert compatible portions of foreign DNA into cell’s own DNA • Conjugation • Bacteria pass copy of their own DNA into another bacterium of same species • Enzymes exchange parts of host’s own DNA for some of the transferred DNA

33. Recombination • Sexual reproduction • Occurs in cells of eukaryotes • Most common source of recombination • Meiosis • Crossing over • Happens at many random points along most chromosomes • No two gametes are likely to have same combination of parental chromosome segments

34. Hybridization • Mating between two different species • Process called hybridization • Progeny are called hybrids • Characteristics of hybrid plants • Often cannot reproduce sexually • Mismatch between chromosomes disrupts meiosis • May be vigorous • May multiply by asexual reproduction

35. Hybridization • Introgression • Process by which hybrid plants can transfer genes between the two parent species • Transfer requires back-crossing • Biologists uncertain as to how often hybridization occurs among plants on the whole • Some fear hybridization and introgression may allow genes from genetically engineered plants to escape into wild populations

36. Endosymbiosis • Cells of one species reside inside cells of another species • If endosymbiosis lasts for many generations, DNA may pass from guest species to the host species • Adds to host’s nuclear DNA • Leaves guest as a dependent organelle • Examples: mitochondria and chloroplasts

37. Endosymbiosis • Primary endosymbiosis • Example: origin of mitochondria and chloroplasts from bacteria • Secondary endosymbiosis • Example: eukaryotic predators gained chloroplasts through endosymbiotic partnership with eukaryotes that already had chloroplasts • Led to brown algae and certain other protists

38. Natural Selection • Guides evolution • Natural selective agents can be abiotic or biotic • Biotic factors • Examples: Competing organisms, predators, prey • Abiotic factors • Examples: Climate, water supply, light

39. Directional Selection • Adaptations – favorable hereditary traits that enhance success in a particular environment • Leads to new adaptations • Example: spines of cacti • Spines • Help plant collect rain water • Dead at maturity

40. Directional Selection

41. Stabilizing Selection • Maintains existing adaptations • Selective forces act equally against variations on both sides of the mean • Example • Each generation of adult cacti has same average spine diameter as generation before

42. Stabilizing Selection

43. Diversifying Selection • Natural selection that increases genetic variation • Can be caused by • Disease agents • Factors that favor two or most distinct types in a population • Example: • Grass growing on mine tailings (rich in lead and zinc) • Same species of grass growing on surrounding normal soil

44. Diversifying Selection • Plants that grow on mine tailings fail to thrive on normal soil • Plants that grow on normal soil fail to grow when transplanted to mine tailings • Presence of mine tailings beside normal soil permits lead and zinc tolerant and intolerant plants to persist simultaneously in population

45. Diversifying Selection

46. Types of Evolution • Divergent evolution • Increase in genetic differences among groups • Convergent evolution • Increase in similarity between two taxa • Occurs when differing populations are exposed to similar environments over many generations

47. Types of Evolution • Coevolution • Interdependent evolution of two or more species • Adaptations of interdependent species selected by mutual interaction • Can result in new species • Example: • Moth-pollinated plants produce nectar at base of long, slender tubes • Ideal for long tongues of moths but beyond reach of other pollinators

48. Types of Evolution • Pollen transfer more efficient because pollinator visits just one plant species • Pollinators get private food supply • Mutual benefit suggests that moth pollination favored evolution of long spurs in the flowers, as well as long tongues in the moths

49. Population Genetics • By 20th century, genetics was advanced enough to show molecular basis of evolution • Question raised concerning heredity and evolution • Why do different versions of the same gene (called alleles) persist in a population, even though one allele is more abundant or is expressed more strongly from the other?

50. Population Genetics • G.H. Hardy and G. Weinberg • 1908 • Simultaneously published model to answer questions about population evolution • Conditions that should apply to an ideal population • Mutations do not occur • Organisms do not migrate between populations • Reproduction is limited to random sexual mating • There is no natural selection • The population is very large