Evolution and Systematics

1
Evolution and Systematics

• Chapter 18

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)
Linnaean Rank Name Ending
Domain Eukarya -a
Kingdom Plantae -
Phylum (Division) Magnoliophyta -ophyta
Class Magnoliopsida -opsida
Subclass Rosidae -idae
Order Brassicales -ales
Family Brassicaceae -aceae
Genus Tropaeolum -
Species name Tropaeolum majus -
Specific epithet 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

Kingdom Description
Animal Move actively and consume prey
Plant Do not move or consume prey
14
Taxonomy

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

Kingdom Examples
Monera Bacteria
Plantae Green plants
Fungi Fungi
Protista Catch-all kingdom composed of all organisms that did not fit into other kingdoms
Animalia Animals
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
Domain Cell Type Description
Eukarya Eukaryotic Membrane bounded organelles, linear chromosomes
Archaea Prokaryotic Found in extreme environments, cell structure and differ from members of Domain Bacteria
Bacteria Prokaryotic Ordinary bacteria, found in every habitat on earth, play major role as decomposers
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
• Darwins 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

• Darwins 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 Darwins term for effect of
environment
25
Evolution

• Darwins 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 cells own DNA
• Conjugation
• Bacteria pass copy of their own DNA into another
  bacterium of same species
• Enzymes exchange parts of hosts 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 hosts 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

51
Population Genetics

• Analysis by Hardy and Weinberg showed under those
  ideal conditions
• Two alleles for same gene remain indefinitely in
  population at fixed ratio, even if one allele is
  dominant over the other
• Called Hardy-Weinberg equilibrium
• Became basis for new discipline known as
  population genetics
• Integrates genetics and evolution

52
Population Genetics
Hardy-Weinberg Equilibrium Factors Leading to Evolution (Disequilibrium)
Mutations do not occur. Mutations convert one allele to another, and therefore alter the ratio of alleles, unless forward and reverse mutations exactly balance.
Organisms do not migrate between populations. If many individuals enter or leave the population, the allele ratio will change unless the migrating individuals have alleles in exactly the same ratio as the overall population.
Reproduction is limited to random sexual mating. If mating is not random, some allele combinations may be reproduced disproportionately often.
There is no natural selection. Natural selection favors the reproduction of individuals with a certain allele combination over others.
The population is very large. If the population is very small, chance can determine which individuals reproduce.
53
Population Genetics

• Population genetics
• Tool to predict changes and explore causes of
  evolution
• Effects of chance on small populations
• Best-adapted individuals do not always leave the
  most offspring
• Random accidents (fire, epidemic) in small
  population may accidentally eliminate all
  individuals that have best allele

54
Population Genetics

• Genetic drift
• Random change in allele ratio
• Founder effect
• Occurs when a few individuals from a large
  population establish a small, isolated population
  
• Founders may have combination of traits that are
  uncommon in old population
• May start new population on new path of evolution
• Often seen in studies of oceanic islands
• Island plants are related to mainland species,
  but traits differ in many ways

55
Speciation

• Process which splits one species into two
• Involves the following processes
• Reproductive isolation and directional selection
• Block to gene exchange
• Geographic isolation
• Geographical barriers prevent populations from
  meeting to exchange genes

56
Speciation

• Polyploidy
• Possession of more than two chromosome sets per
  cell
• Important source of new species in plants
• New polyploid plant is reproductively isolated
  because it cannot exchange genes with its diploid
  relatives
• Hybridization
• another source of reproductive isolation that can
  lead to speciation

57
Speciation

• New hybrids often sterile
• Fertility can be restored if cell at tip of
  hybrid plant becomes polyploid and initiates
  polyploid shoot that forms gametes

58
Macroevolution Microevolution
Consists of changes large enough to represent the emergence of a new life form Consists of changes too small to alter the fundamental nature of the species
More difficult to observe May Be sum of many microevolutionary changes over long periods Involve larger abrupt changes, such as chromosome rearrangements Majority of modern biologists believe macroevolution generated all modern forms of life from microscopic forms that first populated Earth some 3.8 billion years ago. Rapid, easy to observe, easy to produce artificially in the laboratory
59
Phylogenetic Systematics

• Recent developments making phylogenetic
  systematics active field
• Cladistics
• Invention of fast, inexpensive computers to make
  it practical to analyze large amounts of data
• Invention of quick ways to read information
  stored in DNA

60
Phylogenetic Systematics

• Phylogenetic tree
• Diagram showing evolutionary relationships
• Tips of branches
• Most recent products of evolution along each
  branch
• Each branch point
• Act of speciation (where one species divides into
  two)

61
Phylogenetic Systematics

• Some reasons for studying systematics
• Practical rewards for knowing how evolution led
  to present-day species
• Search for new medicines
• Slow growing plant produces compound that cures
  colon cancer
• Look for faster growing relatives of plant for
  alternative sources of compound
• Ways to stop parasites that attack food plants
• Experiment with relatives of parasite that can be
  grown without a host

62
Cladistics

• Cladistics
• Klados tree branch
• Set of quantitative methods and concepts for
  exploring evolutionary relationships among taxa
• Compares modern species to determine most
  probably point in evolution where each species
  branched off from evolving group

63
Cladistics

• Clade
• Branch in tree of life
• Consists of an originating taxon and all its
  descendant taxa
• Cladogram
• Phylogenetic tree produced by cladistics
• Rarely include more than a small sampling of
  species that evolved from the ancestor
• Only species that contributed data to study are
  listed

64
Cladistics

• Node
• Branch point where ancestral species split to
  produce two new species
• Ancestor itself ceased to exist
• Oldest node called root of cladogram

65
Cladistics

• Types of cladograms
• Rooted
• Identify node in cladogram that occurred first
• Shows direction of evolution throughout clade
• Several different ways to draw cladogram to show
  branching
• Reveals sequence in which important character
  states evolved

66
Cladistics

• Unrooted
• Do not show which node is closest to the root
• Leave direction of evolution between each pair of
  nodes unspecified
• Number of possible unrooted cladograms depends
  only on the number of species

67
Cladistics

• Alternative cladograms
• Equally valid as long as they agree on number of
  nodes that separate any two taxa
• Differences in orientation of branches
  unimportant
• Differ in how many steps of evolution stand
  between each pair of species

68
Cladistics

• Cladistics compares species with respect to
  various characters
• To be useful character must occur in all species
  being considered
• Details called character state
• Morphological characters
• Related to body form
• Molecular characters
• Chemical traits
• Examples
• Structure of segment of DNA
• Ability to make a particular kind of molecule

69
Cladistics

• Homologous traits
• Alternative states of the same character
• Arose from the same ancestral trait
• Example
• Wings of bird and forelegs of horse

70
Cladistics

• Analogous traits
• Have similar form or function but evolved from
  different structures
• Not alternate states of same character
• States of different characters
• Example
• Wings of insects and wings of birds

71
Cladistics

• Character matrix
• Prepared table that compares characters among
  species
• Taxa listed along left margin
• Characters listed across top
• Boxes show state of each character for each
  species

72
Cladistics

• Principle of Parsimony
• Postulates that the cladogram requiring the
  fewest evolutionary events is most likely to be
  correct
• Cladogram described as parsimonious
• Good hypothesis but can never be sure it is
  correct

73
Cladistics

• Consensus tree or consensus cladogram
• Includes all the points of agreements
• Leaves points of disagreement unresolved as nodes
  from which more than two branches depart

74
Cladistics

• Finding root of cladogram
• Include data on additional taxa called outgroups
  along with character data on the ingroup (set of
  taxa that is target of the study)

75
Cladistics

• Ancestral and derived character states
• Derived character state
• Character that evolved later
• Use of terms ancestral and derived requires care
• Judgment depends on point of view

76
Cladistics

• Cladistics reveals convergent evolution
• Similar character states sometimes arise
  independently in two groups of organisms
• Cladogram can reveal which characters arose
  through convergent evolution

77
Cladistics

• All formally named taxa should be monophyletic
• True clade includes ancestor and all of its
  descendants and nothing else
• Each currently accepted domain and kingdom of
  life is believed to be monophyletic
• Many traditional taxa at lower levels are still
  not monophyletic