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Evolution and the Theory of Natural Selection What is Evolution? The change in gene frequencies in a population over time.

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Presentation on theme: "Evolution and the Theory of Natural Selection What is Evolution? The change in gene frequencies in a population over time."— Presentation transcript:

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2 Evolution and the Theory of Natural Selection

3 What is Evolution? The change in gene frequencies in a population over time

4 Why the controversy?

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7 Intelligent Design vs Evolution

8 The Greek philosopher Aristotle (384-322 BCE) – Viewed species as fixed and unchanging (Scala naturae) Fixed rungs on a ladder of complexity The Old Testament of the Bible – Holds that species were individually designed by God and therefore perfect Carolus Linnaeus (1707-1778) – Interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose – Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”

9 Charles Darwin (1809-1882) Born in England Attended medical school, HATED IT, and dropped out to become a priest Liked to stuff birds instead of dissect humans Didn’t like grave robbing for bodies Boarded the H.M.S. Beagle for a 5 year UNPAID journey as a naturalist (nothing exists outside of natural laws that govern earth)

10 Charles Darwin “Descent with modification” from an ancestral species November 24 th 1859

11 The Origin of Species Occurrence of Evolution Descent with Modification all organisms related through descent from some unknown ancestral population diverse modifications (adaptations) accumulated over time Mechanism of Evolution Natural Selection and Adaptation natural selection is the differential success in reproduction natural selection occurs from the interaction between the environment and the inherit variability in a population variations in a population arise by chance Can selection actually cause substantial change in a population?

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13 Journey of the H.M.S. Beagle

14 Darwin’s Field Research South American flora/fauna distinct from European flora/fauna S. American temperate species were more closely related to S. American tropical species than European temperate species S. American fossils were distinctly S. American Tropical Rainforest of South America

15 Galapagos Islands + most animal species on Galapagos unique to those islands, but resemble S. American continental species + Darwin’s Finches - 13 types + some unique to individual islands + others found on two or more islands close together Darwin proposed: + new species could arise from an ancestral population by gradually accumulating adaptations to a different environment. - Theory of natural selection as the mechanism of adaptive evolution

16 Alfred Russel Wallace (1823-1913) Presented a paper with identical ideas as Darwin on July 1, 1858 at the Linnaean Society meeting Was a botanist who came up with virtually the same concept of natural selection more or less independently through his studies on the Malay archipelago. Darwin panicked because he was not ready with his book yet!

17 Where did Darwin and Wallace get the idea of evolution?

18 Jean Baptiste Lamarck (1744-1829) Lamarck claimed that evolution was driven by "use vs. disuse" A used structure will become larger, stronger and more important. A disused structure will atrophy and become VESTIGIAL. Evolution occurs because organisms have an innate drive to become more complex

19 Theory of “Use vs. Disuse” The long necks of giraffes were due to their stretching for food, and giraffes passed their stretched necks on to their offspring. Similarly, the big, “ripped” muscles developed by the village blacksmith with all his hammering and slinging of heavy metal objects would be expected to be passed on to his offspring.

20 Theory of “Acquired Characteristics” Lamarck claimed that traits acquired during an organism's lifetime could be inherited by that organism's offspring.

21 Georges Cuvier (1769-1832) Created Paleontology (The study of fossils) He noted that deeper layers of sedimentary rock had diversity of organisms far different from present day life found in more recent layers Proposed the idea of extinction based on fossils

22 James Hutton (1726-1797) A Scottish geologist who challenged Cuvier's view in 1795 with his idea of GRADUALISM Proposed that large changes in the earth's surface could be caused by slow, constant processes e.g. erosion by a river

23 Charles Lyell (1797-1875) Earth processes had been going on constantly, and could explain the appearance of the earth. This theory, uniformitarianism, was a strong basis for Darwin's later theory of natural selection.

24 Thomas Malthus (1766-1834) Suggested that much of humanity's suffering (disease, famine, homelessness and war) was the inevitable result of overpopulation: humans reproduced more quickly than their food supply could support them. Malthus showed that populations, if allowed to grow unchecked, increase at a geometric rate.

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26 Darwin made some profound observations, from which Ernst Mayr inferred some conclusions... Observation #1. All species have huge potential fertility Observation #2. Except for seasonal fluctuations, populations tend to maintain a stable size. Observation #3. Environmental resources are limited.

27 Inference #1 The production of more individuals than the environment can support leads to a "struggle for existence," with only a fraction of offspring surviving in each generation.

28 Observations Observation #4: No two individuals in a population are exactly alike Observation #5: Much of the observed variation in a population is heritable

29 Inference #2 Survival in this "struggle for existence is not random, but depends, in part, on the hereditary makeup of the survivors. Those individuals who inherit characteristics that allow them to best exploit their environment are likely to leave more offspring than individuals who are less well suited to their environment.

30 Inference #3 Unequal reproduction between suited and unsuited organisms will eventually cause a gradual change in a population, with characteristics favorable to that particular environment accumulating over the generations.

31 SO WHAT IS THIS THEORY OF NATURAL SELECTION? It can be broken down into four basic tenets, or ideas

32 Theory of Natural Selection 1. Organisms are capable of producing huge numbers of offspring. 2. Those offspring are variable in appearance and function, and some of those variations are heritable.

33 Theory of Natural Selection 3. Environmental resources are limited, and those varied offspring must compete for their share. 4. Survival and reproduction of the varied offspring is not random. Those individuals whose inherited characteristics make them better able to compete for resources will live longer and leave more offspring than those not as able to compete for those limited resources.

34 Natural selection is differential success in reproduction – That results from the interaction between individuals that vary in heritable traits and their environment

35 Natural Selection Definition Natural selection is differential success in reproduction Selection can only edit existing variations

36 Evolution 1.Theory - an accepted hypothesis that has been tested over and over again without yet being disproved 2.Definition - Evolution is the change in the overall genetic makeup of a population over time 3.Three Basic Components a. Individuals cannot evolve. Populations evolve. b. Natural selection is the mechanism of evolution. c. Evolution occurs by chance (NOT GOAL ORIENTED).

37 Evolution Populations are a group of interbreeding individuals belonging to the same species and sharing a common geographic area Natural selection favors individuals, so multiple generations must be examined

38 What is speciation and who studies it? Speciation is the creation of a new species Scientists who study the processes and mechanisms that lead to such speciation events are called EVOLUTIONARY BIOLOGISTS.

39 Species –species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations

40 (b) Cladogenesis (a) Anagenesis Macroevolution the origin of new taxonomic groups (new species, etc.) + Anagenesis - phyletic evolution - transformation of one species to another + Cladogenesis - branching evolution - new species arise from a population that buds from a parent species + increases biodiversity

41 Allopatric speciation. A population forms a new species while geographically isolated from its parent population. Sympatric speciation. A small population becomes a new species without geographic separation. Speciation can occur in two ways – Allopatric speciation – Sympatric speciation

42 Allopatric Speciation A population becomes physically separated from the rest of the species by a geographical barrier that prevents interbreeding. Because gene flow is disrupted by this physical barrier, new species will form.

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44 A. harrisi A. leucurus

45 Sympatric Speciation Two populations are geographically close to each other, but they are reproductively isolated from each other by different habitats, mating seasons, etc. Polyploidy – Is the presence of extra sets of chromosomes in cells due to accidents during cell division – Has caused the evolution of some plant species

46 2n = 6 4n = 12 2n2n 4n4n Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. Gametes produced by flowers on this branch will be diploid. Offspring with tetraploid karyotypes may be viable and fertile—a new biological species. An autopolyploid – Is an individual that has more than two chromosome sets, all derived from a single species

47 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Viable fertile hybrid (allopolyploid) Normal gamete n = 3 Normal gamete n = 3 Species A 2n = 4 Species B 2n = 6 2n = 10 An allopolyploid – Is a species with multiple sets of chromosomes derived from different species

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49 Reproductive Barriers A reproductive barrier is any factor that prevents two species from producing fertile hybrids, thus contributing to reproductive isolation. Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation

50 Prezygotic barriers – Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate Postzygotic barriers – Often prevent the hybrid zygote from developing into a viable, fertile adult Reproductive Barriers

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52 Prezygotic barriers impede mating or hinder fertilization if mating does occur Individuals of different species Mating attempt Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation HABITAT ISOLATION TEMPORAL ISOLATIONBEHAVIORAL ISOLATION MECHANICAL ISOLATION (b) (a) (c) (d) (e) (f) (g) Prezygotic and postzygotic barriers

53 Viable fertile offspring Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Fertilization Gametic isolation GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN (h) (i) (j) (k) (l) (m) Prezygotic and postzygotic barriers

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55 Adaptive Radiation Adaptive Radiation - Evolutionary process in which the original species gives rise to many new species, each of which is adapted to a new habitat and a new way of life. E.g. Darwin's Finches

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58 Adaptive Radiation of Hominids

59 Evidence for Evolution HOMOLOGY is a characteristic shared by two species (or other taxa) that is similar because of common ancestry. Artificial Selection Farmers had been conducting this controlled breeding of livestock and crops for years in order to obtain the most milk from cows or the best cobs from corn plants.

60 Evidence for Evolution Paleontology - Study of Fossils a. Fossil - preserved evidence of past life b. Radioactive Dating - method by which fossil age can be determined by the amount of organic matter remaining in the specimen. This is possible because some substances break down at a known rate (half-life).

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62 Types of homology morphological homology – species placed in the same taxonomic category show anatomical similarities. ontogenetic homology - species placed in the same taxonomic category show developmental (embryological) similarities. molecular homology - species placed in the same taxonomic category show similarities in DNA and RNA.

63 MORPHOLOGICAL HOMOLOGY Structures derived from a common ancestral structure are called: HOMOLOGOUS STRUCTURES

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66 Ontogenetic Homology The human embryo has gills, a post-anal tail, webbing between the toes & fingers, & spends its entire time floating and developing in amniotic fluid has similar salt concentration as ocean water

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68 Figure 22.15 Pharyngeal pouches Post-anal tail Chick embryo Human embryo ontogeny recapitulates phylogeny

69 MORPHOLOGICAL HOMOLOGY A structure that serves the same function in two taxa, but is NOT derived from a common ancestral structure is said to be an ANALOGOUS STRUCTURE

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71 Sugar glider AUSTRALIA NORTH AMERICA Flying squirrel Some similar mammals that have adapted to similar environments – Have evolved independently from different ancestors

72 Examples of Analogous structures: wings of bat, bird, and butterfly walking limbs of insects and vertebrates cranium of vertebrates and exoskeleton head of insects 4 chambered heart in birds & mammals

73 Molecular Homology

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76 Types of Evolution Divergent Evolution - Method of evolution accounting for the presence of homologous structures. Multiple species of organisms descended from the same common ancestor at some point in the past. Convergent Evolution - Method of evolution accounting for the presence of analogous structures. Organisms of different species often live in similar environments, thus explaining the presence of features with similar functions.

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78 An ongoing process Evolution can be considered a process of "remodeling" a population over the course of many generations, with the driving force being the natural selection factors that favor one form over another in specific environments.

79 Vestigial Structures Have marginal, if any use to the organisms in which they occur. EXAMPLES: femurs in pythonid snakes and pelvis in cetaceans (whales) appendix in humans coccyx in great apes

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83 Rate of Evolution Gradual evolution occurs where the increment of change is small compared to that of time. Punctuated evolution occurs where the increment of change is very large compared to that of time in discrete intervals, while most of the time there is virtually no change at all.

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86 Fitness – Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

87 Natural Selection in Action Industrial melanism

88 Natural Selection in Action Camouflage

89 Figure 22.11 (a)A flower mantid in Malaysia (b)A stick mantid in Africa If an environment changes over time – Natural selection may result in adaptation to these new conditions

90 Natural Selection in Action Mimicry Coral vs. King Snakes: Red on yellow, kill a fellow, red on black won’t hurt Jack

91 Natural Selection in Action Mimicry Monarch or Viceroy Butterfly

92 Natural Selection in Action Warning Coloration

93 Natural Selection in Action Disruptive Coloration

94 Natural Selection in Action Counter Shading

95 Natural Selection in Action Eye spots

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97 Causes of Evolution 1.Mutations - random changes in genetic material at the level of the DNA nucleotides or entire chromosomes 2.Natural Selection - most important cause of evolution; measured in terms of an organism's fitness, which is its ability to produce surviving offspring Modes of Selection a. Stabilizing Selection - average phenotypes have a selective advantage over the extreme phenotypes b. Directional Selection - phenotype at one extreme has a selective advantage over those at the other extreme c. Disruptive Selection - both extreme phenotypes are favored over the intermediate phenotypes

98 In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals Them from predators. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved population Frequency of individuals Modes of Selection

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100 Causes of Evolution 3. Mating Preferences - Organisms usually do not choose their mates at random, thus the selection process can cause evolution 4. Gene Flow - Transfer of genes between different populations of organisms. This situation leads to increased similarity between the two populations (Tends to reduce differences between populations over time) 5. Genetic Drift (Founder Effect & Bottleneck) - Situation that results in changes to a population's gene pool caused by random events, not natural selection. This situation can have drastic effects on small populations of individuals. Common on islands.

101 Gene Flow

102 Genetic Drift

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105 Founder Effect

106 Bottleneck Effect

107 Bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Understanding the bottleneck effect – Can increase understanding of how human activity affects other species

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109 Macroevolution -Evolutionary change above the species level e.g. the appearance of feathers on dinosaurs Macroevolutionary change – Is the cumulative change during thousands of small speciation episodes Microevolution – Is change in the genetic makeup of a population from generation to generation Note the Difference

110 Population geneticists – Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level Average heterozygosity – Measures the average percent of loci that are heterozygous in a population Population genetics – Is the study of how populations change genetically over time

111 Three major factors alter allele frequencies and bring about most evolutionary change – Natural selection – Genetic drift – Gene flow

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113 Figure 23.4 Generation 1 C R genotype C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

114 Hardy-Weinberg Theorem genetic structure of a non-evolving population remains constant + sexual recombination cannot alter the relative frequencies of alleles - Hardy-Weinberg equilibrium Hardy-Weinberg equation p 2 + 2pq + q 2 = 1 p 2 : frequency of AA genotype 2pq: frequency of Aa genotype q 2 : frequency of aa genotype - p: frequency of A allele - q: frequency of a allele

115 Hardy-Weinberg HW law states --> original of a genotypes alleles remains CONSTANT HW Equilibrium... is defined algebraically any gene with 2 allelic forms... A and a let frequency of one allele (A) = p & frequency of other allele (a) = q then by definition, p + q = 1 HW equation... (p + q)2 = p2 + 2 pq + q2 = 1 AA Aa aa

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118 mechanisms that help to preserve genetic variation in a population Diploidy – Maintains genetic variation in the form of hidden recessive alleles Heterozygote Advantage Individuals who are heterozygous at a particular locus have greater fitness than homozygotes Natural selection – Will tend to maintain two or more alleles at that locus

119 Heterozygote Advantage AA = No sickle (Dead from malaria) Aa = sickle trait aa = sickle disease (Dead) Plasmodium falciparum

120 Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4 Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Sexual reproduction

121 If sexual reproduction is a handicap, why has it persisted? – It produces genetic variation that may aid in disease resistance

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123 Phylogeny The evolutionary history of a species or group of related species depicted as a branching tree Each branch represents a new species which inherits many (primitive) traits from the ancestor but also has a new (derived) trait which appear for the 1 st time

124 Systematics – An analytical approach to understanding the diversity and relationships of organisms, both present-day and extinct Morphological, biochemical, and molecular comparisons are used to infer evolutionary relationships

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126 The fossil record – Is based on the sequence in which fossils have accumulated in such strata Fossils reveal – Ancestral characteristics that may have been lost over time

127 Diversity of Life Learned Through the Fossil Record Mass Extinctions extinction is inevitable in a changing world + extinctions open up new adaptive zones - new living conditions, resources, and opportunities

128 Dating Fossils Relative Dating tells the order in which groups of species were present in a sequence of strata (before/after, early/late) + index fossils - fossils that permit the relative dating of rocks within a narrow time span Absolute Dating dating that provides the age of fossils in years + radiometric dating - use of radioactive isotopes to date specimens (Carbon-14)

129 Dinosaur bones being excavated from sandstone Tusks of a 23,000-year-old mammoth, frozen whole in Siberian ice Boy standing in a 150-million-year-old dinosaur track in Colorado Casts of ammonites, about 375 million years old Insects preserved whole in amber Petrified tree in Arizona, about 190 million years old Leaf fossil, about 40 million years old

130 In addition to fossil organisms – Phylogenetic history can be inferred from certain morphological and molecular similarities among living organisms In general, organisms that share very similar morphologies or similar DNA sequences – Are likely to be more closely related than organisms with vastly different structures or sequences

131 C C A T C A G A G T C C G T A Deletion Insertion C C A T C A A G T C C C C A T G T A C A G A G T C C C C A T C A A G T C C C C A T G T A C A G A G T C C 1Ancestral homologous DNA segments are identical as species 1 and species 2 begin to diverge from their common ancestor. 2Deletion and insertion mutations shift what had been matching sequences in the two species. 3Homologous regions (yellow) do not all align because of these mutations. 4Homologous regions realign after a computer program adds gaps in sequence 1. 1 2 1 2 1 2 1 2 Systematists use computer programs and mathematical tools – When analyzing comparable DNA segments from different organisms

132 Sorting Homology from Analogy A potential misconception in constructing a phylogeny – Is similarity due to convergent evolution, called analogy, rather than shared ancestry Convergent evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary Analogous structures or molecular sequences that evolved independently – Are also called homoplasies

133 Phylogenetic systematics connect classification with evolutionary history Taxonomy – Is the ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences Binomial nomenclature – Is the two-part format of the scientific name of an organism – Was developed by Carolus Linnaeus

134 Classification based on physical and structural similarities Carolus Linnaeus (1707-1778) Created binomial nomenclature (2 word naming system) 1 st word = Genus (genera if plural) = a group of similar species 2 nd word = specific epithet = Species Scientific name = Genus + specific epithet e.g. Homo sapiens

135 Rules for writing species names 1.Latin is the language of scientific names (Latin is no longer spoken, so it does not change) 2.Italicize in print and underline when hand written 3.1 st letter of the genus is CAPITALIZED & 1 st letter of specific epithet is lowercase

136 Canis lupus = Grey wolf Canis latrans = Coyote

137 Cougar? Puma? Panther? Catamount? Mountain lion? Or… Felis concolor?

138 Domain Did Kingdom Kinky Phylum Phil Class Come Order Over Family For Genus Good Species Sex

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140 All Living Organisms are grouped into... 3 DOMAINS EUBACTERIA - true bacteria ARCHAEA - ancient prokaryotes EUCARYA - modern eukaryotes

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142 Six Kingdoms Eubacteria · Prokaryotic · True bacteria · RNA is simple · Have true cell walls · Unicellular Archaebacteria · Prokaryotic · RNA more complex · Unicellular Protista · Eukoryotic · Autotrophs and heterotrophs · Lacks organs systems · Lives in moist environments · Unicellular or multicellular Fungi · Eukaryotic · Heterotrophs · Unicellular or multicellular · Absorbs nutrients from organic material in its environment · Unicellular or multicellular

143 Six Kingdoms Plantae · Eukaryotic · Autotrophs · Multicellular · Photosynthetic Animalia · Eukaryotic · Heterotrophs · Multicellular

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145 Systematists depict evolutionary relationships – In branching phylogenetic trees Panthera pardus (leopard) Mephitis mephitis (striped skunk) Lutra lutra (European otter) Canis familiaris (domestic dog) Canis lupus (wolf) Panthera Mephitis Lutra Canis FelidaeMustelidaeCanidae Carnivora Order Family Genus Species

146 Leopard Domestic cat Common ancestor Each branch point – Represents the divergence of two species

147 “ Deeper” branch points – Represent progressively greater amounts of divergence Leopard Domestic cat Common ancestor Wolf

148 Phylogenetic systematics informs the construction of phylogenetic trees based on shared characteristics A cladogram – Is a depiction of patterns of shared characteristics among taxa A clade within a cladogram – Is defined as a group of species that includes an ancestral species and all its descendants Cladistics – Is the study of resemblances among clades Cladistics Vocabulary

149 Character -- Heritable trait possessed by an organism Nodes --The points of branching within a cladogram. Cladistics Vocabulary

150 Clades – Can be nested within larger clades, but not all groupings or organisms qualify as clades MONOPHYLETIC (Only VALID clade) taxon includes all descendent species along with their immediate common ancestor POLYPHYLETIC (b) taxon includes species derived from two different immediate ancestors PARAPHYLETIC (c) taxon includes species A without incorporating all other descendants

151 Phylogeny - evolutionary history of a group of organisms Cladistics – The study of evolutionary relationships between groups to construct their family tree based on characters NOTDerived characters – Characteristics which appear in recent parts of a lineage but NOT in its older members (Evolutionary innovation) Evolutionary Classification

152 Most recent common ancestor – The ancestral organism from which a group of descendants arose.

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155 A shared primitive character – Is a homologous structure that predates the branching of a particular clade from other members of that clade – Is shared beyond the taxon we are trying to define Cladistics Vocabulary A shared derived character – Is an evolutionary novelty unique to a particular clade

156 Systematists use a method called outgroup comparison – To differentiate between shared derived and shared primitive characteristics Outgroup comparison – Is based on the assumption that homologies present in both the outgroup and ingroup must be primitive characters that predate the divergence of both groups from a common ancestor

157 Ingroup -- In a cladistic analysis, the set of taxa which are hypothesized to be more closely related to each other than any are to the outgroup. Cladistics Vocabulary

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159 Characters & Character Table

160 Systematists – Can never be sure of finding the single best tree in a large data set – Narrow the possibilities by applying the principles of maximum parsimony and maximum likelihood The most parsimonious tree is the one that requires the fewest evolutionary events to have occurred in the form of shared derived characters

161 Human MushroomTulip 40% 0 30% 0 Human Mushroom Tulip (a) Percentage differences between sequences 0 Applying parsimony to a problem in molecular systematics

162 The principle of maximum likelihood – States that, given certain rules about how DNA changes over time, a tree can be found that reflects the most likely sequence of evolutionary events Sometimes there is compelling evidence – That the best hypothesis is not the most parsimonious Lizard Four-chambered heart Bird Mammal Lizard Four-chambered heart Bird Mammal Four-chambered heart (a) Mammal-bird clade (b) Lizard-bird clade

163 Gene duplication – Is one of the most important types of mutation in evolution because it increases the number of genes in the genome, providing further opportunities for evolutionary changes Homeotic or Hox genes, when duplicated can lead to new appendage arrangement (Vertebrate Evolution from Invertebrates)

164 The tree of life – Is divided into three great clades called domains: Bacteria, Archaea, and Eukarya The early history of these domains is not yet clear Bacteria Archaea Eukarya

165 The vertebrate Hox complex contains duplicates of many of the same genes as the single invertebrate cluster, in virtually the same linear order on chromosomes, and they direct the sequential development of the same body regions. Thus, scientists infer that the four clusters of the vertebrate Hox complex are homologous to the single cluster in invertebrates. 5 First Hox duplication Second Hox duplication Vertebrates (with jaws) with four Hox clusters Hypothetical early vertebrates (jawless) with two Hox clusters Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster Most invertebrates have one cluster of homeotic genes (the Hox complex), shown here as colored bands on a chromosome. Hox genes direct development of major body parts. 1 A mutation (duplication) of the single Hox complex occurred about 520 million years ago and may have provided genetic material associated with the origin of the first vertebrates. 2 In an early vertebrate, the duplicate set of genes took on entirely new roles, such as directing the development of a backbone. 3 A second duplication of the Hox complex, yielding the four clusters found in most present-day vertebrates, occurred later, about 425 million years ago. This duplication, probably the result of a polyploidy event, allowed the development of even greater structural complexity, such as jaws and limbs. 4 The evolution of vertebrates from invertebrate animals – Was associated with alterations in Hox genes


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