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Evolution and the Theory of Natural Selection

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1 Evolution and the Theory of Natural Selection

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

3 Why the controversy?



6 Intelligent Design vs Evolution

7 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 ( ) 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”

8 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)

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

10 The Origin of Species Occurrence of Evolution Mechanism 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?


12 Journey of the H.M.S. Beagle

13 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

14 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

15 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!

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

17 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

18 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.

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

20 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

21 James Hutton ( ) 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

22 Charles Lyell ( ) 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.

23 Thomas Malthus ( ) 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.


25 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.

26 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.

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

28 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.

29 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.

It can be broken down into four basic tenets, or ideas

31 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.

32 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.

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

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

35 Evolution Theory - an accepted hypothesis that has been tested over and over again without yet being disproved Definition - Evolution is the change in the overall genetic makeup of a population over time 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).

36 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

37 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.

38 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

39 the origin of new taxonomic groups (new species, etc.) + 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 (b) Cladogenesis (a) Anagenesis

40 Speciation can occur in two ways
Allopatric speciation Sympatric speciation 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.

41 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.


43 A. harrisi A. leucurus

44 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

45 An autopolyploid Is an individual that has more than two chromosome sets, all derived from a single species 2n = 6 4n = 12 2n 4n 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.

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


48 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

49 Reproductive Barriers
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


51 Individuals of different species
Prezygotic and postzygotic barriers 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 ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION (b) (a) (c) (d) (e) (f) (g)

52 Prezygotic and postzygotic barriers
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)


54 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 



57 Adaptive Radiation of Hominids

58 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.

59 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).


61 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.

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



65 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


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

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


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

71 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

72 Molecular Homology



75 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.


77 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.

78 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




82 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.



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

86 Natural Selection in Action
Industrial melanism

87 Natural Selection in Action

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

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

90 Natural Selection in Action
Mimicry Monarch or Viceroy Butterfly

91 Natural Selection in Action
Warning Coloration

92 Natural Selection in Action
Disruptive Coloration

93 Natural Selection in Action
Counter Shading

94 Natural Selection in Action
Eye spots


96 Causes of Evolution Mutations - random changes in genetic material at the level of the DNA nucleotides or entire chromosomes 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

97 Modes of Selection Original population Frequency of individuals
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 Frequency of individuals


99 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.

100 Gene Flow

101 Genetic Drift



104 Founder Effect

105 Bottleneck Effect

106 Understanding the bottleneck effect
Can increase understanding of how human activity affects other species 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.


108 Note the Difference 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

109 Population genetics Is the study of how populations change genetically over time 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

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


112 Figure 23.4 Generation 1 CRCR CWCW genotype Plants mate 2 All CRCW
(all pink flowers) 50% CR gametes 50% CW Come together at random 2 3 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

113 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 pq + q2 = 1 p2: frequency of AA genotype 2pq: frequency of Aa genotype q2: frequency of aa genotype - p: frequency of A allele - q: frequency of a allele

114 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 = p pq q2 = 1 AA Aa aa



117 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

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

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

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


122 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 1st time

123 Systematics Morphological, biochemical, and molecular
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


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

126 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

127 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)

128 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

129 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

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

131 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

132 Binomial nomenclature
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

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

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

135 Writing Species Names Canis latrans = Coyote Canis lupus = Grey wolf

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

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


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


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

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


144 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 Felidae Mustelidae Canidae Carnivora Order Family Genus Species

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

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

147 Cladistics Vocabulary
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

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

149 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

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

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



154 Cladistics Vocabulary
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 A shared derived character Is an evolutionary novelty unique to a particular clade

155 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

156 Cladistics Vocabulary
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.


158 Characters & Character Table

159 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

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

161 Sometimes there is compelling evidence
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 Lizard Four-chambered heart Bird Mammal (a) Mammal-bird clade (b) Lizard-bird clade Sometimes there is compelling evidence That the best hypothesis is not the most parsimonious

162 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)

163 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 Archaea Bacteria Eukarya

164 The evolution of vertebrates from invertebrate animals
Was associated with alterations in Hox genes 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 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

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