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Evolution and Systematics Chapter 18 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”) 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 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 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 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 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 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 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 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 - 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 Taxonomy • Taxon (plural, taxa) – Taxonomic group at any level – Examples: species, kingdom Taxonomy • Original taxonomic plan – Two kingdoms • Plant • Animal • Examples of problems with this scheme – Some microscopic organisms have both plantlike and animal-like characteristics – Fungi have more in common with animals than plants Kingdom Animal Plant Description Move actively and consume prey Do not move or consume prey 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 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 Taxonomy • Carl Woese – Found prokaryotes included two distinct groups of organisms – Probably evolved separately – Evidence came from analysis of ribosomal RNA called rDNA 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 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 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 on earth, play major role as decomposers Ordinary bacteria, found in every habitat 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 treesized and exhibited secondary growth and wood – Modern horsetails are herbs 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 Evolution • Darwin kept thoughts to himself until he received letter from Wallace stating same ideas • Darwin – 1859 – Published The Origin of Species 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. 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 Evolution • Darwin’s ideas suggested – No ideal body form for each species – Forms can change as environment changes 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 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 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 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 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) 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 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 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 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 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 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 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 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 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 Directional Selection 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 Stabilizing Selection 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 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 Diversifying Selection 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 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 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 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? 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 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 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. 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 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 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 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 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 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 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 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) 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 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 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 Cladistics – Node • Branch point where ancestral species split to produce two new species • Ancestor itself ceased to exist • Oldest node called root of cladogram 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 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 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 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 Cladistics • Homologous traits – Alternative states of the same character – Arose from the same ancestral trait – Example • Wings of bird and forelegs of horse 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 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 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 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 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) 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 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 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