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Evolution and Systematics. Chapter 19. 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
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Evolution and Systematics Chapter 19
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)
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 plant-like and animal-like characteristics • Fungi have more in common with animals than plants
Taxonomy • Early 20th century biologists divided plant kingdom into four new kingdoms • Monera • Fungi • Plantae • Protista
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
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
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
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
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
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