Ch 19 Evolution and Classification

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University of Toronto Scarborough
Biological Sciences
Ivana Stehlik

Ch 19 Evolution and Classification  Determining evolutionary relationships btw organisms and deciding if similar structures or similar-looking structures are grounds for grouping species together  Convergent (or parallel) evolution: tendency among organisms living under the same conditions to develop similar body forms o Parallel or convergent depending on evolutionary relatedness of organisms involved o Convergent evol. refers to more distantly related organisms o Parallel evol. refers to more closely related ones  Similarity(resemblance) in appearance does not always mean relatedness  biologists classify (and name) organisms based on features o morphological traits, chromosomal anatomy, gene sequences, physiological functioning, morphology of sub-cellular structures and behaviour patterns 19.2 Systematic Biology: An Overview (Pg 422)  Systematics : analytical approach for studying the diversity of life and evolutionary relationships o identify, describe, classify and name organisms based on evolutionary relationships  Two goals of systematic: reconstruct phylogeny and taxonomy  Phylogeny: evolutionary history of a group of organisms o Phylogenetic tree  Formal hypotheses identifying likely relationships among species  Allows us to distinguish similarities inherited from a common ancestor from those that evolved independently in response to similar environments  Taxonomy: identification and naming of species and their placement in a classification o ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences o uses binomial nomenclature  two-part format of the scientific name of an organism  developed by Carolus Linnaeus (mid-1700’s)  binomial name of an organism or “scientific epithet”  is the genus and species  is latinized  is italicized or underlined o Classification: arrangement of organisms into hierarchical groups that reflect their relatedness o Most Systematists want classifications to mirror Phylogenetic history and adaptive radiation (evolutionary history) of group of organisms  Gynogenetic: the process whereby a sperm fertilizes an ovum, but does not contribute genetic material to the resultant zygote.  Androgynous: Partly male and partly female in appearance; of indeterminate sex. o Having the physical characteristics of both sexes; hermaphrodite. 19.3 The Linnaean System of Classification (Pg 424)  Established by Carolus Linnaeus  Taxonomic hierarchy: arranging organisms into ever more inclusive categories. o Species, genus, family, order, class, phylum, kingdom, domain  King Philip came over for great spaghetti. o Life on earth is classified into 3 domains. o Taxon: organisms included within any category of taxonomic hierarchy  Next step is to determine which species are most closely related to one another. o Want to know which characteristics some species have in common and can be used to place the species in the same genus and family 19.4 From Classification to Phylogeny (Pg 425)  for at least 200 years (before Darwin), biologists relied on organismal traits (mainly morphology) when analyzing and classifying organisms - taxonomy  after Darwin – biologists wanted to include evolutionary history of a species = phylogeny  biologists now use systematics o an analytical approach to understand the diversity and relationships of organisms  After Linnaeus, they developed phylogenies based on chromosomal anatomy, details of physiological functioning, morphology of subcellular structures, cells, organ systems and whole organisms, and patterns of behavior  Today systematists also use molecular sequences of nucleic acids and proteins as additional characters when deriving phylogenies  19.5 Evaluating Systematic Characters (Pg 425)  systematics - provides essential information for all of the biological sciences o provides data for testing for homologous vs. analogous relationships  systematists study traits in which phenotypic variation reflects genetic differences. o Exclude diff caused by env. Conditions  Systematic characters: o Must be genetically independent, reflecting diff parts of organisms’ genomes  Necessary because diff organismal characters can have the same genetic basis.  Want to use each genetic variation only once in an analysis  Example: two foots, toes (pads on lizards). Both right and left toe coded by same genes, therefore choose only one as systematic character  Homologous characters: useful in preparing phylogenies o Emerge from comparable embryonic structures and grow in similar ways during development  Systematists have put great stock in embryological indications of homology on the assumption that evolution has conserved the pattern of embryonic development in related organisms. o Such phenotypic similarities btw organisms reflect underlying genetic similarities o Systematic analyses rely on the comparison of homologous characters as indicators of common ancestry and genetic relatedness o Homologous structures where their function has changed can differ considerably among species  Analogous characters: homoplasious (homoplasies): phenotypic similarities that evolved independently in different lineages. o Similar function in diff species o Systematists exclude homoplasies from their analyses because homoplasies provide no information about shared ancestry.  Mosaic evolution o Refers to reality that in all evolutionary lineages, some characteristics evolve slowly, whereas others evolve rapidly. o It is pervasive: (esp. of an unwelcome influence or physical effect) Spreading widely throughout an area or a group of people. o Every species displays a mixture of ancestral characters (old forms of traits) and derived characters (new forms of traits). o A "derived character" is a trait that arose in the most recent common ancestor of a particular lineage and was passed along to its descendants.  Among a given group of organisms, the shared derived characters are generally the less common characters.  The evolutionary interpretation is that these characters of organisms are more recently evolved. They are contrasted with primitive characters. Shared derived characters should have the same structure and function. o Derived characters provide the most useful information about evolutionary relationships because once a derived character becomes established, it is usually present in all of that species’ descendants.  Unless they are lost or replaced by newer characters over evolutionary time, derived characters can serve as markers for entire evolutionary lineages.  Systematists score characters as either ancestral or derived only when comparing them among organisms o Any particular character is derived only in relation to what occurs in other organisms  Older version of same character or an entirely new trait, absence of it altogether o Example: most species of animals lack a vertebral column (backbone)  Systematists compare vertebrates with all animals lacking vertebral column  Absence of vertebral column = ancestral condition  Presence of vertebral column= derived  Fossils of earliest animals lack backbones. o Systematists distinguish between ancestral and derived characters to ascertain in which direction a character has evolved.  Outgroup comparison: comparing the group under study with more distantly related species not otherwise included in the analysis o Used to identify ancestral and derived characters o Example: butterflies mostly have 6 walking legs, two families have 4 walking and 2 nonwalking legs  Outgroup comparison with other insects (most which have 6 walking legs as adults) show that 6 legs is the ancestral character and 4 walking legs and 2 non walking legs is a derived character. 19.6 Phylogenetic Inferences and Classification (Pg 427)  Phylogenetic trees portray the evolutionary diversification of lineages as a hierarchy that reflects the branching pattern of evolution o Each branch rep the descendants of a single ancestral species  When converting Phylogenetic tree into a classification, Systematists use the principle of monophyly  Monophyletic taxa: those derived from a single ancestral species  Polyphyletic taxa: includes species from separate evolutionary lineages  Paraphyletic taxon: includes an ancestor and some but not all of its descendants o Eg class reptiles  Most Systematists strive to create parsimonious Phylogenetic hypothesis. o Include fewest possible evolutionary changes to account for diversity with
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