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BIO153 Lecture 2.pdf

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Christoph Richter

BIO153: Lecture 2 Ordering Diversity January 9, 2009 Topics: ▯ species concepts ▯ taxonomy & the Linnean system ▯ introduction to phylogenetic analysis: types of characters; types of groups The species concept ▯ The species is the fundamental unit in classifying the diversity of life ▯ However, it’s tricky to define! ▯ Criteria for recognition of species: 1. common descent (must trace ancestry to a common ancestral population) 2. smallest distinct grouping 3. reproductive isolation from other organisms* *what about asexual organisms? Reproductive isolation is defined as occupation of a particular ecological niche so that the population responds as a unit to evolutionary processes such as selection and drift. ▯ There have been many attempts to provide a species definition that works for all types of organisms; this is still an unresolved issue in modern biology. ▯ Modern biologists tend to use the phylogenetic species concept—“the irreducible (basal) grouping of organisms diagnosably distinct from other such groupings and within which there is a parental pattern of ancestry and descent.” ▯ This approach focuses on reconstructing the historical pattern of evolution by tracing descent relationships 1 Taxonomy = “arrangement law” ▯ taxonomy: formal system for naming and classifying species ▯ Linnean system for naming species: binomial nomenclature ▯ Form: Genus + species (authority) e.g. Virginia opossum: Didelphis virginiana (Linnaeus 1758) Didelphis = “two-wombed” virginiana = “of Virginia” (*** A note on the use of bionomial nomenclature in BIO153: We are fussy about the proper use of the Linnean system, so please use it correctly! 1. The name of the genus is ALWAYS capitalized (e.g. Didelphis). 2. The species epithet (species name) is NEVER capitalized (e.g. virginiana). 3. If typed out, a scientific binomial should be italicized. If handwritten, the binomial should be underlined. (THIS APPLIES TO BIO153 TESTS!) 4. Once you have given the full binomial, you can abbreviate the genus name: “Didelphis virginianais the only marsupial native to North America. D. virginianais nocturnal…. 5. You are not responsible for knowing or indicating the scientific authority for any species name.***) 2 The Linnean ranks: (Domain) (not a mandatory rank) Kingdom Phylum (animals)/ Division (plants) Class Order Family Genus Species ▯ ranks are nested & hierarchical ▯ major ranks are called taxa (singular = taxon) ▯ 7 mandatory ranks ▯ There may be intermediate rankings: e.g., between Order and Family, there may be 2 additional ranks – Suborder and Superfamily). For example: the taxonomic categories (taxa) to which humans belong: Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Mammalia Subclass: Eutheria Order: Primates Suborder: Anthropoidea Family: Hominidae Genus: Homo Species: sapiens The nestedness of the Linnean ranks reflects evolutionary relationships: e.g. a domestic dog (Canis familiaris) shares a more recent common ancestor with a wolf (Canis lupus) than it does with an arctic fox (Alopex lagopus). This is shown by the fact that dogs and wolves are in the same genus, while dogs, wolves and arctic foxes are in the 3 same family, but different genera (genus pl. = genera). Phylogenetics: ▯ From the Greek: phyle = tribe and genesis = birth or origin ▯ study of evolutionary relationships among groups of organisms ▯ goal: describe nested, hierarchical groups showing descent relationships ▯ Phylogenetics describes the tree of life! Phylogenetic tree (cladogram): diagrammatic representation of a hypothesis about evolutionary relationships among a group of organisms present A CC B branch nodde imee node indicates a commmmoon anncestor past AC B DDDD Clade = monophyletic group EEFFFF ▯ clade: Greekados = branch GGGG ▯ mono = one; phyle = tribe ▯ a group of organisms composed of an ancestor and all its descendents 4 How do we assign organisms into monophyletic groups? ▯ examine features of organisms to deduce relationships ▯ features can be anatomical, physiological, behavioural; chromosomal features, molecular sequences… ▯ these features/traits are formally called characters Why is it useful to examine characters? Often, traits (characters) that are similar among organisms indicate that those organisms have a close evolutionary relationship (a recent common ancestor), and traits that differ among organisms indicate that those organisms are not closely related (share only a distant common ancestor). Recall: Darwin’s 1st Big Idea: Evolutionary change by “descent with modification": we expect that: ▯ organisms that are closely related will share many characters ▯ conversely, organisms that are distantly related will share few characters However we must also consider Darwin’s 2nd Big Idea: NATURAL SELECTION, which produces adaptation The result of selection: closely related organisms may differ, and distantly related organisms may be similar! 5 Adaptation to similar circumstances makes similar
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