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Topic 1: Classification; Diversity of Life (Ch BIOL 1030 – TOPIC 1 LECTURE NOTES Topic 1: Classification and the Diversity of Life (Chapters 25, 26.6) I. Backgroun d review (Biology 1020 material) A. Scientific Method 1. observations 2. scientific model • explains observations • makes testable predictions 3. test predictions (can confirm predictions) 4. reject, revise, or tentatively accept scientific model 5. caveats: • Scientific models can only be proven false, never proven true. • Correlation does not equal causation. • Testable predictions cannot include the supernatural (the supernatural cannot, by definition, be tested scientifically); thus, the supernatural is outside the realm of science. • The term “theory” has a very different meaning in science than in most everyday conversations. 6. terms: • hypothesis – model that has not been tested or has only been tested some • theory – model that has been tested extensively and is accepted by most scientists in that field • law – usually a very well-established theory that explains a wide body of observations B. Theory of Evolution: The Modern Synthesis 1. Evolutionary relationships between organisms provides the theoretical framework for modern classification systems; as such, it is the major organizing principle underlying the structure of most of this course 2. In POPULATIONS, new mutations (random) and recombination of current variations (random) occurs. 3. Populations encounter EVOLUTIONARY MECHANISMS: • natural selection (greater reproduction by the “fittest”) • genetic drift (random, greater for small populations) • gene flow (genetic exchange with other populations) • mutations (new changes in genetic material) 4. Evolutionary mechanisms cause MICROEVOLUTION: changes in population genotype and allele frequencies for the next generation. 5. Adding any REPRODUCTIVE ISOLATION MECHANISM allows MACROEVOLUTION (speciation). • Examples of reproductive isolation mechanisms include physical separation, selective mating, and sterile offspring. 6. Speciation can be rapid (punctuated equilibrium) or gradual; relative amounts of these are debated but both appear to occur. 1 of 7 BIOL 1030 – TOPIC 1 LECTURE NOTES II. Classi fication of organisms A. Biologists use a binomial system for classifying organisms. 1. taxonomy - the science of classifying and naming organisms. 2. Carolus Linnaeus (18th century biologist) developed a system of classification that is the basis of what is used today • binomial system: today each species’ official scientific name is made of 2 words (bi=“2” nomen=“name”) • names are Latin . same language used universally in biology . dead language – not changing . names of people can be “Latinized” for use in naming 3. species - basic unit of classification or taxonomy (more on this later) • if sexual, a group of organisms that can interbreed and produce fertile offspring • if asexual, grouped based on similarities (DNA sequence is best) • about 1.8 million living species have been described, likely millions more 4. genus - a group of closely related species. 5. together the genus and specific epithet names make up the binomial name used to name a species • the Genus name is always capitalized, and the specific epithet is never capitalized. • the Genus and specific epithet are always together, and italicized (or underlined). • example: Homo sapiens or Homo sapiens B. Taxonomic classification is hierarchical. 1. A group of related genera make up a Family. 2. Related families make up an Order. 3. Related orders are grouped into a Class. 4. Related classes are grouped into a Phylum or Division. 5. Related phyla or divisions are grouped into a Kingdom. 6. Related kingdoms are grouped into a Domain, the highest level of classification in the modern system. 7. The gold standard for “related” is based on DNA sequence similarities, but other criteria are used as well (we don’t have the complete DNA sequence of all known species) III. What is a species? A. Species: “Kind of living thing” B. Word “species” is both plural and singular C. relatively easy to define for sexual organisms, hard for asexual organisms and extinct species 1. biological species concept (for sexual organisms) – one or more populations whose members are capable of interbreeding and producing fertile offspring, and whose members are reproductively isolated from other such groups • not always clear-cut, because some can interbreed under “artificial” conditions but don’t appear to do so in nature • sometimes, “race” and “subspecies” designations are used, but often different specific epithets are used when there are clear morphological differences involved 2 of 7 BIOL 1030 – TOPIC 1 LECTURE NOTES 2. asexual species – definition based on biochemical (think DNA sequence) and morphological differences; no solid rules • also includes use of “race,” “subspecies,” and “strain” designations • in asexual species, microevolution over time directly leads to macroevolution (speciation) 3. evolutionary species concept – a single line of descent (lineage) that maintains its distinctive identity from other lineages; works for all species, but it can be hard to clearly define “distinctive identity” D. So how many species are there? 1. no one knows for sure, best guess is about 10 million, but only about 1.8 million have been described by humans 2. most are tropical 3. human activities (particularly in the tropics) are certainly destroying many species before they can even be described; we are undergoing the sixth mass extinction event in the history of life on earth (and the first one driven by the activities of man) IV. Classi fication: constructing phylogenies A. classification is largely based on inferred evolutionary relationships between organisms; the two major approaches to this are cladistics and traditional taxonomy 1. phylogeny – evolutionary tree; explanation of evolutionary relationships among groups (what evolved from what, in what order, and when) 2. systematics – study and reconstruction of phylogenies 3. groups of organisms may be: • monophyletic (includes most recent common ancestor and all descendants) • paraphyletic (includes most recent common ancestor BUT not all descendants) • polyphyletic (does not include most recent common ancestor) 4. both cladistics and traditional taxonomy avoid polyphyletic groups; cladistics also avoids paraphyletic groups EXPLANATION What do terms monophyletic, paraphyletic and polyphyletic mean? These terms are used to describe groupings of organisms, and indicate the extent to which they can be considered as ``natural groups''. They are best explained using examples, so consider the following family-tree diagram: Aves / / Crocodilia / Mammalia \ Dinosauria \ \ / \ \ / \ \ / Synapsida Reptilia \ / \ / \ / Amniota Here are examples of all three types of group: 3 of 7 BIOL 1030 – TOPIC 1 LECTURE NOTES • Consider the group consisting of all the animals in this diagram - that is, Amniota. This group is monophyletic because it consists of a single animal together with all of its descendants. The Dinosauria, including the modern birds, is another monophyletic group, sometimes defined as the most recent common ancestor of Igunanodon and Megalosaurus together with all its descendants. Monophyletic groups are also called clades, and are generally considered as the only ``natural'' kind of group. They are very important in phylogenetic classification. • Now consider the group consisting of the non-avian dinosaurs (which is what people usually mean by the informal term ``dinosaurs''). This is a paraphyletic group, because it can't be defined simply as ``this animal plus all its descendants'', but must be described as one clade minus another: in this case, Dinosauria minus Aves. The ``non-avian dinosaurs'' make up a singly paraphyletic group because only one clade need be omitted from its base definition. Groups may also be doubly paraphyletic, thrice paraphyletic, etc., depending on how many sub-clades they omit. • Finally, consider the group of ``warm-blooded animals'', which consists of Mammalia and Aves. This is a polyphyletic group - a totally unnatural assemblage - because it can't even be expressed as a paraphyletic group, that is, a clade minus one or more of its subclades. Such groups are not used at all in phylogenetic work since they are a purely artificial construct. In terms of common descent, a ``warm-blooded animals'' grouping makes no more sense than a Synapsida-plus-Crocodilia group - though this is not to say the notion of a warm-blooded group may not be useful in some informal discussions. So far, so straightforward. The only wrinkle in this scheme is that some workers use the word ``monophyletic'' in a sense that includes what we have described here as paraphyletic groups. These people then use ``holophyletic'' to describe what are usually called monophyletic groups. It's tempting in the face of this ambiguity just to abandon the word ``monophyletic'' and use a holophyletic/paraphyletic dichotomy, but this terminological abuse is probably not widespread enough to merit such extreme measures. It's just something to be on the watch for. Exercise: Determine which of these groupings are mono, para and polyphyletic (Refer Textbook for more details). B. cladistics groups organisms on the basis of unique shared characters inherited from common ancestor, or derived character 1. clade – group of organisms related by descent 2. synapomorphy – a derived character that is unique to and thus defines a particular clade 3. cladogram – branching diagram based on cladistic analysis that represents a phylogeny • cladograms
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