HC BIOL 103 - Speciation and Taxonomy

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Speciation and Taxonomy 1) Species: a) The basic lower unit of classification, consisting of a population of closely related and similar organisms. 2) Biological Species - Individuals capable of successful interbreeding and producing fertile offspring with each other but not with other species. a) Ernst Mayr: “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” b) No matter how extensive the phenotypic variation, populations belong to the same species for as long as their individuals have the form, physiology and behavior that allow them to interbreed and produce fertile offspring. 3) Speciation: The multiplication of species through division of one species into two or more as a result of the genetic divergence of reproductively isolated populations. a) Speciation can vary from case to case in its i) Duration-some take longer than other ii) Details- the extent of change is not always the same b) Reproductive isolation leads to speciation. i) Note: Reproductive isolation does not evolve purposefully to promote formation of a species or maintain its identity. Any structural, functional or behavioral difference that favors reproductive isolation simply is a by-product of genetic change. c) After reproductive isolation genetic divergence can result from i) Mutation ii) Natural Selection iii) Genetic Drift d) Reproductive Isolating Mechanisms: Any heritable feature of (form, functioning or behavior) that prevents interbreeding between one or more genetically divergent populations. i) Prezygotic Isolation-Occurs before fertilization (1) Ecological Isolation- Potential mates occupy different local habitats within the same area. (2) Temporal Isolation- Potential mates occupy overlapping ranges but reproduce at different times. (3) Behavioral Isolation- Potential mates meet but cannot figure out what to do about it. (4) Gametic Mortality- Sperm is transferred, but egg is not fertilized ii) Postzygotic Isolation-Occurs after fertilization (1) Zygotic Mortality- Egg is fertilized, but zygote or embryo dies. (2) Hybrid Inviability- First generation hybrid forms but shows very low fitness (3) Hybrid Infertility- Hybrid is sterile or partially so. 4) Sympatric Speciation- Isolation within a population a) New species arises in the midst of the original population.b) Isolation may be due to ecological separation on a small scale, but enough to influence sexual selection among potential mates and over generations to provide the reproductive isolation that can lead to speciation. i) Ex: Lake Victoria Cichlids c) Speciation by polyploidy- Successful hybridization to form new species i) Polyploidy- Change in the chromosome number ii) Common in plants iii) Usually polyploidy results in dosage problems, but this can sometimes be beneficial. 5) Allopatric Speciation- Geographical isolation a) Thought to be most common means of speciation b) A geographic barrier, such as a river or a mountain range, causes the splitting of a population such that individuals of these now-separate populations can no longer interbreed. The process of speciation is complete when individuals of the two populations no longer will interbreed even if changing circumstances put them back in the same area. i) Ex: Grey Squirrels around the Grand Canyon c) Pioneering individuals may colonize a new habitat, such as an oceanic island. (1) Ex: Hawaiian Honeycreeper 6) Patterns of Speciation and Evolution a) Phylogeny- The evolutionary relationships among living, or extinct, organisms. b) Evolutionary Trees- Summarize information about the continuity of relationships among species; summarize phylogenies i) Branching in the tree represents speciation. ii) Angling of a branch represents gradual change in a lineage= phyletic evolution. iii) A straight line represents no change in a lineage. iv) Branches ending before the present represents extinction. v) Adaptive radiation- Burst of divergence (branches) from a single lineage. 7) Systematics and Taxonomy a) Systematics-Scientific study of the diversity of organisms b) Taxon (pl.=taxa) – Any group of organisms treated as a unit in the classification system c) Taxonomy- The theory and practice of classifying organisms 8) The Linnaean System a) Uses binomial nomenclature (two names, a genus and a species) to identify a particular species. i) Homo sapiens b) Species are grouped into taxonomic categories from Most Inclusive to Most Exclusive i) Kingdom- Most Inclusive (1) Phylum (a) Class (i) Order1. Family a. Genus i. Species- Most Exclusive 9) Cladistics- Classification based solely on evolutionary relationships. a) Classification of organisms matches their evolutionary history and expresses the history in branching trees known as cladogram. b) Clade-The entire portion of a phylogeny that is descended form common ancestor. c) How do we construct evolutionary trees using cladistics? i) Identify ancestral and derived traits. (1) Ancestral traits- Traits shared with a common ancestor (a) Most mammals have four limbs, having inherited this from common ancestor. (2) Derived traits- A trait that differs from the ancestral trait in a lineage. (a) Homologous Traits- Traits derived from a common ancestor. (i) Most mammalian limbs terminate in five digits, but in the hooved animals, there is only one. This trait, having only one digit, is a homologous trait and could be used to group all the hooved animals together. (b) Difficulties in determining homologous traits. (i) Not all resemblances are products of common ancestry. (ii) Homoplasy- Some traits are the product of convergent evolution, the evolution of the same trait in different lineages 1. Australian mammals vs. N. American Mammals d) Species that share a common ancestor should share many homologous traits. e) Therefore, if two species share the same trait, systematists should, until proven otherwise, assume that the trait is homologous, i.e. none of the traits are the product of convergent evolution. f) Constructing a Cladogram i) Assumptions (1) Evolution of traits is irreversible, i.e. an ancestral trait can change into a derived one, but not the reverse. (2) Each trait can change only once in a lineage. ii) Determine an outgroup, a taxon that is closely related to the group whose phylogeny is being constructed, but that branched off from the lineage of the group below its base on the evolutionary


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HC BIOL 103 - Speciation and Taxonomy

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