Systematics-Lecture-2.pdf

Lecture 2: Taxonomic Hierarchy and Species Genus Concept It is the next principal category above species in the taxonomic hierarchy. Taxonomic Hierarchy...

Lecture 2: Taxonomic Hierarchy and Species Genus Concept It is the next principal category above species in the taxonomic hierarchy. Taxonomic Hierarchy Those species which share certain common The arrangement of organism into these ranking characters are placed together in a larger group orders or taxonomic categories (Division, class, called genus (plural: genera). order, family, genus, species) in an ascending It is more difficult to define a genus as its series ranging from the lowest to the highest description must include all the closely related category species grouped within a genus. arrangement is also called Linnaean hierarchy Linnaeus thought that there were as many (after Carolus Linnaeus). genera as there are ‘different constructed fruit Taxonomic structure - system in which bodies among species’ (Linnaeus 1736:159). taxonomic groups are assigned to different taxonomic categories, depending upon the levels Family of diversity of the organism in relation to plants was first used by Magnol (1689). Taxon represents a more natural and real grouping. a category or taxonomic group of any rank More information is usually available about a within the system of classification. family, and it is comparatively easy to recognize used to indicate the rank of a group as well as a family than the lower categories like genus or the organisms contained within that group. species. can be used at the level of variety, species, genus, family or any other higher category 14.1 The origin of species is the source of biological diversity Species Microevolution is the change in the gene pool of the fundamental category of the taxonomic a population from one generation to the next hierarchy. Speciation is the process by which one species defined as “a group of individuals which splits into two or more species resemble each other by certain criteria and Each time speciation occurs, the diversity of life distinct from other such groups of individuals”. increases Species literally means ‘appearance”, from the Over the course of 3.5 billion years, Latin word specere i.e., “to look at, see”. o An ancestral species gave rise to two or may be asexual species (those reproducing more different species, through asexual means) or cryptic species (those o Which then branched to new lineages, that are not morphologically distinguishable o Which branched again, from another species). o Until we arrive at the millions of species may be synchronic (a group of contemporary that live, or once lived, on Earth organisms, living at the same general point in The word species is from the Latin for “kind” or time) or diachronic (a group of organisms that “appearance” temporally span versus a lineage of ancestral- Although the basic idea of species as distinct life- descendant populations, extending through a form seems intuitive, devising a more formal period). definition is not easy and raises questions lowest unit of biological classification and the In many cases, the differences between two building blocks of classification species are obvious. In other cases, the differences between two species are not so obvious NOTES NI MAEVIN GEM How does a new species emerge? Adaptive radiation New species evolve from preexisting species The evolution of many diverse species from a Speciation process: common ancestor o Cladogenetic Islands that have physically diverse habitats and o Reticulate that are far enough apart to permit populations o Phyletic to evolve in isolation but close enough to allow occasional dispersions to occur are often the sites of multiple speciation events. The Galapagos Islands currently have 14 species of closely related finches, called Darwin’s finches, because Darwin collected them during his around-the-world voyage on the Beagle These birds o Share many finchlike traits o Differ in their feeding habits and their beaks, specialized for what they eat, and o Arose through adaptive radiation Species Concepts Emphasizing Process Emphasizing product of evolution Other species types Concepts Emphasizing Process Biological species Recognition species Evolutionary species Cohesion species NOTES NI MAEVIN GEM Biological/ Isolation Species Pre-mating – prevents mating from occurring Groups of interbreeding 1. ecological or habitat isolation natural populations - different species have a genetic basis for which are reproductively occupying different habitats (e.g., dry versus isolated from other such wet soils) and therefore do not normally groups (Mayr, 1963) come into contact to successfully breed The biological species 2. temporal isolation concept defines a - different species breed at different times, species as a group of e.g., spring versus fall or day versus night populations whose blooming flowering plants members have the potential to interbreed in 3. behavioral/ethological isolation nature and produce fertile offspring (offspring - different species have different behaviors that that themselves can reproduce) inhibit interbreeding, such as (for flowering Thus members of a biological species are united plants) different pollination mechanisms, by being reproductively compatible, at least preventing the transfer of pollen between potentially different flower types Barriers to gene exchange often referred to as “reproductive isolating mechanisms” derived suites of characters in one or more lineages that inhibit or prevent interbreeding Reproductive barriers - serve to isolate the gene pools of species - prevent interbreeding. Depending on whether they function before or after zygotes form, reproductive barriers are categorized as - prezygotic or - postzygotic. Reproductive Isolation prevents genetic exchange (gene flow) and maintains a boundary between species But there are some pairs of clearly distinct species that do occasionally interbreed o The resulting offspring are called hybrid o An example is the grizzly bear (Ursus arctos) and the polar bear (Ursus maritimus), whose hybrid offspring have been called “grolar bears”. NOTES NI MAEVIN GEM  Post-zygotic – occurring after a zygote forms 1. hybrid inviability - hybrids between species cannot develop properly to adulthood; interaction of parental genes impairs the hybrid’s development or survival. 2. hybrid sterility - hybrids develop to adulthood but are sterile or have reduced fertility; hybrids are vigorous but cannot produce viable offspring 3. hybrid breakdown - hybrids are fertile but their F2 generations have reduced viability or fertility; hybrids are viable and fertile, but their offspring are feeble or sterile NOTES NI MAEVIN GEM  Recognition species Refer to sexually reproducing systems that are maintained by genetically-based features that promote reproduction (Paterson, 1985) Promotes Gene flow (fertility, behavior, gamete propagation and recognition) Evolutionary species A single continuous lineage of ancestral- descendant populations that Post-mating – allowing mating (equivalent to retains its identity pollination in seed plants), but not zygote from other such formation lineages and has its 1. mechanical or physiological isolation own evolutionary - self-incompatibility mechanisms, for example, tendencies and (in flowering plants) pollen landing on the historical fate stigma of another species will not germinate (Wiley, 1978) or will not form a functional pollen tube 2. gametic isolation Cohesion species - sperm and egg come into contact, but these Largest or most inclusive gametes are incompatible; thus, the sperm group of individuals that cannot successfully fertilize the egg maintains genetic and phenotypic cohesion Limitations (Templeton, 1989). Asexual reproduction How species are Spatially disjunct maintained and how they populations/species: diverge (speciate) o Reproductively Cohesion mechanisms isolated due to spatial (gene flow, genetic drift, segregation, but may natural selection) be reproductively compatible Concepts Emphasizing Product of Evolution o Lab tests Taxonomic or morphological species Hybridization: different Phylogenetic species species often hybridize naturally (very common Genealogical species in plants) Fossils: impossible to know for sure if similar looking species interbreed NOTES NI MAEVIN GEM Morphological / taxonomic Species Smallest group or class of individuals that are similar to one another in one or more features and different from other such groups Species can be distinguished from each other by morphological traits “Classical species concept” Morphological or morphospecies concept is the oldest species concept and uses Phylogenetic species concept differences in morphological characters to uses molecular markers (usually DNA sequences) distinguish species. to identify closely related taxa. In former times species were considered as It is e.g. the standard species concept in inalterable units. bacteria, but has been tested also for eukaryotic After Charles Darwin published his book "On the taxa (Leliaert et al. 2009) Origin of Species by Means of Natural Selection" The phylogenetic species concept defines a (1859), the morphospecies concept has been species as the smallest group of individuals that applied under the share a common ancestor and thus form one assumption that similar branch of the tree of life to identical Biologists trace the phylogenetic history of a morphological species by comparing its characters would reflect o Morphology relatedness o DNA sequences o Biochemical pathways Limitations of However, agreeing on the amount of difference Morphological Species required to establish separate species remains a High plasticity challenge Undetected speciation Phylogenetic Species Character-based, defined as the smallest group of populations or lineages that are diagnosable by a unique combination of character states in comparable individuals (Nixon & Wheeler, 1980) “the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent” (Cracraft 1983) Clusters must be: - 1) Monophyletic (ancestor + all descendants) - 2) derived through an evolutionary process of descent from an ancestral lineage - 3) diagnosable through examination of derived characters NOTES NI MAEVIN GEM Phylogenetic Species Other Species Types Autapomorphic species Asexual species - (termed “apomorphic species” here) because o Reproducing of the necessity of recognizing an apomorphy without sex, yet for the species lineage alone. are recognizable Metaspecies morphological - is defined as two or more lineages segments or genetic units (e.g., from samples of separate populations) o Ex. Apomictic that can be resolved as neither monophyletic populations that or paraphyletic (Donoghue 1988). develop seed Paraphyletic species without - similar to the “plesiospecies” of Olmstead fertilization, 1995) may represent temporarily separate wherein the lineage segments that have diverged relative embryo is a to one another clone of the original parent Cryptic species o Not morphologically distinguishable from another species o Genetically different Limitations from other Can inflate the number of species if species populations designations are based on a few characters o Ex. Deinandra species The genetics of a species may not always fully explain the differences in species. Genealogical species All members of the group are more closely related to one another than to any organisms outside the group (Baum & Shaw, 1995) The ecological species concept Defines a species by its ecological niche Focuses on unique adaptations to particular roles in a biological community For example, two species may be similar in appearance but distinguishable based on what they eat or the depth of water in which they are usually found NOTES NI MAEVIN GEM Mechanisms of Speciation- Allopatric and Sympatric speciation Sympatric speciation takes place without geographic isolation Allopatric speciation – geographic isolation leads occurs when a new species arises within the to speciation same geographic area as its parent species. the initial block to gene flow may come from a Gene flow between populations geographic barrier that isolates a population. may be reduced by A key event in the origin of a new species is the – polyploidy, separation of a population from other – habitat differentiation, or populations of the same species. – sexual selection. o With its gene pool isolated, the splinter population can follow its own evolutionary Biogeography Relationships course. between pairs of Species o Changes in allele frequencies caused by Allopatry natural selection, genetic drift, and Sympatry mutation will not be diluted by alleles Parapatry entering from other populations (gene flow). Several geologic processes can isolate populations. Allopatry A mountain range may emerge and gradually Denotes the split a population of organisms that can inhabit occurrence of 2 only lowlands. taxonomically A large lake may subside until there are several related species in smaller lakes, isolating certain fish populations. geographically Continents themselves can split and move apart. separated of Allopatric speciation can also occur when distribution individuals colonize a remote area and become Living in different countries geographically isolated from the parent population NOTES NI MAEVIN GEM Sympatry The origin of most plant species can Denotes the be traced to polyploid speciation occurrence of 2 Plant biologists estimate 80% of all living plant taxonomically species are descendants of ancestors that related species in formed by polyploid speciation the same general Hybridization between two species accounts for area most of these species, perhaps because of the Living in the adaptive advantage of the diverse genes a hybrid same country inherits from different parental species Polyploid plants include o Cotton o Oats o Potatoes Parapatry o Bananas Denotes the partial overlapping of 2 otherwise o Peanuts allopatric areas o Barley o Plums o Apples o Sugarcane o Coffee o Wheat Polyploidy Wheat o has been domesticated for at least 10,000 In Plants years and Many plant species have originated from o is the most widely cultivated plant in the sympatric speciation that occurs when accidents world. during cell division result in extra sets of Bread wheat, Triticum aestivum, is chromosomes. o a polyploid with 42 chromosomes and New species formed in this way are polyploid, in o the result of hybridization and polyploidy. that their cells have more than two complete sets of chromosomes. Sympatric speciation can result from polyploidy – within a species (by self-fertilization) or – between two species (by hybridization). NOTES NI MAEVIN GEM Taxon Rarity Questions Addressing the Importance of Extinct species or taxon Biodiversity - is one no longer known to exist in the wild. Extirpated species or taxon 1. Biodiversity: What is it, where is it, and why is it - extinct within a particular portion of its range important? but present within other parts of its range. 2. Why is biodiversity loss a concern? Keystone species 3. What are the current trends and drivers of - is one that, by its absence from an ecosystem, biodiversity loss? results in the disappearance (directly or 4. What is the future for biodiversity and indirectly) of several other species, causing an ecosystem services under plausible scenarios? “extinction cascade.” 5. What response options can conserve Endangered taxon biodiversity and promote human well-being? - a species, subspecies, or distinct population 6. What are the prospects for reducing the rate of segment) is one that is in danger of becoming loss of biodiversity by 2010 or beyond and what extinct in the near future within all or a are the implications for the Convention on significant part of its geographical range due Biological Diversity? to one or more causal factors. Threatened taxon How does plant systematics relate to - thought likely to become endangered in the conservation biology? wild in the near future within all or part of its Taxon diagnosis – endangered or threatened geographical range if the same causal factors Floristic surveys – species and infraspecies continue to operate. range, disjunct population, endemic, “hotspots” Evaluation of taxon rarity – voucher specimen; Reasons for Saving Biodiversity extinct species, extirpated species, keystone Aesthetic species, endangered taxon, threatened taxon - gain much joy from viewing or studying Use of phylogenetic information in evaluating nature conservation decisions (Phylogenetic Analysis) – - It is worth preserving and nurturing the paleoendemics or neoendemics natural world for our direct enjoyment and those of future generations. Conservation Biology and Stewardship Economic reason Things to Do: - untold economic importance to humans. Act to train systematists to describe, inventory, - Undiscovered or not utilized plant species and map the biotic diversity that still exists may have future important uses as sources of Act to help direct more money into these food, fiber, fuel, rubber, wood, etc. training efforts, as well as to museums, botanic - Medicinal uses garden, zoos, and herbaria which maintain Maintenance of Genetic Diversity collections of species - Native plants that are close relatives to Act to prioritize our own research programs, to important cultivated agricultural plant species preferentially study plant groups containing may be important in contributing genes, endangered species, local endemics in either directly or via interbreeding or genetic vulnerable environments, paleoendemic relicts, engineering, for valued traits such as disease or neoendemics representing recent speciation resistance, increased vigor and yield, or food Act to strive educate others regarding nature, quality conservation, and biodiversity and to cultivate the aesthetics of enjoying and revering the natural world. NOTES NI MAEVIN GEM