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TAMU BIOL 112 - CH24

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Chapter 24 The Origin of SpeciesWays to define a species- A natural and basic unit of classification- Originally Latin for “kind”- Morphological species is a set of organisms that LOOK alikeo But looks may deceive- Ecological species is a set of organisms adapted to a particular set of resources (Niche). (useful in defining prokaryotes)- Phylogenetic species is the smallest distinct set of organisms that share a common ancestor (cougars in north and south America)- BIOLOGICAL species is the member of all the populations that potentially interbreed in nature and do not successfully interbreed with other such groups. It has been used since the Hardy-Weinberg principle, about 100 years. Its focus is on reproductionBiological Species ConceptMembers of the same species- Share the same gene poolo There is gene flow between populations (it doesn’t take much to count)- Are reproductively isolated from other species by natural barrierso Prezygotic is a natural feature that keeps a individuals from meeting and forming offspringo Postzygotic barriers keep the formed hybrid from reproducingPrezygotic Barriers prevent the formation of a hybrid- Habitat (ecological) isolation-distant or different ecosystem- Temporal isolation-different mating seasons (frogs)- Behavior isolation (applies only to animals)-courtship, calls (blue-footed v. red-footed booby)- Mechanical isolation-anatomy won’t fit (love bugs)- Gametic isolation-gametes don’t match, zygote doesn’t formPostzygotic Barriers act after the hybrid is formed- Reduced hybrid viability- embryo fails to develop or is weak- Reduced hybrid fertility- the hybrid survives, but is sterile- Hybrid breakdown-the 2nd generation of hybrids are feeble or sterileProblems with the Biological Species Concept- We cannot classify fossil species in this way and must use the morphological concept- It does not take asexual reproduction into account- Sometimes gene flow and the production of a viable hybrid happensHow can new species form?Speciation and Evolution- Microevolution with gene flow can change the appearance of species over time, BUT thisdoes not make more species- Speciation is the creation of new species from existing ones- There are two ways allopatric and sympatric speciationAllopatric speciation- Begins with the geological isolation of a populationo The effectiveness of the geographical barrier depends on the species (birds vs. beetles)- Time and isolation allows for divergence, through natural selection and genetic drifto Non/Adaptive microevolution- Requires the evolution of a reproductive barrier- Ex-Squirrels at the grand canyon- Evidenceo Reduced success in lab matings between allopatric salamanderso Fish from isolated populations prefer to mate with their own “shape”o Fruit flies adapted to different foods diverged enough to prefer like-adapted mateso Endemic island species resemble the nearest mainland species (cormorants)- If prezygotic barriers do NOT ariseo The hybrids result at contact areas (hybrid zone)o Possible outcomes (specific to allopatric) Reinforcements-gene pools separate further, creating fewer hybrids Fusion- gene pools merge, no speciation occurs Stability-hybrids continue to be formed, but other factors keep the gene pool isolatedSympatric Speciation- Speciation in the same geographic areao Gene pools are isolated in other wayso Much less common (we think) especially in animals- Due to habitat differentiationo Population isolation by use of different resourceso Ex- Apple maggot fly, fig wasp- Due to sexual selectiono Experiment 24.12, fish color patterns- Due to polyploidyo New species with multiple sets of chromosomes (4n, 6n, etc.)o Arises from cell divergence errorso Occurs often in plantso Autopolyploidy-from same specieso Allopolyploidy- polyploidy hybrid of 2 specieso 30-70% of plants polyploidy (very rare in animals)Patterns in the fossil record- Gradual-small divergences and change over a long period of time- Punctuated-species are the same for a long time, then there is rapid changeo Ex- trilobites’ ribs in the exoskeleton stay much the same after a new group appears.How many genes make a species?Amount of genetic variation varies- 1, few or many genes may separate a specieso Japanese snail-one gene causes different spiral directions and thus mechanical isolationo Flower color- one gene changes pollinator preference (see


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TAMU BIOL 112 - CH24

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