Week 2 notes
Week 2 notes 70916 - BIOL 103 - 001
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This 4 page Class Notes was uploaded by Aimee Castillon on Wednesday September 2, 2015. The Class Notes belongs to 70916 - BIOL 103 - 001 at George Mason University taught by Gwendolyne Y Fondufe (P) in Fall 2015. Since its upload, it has received 96 views. For similar materials see Introductory Biology I in Biology at George Mason University.
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Date Created: 09/02/15
George Mason University Aimee Castillon student email address BIOL 103 0 Fal2014 U gm 7 g fvgicl 17le F If f7 glang l l Hg 72 i 39 K r L m A VV 39L 5 f 02 71127312 Viva 1 I v 1 xll liMliiiiyf llligllguglquot l x gll l l llg t igill Defining Species o Microevolution change in the gene pool of a population from one gen to the next 0 speciation process by which one species splits into 2 or more species 0 every time this occurs the diversity of life increases 0 the many millions of species on Earth have all arisen from ancestral life form that lived around 35 billion years ago 0 over the course of 35 billion years 0 an ancestral species first gave rise to two or more diff species 0 then branched to new lineages 0 then branched again 0 until we arrived at the millions of species that live or lived on Earth There are several ways to define a species 0 species is Latin for kind or appearance devising formal def of species isn39t easy and raises s such as 0 how similar are members of same species 0 whereas indv ls of many species exhibit fairly limited variation in physical appearance certain other species seem extremely varied o biological species concept group of populations whose members have the potential to interbreed in nature and produce fertile offspring 0 Therefore members of a species are similar bc they reproduce w each other 0 reproductive isolation prevents members of diff species from mating w each other prevents gene flow bt species and maintain separate species 0 therefore species are distinct from each other bc they don t share the same gene pool 0 biological species concept can be problematic 0 some pairs of clearly distinct species occasionally interbreed and produce hybrids le grizzly bears polar bears grolar bears o reproductive isolation cannot usually be determined for extinct organisms known only from fossils o doesn t apply to prokaryotes or other organisms that reproduce only asexually 0 Therefore alternate species concepts can be useful 0 Morphological species concept classifies organisms based on observable physical traits and can be applied to asexual organisms and fossHs 0 however there s subjectivity in deciding which traits to use 0 Ecological species concept defines species by its ecological niche and focuses on unique adaptations to particular roles in a biological community 0 For example two species may be similar in appearance but distinguishable based on what they eat or where they live 0 Phylogenetic species concept defines a species as the smallest group of individuals that shares a common ancestor and thus forms one branch of the tree of life 0 biologists trace phylogenetic history of species by comparing its morphology DNA sequence or biochem pathways 0 however defining amount of diff required to distinguish separate species is a problem 0 Reproductive barriers serve to isolate the gene pools of species and prevent interbreeding 0 depending on whether they function before or after zygotes form barriers are categorized as o prezygotic I 5 types 0 habitat isolation two species live in same area but not the same species 0 temporal isolation two species breed at different times 0 behavioral isolation there s little or no mate recog bt females and males of diff species 0 mechanical isolation female and male sex organs aren t compatible 0 gametic isolation female and male gametes aren t compatible 0 postzygotic I 3 types 0 reduced hybrid viability most hybrid offspring don t develop fully or don t survive 0 reduced hybrid fertility hybrid offspring are vigorous but can t reproduce viable offspring 0 hybrid breakdown firstgen hybrids are viable and fertile but the offspring of the hybrids are feeble or sterile Mechanism of Speciation Allopatric speciation o Allopatric speciation populations of the same species are geographically separated isolating their gene pools 0 gene flow bt the population is prevented o Populations separated by a geographic barrier are called allopatric populations Sympatric speciation takes place wlo geographic isolation o sympatric speciation new species arises win same geographic area as a parent species 0 how can reproductive isolation develop when members of sympatric populations remain in contact w each other 0 gene flow bt populations may b reduced by I polyploidy cells have more than two complete sets of chromosomes 0 diploid having 2 complete sets of chromosomes I habitat differentiation or I sexual selection 0 failure of cell division after chromosome duplication could double a cell s chromosome number from diploid 2n to tetraploid 4n 0 if selffertilization occurs the resulting tetraploid zygotes would develop into plants that can reproduce fertile tetraploid offspring by selffertilization or by meeting w other tetraploids o a tetraploid can t produce fertile offspring by mating w a parent diploid plant 0 formation of a tetraploid therefore is an instantaneous speciation event new species which is reproductively isolated 0 most polyploid species arise when 2 different species interbreed and produce hybrid offspring normally the hybrid is sterile bc its chromosomes can t pair although far more common in plants polyploid speciation occurs occasionally in animals 0 sympatric speciation in animals is more likely to happen through habitat differentiation or sexual selection than by poypoidy 0 Le cichlid fish 0 adaptation for exploiting diff food sources may have evolved in different subgroups of the original cichlid population 0 if these sources were in different habitats mating bt the populations would become rare 0 the gene pools would become isolated as each population became adapted to a different resource 0 Sexual selection in which females choose their mates based on appearance coloration in this case may have also been a factor in the speciation of these brightly colored fish 0 Such mate choice can contribute to reproductively isolating populations thus keeping the gene pools of newly forming species separate 0 Both habitat differentiation and sexual selection contribute to forming reproductive barriers between allopatric species Evolution Connection Most plant species trace their origin to polyploid speciation 0 plant biologists estimate that 80 of all living plant species are descendants of ancestors that formed by polyploid speciation hybridization bt two species accounts for most of these species 0 Le wheat cotton peanuts etc Isolated islands are often showcases of speciation 0 most of species on Earth are thought to have originated by allotrapic speciation o isolated island chains offer some of the best evidence of this type of speciation 0 multiple speciation events are more likely to occur in island chains that have 0 physically diverse habitats o islands far enough apart to permit populations to evolve in isolation and o islands close enough to each other to allow occasional dispersions bt them 0 evolution of many diverse species from a common ancestor is adaptative radiation 0 Le Galapagos Islands 14 closely related finches Danvin s finches Hybrid zones provide opportunities to study reproductive isolation 0 hybrid zones regions in which members of different species meet and mate to produce at least some hybrid offspring o 3 possible outcomes I reinforcement strenghtening of reprod barriers 0 when hybrid offspring are less fit than members of both parent species 0 natural selection to strenghten or reinforce reproductive barriers thus reducing the formation of unfit hybrids and o barriers bt species should b stronger where the species overlap I fusion weakening of reprod barriers 0 gene flow bt species increases and gene pool of the two species become increasingly alike 0 in effect speciation process reverses eventually causing two hybridizing species to fuse into single species 0 Le cichlid species in Lake Victoria I stability continued formation of hybrid individuals 0 in fact many hybrid zones are stable as hybrid offspring continue to be produced Speciation can occur rapidly or slowly o punctuated equilibria model draws on the fossil record which states 0 change most as they arise from an ancestral species and then 0 experience relatively little change for the rest of their existence 0 what is the total length of time bt speciation events bt formation of a species and subsequent divergence of that species 0 it varies considerably 0 Data suggests that on average millions of years may pass before a newly formed species will itself give rise to another new species
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