Chapter 24: Origins of Species
Chapter 24: Origins of Species BYS 120
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This 6 page Class Notes was uploaded by Amelia Notetaker on Tuesday December 1, 2015. The Class Notes belongs to BYS 120 at University of Alabama - Huntsville taught by Dr. Luciano Matzkin in Fall 2015. Since its upload, it has received 12 views. For similar materials see Organismal Biology in Biological Sciences at University of Alabama - Huntsville.
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Date Created: 12/01/15
Chapter 24 Lecture Notes • The origin of species ○ Speciation process in which one species splits into 2 or more species • What is a species? ○ Many definitions of species concepts § Biological species concept □ Species is a group of individuals that can successfully interbreed in nature and produce viable and fertile offspring ® Gene flow (mating within a species) ® No gene flow (mating between species) ○ Gene flow keeps populations together as a single species ○ Reproductive isolation: barrier that keeps species apart § Prevents formation of species hybrids in nature (gene flow) • Reproductive barriers ○ Before fertilization § Prezygotic barrier or isolation ○ After fertilization § Postzygotic barrier or isolation ○ Species can be isolated reproductively due to several isolating mechanisms • Habitat isolation ○ Species occupy different habitats, therefore they don't have the chance to mate § Insects using different plants in the same area • Temporal isolation ○ Species are reproductively active at different times, then they can't hybridize § Flowering time • Behavioral isolation ○ Different in courtship rituals will prevent hybridization § Blue-footed boobies dance • Mechanical isolation ○ Prevents successful mating § Bradybaena snails • Gametic isolation ○ Difference in egg and/sperm prevent fertilization § Sea urchins • Reduces hybrid viability ○ Fertilization occurs, but hybrid dies in development or environment § Ensating salamanders • Reduced hybrid fertility ○ Hybrid individuals are sterile § Drosophila majavensius and Drosophila arizonae • Hybrid breakdown ○ Hybrids are viable and fertile, but have a reduced or no mating success § Rice strains and lake white fish • Issues with biological species concept ○ Sometimes species can produce a fully viable and fertile offspring. How is it possible? § Grizzle and polar bear=grolar bear § Lion and tiger=liger ○ Many exceptions to biological species concept • Morphological species concept ○ Individuals with similar morphology are a single species ○ Works on sexual and asexual species but can be subjective § Which character is more important? • Ecological species concept ○ Ecology of individuals defines species § Food, shelter, physiology ○ Works on sexual and asexual species, but need to have a very clear understanding of the ecology of organisms • Phylogenetic species concept ○ Difference at genetic level defines species ○ Works on sexual and asexual species but need to defines degree of difference allowed to still be a single species • Two populations of fish are debated whether or not they are the same species or not. What would those who agree with the biological species concept do? ○ Put one population of fish in the other population's natural environment and observe if they successfully mate • Speciation ○ Ultimately occurs when two gene pools become isolated ○ Isolating mechanism describes how two species are prevented from coming back together ○ Initial separation can occur in § Allopathy § Sympathy • Sympatric speciation ○ Occurs between populations living in the same geographic area ○ How can you have divergence? § Forces splitting gene pools must be stronger than the homogenizing effect of gene flow • Allopatric speciation ○ Occurs when one population is split by a geographical feat. § The grand canyon, lake, etc • Sympatric speciation habitat differentiation ○ Shifts in resource utilization could be a strong force ○ Apple maggots (rhagoletis pomonella) § Apples introduced in 1850's ○ Created maggots that ate apples § Can mate with other fruit consuming maggots and are different, but are the same species • Sympatric speciation: sexual selection ○ Mate preference can also drive gene pools ○ Given variation in mates results in disruptive selection ○ In lake Victoria there are over 600 species in cichlids originating less than 100,000 years ago ○ Female choice based on coloration is very strong • Sympatric speciation: polyploidy ○ Very common in plants, about 80% ○ Polyploidy: occurrence in extra chromosomes due to an error in cell division ○ Rare in animals ○ Autopolyploids: when the extra set(s) of chromosomes originate from a single species ○ Allopolyploids: when extra set(s) of chromosomes originate from different species § Common with crop plants • How can a population's characteristics make allopatric speciation more possible? ○ Existing on the edge of the home range • Genetic model of speciation ○ Dobzhansky-Muller model § One population is split by a geographical feat § The now two different populations evolve and change on their own □ Mutations occur § These new genes become fixed in the two populations § Suddenly, the geographical feat is gone and the two populations attempt to mate □ They are unable to produce a viable, fertile offspring due to the differences in their mutations • Hybrid zones ○ Regions where contact between two different species produces hybrid inidividuals • Hybrid zones: case study ○ Yellow bellied toad and fire bellied toads • Presence of hybrid zone ○ Reinforcement: gets narrower and narrower until it eventually disappears § Keeps hybrids from forming ○ Fusion: 2 species become one ○ Stability: persists over time and is maintained § Maybe hybrids do better? • Reinforcement ○ Natural selection will reduce (or eliminate) the size of the hybrid zone ○ If hybrids don't perform as well as parental species, then there will be selection against creating hybrids § Reinforcing separation between species ○ Female D. megavensis from sonora weren't mating with D. drizonae males ○ Female D. megavensis from mojave will mate with D. arizonae more often • Fusion ○ If selection (reinforcement) against hybrids are week, then it's likely that the two gene pools will fuse • Stability ○ Although selection might be resent against hybrids, it's balanced by migration • Speciation rates ○ Rate of new species formation varies widely across different lineages ○ This variation in speciation rates can also be observed in fossil records • Fossil records and speciation rates ○ The fossil record can demonstrate gradual change ○ Times of punctuated equilibric • Genetics of speciation ○ Does this speciation gene exist? ○ Most speciation events are a product of change at many genes § Some have greater effect • EX: Mimulus ○ Monkey flower § Mimulus luvisii and M. cardinals ○ M.L.= bumble bees ○ M.C.=humming birds ○ Crossed two species and selected for favored color § Got M.C. to have M.L. color and vise versa ○ Flower color (one major gene) had significant effect on type of pollinator visits