Lecture 21 Speciation II Mechanisms Core Concepts 1 Populations isolated from each other will tend to diverge slowly due to drift and selection 2 When divergent populations come in contact if hybrids have lower fitness there can be selection for assortative mating reinforcement to accelerate the speciation process 3 Gene flow between species is prevented by a variety of mechanisms acting as both pre and postzygotically 4 While most speciation is thought to be a long slow process some processes can cause almost instantaneous speciation 4 5 mechanisms of HW Equilibrium lead to divergence and speciation Selection Genetic Drift Non random mating Mutation Only migration retards the progress or stops it completely Two ways to disrupt gene flow 1 Reproductive isolation 2 Geographic isolation Usually a 3 step process 1 Isolation of populations Reduce gene flow 2 Divergence of populations selection drift 3 Evolution of reproductive barriers Step 1 The Geography of Isolation Purpose reduce gene flow Allopatric completely isolated Most common Parapatric two different areas niches not completely separate Sympatric one group is inside another Paripatric peripheral isolates Dispersal movement to a new habitable location Does not mean that every species moved at the same time Vicariance something happens in the environment that forms a barrier Changing the habitat for every species at the same time in the same way Expect the same phylogenetic pattern E g Isthmus of Panama Land Bridge Opened up path for mice Closed bridge for water dwelling animals Pacific vs Atlantic Allopatric and parapatric are common and most likely Step 2 Divergence Purpose Selection Local Adaptations or Drift Gene flow is disrupted Becoming different from each other Due to selection or drift May or may not be able to interbreed Slow accumulation of genetic differences 1 3 million years for fruit flies Step 3 Post geographic Isolation Mechanisms Purpose Forming reproductive barriers Needed in order to make permanent 1 Prezygotic Premating Temporal or Spatial Isolation the individuals are not reproducing at the same time or the same space able to fertilize in lab e g Orchid flowers on different days e g Release of sperm egg by Echinoderms at diff hours of day Behavioral Isolation display is not the same e g Songs Mechanical Isolation sexual parts present physical barriers e g Pollen packages at different positions on orchids Gametic Incompatibility receptors on gametes different biochemical barriers chemical differences prevent fertilization 2 Postzygotic Postmating Zygote Mortality Zygote dies Due to unequal chromosomes Hybrid Inviability dies before sexual maturity F1 hybrid has recued survival Hybrid Sterility not able to produce viable gametes e g Donkey ass mule mules are sterile Hybrid Breakdown F2 on backcross Reinforcement a special type of selection that states that once a barrier evolves it is a disadvantage to reproduce with another species Therefore selection will favor the evolution of traits that allow the two species to recognize each other as different Pre zygote isolation evolves in the presence of postzygotic isolation to prevent it from occurring Both pre and postzygotic mechanisms evolve slowly under normal conditions with complete reproductive isolation taking 1 5 3 5 million years in Drosohila Others have suggested much higher rates e g recent salmon some fruit flies and African cichlids that is most adaptive radiations As predicted prezygotic isolating mechanisms evolve much faster in sympatric species than allopatric species because there is no other way like a geographical boundary to prevent them from mating otherwise Evidence Comparing allopatric species different areas no potential mating Prezygotic postzygotic about the same rate Comparing sympatric species same areas potential mating but isolation Prezygotic evolves very fast Some species are found mid process making it difficult to decide on the number of species Speciation takes many years We have been watching for 50 years 1 of time typically required But you can do it in the year artificially in the lab Recent studies show that the first stages of speciation in a salmon and in a fruit fly have evolved in just 50 years and 6 months respectively In lab experiments 11 of 14 populations under strong selection for an adaptive trait also incidentally evolved prezygotic isolating mechanisms i e nearly complete speciation in the lab Instantaneous Isolating Mechanisms Chromosomal rearrangements disrupt pairing during meiosis Polyploidy Hybridization Lecture 22 Phylogenetics I Theory Core Concepts 1 A history of descent with modification and cladogenesis will necessarily produce a branching hierarchy 2 Synapomorphies are evidence of relationship and serve to define clades 3 The principle of parsimony is one way to choose among alternative hypothesis of relationship Evolutionary School Mayr Classified organisms by genealogical adaptive similarity No objective set of criteria Phenetic School Measures overall similarity Measure everything and puts it into a computer and it clusters things into classifications Very objective very repeatable Very nice people Cladistic School Phylogeny is the organizing principle Two things may look similar but does that not mean that they are the most closely related Emotional highly involved people Statistical School Uses statistics to find clades Ultimately it was not the personality that counted The data is more important It is more meaningful to base our system off of relatedness and biological shared history than similarity by morphology Where do we find the evidence for phylogeny Fossil Record Yes but can leave gaps Many species also have no living relatives Plethora of data is too large Apomorphy Derived trait Synapomorphy Shared derived trait multiple ancestors Goal is to look for the most common Symplesiomorphy shared ancestral primitive trait Not informative about phylogeny because it is an absent trait not a present one Problem arises when traits converge or reverse Convergence traits evolve multiple times Reversal evolve back to ancestral trait Homoplasy character conflict or incongruence sharing a trait but not related What do you do when the systems conflict Parsimony choose the simplest explanation Minimizing Ad Hoc hypothesis the extra hypothesis you add We thought it was the same trait which is why we coded it in the computer that way however our analysis says that it can t be it must have evolved twice