Speciation Speciation -the central process in evol

Species have unique combinations of features when similar, usually closely related Species are reproductively isolated when closely related, can hybridize general scheme of speciation -biol diversity reveals 2 things: 1 many spp have unique combinations of features we use to distinguish them when differences hard to detect, the are usually closely related 2 almost all spp are reprod isolated from each other – closely related can hybridize -therefore copies of genes in 1 spp cannot reach descendants of indiv from other spp, each spp has its own gene pool (some leakage with closely related pools) -speciation requires splitting of 1 gene pool into 2+ separated pools and diversification into 2+ phenotypically different forms -genetic separation and phenotypic differentiation

Allopatric & sympatric speciation Allopatric speciation populations evolve in geographic isolation Sympatric speciation populations evolve in geographic proximity geographic isolation -allopatry vs sympatry allopatric speciation -popln’s become geographically isolated, are exposed to divergent selection, evolve independently -accumulate so many genetic differences that they become reprod isolated if they come into contact again -if isolation is complete, speciation has occurred -can be geographically isolated by water, mtns, lava, savannah etc -recap – assumes complete geological separation

Allopatric speciation Geographic isolation produces genetic isolation origin of reprod isolation -physical separation of allopatric poplns guarantees zero gene flow -divergence built up in allopatry has inherent reprod isolation – that is only apparent in 2ary sympatry -what about allopatric poplns that have only partial reprod isolation in 2ary sympatry? -like the apple flies, how much gene flow will stop the speciation process?

Allopatric speciation Populations can become geographically isolated when individuals colonize a new habitat

Allopatric speciation Populations can also become geographically isolated when a species’ former range is split into two or more distinct areas -isthmus of Panama -lava flow - Kilauea

Sympatric speciation Natural selection can cause populations to diverge even when a small amount of gene flow occurs sympatric speciation -geological or ecological conditions that cause partial isolation of subpopln’s -still have some gene flow with parent popln -most likely form involves selection to use different resources – may shift hosts -eg. hawthorn/apple fly, Rhagoletis polmonella -used to lay eggs on hawthorn -1864 found in apples -can now distinguish apple & hawthorn poplns that differ genetically -females prefer to lay eggs in type of fruit from which they came -males prefer to mate on type of fruit from they came -thus self-reinforcing -but still interbreed freely either apple/hawthorn -speciation not yet complete, but genetic differentiation and partial reprod isolation of R. polmonella did occur sympatrically -whether it leads to speciation is still open -incipient spp

Changes in chromosome number isolate populations genetically Sympatric speciation Changes in chromosome number isolate populations genetically Speciation triggered by changes in chromosome number has been especially important in plants (polyploidization) sudden sympatric speciation by polyploidization -polyploidization – doubling the complete set of chromosomes due to irregularities in cell division -further normal cell divisions xfer polyploidy condition to all descendent cells -once arisen, polyploidy cannot mate with parents, because would produce triploid offspring -particularly important for flowering plants -not as much in animals

Mechanisms of divergence Genetic drift Natural selection Sexual selection Reinforcement polyploidy,dispersal, vicariance produce the conditions for speciation

Mechanisms of divergence Genetic drift Drift can produce rapid genetic divergence in small, isolated populations genetic drift -genetic drift effects most pronounced in small poplns most spp originate with small popln size -small # of indiv in colonization event -model this -start with small popln of non-random from ancestral popln -drift in the small popln -random loss of alleles, random fixation of others + new alleles -so isolated popln undergoes rapid genetic divergence from ancestral popln -role of drift is controversial -bottlenecking eliminates only very rare alleles -popln has to be extremely small & remain small for a signif period for drift to change allele freqs -now recognize natural selection as more important

Mechanisms of divergence Natural selection Natural selection can produce genetic divergence in populations subjected to different pressures natural selection -look at apple flies again -selection acting on preferences for different fruit drives flies apart

Mechanisms of divergence Sexual selection Sexual selection acts on characters involved in mate choice sexual selection sexual selection may cause prezygotic reprod isolation -eg HI drosophila 800spp in HI, 2800 total worldwide -eg African cichlids -differences among indiv in their ability to obtain mates -females prefer males with certain characters which leads to self-reinforcing loop -more later

Mechanisms of divergence Sexual selection

Mechanisms of divergence Sexual selection Changes in sexual selection may isolate populations and cause rapid divergence

Mechanisms of divergence Reinforcement Reinforcement leads to assortative mating and the prezygotic isolation of populations reinforcement -2ary reinforcement – mechanisms would help to keep spp separate -mechanisms selected to prevent non-productive reprod -reinforcement may not have an important role

Mechanisms of divergence Reinforcement Prezygotic isolation 2 reinforcement probably unimportant in generating prezygotic isolation -prezygotic isolation often stronger when indiv are sympatric vs allopatric -consistent with idea that prezygotic isolation evolved to prevent production of hybrids -importance of reinforcement is doubtful -low genetic distance = recent divergence - sexual isolation greater among sympatric than allopatric forms Genetic distance Allopatric Sympatric