Speciation Modes of Speciation

Speciation: Modes of Speciation Modes of speciation Allopatric, parapatric and sympatric speciation Polyploid or instantaneous speciation ALLOTPATRIC SPECIATION- barrier is removed or new species disperse over it, re-establishing sympatry PERIPATRIC – range expansion re-establishes sympatry PARAPATRIC – range expansion leads to sympatry SYMPATRIC – genetic differences results in reproductive isolation ALLOPATRIC The evolution of genetic reproductive barriers between populations that are geographically separated by a physical barrier. Sever reduction in the movement of individuals/gametes; not by geographic distance (i.e low dispersal) Selective pressure and genetic drift operating over time will induce incipient and subsequent speciation between reproductively compatible but geographically isolated populations NATURAL SELECTION AND SPECIATION Speciation is usually a by-product of the divergent selection pressure that occurs during allopatry. Which may be ecological selection or sexual selection. Showing the reproductive isolation is the result of adaptive divergence required evidence that the two processes are genetically related and causally related to each other Ecological speciation also demonstrates this – barriers to gene flow barriers caused by divergent ecology i.e Minulus lewsii become adapted to pollinators Sexual isolation in this case is more pronounced between ecologically divergent groups rather than geographically distant groups. Modes of sexual selection of male traits by females show that divergent traits/preferences can evolve n different populations of an ancestral species, resulting in speciation. PARAPATRIC SPECIATION Can theoretically occur of gene flow between populations that occupy adjacent regions with different selective pressures, is much weaker than divergent selection for different gene combinations. Strong selection at a sharp border prevents gene exchange cause by the reproductive failure of individuals with the “wrong” genotype or phenotype to migrate across SYMPATRIC SPECIATION Speciation would only be sympatric if a biological barrier to gene exchange arose within an initially randomly mating population without any spatial segregation of the incipient species Disruptive selection must play a critical role. Evidence has been documented in a range of taxa that inhabit small islands POLYPLOIDY AND SPECIATION An organism with two or more complements of chromosomes A tetraploid, for example, has 4 chromosome compliments in its somatic cells (4n); hexaploid (6n) Polyploid populations are reproductively isolated by postzygotic barriers from their diploid progenitors and are therefore distinct biological species,] Instantaneous speciation by a single genetic event normally by a failure in reductive division at meiosis. Polyploidy are rare among sexually reproducing animals, although many parthenogenetic polyploidy anumals have been describes. (moths, lizards, weevils) Human triploids and tetraploids are aborted or die shortly after birth. AUTOPOLYPLOID formed by the union of unreduced gametes from genetically and chromosomally compatible individuals (same species) ALLOPOLYPLOID is a derivative of a partly sterile diploid hybrid. SPECIATION RATE Transition time - the time required for (nearly) complete reproductive isolation to evolve, once the process has started BIOLOGICAL SPECIATION INTERVAL – the average time between the origin of a new species and when that species branches (speciates) again.