Lecture 27 11 07 2012 Sympatric Speciation via polyploidy increase in the number of complete sets of chromosomes Very common in plants less so in animals o Asexual reproduction more common in plants o Self fertilization more common in plants o Plants don t have a germ line Can result in rapid speciation 2 basic modes o Autopolyploidy o Allopolyploidy The tetraploid offspring cannot backcross to the parent Something happens in a particular plant that makes a plant tetraploid A single individual can be the parent of two different species reproductive isolation Inability for 2n and 1n zygotes to reproduce Start with two different species A hybrid of two species is usually sterile because its chromosomes are not homologous and cannot pair during meiosis They may be able to reproduce asexually though Fertile hybrids allopolyploids The new species has a diploid chromosome number equal to the sum of the diploid chromosome numbers of the two parent species 2 new allopolyploid species were spontaneously generated in the northwestern US by hybridization events involving 3 introduced diploids Happened very quickly within 50 years Plant polyploids are very important in agriculture Modern domesticated wheat is an allohexaploid 6n produced by 2 hybridization events over the past 8000 years in the Middle east Testing speciation If individuals from the formerly isolated populations begin interbreeding and the hybrids show no reduction in fitness the cohesiveness of the original species is returned If the two populations remain reproductively isolated a speciation If the populations hybridize and the hybrids show a higher fitness in an environment different from the parental species they may event has occurred persist as Hybrid Zone o Plains pocket gophers in eastern Nebraska have a dark pelage which hides them rom predators against the black soils there Gophers in western Nebraska have a lighter pelage similar to the sandier soils there In the area where the two soil types meet a band of intermediate colored soil is home to hybrids o Introgression movement of allele from one species to another via hybridization much more frequent with neutral alleles Darwin s model of evolution proposed a single origin for all life on earth Gives a central role to the inference of ancestor descendant genealogical relationships in classifying all life forms living and dead The book titled the origin of species o Had one figure in the whole book representing the divergence of variants within a species and showing more difference in a single lineage Darwin expanded on the tree metaphor explaining that limbs divided into great branches were themselves once when the tree was small budding twigs and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate genealogically to groups Phylogeny the study of the connections between all groups of organisms as understood by ancestor descendant relationships Greek for tribe and origin One part of the larger field of systematics which also includes Studies the diversity and relationships of organisms both living and taxonomy extinct Phylogeneticists express the relationships among groups of organisms through tree like diagrams cladograms that are inferred genealogies of lineages and species from data A hypothesis about the evolutionary history of a group estimated o This data is obtained rom Fossil record Morphologies of living organisms Genomes of living organisms Reconstructing evolutionary relationships A cladogram of phylogenic tree is a graphical representations of evolutionary history that describes the sequence of speciation events Plesiomorphic ancestral characteristics ancient genes that control central biochemical pathways that are found in all organisms and have been inherited from a common ancestor Apomorphic derived characteristics subsequent evolutionary innovations that account for the differences among organisms Phylogeneticists are particularly interested in identifying homologous entities either morphological or molecular i e structures or genes that are similar because of descent from a common ancestor that had the first version of that structure or gene It is important to be able to infer where in the tree of life a particular structure or gene originated o i e the tetrapod vertebrate forelimb arose in the exclusive common anscestor of the group and what the ancestral condition was likely to have been Then you find the derived trait in each of the lineages o Variation in the structure or gene may be used to infer genealogical relationships within the group the more similar these variants are to each other the closer the inferred genealogical relationship Variation in homologous genes or proteins may be similarly used to infer genealogical relationships Changes from the ancestral state are apomorphic derived Synapomorphies shared derived changes among different groupings o The central method of phylogenetics is to identify synapomorphies that exclusively diagnose branches on the tree of life o Can be morphological or molecular 5 different vertebrates leopard tuna salamander lamprey and Example turtle The 5 different vertebrates are scores for the presence 1 or absence 0 of 5 morphological characters The data is assembled in the form of a matrix o The matrix includes an outgroup a close relative of vertebrates lancelet that acts as a reference lineage defining the ingroup synopomorphies Then form a phylogeny tree o At each split in the tree there is a new derived triat that is shared bye all clades above the split Reading phylogenic trees Populations through time are represented by branches and nodes show where ancestral groups split into descendant groups Sister taxa a taxon is any named group of organisms are the Polytomy is a node where more than two descendant groups Tips are branch endpoints and represent living groups or a group s adjacent branches branch off end in extinction Read trees from root to tips not left to right Traits may be similar for two reasons Homology similarity due to inheritance from a common ancestor Homoplasy structures evolved independently convergent evolution similar selection pressure imposed by similar environment led to similar structures in two different but not closely related evolutionary lineages Ex wings in birds bats and extinct pterosaurs are homoplasious characters not homologous characters Monophyletic group