Chapter 24 Origin of Species Speciation when 1 species splits into 2 or more species o Forms a conceptual bridge between microevolution and macroevolution Microevoltuion changes over time in allele frequencies in a population Macroevolution broad pattern of evolution above the species level Evolution of new groups of species ex mammals Biological species concept emphasizes reproductive isolation o Biological species concept defines a species as a group of populations with individuals who can interbreed and produce fertile offspring that don t produce viable fertile offspring with members of other such groups Gene flow holds the group together o Reproductive isolation existence of biological factors barriers that impede members of 2 species from interbreeding and producing viable fertile offspring These barriers prevent hybrids o Hybrid offspring from interspecific mating Barriers to more closely related species o Prezygotic barriers before the zygote block fertilization from even occurring Impeding numbers of different species from trying to mate Preventing mating from being successfully completed Hindering fertilization Habitat isolation same area different habitats so they might not ever come in contact with each other Temporal isolation species might breed during difference times days seasons or years Behavior isolation courtship rituals mate recognition way to ID potential mates of the same species Mechanical isolation mating is attempted but morphological differences prevent successful completion key doesn t fit the lock Gametic isolation sperm may not be able to fertilize eggs of other species Might not survive reproductive tract Biochemical mechanisms prevent penetration at the membrane o Postzygotic barriers after the fertilization Developmental errors reduce survival among hybrid embryos Problems after birth infertile deceased survival long enough to reproduce Reduced hybrid viability genes of parents interact in ways that impair hybrids development or survival in its development Reduced hybrid fertility if chromosomes of parents differ in number or structure meiosis in hybrids might make abnormal gametes genes cannot flow between parent species Hybrid breakdown some of the 1st generation hybrids are fertile but if they interbreed or breed with parent species the next generation is feeble Limitation of the biological Species concept No way to look at reproductive isolation of fossils Doesn t apply to asexual organisms prokaryotes Species are designated by isolation o Some species are morphologically and ecologically distinct yet gene flow occurs between them Grolar bears Biological species concept overemphasized gene flow and downplays natural selection Other Definitions of Species Morphological species concept species classified by body shape and other structural features o Applicable to sexual and asexual organisms o Usable without gene flow information o However subjective researchers may disagree on which structural features distinguish a species Ecological species concept species viewed in terms of its ecological niche o Niche sum of how members of a species interact with the nonliving and living parts of their environment o Applies to sexual and asexual organisms o Emphasizes the role of disruptive natural selection as organisms adapt to different environmental conditions Phylogenetic species concept species is the smallest group of individuals that share a common ancestor 1 branch on the tree of life Speciation can take place with or without geographic separation Allopatric speciation gene flow is interrupted when a population is divided into geographically isolated sub populations o Process separate gene pools different mutations natural selection and genetic drift alter allele frequencies o Areas isolated by geographical barriers often have more species islands Sympatric speciation speciation occurs in populations that live in the same geographic area o Caused by o Polyploidy extra sets of chromosomes that come from accidents during cell division 80 of plant species today Autopolyploidy individual that has more than 2 chromosome sets that are all derived from a single species Reproductively isolated from original population Could self pollinate or mate with other tetraploids Allopolyploidy when hybrids can propagate asexually and through reproduction with other hybrids new species o Habitat differentiation Genetic factors allow a sub population to exploit a habitat resource not used by parent population Apple maggot fly apple trees vs hawthorn plant o Prezygotic restriction temporal because apple flowers at a different time o Post zygotic alleles that are beneficial on one plant are deleterious on the other o Sexual selection Cichlids Subgroups adapted to different food sources Most differ in color of males Hybrid Zone Reveal factors that Cause Reproductive Isolation Hybrid zone region in which 2 species meet and mate Patterns in hybrid zones o Rarely serve as ways to get alleles from one population to another because hybrids have lowered strength reproduction o Outside the hybrid zone other obstacles to gene flow may be provided by natural selection in the different environments in which the parent species live Over time 3 possible outcomes o Reinforcement strengthening reproductive barriers Natural selection strengthens prezygotic barriers to reproduction o Fusion weakening reproductive barriers Barriers are not strong Gene pools might become more and more alike Fuse into 1 species o Stability continued formation of hybrid individuals Hybrids continue to be produced Sometimes occurs when hybrids survive reproduce better than parent species Sometimes when hybrids are selected against When parent species continue to migrate into the hybrid zone Speciation can occur rapidly or slowly and can come from changes in many or few genes Timing of speciation o Fossil record gives morphological data static equilibria periods of apparent stasis punctuated by sudden change Might be because layers of sediment build up slowly o Speciation rates Punctuated pattern indicates speciation is rapid Varies Organisms have no speciation clock Gradualism divergence is slow Punctuationalism divergence is fast o Learned from snail species in an area where a lake would go through dry seasons where populations would be isolated for a short while o Genetics of speciation o Sometimes only a few genes have to be changed Large number of genes and gene interactions