I. Macroevolution is essentially the formation of new species (speciation) and accompanying eventsII. Reproductive isolation can occur in a variety of waysIII. Reproductive isolation is the key to cladogenic speciationIV. Cladogenic speciation has two modes: allopatric speciation and sympatric speciationV. Macroevolution involves both rapid and gradual divergenceBIOL 1020 – CHAPTER 24 LECTURE NOTESChapter 24: The Origin of Species (Macroevolution)I. Macroevolution is essentially the formation of new species (speciation) and accompanying eventsA. species: “Kind of living thing”; the word “species” is both plural and singularB. relatively easy to define for sexual organisms, hard for asexual organisms and extinct species1. biological species concept (for sexual organisms)- a species is one or more populations whose members are capable of interbreeding and producing fertile offspring, and whose members are reproductively isolated from other such groups- not always clear-cut, because some can interbreed under “artificial” conditions but don’t appear to do so in nature- sometimes, “race” and “subspecies” designations are used, but often different specific epithets are used when there are clear morphological differences involved2. asexual species – definition based on biochemical (think DNA sequence) and morphological differences; no solid rules- also includes use of “race,” “subspecies,” and “strain” designations- in asexual species, microevolution over time directly leads to macroevolution (speciation)3. phylogenetic species concept or evolutionary species concept - a species is a single line of descent (lineage with a unique genetic history) that maintains its distinctive identity from other lineages- this is a more comprehensive concepts (it works for both sexual and asexual species)- but, it can be hard to clearly define and agree upon “distinctive identity”C. the basis of macroevolution in sexual species is microevolution coupled with reproductive isolationII. Reproductive isolation can occur in a variety of waysA. reproductive isolation is any means of preventing gene flow between two species; for a new species to evolve from an existing (sexually reproducing) species, there must be a reproductive isolating mechanism in placeB. reproductive isolating mechanisms can be classified as either prezygotic or postzygotic1. prezygotic barriers – prevent fertilization (zygote formation) between gametes from two species- habitat isolation (or ecological isolation) – isolation by differences in habitat occupied at the time of mating; examples: some garter snakes; some flycatchers- temporal isolation – isolation by differences in timing of mating; examples: mating seasons in some skunks; flowering time in some plants; mating time in some fruit flies- behavioral isolation – differences in behavior that cause reproductive isolation; examples: mating calls, courtship patterns, and other mating rituals; can be an aspect of sexual selection (selective pressure that comes from mating choices)- mechanical isolation – differences in physical aspects make successful mating impossible; examples: many flower species; dragonflies may attempt interspecies mating, but the physical structure of their genitalia prevents successful mating- gametic isolation – mating occurs, but the sperm and egg can not fuse; examples: sperm cannot penetrate the egg of the different species, such as between sea urchins species, as well as in many other animal species; in plants, often pollen grains do not get the proper signal to germinate when on a stigma of a different species2. postzygotic barriers – reproductive isolation after fertilization has occurred- hybrid inviability – (the most common type of postzygotic barrier) normal development of the zygote formed fromthe mating of two species does not occur and the embryo is aborted, or if development is completed the offspring is very frail; examples: crosses between different iris species, cross between bullfrog and leopard frog; crosses between some salamander subspecies- hybrid sterility – a zygote of a hybrid proceeds through normal development, but is reproductively sterile sometimes due to other barriers such as behavioral isolation (don’t make the right mating call, etc.) most often due to problems in meiosis (example: male donkey [n=31] x female horse [n=32] generates sterile mule [2n=63, not an even number, pairing problems during meiosis])- hybrid breakdown – a zygote of a hybrid proceeds through normal development, and the interspecific hybrid reproduces, but the F2 generation and beyond have problems with reproduction; examples: crosses between sunflower species result in 80% hybrid breakdown in the F2 generation; crosses between some rice strainsC. the genetic basis for some reproductive barriers have been identified (the basis will vary depending on the species involved)III. Reproductive isolation is the key to cladogenic speciationD. speciation (the evolution of new species) has two general forms, anagenic and cladogenicE. anagenic speciation is the gradual change of one species into a new form, with the “new” species form replacing the “old” form; this is essentially microevolution on the whole species level – the number of species does not changeF. cladogenic speciation, or branching evolution, occurs when two or more species are present where only one existed before; these species share a common ancestor1. a cluster of species that share a common ancestor is a clade2. cladogenic speciation increases the number of species3. two separate species are said to diverge from the point where they shared a common ancestor – the gene pools of these species are separated from the point of divergence on1 of 2BIOL 1020 – CHAPTER 24 LECTURE NOTESG. cladogenic speciation occurs when a population is different enough from its ancestral species so that no genetic exchange can occur between them1. the appearance of a reproductive isolating mechanism produces two or more species where once there was one2. The question of macroevolution becomes, what creates reproductive isolation?3. there is no easy way to define when cladogenic speciation is complete – often some level of hybridization can persist when there is essentially no gene flow between the “species” or “subspecies” involvedIV. Cladogenic speciation has two modes: allopatric speciation and sympatric speciationH.