Chapter 24 The Origin of SpeciesLecture Outline Overview That Mystery of Mysteries Darwin visited the Gal pagos Islands and found them filled with plants and animals that lived nowhere else in the world He realized that he was observing newly emerged species on these young islands Speciation the origin of new species is at the focal point of evolutionary theory because the appearance of new species is the source of biological diversity Speciation addresses the question of how new species originate and develop through the subdivision and subsequent divergence of gene pools Microevolution is the study of adaptive change in a population Macroevolution addresses evolutionary changes above the species level It deals with questions such as the appearance of evolutionary novelties e g feathers and flight in birds that can be used to define higher taxa Two patterns of evolutionary change anagenesis and cladogenesis Anagenesis phyletic evolution is the accumulation of changes that gradually transform a given species into a species with different characteristic Cladogenesis branching evolution is the splitting a gene pool into two or more separate pools which each give rise to one or more new speices Only cladogenesis can promote biological diversity by increasing the number of species Concept 24 1 The biological species concept emphasizes reproductive isolation Species is a Latin word meaning kind or appearance Traditionally morphological differences have been used to distinguish species Today differences in body function biochemistry behavior and genetic makeup are also used to differentiate species Are organisms truly divided into the discrete units we called species or is this classification an arbitrary attempt to impose order on the natural world The Biological Species Concept In 1942 Ernst Mayr proposed the biological species concept The biological species concept defines a species as a population or group of populations whose members have the potential to interbreed and produce viable fertile offspring but who cannot produce viable fertile offspring with members of other populations The members of a biological species are united by being reproductively compatible potentially In contrast humans have considerable diversity but we all belong to the same species because of our capacity to interbreed Reproductive Isolation Because the distinction between biological species depends on reproductive incompatibility the concept hinges on reproductive isolation the existence of biological factores barriers that impede members of two species from producing viable fertile hybrids A single barrier may not block all genetic exchange between species but a combination of several barriers can effectively isolate a species gene pool Reproductive isolation prevents populations belonging to different species from interbreeding even if their ranges overlap Reproductive barriers can be categorized as prezygotic or postzygotic depending on whether they function before or after the formation of zygotes Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate These barriers include habitat isolation behavioral isolation temporal isolation mechanical isolation and gametic isolation If a sperm cell from one species does overcome prezygotic barriers and fertilize an ovum of another species postzygotic barriers often prevent the hybrid zygote from developing into a viable fertile adult These barriers include reduced hybrid viability reduced hybrid fertility and hybrid breakdown Other Definitions of Species Ex a species that is a parasite may be defined in part by its adaptations to a specific organism This concept accommodates asexual and sexual species The ecological species concept defines a species in terms of its ecological niche the set of environmental resources that a species uses and its role in a biological community The paleontological species concept focuses on morphologically discrete species known only from the fossil record There is little or no information about the mating capability of fossil species and the biological species concept is not useful for them The phylogenetic species concept defines a species as a set of organisms with a unique genetic history Biologists compare the physical characteristics or molecular sequences of species to those of other organisms to distinguish groups of individuals that are sufficiently different to be considered separate species Sibling species are species that appear so similar that they cannot be distinguished on morphological grounds Scientists apply the biological species concept to determine if the phylogenetic distinction is confirmed by reproductive incompatibility The morphological species concept the oldest and still most practical defines a species by a unique set of structural features The morphological species concept has certain advantages It can be applied to asexual and sexual species and it can be useful even without information about the extent of gene flow However this definition relies on subjective criteria and researchers sometimes disagree about which structural features identify a species In practice scientists use the morphological species concept to distinguish most species Each species concept may be useful depending on the situation and the types of questions we are asking Concept 24 2 Speciation can take place with or without geographic separation Speciation can occur in two main ways depending on how gene flow between the populations is interrupted In allopatric speciation geographic separation of populations restricts gene flow In sympatric speciation speciation occurs in geographically overlapping populations when biological factors such as chromosomal changes and nonrandom mating reduce gene flow Allopatric Other Country speciation In allopatric speciation gene flow is interrupted when a population is divided into geographically isolated subpopulations Several geological processes can fragment a population into two or more isolated populations Mountain ranges glaciers land bridges or splintering of lakes may divide one population into isolated groups Allopatric speciation can also occur without geographic remodeling such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population For example mainland organisms that colonized the Gal pagos Islands were isolated from mainland populations How