Chapter 24 The Origin of Species 1 Chapter 24 The Origin of Species That Mystery of Mysteries existing ones life Speciation The process by which one species splits into two or more species Speciation is responsible for the tremendous diversity of life repeatedly yielding new species that differ from Speciation explains not only differences between species but also the similarities between them the unity of When one species splits into two the species that result share many characteristics because they are descended from this common ancestral species Speciation also forms a conceptual bridge between microevolution changes over time in allele frequencies in a population and macroevolution the broad pattern of evolution above the species level Macroevolutionary change is the origin of new groups of organisms such as mammals or flowering plants through a series of speciation events Microevolutionary mechanisms consist of mutation natural selection genetic drift and gene flow Concept 24 1 The biological species concept emphasizes reproductive isolation The Biological Species Concept Morphologically distinct species are indeed discrete groups differing in many ways besides their body form Species a group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring but do not produce viable fertile offspring with members of other such groups Being reproductively compatible unites the members of a biological species Gene flow the transfer of alleles between populations Typically gene flow occurs between the different populations of species This ongoing exchange of alleles tends to hold the populations together genetically Reproductive Isolation The formation of a new species hinges on reproductive isolation the existence of biological factors barriers that impede members of two species from interbreeding and producing viable fertile offspring Such barriers block gene flow between the species and limit he formation of hybrids offspring that result from an interspecific mating The reproductive barriers between more closely related species are not so obvious These barriers can be classified according to whether they contribute to reproductive isolation before or after fertilization Prezygotic barriers before the zygote block fertilization from occurring Such barriers typically act in one of three ways Impeding members of different species from attempting to mate Preventing an attempted mating from being completely successful Hindering fertilization if mating is completed successfully Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation two species that occupy different habitats within the same area may encounter each other rarely if at all even though they are not isolated by obvious physical barriers such as mountain ranges Chapter 24 The Origin of Species 2 Temporal isolation species that breed during different times of the day different seasons or different years cannot mix their gametes Behavior isolation courtship rituals that attract mates and other behaviors unique to a species are effective reproductive barriers even between closely related species Such behavioral rituals enable mate recognition a way to identify potential mates of the same species successful completion Mechanical isolation mating attempted but morphological differences prevent is Gametic isolation Sperm of one species may not be able to fertilize the eggs of another species For instance the sperm may not be able to survive in the reproductive tract of females of the other species or biochemical mechanisms may prevent the sperm from penetrating the membrane surrounding the other species eggs Postzygotic barriers after the zygote occur when a sperm cell from one species overcomes prezygotic barriers and fertilizes an ovum from another species It may contribute to reproductive isolation after the hybrid zygote is formed Postzygotic barriers prevent a hybrid zygote from developing into a viable fertile adult Reduced hybrid viability the genes of different parent species may interact in ways that impair the hybrid s development or survival in its environment Reduced hybrid fertility Even if hybrids are vigorous they may be sterile If the chromosomes of the two parent species differ in number or structure meiosis in the hybrids may fail to produce normal gametes Since the infertile hybrids cannot produce offspring when they mate with either parent species genes cannot flow freely between the species Hybrid breakdown some first generation hybrids are viable and fertile but when they mate with one another or with either parent species offspring of the next generation Limitation of the biological species concept are feeble or sterile One strength of the biological species concept is that it directs our attention to a way by which speciation can occur by the evolution of reproductive isolation The biological species concept also does not apply to organisms that reproduce asexually all or most of the time such as prokaryotes In the biological species concept species are designated by the absence of gene flow But there are many pairs of species that are morphologically and ecologically distinct and yet gene flow occurs between them between them Natural selection can cause such species grolar bears to remain distinct even though some gene flow occurs This observation has led some researchers to argue that the biological species concept overemphasizes gene flow and downplays the role of natural selection Other Definitions of Species The biological species concept emphasizes the separateness of species from one another due to reproductive barriers Morphological species concept characterizes a species by body shape and other structural features The morphological species concept can be applied to asexual and sexual organisms This definition relies on subjective criteria researchers may disagree on which structural features distinguish a Chapter 24 The Origin of Species 3 Ecological species concept views a species in terms of its ecological niche the sum of how members of the species interact with the nonliving and living parts of their environment The ecological species concept can accommodate asexual as well as sexual species It emphasizes the role of disruptive natural selection as organisms adapt to different environmental species conditions Phylogenetic species concept defines a species as the smallest group of individuals that share