Chapter 24 The Origin of Species Speciation is the source of diversity that we see in life Forms a conceptual bridge between microevolution and macroevolution Microevolution changes over time in the allele frequency Macroevolution the broad pattern of evolution above the species level An example of this is the creation of a new group of organisms such as mammals through speciation 24 1 THE BIOLOGICAL SPECIES CONCEPT EMPHASIZES REPRODUCITIVE ISOLATION The Biological Species Concept The biological species concept classifies a species as a group of populations whose members have the potential to interbreed and produce viable fertile offspring but do not produce fertile viable offspring with other species GENE FLOW holds the populations of species together The absence of gene flow leads to the formation of new species Reproductive Isolation Biological species are defined in terms of reproductive compatibility and the formation of a new species hinges on reproductive isolation Reproductive isolation blocks gene flow between species and limits the formation of hybrids A combination of barriers can isolate a species gene pool completely These two barriers fall into two categories pre zygotic and post zygotic barriers Pre zygotic barriers block fertilization from occurring If gametes from different species overcome pre zygotic barriers the hybrid zygote may face post zygotic barrier and may contribute to reproductive isolation Pre zygotic Barriers Habitat Isolation Two species occupy different habitats within the same area and may encounter each other rarely They may not be isolated by physical barriers e g mountains An example of this is two species of garter snakes One lives on land the other lives in water Temporal Isolation Species that breed during different times of the day seasons or year cannot mix gametes Behavioral Isolation Courtship rituals and other behaviors that attract mates are effective reproductive barriers Behavioral rituals enable mate recognition Mechanical Isolation Mating is attempted but morphological differences in body and structure prevent successful completion Gametic Isolation Sperm of one species may not be able to fertilize the eggs of another species For example sperm may not be able to survive in the reproductive tract of females from different species Biochemical mechanisms may prevent the membrane surrounding the other species eggs Gametic Isolation separates closely related species of aquatic animals that release sperm and eggs into the sea Post zygotic Barriers Reduced Hybrid Viability The genes of different parents species may interact in ways that impair the hybrid s development or survival in it s environment Reduced Hybrid Fertility Hybrids may be vigorous but may be sterile The chromosomes of two different parent species differ in number or structure and meiosis may fail to reproduce normal gametes As a result hybrids cannot pass on their genes 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 are feeble or sterile Limitations of the Biological Species Concept Strength of the Biological Species Concept is that it allows use to see the way speciation can occur by the evolution of reproductive isolation However the biological species concept cannot evaluate the reproductive isolation of fossils and does not apply to organisms that reproduce asexually Furthermore the biological species concept is designated by the absence of gene flow There are many pairs of species that are morphologically and ecologically distinct yet gene flow is present Polar Bears Grizzly Bears Grolar Bears Other Definitions of Species There are more than 20 other species definitions Each definition is useful to the question asked and the situation asked For example the biological species concept is very useful on the focus of reproductive barriers Morphological Species Concept distinguishes a species by body shape and other structural features Can be applied to sexual and asexual organism Can be useful without information on gene flow However researches may disagree on which structural features distinguish a species Ecological Species Concept Defines Species in terms of its ecological niche the sum of how members of the species interact with the nonliving parts of their environment This can accommodate asexual and sexual organisms Phylogenetic Species Concept Defines a species as the smallest group of individuals that share a common ancestor forming one branch on the tree of life The difficulty with this species concept is determining the degree of difference required to indicate separate species 24 2 Speciation can take place with or without geographical separation In allopatric speciation the populations are geographically isolated In sympatric speciation they are not ALLOPATRIC OTHER COUNTRY SPECIATION In allopatric speciation gene flow is interrupted when a population is divided into geographically isolated subpopulations It can also occur without geographic remodeling such as when individuals colonize a remote area and the descendants become isolated THE PROCESS OF ALLOPATRIC SPECIATON Allopatric requires geographic separation which ultimately allows the gene pools to survive Different mutations arise and natural selection and genetic drift may alter allele frequencies in the populations Eventually reproductive isolation may evolve out of genetic divergence EVIDENCE OF ALLOPATRIC SPECIATION There are 30 species of snapping shrimp that live of the Isthmus of Panama 15 live on the Atlantic Side and 15 live on the Pacific Side Each of the 15 species on the Pacific Side has a sister species on the Atlantic Side This provides strong evidence that they were geographically separated The divergence times according to genetic analysis is constituent to when the isthmus developed and divided the shrimp SYMPATRIC SAME COUNTRY SPECIATION In sympatric speciation speciation occurs in populations that live in the same geographical area Sympatric Speciation can be reducing gene flow by factors such as polyploidy sexual selection and habitat differentiation POLYPLOIDY A species may originate from an accident during cell division that results in extra sets of chromosomes a condition called polyploidy Polyploidy is very common in plants Two distinct forms of polyploidy have been observed in plant populations Autopolyploid and allopolyploid Autopolyploid An individual