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Chapter 24: The Origin of Species 24.1: The biological species concept emphasizes reproductive isolation That “Mystery of Mysteries” Speciation = process by which one species splits into two or more species o Responsible for diversity of life, yielding new species o Explains differences and similarities between species o When one species splits in two the species that result share many characteristics because they are descended from a common ancestor Microevolution = changes over time in allele frequencies Macroevolution = broad pattern of evolution above species level The biological species concept emphasizes reproductive isolation Compare morphology (body form), physiology, biochemistry, and DNA sequences to determine order out of life forms Morphologically distinct species are discrete groups, differing in many ways beside body forms The Biological Species Concept = definition of species Species = group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring o Don’t produce viable, fertile offspring with members of other groups o Members of species are united by being reproductively compatible Absence of gene flow plays role in formation of new species and keeping different species apart Reproductive isolation = existence of biological factors that impede members of two species from interbreeding and producing viable, fertile offspring o Limits formation of hybrids = offspring resulting from interspecific mating o Prezygotic barriers block fertilization from occurring Impede members from different species from trying to mate Preventing attempted mating from being successful Hindering fertilization of mating is completed successfully 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 Temporal Isolation = species that breed during different times of day, different seasons, or different years cannot mix gametes Behavioral Isolation = courtship rituals that attract mates and other behaviors unique to a species are effective reproductive barriers because they enable mate recognition Mechanical Isolation = mating is attempted but morphological differences prevent successful completion Gametic Isolation = sperm of one species may not be able to fertilize eggs of another species- sperm cant survive in reproductive tract or biochemical mechanisms prevent sperm from penetrating egg’s membrane o Postzygotic barriers = when prezygotic barriers are overcome, contributes to reproductive isolation after hybrid zygote is formed Developmental errors reducing survival of embryos Problems after birth cause infertile hybrids Reduced Hybrid Viability = genes of different parent species interact in ways that impair hybrid’s development/survival Reduced hybrid fertility = hybrids are sterile, chromosomes of two parent species differ in number or structure meiosis fails to produce normal gametes Hybrid breakdown = first generation hybrids are viable and fertile but next generation are sterile Limitations of the Biological Species Concept o Limit on number of species to which concept can be applied o Can’t evaluate reproductive isolation of fossils o Doesn’t apply to organisms that reproduce asexually o Species are designated by absence of gene flow, but there are species that are morphologically and ecologically distinct but gene flow occurs between them (Ex: polar bear and grizzly bear) Other Definitions of Species emphasize unity within a species Morphological species concept characterizes species structural features o Can be applied to asexual and sexual organisms and used without info on gene flow o Disadvantage: relies on subjective criteria Ecological species concept views species in terms of ecological niche o Sum of how members of the species interact with nonliving and living parts of environment o Accommodates asexual and sexual species o Emphasizes role of disruptive natural selection Phylogenetic species concept defines species as smallest group of individuals that share a common ancestor (one branch on tree of life) o Trace phylogenetic history of species by comparing its characteristics (morphology/molecular sequences) with other organisms o Distinguishes groups of individuals that are sufficiently different to be considered separate species o Difficult to determine degree of difference required to be separate 24.2: Speciation can take place with or without geographic separation Allopatric speciation = when gene flow is interrupted when a population is divided into geographically isolated subpopulations Can occur through geologic remodeling or when descendants become geographically isolated from parent population Process of Allopatric Speciation o How formidable a geographic barrier must be depends on ability of organisms to move about o Once geographic separation occurs, separated gene polls may diverge o Different mutations arise, natural selection and genetic drift may alter allele frequencies in different ways o Reproductive isolation may arise as a by-product of selection or drift o Ex: mosquitofish on Andros Island became isolated in ponds, ponds have similar environment but some have predatory fishes so the fish that have body shape that lets them swim faster in ponds with predatory fishes have different shape than fish in ponds without the predators Fish like to mate with fish with similar body type Evidence of Allopatric Speciation o Regions that are isolated or highly subdivided by barriers have more species than similar regions without such features o Lab/field tests provide evidence that reproductive isolation between two populations increases as distance between them increases o Reproductive isolation also occurs when populations are isolated experimentally and subjected to different environments o 30 species of snapping shrimp that live on different sides of Isthmus of Panama, land bridge that connects South and North America 15 on one side, 15 on the other = sibling species that arose due to geographic separation Sympatric speciation = when reproductive barriers and speciation occurs in populations that live in the same geographic area Less common than allopatric speciation Factors/mechanisms can also promote allopatric speciation Polyploidy = accident during cell division that results in extra sets of chromosomes o More common in plants (~80% of plant species today are descended from ancestors from polyploidy speciation) o Autopolyploid = individual with more than two chromosome sets that are all derived from a single species (2n, 3n, 4n, etc.) Can produce fertile offspring by selfpollinating or by mating with other mutants with the same chromosome number Reproductively isolated from organisms of different chromosome number because the mutated offspring have reduced fertility Can generate reproductive isolation in just one generation without geographic separation o Allopolyploid = fertile polyploidy Occurs when two different species interbreeds and produces hybrid offspring Most hybrid are sterile because set of chromosomes can’t pair, but infertile hybrid can propagate asexually After subsequent generations a sterile hybrid becomes fertile, but cannot interbreed with either parent species = new species Ex: species of weeds and wheat for bread arose through allopolyploidy Habitat Differentiation occurs when genetic factors enable a subpopulation to exploit a habitat/resource not used by parent population o Ex: maggot fly originally fed on hawthorn tree, then some fed on apple trees natural selection favors apple feeding flies which now have reproductive isolation from hawthorn feeding flies Alleles that benefit one type of fly and harm the other = postzygotic barrier further limiting gene flow Sexual Selection drives sympatric speciation o Ex: Africa’s Lake Victoria had 600+ species of cichlids which originated within last 100,000 years from small number of species from other rivers/lakes, number of species may have increased due to females selecting males based on appearance 24.3: Hybrid zones reveal factors that cause reproductive isolation Patterns Within Hybrid Zones Hybrid zone = a region where members of different species meet and mate producing offspring of mixed ancestry o Occurs when species with incomplete reproductive barriers come in contact with each other Some form as narrow bands of geographic area o Ex: two species of toads, hybrid zone where high altitude of one toad meets low altitude of other toad altering pattern of allele frequencies Obstacle to gene flow causes pattern of allele frequencies across hybrid zone o Ex: hybrid toads have increased rates of embryonic mortality and morphological abnormalities = poor survival and reproduction Other hybrid zones have more complicated spatial patterns o Ex: plant species only occur in locations that have a particular set of environmental conditions- favorable patches with such conditions can be scattered randomly across landscape but isolated = hybrid zone Hybrid Zones over Time Hybrids may become reproductively isolated from their parents and form a new species If not, other outcomes: o Fusion of species: reproductive barriers are weakened o Stability: hybrids continue to be produced Reinforcement = Strengthening Reproductive Barriers o When hybrids are less fit than parent species prezygotic barriers are strengthened, reducing formation of unfit hybrids o Barriers to reproduction should be stronger for sympatric than allopatric populations o Ex: allopatric populations of flycatcher birds resemble each other while in sympatric populations they look very different so females make mistakes in choosing in allopatric populations but not in sympatric populations reproductive barriers stronger in sympatric populations Fusion = Weakening of Reproductive Barriers o So much gene flow occurs that reproductive barriers are weakened and the gene pools of the two species become increasingly alike o Causes two hybridizing species to fuse into a single species o Ex: Lake Victoria cichlids: 200 of former 600 species vanished in some causes through species fusion because while females prefer to mate with fish of their own species of a specific color, murky water caused by pollution may have made females unable to distinguish o Ex: polar bears evolved from North American grizzly bears, recently global warming has reduced ice packs where polar bears hunt so they go to land where they encounter grizzly bears = hybrid zones Stability = Continued Formation of Hybrid Individuals o Occurs because hybrids survive or reproduce better than members of either parent species o Also occurs in cases where hybrids are selected against o Ex: Bombina hybrid zone where hybrid toads are more unfit but reproductive reinforcement doesn’t occur because the hybrid zone is so narrow that parents migrate into the zone = lots of gene flow, continued production of hybrids overwhelming selection for reproductive isolation 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes The Time Course of Speciation Gather info about how long it takes new species to form from broad patterns in fossil record and from studies that use morphological data like fossils or molecular data Punctuated equilibria = periods of stasis punctuated by sudden change o Fossil record shows episodes where new species appear suddenly in stratum, persist unchanged through strata, and then disappear o Once process of speciation begins it can be completed rapidly o Ex: rapid speciation of wild sunflower shown through genetic evidence indicating that species originated by hybridization of two other sunflower species because natural selection produced extensive genetic changes in hybrid populations over short periods of time o Other ex’s: apple maggot fly, Lake Victoria fish, fruit flies Other species show more gradual change (through fossils) = gradualism Length of time between speciation events o Interval consists of time that elapses before populations of a newly formed species start to diverge from one another + time it takes for speciation to be complete once divergence begins o Total time between speciation events varies o Millions of years may pass before newly formed species will itself give rise to another new species o Speciation begins after gene flow between populations is interrupted by changing environmental conditions, unpredictable events, etc. Populations must also diverge genetically to become reproductively isolated before other events resume gene flow Studying the Genetics of Speciation Studies of ongoing speciation like hybrid zones can reveal traits that cause reproductive isolation Identifying genes that control those traits can help explore how many genes change when new species form Can be influenced by small amount of genes o Ex: one gene in a snail can create a mechanical barrier to reproduction o Ex: small number of genes controls prezygotic and postzygotic barriers to different species of monkey flower which are visited by different pollinators preferring different colors hybridization can impact pollinator preference Can be influenced by large amount of genes o Ex: hybrid sterility of two types of fruit fly due to at least four loci o Ex: sunflower postzygotic isolation influenced by 26 segments From Speciation to Macroevolution Speciation can begin with small differences which accumulate and become more pronounced leading to formation of new groups of organisms One group of organisms may increase in size by producing new species while other groups may shrink, loosing species Cumulative effects of speciation and extinction help shape evolutionary changes in fossil record