Species and Their Formation - Laboratory of Visual Systems

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486 CHAPTER TWENTY-FOURlations of wind-pollinated plants are isolated at the maximumdistance pollen can be blown by the wind, but individualplants are effectively isolated at much shorter distances.Among animal-pollinated plants, the width of the barrier isthe distance that animals can travel while carrying pollen orseeds. Even animals with great powers of dispersal are oftenreluctant to cross narrow strips of unsuitable habitat. For animalsthat cannot swim or fly, narrow water-filled gaps maybe effective barriers. However, gene flow can sometimes beinterrupted even in the absence of physical barriers.MeiosisHaploid gametes(one copy of eachchromosome)Sympatric speciation occurs without physical barriersAlthough physical isolation is usually required for speciation,under some circumstances speciation can occur withoutit. Such a partition of a gene pool is called sympatric speciation(sym-, “with”). The most common means of sympatricspeciation is polyploidy, the production within an individualof duplicate sets of chromosomes. Polyploidy can ariseeither from chromosome duplication in a single species(autopolyploidy) or from the combining of the chromosomesof two different species (allopolyploidy).An autopolyploid individual originates when (for example)cells that are normally diploid (with two sets of chromosomes)accidentally duplicate their chromosomes, resultingin a tetraploid (four sets of chromosomes) individual.Tetraploid and diploid plants of the same species are reproductivelyisolated because their triploid offspring are essentiallysterile.Even if triploid individuals survive to reproductive maturity,they cannot produce viable gametes because theirchromosomes do not synapse correctly during meiosis (Figure24.7). So a tetraploid plant cannot produce viable off-The diploidparent has twocopies of eachchromosomeMatingMeiosisMost gametes producedby the triploid hybridare not viable becausethey have an incorrectnumber of chromosomes.The F 1 offspring istriploid (three copiesof each chromosome)MeiosisDiploid gametes(two copies ofeach chromosome)The tetraploidparent has fourcopies of eachchromosome.24.7 Tetraploids Are Soon Reproductively Isolated from DiploidsEven if the triploid offspring of diploid and tetraploid parents reachessexual maturity, most of the gametes it produces have inviable numbersof chromosomes. Such triploid individuals are effectively sterile.(For simplicity, the diagram shows only three chromosomes; mostspecies have many more than that.)
SPECIES AND THEIR FORMATION 487spring by mating with a diploid individual—but it can do soif it self-fertilizes or mates with another tetraploid.Allopolyploids may be produced when individuals of twodifferent (but closely related) species interbreed, or hybridize.Allopolyploids are usually fertile because each of thechromosomes has a nearly identical partner with which topair during meiosis.New species arise by means of polyploidy much more easilyamong plants than among animals because plants ofmany species can reproduce by self-fertilization. In addition,if polyploidy arises in several offspring of a single parent, thesiblings can fertilize one another. Speciation by polyploidyhas been very important in the evolution of flowering plants.Botanists estimate that about 70 percent of flowering plantspecies and 95 percent of fern species are polyploids. Most ofthese arose as a result of hybridization between two species,followed by self-fertilization.How easily allopolyploidy can produce new species is illustratedby the salsifies (Tragopogon), members of the sunflowerfamily. Salsifies are weedy plants that thrive in disturbedareas around towns. People have inadvertentlyspread them around the world from their ancestral ranges inEurasia. Three diploid species of salsify were introduced intoNorth America early in the twentieth century: T. porrifolius,T. pratensis, and T. dubius. Two tetraploid hybrids—T. mirusand T. miscellus—between the original three diploid specieswere first discovered in 1950. The hybrids have spread sincetheir discovery and today are more widespread than theirdiploid parents (Figure 24.8).Studies of their genetic material have shown that both salsifyhybrids have formed more than once. Some populationsof T. miscellus—a hybrid of T. pratensis and T. dubius— havethe chloroplast genome of T. pratensis; other populations havethe chloroplast genome of T. dubius. Such differences amonglocal populations of T. miscellus show that this allopolyploidhas formed independently at least 21 times! Scientists seldomknow the dates and locations of species formation so well. T.mirus, a hybrid of T. porrifolius and T. dubius, has formed 12times. T. porrifolius prefers wet, shady places; T. dubius prefersdry, sunny places. T. mirus, however, can grow in partlyshaded environments where neither parent does well. Thesuccess of these newly formed hybrid species of salsifies illustrateswhy so many species of flowering plants originatedas polyploids.Polyploidy, as we have just seen, can result in a newspecies that is completely reproductively isolated from its parentspecies in one generation. Allopatric speciation proceedsmuch more slowly, and some populations separated by aphysical barrier may never acquire full reproductive isolation.Let’s see how reproductive isolation may become establishedonce two populations have been separated from each other.A tetraploid hybrid has an almostcontinuous range in an areaaround Spokane, Washington.The range oftetraploid hybrids ( )is broader than that ofdiploid parentalspecies ( ).24.8 Polyploids May Outperform Their Parent SpeciesTragopogon species (salsifies) are members of the sunflower family.The map shows the distribution of the three diploid parent speciesand of the two tetraploid hybrid species of Tragopogon in easternWashington and adjacent Idaho.Completing Speciation:Reproductive Isolating MechanismsOnce a barrier to gene flow is established, by whatevermeans, the separated populations may diverge geneticallythrough the action of the evolutionary agents we describedin Chapter 23. Over many generations, differences may accumulatethat reduce the probability that members of the twopopulations could mate and produce viable offspring. In thisway, reproductive isolation can evolve as an incidental byproductof genetic changes in allopatric populations.Geographic isolation does not necessarily lead to reproductiveincompatibility. For example, American sycamoresand European sycamores (also known as plane trees) havebeen physically isolated from one another for at least 20million years. Nevertheless, they are morphologically verysimilar (Figure 24.9), and they can form fertile hybrids,even though they never have an opportunity to do so innature.In other cases, however, genes that result in reproductiveisolation between two evolving lineages spreadquickly through populations as they diverge. In this section,we will examine the ways in which reproductive isolatingmechanisms may arise. In the following section, wewill explore what happens when reproductive isolation isincomplete.
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