Species and Their Formation - Laboratory of Visual Systems

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.