Sympatric speciation occurs without physical barriers

Although physical isolation is usually required for speciation, under some circumstances speciation can occur without it. Such a partition of a gene pool is called sympatric speciation (sym-, "with"). The most common means of sympatric speciation is polyploidy, the production within an individual of duplicate sets of chromosomes. Polyploidy can arise either from chromosome duplication in a single species (autopolyploidy) or from the combining of the chromosomes of 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, resulting in a tetraploid (four sets of chromosomes) individual. Tetraploid and diploid plants of the same species are repro-ductively isolated because their triploid offspring are essentially sterile.

Even if triploid individuals survive to reproductive maturity, they cannot produce viable gametes because their chromosomes do not synapse correctly during meiosis (Figure 24.7). So a tetraploid plant cannot produce viable off-

Tetraploid Meiosis

Most gametes produced by the triploid hybrid are not viable because they have an incorrect number of chromosomes.

The F1 offspring is triploid (three copies of each chromosome)

Diploid gametes y (two copies of each chromosome)

Most gametes produced by the triploid hybrid are not viable because they have an incorrect number of chromosomes.

The F1 offspring is triploid (three copies of each chromosome)

Diploid gametes y (two copies of each chromosome)

24.7 Tetraploids Are Soon Reproductively Isolated from Diploids

Even if the triploid offspring of diploid and tetraploid parents reaches sexual maturity, most of the gametes it produces have inviable numbers of chromosomes. Such triploid individuals are effectively sterile. (For simplicity, the diagram shows only three chromosomes; most species have many more than that.)

spring by mating with a diploid individual—but it can do so if it self-fertilizes or mates with another tetraploid.

Allopolyploids may be produced when individuals of two different (but closely related) species interbreed, or hybridize. Allopolyploids are usually fertile because each of the chromosomes has a nearly identical partner with which to pair during meiosis.

New species arise by means of polyploidy much more easily among plants than among animals because plants of many species can reproduce by self-fertilization. In addition, if polyploidy arises in several offspring of a single parent, the siblings can fertilize one another. Speciation by polyploidy has been very important in the evolution of flowering plants. Botanists estimate that about 70 percent of flowering plant species and 95 percent of fern species are polyploids. Most of these arose as a result of hybridization between two species, followed by self-fertilization.

How easily allopolyploidy can produce new species is illustrated by the salsifies (Tragopogon), members of the sunflower family. Salsifies are weedy plants that thrive in disturbed areas around towns. People have inadvertently spread them around the world from their ancestral ranges in Eurasia. Three diploid species of salsify were introduced into North America early in the twentieth century: T. porrifolius, T. pratensis, and T. dubius. Two tetraploid hybrids—T. mirus and T. miscellus—between the original three diploid species were first discovered in 1950. The hybrids have spread since their discovery and today are more widespread than their diploid parents (Figure 24.8).

Studies of their genetic material have shown that both salsify hybrids have formed more than once. Some populations of T. miscellus—a hybrid of T. pratensis and T. dubius— have the chloroplast genome of T. pratensis; other populations have the chloroplast genome of T. dubius. Such differences among local populations of T. miscellus show that this allopolyploid has formed independently at least 21 times! Scientists seldom know the dates and locations of species formation so well. T. mirus, a hybrid of T. porrifolius and T. dubius, has formed 12 times. T. porrifolius prefers wet, shady places; T. dubius prefers dry, sunny places. T. mirus, however, can grow in partly shaded environments where neither parent does well. The success of these newly formed hybrid species of salsifies illustrates why so many species of flowering plants originated as polyploids.

Polyploidy, as we have just seen, can result in a new species that is completely reproductively isolated from its parent species in one generation. Allopatric speciation proceeds much more slowly, and some populations separated by a physical barrier may never acquire full reproductive isolation. Let's see how reproductive isolation may become established once two populations have been separated from each other.

Hybrid Distribution Tragopogon
The range of tetraploid hybrids ( ) is broader than that of diploid parental species (A).

24.8 Polyploids May Outperform Their Parent Species

Tragopogon species (salsifies) are members of the sunflower family. The map shows the distribution of the three diploid parent species and of the two tetraploid hybrid species of Tragopogon in eastern Washington and adjacent Idaho.

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Responses

  • abraham
    Can a tetraploid plant mate with a diploid plant?
    8 years ago
  • JOHANNA
    Why is polyploidy a good place for sympatric speciation to occur?
    8 years ago

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