When hybridization (the production of offspring from different populations by parents that differ in one or more characteristics) takes place, the hybrids may be significant or important in evolutionary change, depending on how the characteristics of the parents were combined. If, for example, the environment changes (e.g., average temperatures drop or annual precipitation increases), hybrids may have gene combinations
that are better or worse suited to the new environment than those of either parent. Two related species may hybridize occasionally, and when they do, introgression (backcrossing between the hybrids and the parents) may occur. If the back-crossing occurs repeatedly, some characteristics of the parents may eventually disappear from the population if the new combinations of genes in the offspring happen to be better suited to the environment than those of the parents and as natural selection favors the offspring. Both parents and hybrids may, however, also evolve in other ways.
Polyploidy occurs occasionally in nature when, during mitosis or meiosis, a new cell wall fails to develop between two daughter cells, even though the chromosomes have divided. During our discussion of plant breeding in Chapter 14, we noted that this situation can result in a cell with twice the original number of chromosomes. If mitosis were to occur normally after such a cell was formed and the cell divided repeatedly until a complete organism resulted, that organism would have double the original number of chromosomes in all its cells.
The hybrids resulting from a cross between two species are often sterile because the chromosomes do not pair up properly in meiosis. If polyploidy does occur in such a hybrid, however, the extra set of chromosomes present from
Figure 15.10 Fire weed (Epilobium angustifolium)—a polyploid found primarily in mountainous areas of North America below 3,050 meters (10,000 feet). One subspecies has four sets of chromosomes, with a diploid chromosome number of 2n = 36. A second subspecies has 8 sets of chromosomes, with a diploid chromosome of 2n = 72.
each parent provides an opportunity for any one chromosome to pair with its homologue in meiosis, possibly overcoming the problem of sterility. This type of polyploidy apparently occurred frequently in the past (in terms of geological time), and it is believed that more than 40% of flowering plants that exist today originated this way (Fig. 15.10).
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