The Way of Science

(2) Speciation via polyploidy
In plants, this process occurs with high frequency: probably 50-60% of plant species have originated in this way. Compared to animals, plants have a simple anatomy. However, some tissues are unique to plants, and are not found in animals. Meristems, permanently young tissue capable of producing any mature tissues, allow plants to reproduce asexually. These differences allow plants to utilize polyploidy for speciation.

A prerequisite for speciation via polyploidy is the absence of geographic isolation between two related species; occupying the same area is called "sympatry." Suppose that two related and sympatric species have different chromosome numbers and structures. Suppose also that they do not ordinarily interbreed in the wild, perhaps because of different flowering times. Suppose that, in one unusual year, the weather shifts, and some individuals of each species flower simultaneously. Pollination takes place. Given the differences in chromosomes, these hybrid individuals will almost certainly be sterile ("mules"). In animals, this sterility would be a dead end. In many plants, asexual reproduction may give a "mule" a long life. During this time, the sterility may be overcome by a single event in a meristem. These growing tips, which can produce new stems, leaves, and reproductive organs, may contain cells that make a particular kind of error when they divide. This fairly common error can produce new tissue that has twice the number of chromosomes of the sterile parent. This doubling restores fertility, and since only one individual is necessary for sexual reproduction in most plants, a whole new population could be generated from this one polyploid plant. Most significant is that the resulting tetraploids are not only interfertile and vigorous, but are unlikely to successfully "breed back" to their diploid parent species. Voila! A new species.

The process is summarized below (and will be reviewed in class):

[

2n=10 n=5

double crossed with plus

2n=12 n=6

] è

"2n"=11

è

"2n"=22

(fertile; not infertile with parent species)

This process has been duplicated many times in the lab, including the production of a new species that has been transplanted to the wild, where it has persisted and multiplied for decades. In addition, the evidence for this kind of speciation in the wild is conclusive and abundant. Biology majors will find the details in the Genetics course.