Cambridge International A Level Biology Revision Guide

Chapter 17: Selection and evolution
being formed, so that a gamete ends up with two sets of
chromosomes instead of one set. If two such gametes fuse,
then the zygote gets four complete sets of chromosomes. It
is said to be tetraploid.
Tetraploids formed in this way are often sterile. As
there are four of each kind of chromosome, all four try to
‘pair’ up during meiosis I, and get in a terrible muddle. It is
very difficult for the cell to divide by meiosis and produce
new cells each with complete sets of chromosomes.
However, the cell may well be able to grow perfectly well,
and to reproduce asexually. There is nothing to stop
mitosis happening absolutely normally. (Remember that
chromosomes do not need to pair up in mitosis – they
each behave quite independently.) This does quite often
happen in plants but only rarely in animals, largely
because most animals do not reproduce asexually.
Just occasionally, this tetraploid plant may manage to
produce gametes. They will be diploid gametes. If one of
these should fuse with a gamete from the normal, diploid,
plant, then the resulting zygote will be triploid. Once
again, it may be able to grow normally, but it will certainly
be sterile. There is no way in which it can produce gametes,
because it cannot share the three sets of chromosomes out
evenly between the daughter cells.
So, the original diploid plant and the tetraploid that
was produced from it cannot interbreed successfully. They
can be considered to be different species. A new species
has arisen in just one generation.
The kind of polyploid just described contained four
sets of chromosomes all from the same species. It is said to
be an autopolyploid. (‘Auto’ means ‘self ’.) Polyploids can
also be formed that contain, say, two sets of chromosomes
from one species and two sets from another closely
related species. They are called allopolyploids. (‘Allo’
means ‘other’ or ‘different’.) Meiosis actually happens
more easily in an allotetraploid than in an autotetraploid,
because the chromosomes from each species are not quite
identical. So the two chromosomes from one species
pair up with each other, while the two chromosomes
from the other species pair up. This produces a much
less muddled situation than in an autopolyploid, where
the chromosomes try to get together in fours, so it is
much more likely that meiosis can come to a successful
conclusion. The allopolyploid may well be able to produce
plenty of gametes. It is fertile.
Once again, however, the allopolyploid cannot
interbreed with individuals from its parent species, for the
same reasons as the autopolyploid. It is a new species.
One well-documented instance of speciation through
allopolyploidy is the cord grass Spartina anglica. This is a
vigorous grass that grows in salt marshes.
Before 1830, the species of Spartina that grew in
these places in England was S. maritima. Then, in 1829, a
different species called S. alterniflora was imported from
America (Figure 17.24). S. maritima and S. alterniflora
hybridised (interbred), producing a new species called
S. townsendii (Figure 17.25). This is a diploid plant, with
one set of chromosomes from S. maritima and one set
from S. alterniflora. It is sterile, because the two sets of
chromosomes from its parents cannot pair up, so it cannot
undergo meiosis successfully. Nor can S. townsendii
interbreed with either of its two parents, which is what
makes it a different species. Although it is sterile, it has
been able to spread rapidly, reproducing asexually by
producing long underground stems called rhizomes, from
which new plants can grow.
Figure 17.24 Spartina alterniflora.
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Figure 17.25 Spartina townsendii.