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mosomes come in groups of five rather than in diploid pairs.
Meiosis cannot occur in these plants, because very early in
meiosis the chromosomes are divided into two equal groups,
and an odd number of chromosomes cannot divide into two
equal groups. Therefore a pentaploid plant is sexually sterile.
It cannot produce seeds; it propagates by means of bulbs.
Some plants produce seeds asexually (without meiosis or fertilization).
Asexual propagation by means of diploid eggs or
ovules, which are essentially clones of the mother, is called
parthenogenesis (“partheno” refers to the Greek goddess
Athena who supposedly grew out of Zeus’s head). One example
is the common dandelion, Taraxacum oficinale. It is triploid,
which means that its chromosomes come in groups of
three rather than in diploid pairs. Both of these plant species
produce flowers as if they were reproducing sexually, as their
immediate ancestors did and as their close relatives still do.
Most plants, however, reproduce sexually either along with
or instead of asexual propagation.
Parthenogenetic animal species are even more uncommon
than asexually propagated plants. Most parthenogenetic
animal populations appear to be evolutionary dead
ends, on a few of the outermost twigs of the tree of life.
However, a few animal species, such as some mites and the
bdelloid rotifers, appear to have persisted for a long time
without sexual reproduction. The bdelloid rotifers have been
asexual for a long enough evolutionary time that they have
evolved into 360 asexual species. Genetic analysis confirms
that this lineage has probably been asexual throughout its
evolutionary history. Some animals, such as many species of
aphids, propagate themselves asexually during the summer
and undergo sexual reproduction in the autumn. Within the
fungus kingdom, most species have sexual reproduction,
although in many fungi sexual reproduction has never been
observed.
Parthenogenesis is not possible in some lineages. Mammals
cannot be parthenogenetic. Because some maternal
alleles are methylated, only the father’s allele is functional,
thus a parthenogen will be missing these genes and cannot
survive. Conifers cannot be parthenogenetic because the chloroplasts
are passed on through pollen.
One important difference between asexual propagation
and sexual reproduction is that sexually produced offspring
have a different combination of genes than the parents. In
asexual propagation, the “parents” and the propagules are
genetically identical to one another, and all the propagules
are identical to one another. In contrast, each sexually produced
offspring is genetically unique. This occurs because
diploid organisms have pairs of chromosomes, one member
of each pair coming from the mother, the other coming from
the father. When meiosis separates the pairs, the chromosomes
that came from the father and those that came from
the mother are usually distributed randomly among the
spores or gametes. Therefore the genes on the different chromosomes
experience independent assortment. In humans,
with 23 pairs of chromosomes, meiosis can distribute the
chromosomes into different sperm cells or different ova in 2
to the 23rd power, or more than eight million, different ways.
sex, evolution of
When an egg and a sperm come back together in fertilization,
any of these eight million kinds of sperm could unite with any
of the eight million kinds of eggs. The genes carried in the
cells of one individual can mix with those carried by another
individual. Although each individual can carry, at most, two
different versions (or alleles) of a gene, a population can have
many versions of the gene; and sexual reproduction mixes
them all together. Although new mutations (somatic mutations)
can produce new alleles in the cells of asexually propagated
organisms, they cannot produce new combinations of
alleles.
But what are the advantages of such sexual genetic mixing?
There must be a strong advantage to sexual reproduction,
since nearly every species has it. The advantages must be
major, to compensate for the disadvantages of sexual reproduction:
• Sexual reproduction seems hopelessly vulnerable. Sperm or
pollen must find eggs or ovules, and most of them fail.
• The average individual in the population can produce only
half as many offspring sexually as it would be able to asexually.
• Good combinations of genes are broken up; this is called
recombinational load.
• Most plants are capable of a great deal of developmental
plasticity, in which they adjust their gene expression and
their growth to the prevailing environment. Even genetically
identical plants, therefore, can have much larger leaves if
grown in moist, shady conditions than in dry, sunny conditions
(see adaptation). Animals have less plasticity than
plants. Nevertheless, plasticity in both animals and plants
would seem to assure that only major genetic variations
would make an immediate difference to survival of the offspring
in the next generation. Most of the genetic variation
among offspring of one parent is minor, rather than major,
genetic variation. Therefore, in the short term, the asexual
species would seem to have an advantage over the sexual
species.
Where, then, is the tremendous advantage that sexual
reproduction confers upon the species that possess it, as
nearly all do? The following categories of advantage have
been proposed:
Variable offspring in unpredictable physical and biological
environments. When the physical environment is
unpredictable, as nearly all environments are, it is impossible
to know which combination of genes will be best in
the next generation. By having a great diversity of combinations,
a population is more likely to persist. The problem
with this explanation is that natural selection does not favor
the best groups but the best individuals (see natural selection;
group selection). But the same reasoning could be
applied to individuals: The individuals that produce the greatest
genetic diversity of offspring are the most likely to have
at least some of those offspring survive in the unpredictable
future. If an organism is perfectly adapted to its current environment,
its clonal propagules would not be perfectly adapted
to the changed environment of the future. As evolutionary