Encyclopedia of Evolution.pdf - Online Reading Center

emained dormant and sprouted after the Earth recovered
from this event.
Botanists understand that the angiosperms evolved
from a gymnosperm ancestor. This had to occur, unless
seeds evolved more than once, which is unlikely. One major
difference between gymnosperms and angiosperms is that
the wood of most angiosperms contains large water-conducting
vessels, while most gymnosperms have only small
conducting tubes. A few gymnosperms, including the now
extinct Bennettitales and the modern group often classified
as Gnetales, have wood with large vessels, while a few
angiosperms have wood without large vessels. The Bennettitales
and Gnetales had or have reproductive structures that
are not flowers but resemble them in some ways. The angiosperms
that have wood without vessels are those that have
also been identified, for other reasons, as the most primitive
angiosperms.
One major difference between angiosperms and gymnosperms
is that most angiosperms have a much faster life
cycle. In gymnosperms, a year may be required for the pollen
nucleus to reach the egg and bring about fertilization.
There are therefore no annual gymnosperms (i.e., that live
only for a year, completing their reproduction at the end of
the same year in which they germinated). In angiosperms,
fertilization can happen in a few days. This observation has
led many researchers to believe that the earliest angiosperms
lived in disturbed environments. These disturbed areas may
have included places that experienced a forest fire or landslide,
seasonal drought, or even damage by dinosaurs. Rapid
growth provides an evolutionary advantage in disturbed
areas. If the earliest angiosperms lived in such environments,
it may not be surprising that no fossils of them have been
found, as such environments are very unsuitable for fossilization
(see fossils and fossilization).
There is also widespread recognition among botanists
that angiosperms have proliferated largely because of coevolution
with animals:
• Although many lineages of angiosperms have re-evolved
wind pollination, most angiosperms have coevolved with
the insects and other animals that pollinate their flowers.
Insects as a whole did not experience an increased burst
of diversity during the Cretaceous period. But insect lineages
in which the adults pollinate flowers have proliferated,
along with the angiosperms, since the Cretaceous
period.
• Although many lineages of angiosperms have re-evolved
wind dispersal of seeds, most angiosperms have coevolved
with mammals and other animals that disperse their seeds.
The fact that many mammals ate fruits, while dinosaurs
appeared to rarely do so, paralleled the decline of dinosaurs
during the Cretaceous and the proliferation of mammals
since the Cretaceous.
The earliest angiosperm fossils do not provide a clear
indication of what the earliest flowers were like, because by
the time of these earliest fossils, angiosperms had already
evolved a great diversity of flowers, from small, simple flowers
to large magnolia-like flowers. Angiosperms radiated
angiosperms, evolution of
into many diverse forms very early in their evolutionary history.
Since botanists have no fossils of the ancestral angiosperm,
they have attempted to reconstruct the earliest flower
by a phylogenetic analysis of existing groups of angiosperms.
Most of these analyses have clearly identified a small woody
plant that today exists only in the cloud forests of New Caledonia,
Amborella trichopoda, as the only surviving member
of the lineage that most closely resembles the ancestor of all
flowering plants. Characteristics of Amborella include the
following:
• Five to eight sepals and petals that are small, greenish-yellow,
and largely separate from one another
• Numerous stamens, largely separate from one another
• Five to six separate carpels, each of which has an ovary
with a single seed. The carpel does not completely close
with tissue during growth; it is sealed by a secreted liquid.
If these characteristics represent what the first flowers
were like, then the following major changes have occurred
during the evolution of the angiosperms:
• In many but not all lineages, the floral parts have fused
together. Two kinds of floral fusion are connate fusion,
in which similar parts have fused, and adnate fusion, in
which different parts have fused. In some lineages, petals
have fused together to form a tube, or to form a landing
platform that accommodates pollinators; in many lineages,
carpels have fused into pistils; in a few, stamens have
fused together. All these are examples of connate fusion. In
some lineages, stamens have fused to petals, an example of
adnate fusion.
• In many but not all lineages, the numbers of floral parts
have changed. In many lineages, the numerous stamens
have been replaced by a small number of stamens, usually
the same as the number of petals. In some lineages, the
number of parts has increased, as in the lineages in which
the flowers have dozens or even hundreds of petals.
In many lineages of flowers, a mosaic of such changes
has occurred. In roses, for example, all of the floral parts are
still separate, the stamens are still numerous, but there are
only five petals. In geraniums, all of the floral parts are still
separate, but there are only five stamens and five petals. Mallow
flowers have five separate petals but have numerous stamens
that have fused together.
The traditional classification of angiosperms divides
them into monocots and dicots. Monocots have characteristics
such as these:
• The embryo has one leaf (mono- for one, -cot for cotyledon).
• The leaves usually have parallel veins.
• The stems usually have scattered bundles of conducting
tubes.
• The flowers usually have parts in threes or multiples of three.
Dicots have characteristics such as these:
• The embryo has two leaves (di- for two, -cot for cotyledon).
• The leaves usually have netlike veins.