diversification within extant plant families occurred during the last 50 million
years ago (Fig. 3) [58, 65].
The numbers of known plants is increasing over the years as new species are
constantly being discovered: For our discussion, we only need to consider the actual
numbers (not speculating on the unknown numbers of undescribed taxa): Species
numbers: Bryophyta: 20,000; Pteridophyta: 13,000; Gymnosperms: 1100 and
Angiosperms: 369,000 (Fig. 5a) [58, 65, 66].
Spore-bearing plants reproduce without the aid of animals. Also, most
gymnosperms are wind-pollinated. Only cycads and Ephedra are visited by insects,
which help to pollinate. In cycads, insects are mostly beetles and not flies and
hymenopterans as in angiosperms.
The key innovation of angiosperms was the formation conspicuous flowers, with
animals playing a role in pollination, whereas wind-pollinated angiosperms have
inconspicuous flowers, such as in grasses. Flowers evolved in a way to attract
animals for pollination. As already discussed above, flowers accumulate aromatic
and colored PSM, such as anthocyanins, flavonoids, and carotenoids, which can
be detected by insects and birds (nectar specialists, such as humming birds, sun
birds). In some flowers, coloration (sometimes only visible in UV light) leads to the
nectar source, which is the reward for the pollinating organism. The nectar of some
plants contains bitter-tasting PSM, such as alkaloids [22, 23]. There is evidence
that these PSMs provide a niche for some pollinators, which are adapted to the
bitter-tasting PSM. An example is Anna’s Hummingbird [24]. In this case, the nectar
of a particular species is exclusive for a few pollinators, which may be of benefit
for the plants, as this improved the likelihood that pollen of the same species is
transferred by a pollinator.
In addition, many flowers produce aromatic and foully smelling fragrances,
which are attractive for many insects (flies, bees, bumble bees, syrphid flies,
butterflies). In some plants, these fragrances are mainly produced at night when
certain kind of moth roam around for pollination [2, 21].
4
Co-evolution of Angiosperms, Animals, and Alkaloids
Among animals, chordates (which include mammals, birds, reptiles) consist of
about 65,000 species in contrast to more than 1,360,000 species of invertebrates
(Fig. 5b). The largest group within the invertebrates are insects with over one million
species. Species numbers are uneven within insects: The major groups are
Coleoptera (beetles) with 360,000–400,000 species, followed by Lepidoptera
(moths and butterflies) with 175,000 species, Diptera (flies) with 153,000 species,
Hymenoptera (bees, wasps, ants) with 115,000 species, and Hemiptera (aphids,
bugs) with 88,000 species [58, 66]. A common theme for the species-rich insect
groups is that many are pollinators and feed on plants at least in one developmental
stage (e.g., as larvae many butterflies are herbivores and nectar feeders as adults).
It has been argued that the large number of insects is a consequence of the
diversification of angiosperms, which constitute more than 94% of all vascular
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
169
years ago (Fig. 3) [58, 65].
The numbers of known plants is increasing over the years as new species are
constantly being discovered: For our discussion, we only need to consider the actual
numbers (not speculating on the unknown numbers of undescribed taxa): Species
numbers: Bryophyta: 20,000; Pteridophyta: 13,000; Gymnosperms: 1100 and
Angiosperms: 369,000 (Fig. 5a) [58, 65, 66].
Spore-bearing plants reproduce without the aid of animals. Also, most
gymnosperms are wind-pollinated. Only cycads and Ephedra are visited by insects,
which help to pollinate. In cycads, insects are mostly beetles and not flies and
hymenopterans as in angiosperms.
The key innovation of angiosperms was the formation conspicuous flowers, with
animals playing a role in pollination, whereas wind-pollinated angiosperms have
inconspicuous flowers, such as in grasses. Flowers evolved in a way to attract
animals for pollination. As already discussed above, flowers accumulate aromatic
and colored PSM, such as anthocyanins, flavonoids, and carotenoids, which can
be detected by insects and birds (nectar specialists, such as humming birds, sun
birds). In some flowers, coloration (sometimes only visible in UV light) leads to the
nectar source, which is the reward for the pollinating organism. The nectar of some
plants contains bitter-tasting PSM, such as alkaloids [22, 23]. There is evidence
that these PSMs provide a niche for some pollinators, which are adapted to the
bitter-tasting PSM. An example is Anna’s Hummingbird [24]. In this case, the nectar
of a particular species is exclusive for a few pollinators, which may be of benefit
for the plants, as this improved the likelihood that pollen of the same species is
transferred by a pollinator.
In addition, many flowers produce aromatic and foully smelling fragrances,
which are attractive for many insects (flies, bees, bumble bees, syrphid flies,
butterflies). In some plants, these fragrances are mainly produced at night when
certain kind of moth roam around for pollination [2, 21].
4
Co-evolution of Angiosperms, Animals, and Alkaloids
Among animals, chordates (which include mammals, birds, reptiles) consist of
about 65,000 species in contrast to more than 1,360,000 species of invertebrates
(Fig. 5b). The largest group within the invertebrates are insects with over one million
species. Species numbers are uneven within insects: The major groups are
Coleoptera (beetles) with 360,000–400,000 species, followed by Lepidoptera
(moths and butterflies) with 175,000 species, Diptera (flies) with 153,000 species,
Hymenoptera (bees, wasps, ants) with 115,000 species, and Hemiptera (aphids,
bugs) with 88,000 species [58, 66]. A common theme for the species-rich insect
groups is that many are pollinators and feed on plants at least in one developmental
stage (e.g., as larvae many butterflies are herbivores and nectar feeders as adults).
It has been argued that the large number of insects is a consequence of the
diversification of angiosperms, which constitute more than 94% of all vascular
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
169