Evolution of Angiosperms
Chapter 20
History of Angiosperms
Arrived relatively late (90 mya)
Eventually dominated
Diversified greatly
How have angiosperms achieved this?
Anthophyta family tree
Anthophyta
Derived from a single ancestor
Flowers
Closed carpels
Three-nucleate microgametophyte
Eight-nucleate megagametophyte
Stamens with two pairs of pollen sacs
Sieve tubes and companion cells in phloem
Archaefructus sinensis
Earliest known flowering plant
125 mya
Proved that showy spirally-arranged flowers are not ancestral form
“Other” plants
97% of all plants are monocots or eudicots
What are the rest?
Magnoliaceae (magnolias)
Lauraceae (laurels)
Piperaceae (peppers)
Aristolochioaceae (pipevines)
Calycanthaceae (spice bushes)
Nymphaeaceae (waterlilies)
Amborellaceae (one species)
Follow the flowers
Flower structure gives cues to angiosperm evolution
Insect pollination accelerated evolution of angiosperms
Four trends in floral evolution
From flowers with few to many parts that are indefinite in number, flowers have evolved toward having few parts that are definite in number
The number of floral whorls has been reduced. The floral axis has become shortened to hide original spiral arrangement of parts. Floral parts have become fused.
Four trends in floral evolution
Ovary has become inferior rather than superior in position and the perianth has become differentiated into a distinct calyx and corolla.
Radial symmetry of early flowers has given way to irregularity in more advanced ones
Asteraceae and Orchiaceae
Most evolutionary specialized flowers
Two largest families
Composites
Epigynous flowers with one ovule are bunched together on a head
Many have two types of flowers
Disk flowers
Ray flowers (usually carpellate, but sometimes sterile)
Flowers are arranged so that entire head has appearance of flower
Orchids
Irregular flowers with many ovules
Entire contents of anther are dispersed as a unit called the pollinium
Lip of orchid serves as landing pad for insects
Dispersal and pollination
Many angiosperms rely on animals for:
Cross pollination
Dispersal of seeds
Coevolution
Insects and flowers have evolved together
Stigma exudes sticky drops of sap
Insects fed on sap and pollinated flower
Insect pollination is more specific and requires less pollen grains
Ovules became covered in order to protect them from pollinators
Coevolution
Bisexual flower makes each pollinator visit more effective
Further specialization occurs when a certain type of pollinator visits
Flies
Bees
Nectar
Honeyguides
Wind pollination
Sexes usually separate
Anthers exposed
Large stigmas
Oak, birches, grasses
Floral pigments
Carotenoids
Found in plastids
Oil soluble
Ultraviolet reflectivity
Floral pigments
Most important pigments in floral coloration are flavonoids
Found in all flowering plants
Anthocyanins
Water soluble
Found in vacuoles
Color depends on acidity of cell sap
Flavonols
Floral pigments
Betacyanins
Red color in beets, Chenopodiales
Fruit evolution
Fruits are dispersal agents
Remember fruits?
Fruits are mature ovaries
Accessory fruits have additional parts (e.g. strawberry)
Parthenogenic fruit develop without seed formation
Types of fruits
Simple fruits – 1 carpel
Most diverse group
Aggregate fruits – separate carpels of one gynoecium (fruitlets)
Multiple fruits – gynoecium of more than one flower
Simple fleshy fruits
Berries have one to several many seeded carpels with inner flesh
Drupes have single seed. Inner layer of seed is stony and tightly adherent to seed
Pomes are found in rose family and is derived from compound inferior ovary
Dry simple fruits
Dehiscent
Seeds released from fruit at maturity
Indehiscent
Seeds remain in the fruit
Dehiscent simple dry fruits
Follicle splits along one side
Legume splits along both sides
Siliques have central membrane
Capsule splits at various locations
Indehiscent simple dry fruits
Achene is small single seeded fruit where seed is attached by funiculus
Winged achenes are samaras
Caryopsis or grain – seed coat is firmly united to fruit wall
Cypsela is derived from inferior ovary
Nuts are derived from a compound ovary
Schizocarps split at maturity into two or more one-seeded portions
Coevolution of fruits and seeds
Wind-borne fruits
Self-propelled fruits
Floating seeds
Animal dispersal
Food (birds, ants)
Hitchhikers
Biochemical coevolution
EVOLUTION
Charles Darwin - The Origin of Species, published 24 Nov 1859, has had a larger impact on Biology than any other work.Taxonomists began to think of having their classification systems reflect evolutionary relationships. This is sometimes very difficult as it may conflict with the need for an easily referable classification.Oldest angiosperm fossils are about 125 million years oldAngiosperm fossils are rare, it has been proposed that early angiosperms evolved in the hills and uplands of the tropics where fossilization does not occur. So, if this is true, we have no way of knowing what the earliest angiosperms looked like. Once angiosperms moved into the lowlands, fossilization occurred and we do have some records of these plants.Angiosperms are clearly the most successful group of land plants. Why?ADAPTIVE TRAITS IN EARLY ANGIOSPERMS1. Tough leaves resistant to drought and cold.2. Vessel members - efficient water-conducting cells3. Tough, resistant seed coatOTHER ADAPTIVE TRAITS OF ANGIOSPERMS1. Precise systems of pollination and seed dispersal2. Deciduous - enables plant to survive times when water is not available, especially imporant in cold climates3. Enormous chemical diversity provides protectionfrom herbivores4. Evolution of herbaceous perennials and annuals. These can survive in more severe habitats than woody plants.Angiosperm evolution and the composition of floras are closely tied in with continental movements. India is a good example. India was originally far south, in a cool temperate climate, moved northward through the southern arid zone, the tropics, and the southern arid zone. Most of India's original plants and animals became extinct and were finally replaced by flora and fauna from Eurasia.
EVOLUTION OF THE ANGIOSPERM FLOWER
Carpel is a folded leaf blade that has been modified in a variety of ways.
Stamens could have evolved from a leaflike structure; an alternative is that they came from slender branch systems bearing terminal sporangiaOrigin of sepals - probably derived directly from leavesOrigin of petals - possibly from leaves in some groups, probably derived from stamens in most groups
GENERAL TRENDS IN FLORAL EVOLUTION
Evolutionary relationships among angiosperms is still actively debated. The high levels of parallel and convergent evolution confound attempts to construct evolutionary trees.