Evolution of Angiosperms

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.