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 old

Angiosperm 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 ANGIOSPERMS
1. Tough leaves resistant to drought and cold.
2. Vessel members - efficient water-conducting cells
3. Tough, resistant seed coat

OTHER ADAPTIVE TRAITS OF ANGIOSPERMS
1. Precise systems of pollination and seed dispersal
2. Deciduous - enables plant to survive times when water is not available, especially imporant in cold climates
3. Enormous chemical diversity provides protection
from herbivores
4. 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 sporangia

Origin of sepals - probably derived directly from leaves

Origin 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.