The Evolution of Seed Plants

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Clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers.

This is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations.

Slide 38

Can Flower Shape Influence Speciation Rate?

Common ancestor

Radial symmetry (N = 4)

Bilateral symmetry (N = 15)

Compare numbers of species

Time since divergence from common ancestor

“Radial” clade

“Bilateral” clade

3,000

2,000

1,000

0

EXPERIMENT

RESULTS

Mean difference in number of species

Slide 39

Human welfare depends greatly on seed plants

No group of plants is more important to human survival than seed plants.

Plants are key sources of food, fuel, wood products, and medicine.

Our reliance on seed plants makes preservation of plant diversity critical.

Slide 40

Products from Seed Plants

Most of our food comes from angiosperms. Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% of the calories consumed by humans.

Modern crops are products of relatively recent genetic change resulting from artificial selection.

Many seed plants provide wood.

Secondary compounds of seed plants are used in medicines.

Slide 41

Slide 42

Threats to Plant Diversity

Destruction of habitat is causing extinction of many plant species.

Loss of plant habitat is often accompanied by loss of the animal species that plants support.

At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years.

Slide 43

Summary

Reduced gametophytes

Microscopic male and female gametophytes (n) are nourished and

protected by the sporophyte (2n)

Five Derived Traits of Seed Plants

Male gametophyte

Female gametophyte

Heterospory

Microspore (gives rise to a male gametophyte)

Megaspore (gives rise to a female gametophyte)

Ovules

Ovule (gymnosperm)

Pollen

Pollen grains make water unnecessary for fertilization

Integument (2n)

Megaspore (2n)

Megasporangium (2n)

Seeds

Seeds: survive better than unprotected spores, can be transported long distances

Integument

Food supply

Embryo

Slide 44

Plant Evolutionary Relationships: Clades