Plant Speciation – Part 1

Spring 2012

Major topics• Variation in plant populations and

species (1)• Gene flow and reproductive isolating

barriers (1)• Speciation mechanisms (2)• Species concepts (2)

Anagenesis

versus

Cladogenesis

A B C ED F

Cladogenesis

Cladogenesis

Cladogenesis

Anagenesis

Cladogenesis

Biological Variation

• All populations of organisms have inherent variation within them – a range of genetic variation of which part is expressed as phenotypic variation

• Influences of the environment can change how this variation is expressed = phenotypic plasticity

• Observations by biologists can be interpreted in a number of ways to determine the similarities or differences in groups of organisms, depending on which characteristics are measured or emphasized

• Need to understand speciation (the process) in order to understand patterns of diversity (and vice versa) and to provide a means to define species

Variation in Plant Populations and Species

One P. jeffreyiecotype isadapted toserpentine soils.

This ecotype hasless overallgenetic diversitythan ecotypesof this species adapted to morefertile soils.

Clinal variation within a species

Genetic variation withinand among species ofnative American canes

Mutation is the ultimate source of all genetic diversity.

Types of mutations• Point mutations (change in one base)• Insertions, deletions, inversions,

duplications of parts of a chromosome• Gains or losses of whole chromosomes

(aneuploidy)• Multiples of whole genomes (the full set of

chromosomes in the nucleus) (polyploidy)

Genetic Recombination

http://www.pbs.org/wgbh/nova/miracle/divide.html

Review of Mitosis and Meiosis

Genetic Drift: chance fixation of genes (alleles) in small populations

Generation 1 Generation 2 Generation 3(5/10 plants leave offspring)

(2/10plantsleaveoffspring)

Gene flow and reproductive isolating barriers

Figure 13.3A, B

Gene flow• Gene flow = exchange of genes (alleles)

between populations• In plants, occurs through the dispersal of

pollen or fruits/seeds• Expected to occur between populations of

the same species, but in plants also occurs between populations of different species (hybridization)

Usually measured in meters, but can occur over longer distances.

Gene flow• Gene flow within and between

populations of a species tends to maintain the cohesiveness of a species

• Lack of gene flow (due to reproductive isolating barriers) between populations is normally required for speciation to occur

Reproductive Isolating Barriers

Pre-mating

Ecological or habitatisolation

Temporal isolation

Behavioral isolation

Post-mating

Mechanical orphysiologicalisolation

Gametic isolation

Post-zygotic

Hybrid inviability

Hybrid sterility

Hybrid breakdown

See Table 19.1

Prairie Woodland

White lady’s slipper Yellow lady’s slipper

Habitat isolation

Temporal Isolation

Red = staminate plantsBlue = carpellate plants

Behavioral Isolation: Adaptation to different pollinators

Two species of orchids each with a different bee pollinator.

The labellum may be a “key” innovation driving diversification.

Variation in the orchid labellum

Two species of Aquilegia(Ranunculaceae)

Aquilegia formosamesic sites < 3,050 m

Aquilegia pubescensexposed, xeric sites > 2,750 m

Columbines(Aquilegia)in California

Habitat isolation & floral isolation

Post-mating: Mechanical or physiological isolation

See Ch. 13: 574-576.

Beetle pollination is relatively unspecialized andprobably ancestral for angiosperms.

Bee Pollination

-showy, colorful (blue, purple, yellow) flowers-fragrant-day-flowering-bilateral landing platform-nectar and/or pollen rewards

Moth pollination

-white or pale, usually large flowers-sweet, strong scent-nectar reward-no nectar guides but may have nectar spurs-night- or dusk-flowering

http://vimeo.com/7048122

Pistil-packing mama

Yuccaand theYucca moth

Fly pollination

-brown or maroon flowers-fetid odor (rotting meat)-day- or night-flowering-usually no reward-some operate as trap flowers

Bird pollination

-brightly colored, often red flowers-no scent-day-flowering-usually copious nectar reward-often tubular corolla, often with an inferior ovary

Bat pollination

-usually large, whitish or colorful flowers-musky, strong scent-night-flowering-usually copious nectar and/or pollen reward

Moraceae – The Fig and The Fig Wasp

Wind pollination

-small, numerous often unisexual flowers with reduced or absent perianth-large quantities of pollen, individual grains smooth-feathery styles-no scent

Breeding Systems• Outcrossing = fertilization between

different individuals; depends on ability to screen pollen by the stigma and style (incompatibility)

• Uniparental reproduction– Self-fertilization (pollen from a flower fertilizes

ovules of the same individual)– Agamospermy (production of seed without

fertilization)

Breeding systems are not necessarily mutually exclusive!

Early season, open, cross-pollinated flowers in Viola

Later season, closed, self-pollinated flowers in Viola

http://www.youtube.com/watch?v=4YQ5q1cjEU4