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Evolution and Diversity in Plants I - E col 182 – 4-7-2005. Re-downloaded at 7:10am on 4-7. Big Questions. What have been the important constraints and / or principles that have shaped the evolution of plants. Diversification Form and function.
Re-downloaded at 7:10am on 4-7
R.A. Fisher (1958)
Fundamental Theorem of Natural Selection
“Rate of increase in the mean fitness of a population is proportional
to the genetic variance in fitness”
In order for there to be evolution there must be genetic variation
Major ways genetic variation is introduced into populations
(1) Mutation (UV, random error)
(2) Genetic recombination (meiosis) – including ‘crossing-over’
(3) Immigration (into population)
But plants ‘do’ two additional ‘tricks’ that enhance genetic variation
(4) Polyploidy – an organism that has more than one complete set of the normal chromosome compliment
- most animals are diploids, many plants are polyploids
- occurs through processes such as chromosome duplication
(5) Hybridization – crossing of closely related taxa (usually between species within a genus)
Multicellularity evolved more than once!
-for plants, prokaryotic unicellular algae → multicellular algae → embryophytes
Multicellularity has several interesting advantages
Cells can be specialized – division of labor (requires communication and transport)
Organism can increase surface area for environmental exchange (access to more resources)
Organism can increase in size – better buffering of environmental extremes – live longer – access to ‘additional’ resources
-Single living protoplast of adjoining cells.
-Cell membranes (which line plasmodesmata)
are continuous from one cell to the next
-Water and small molecules may pass with relative ease (essentially through the whole plant).
Material flow may be modified by altering number and location of plasmodesmata
Figure 29.2 in the book
Sporophyte generation – from the zygote through the adult, multicellular, diploid plant.
Gametophyte generation - from the spore through the adult, multicellular, haploid plant to the gamete.
Charophytes (a group of green algae) appear to be the closest living relative of Embryophytes
These organisms now occupy the margins of ponds or marshes (meaning that the ‘jump’ to a terrestrial environment was in close proximity)
Rhizoids absorb water with filaments found on the lower surfaces gametophytes.
Several genera have both sexual and asexual reproduction
Asexual reproduction - by simple fragmentation of the gametophyte.
These groups all possess stomata that allow the uptake of CO2 and the release of O2, but they can be closed to prevent excessive water loss (in some groups).
Hydroid cells, in many mosses, are a likely progenitor of the water-conducting cells of the tracheophytes.
When hydroid cells die, they leave a tiny channel through which water can flow.
Roots had their origins as branches, either as rhizomes or aboveground portion of stems.
Early roots were simple structures that penetrated soil, branching and anchoring the plant (absorbing water and minerals?)
Belowground and aboveground environments are quite different.
The megaphyll is larger, and more complex found in ferns and seed plants.
May have arose from flattening of stems and development of overtopping (one branch differentiates from and extends beyond rest).