• within individual trees
• within populations of a given species
• between populations of a species within a given region
• between populations in different regions
• between species in the same genus

Some of this variation is due to environment and some to genetic make-up
Last lecture: process leading to genetic differences among populations = Evolution
Today: process leads to different species = Natural Selection and Geographic Isolation

First let's define what a species is.
We have general idea that they have some identity as a unit of biological organization-- consistent characteristics, manageable unit

Formal Definition: holds true for both plants and animals

Groups of actually or potentially interbreeding* populations** that are reproductively isolated*** from other such groups (Ernst Mayr-- famous evolutionary biologist at Harvard)

LET'S ASK 2 QUESTIONS--

1. HOW DO SPECIES ARISE?
2. HOW ARE THEY MAINTAINED OVER TIME?

• natural populations are made up of genetically different individuals
• environments of natural populations differ
• natural selection will act on different populations,
• resulting in different gene frequencies in different populations-- because the environments of different populations differ (eg. date of last frost would select for individuals with different bud breaks)
• 
• if populations are separated by geographic barriers that stop gene flow between populations (migration), they populations can continue to diverge genetically

  

  Geography of earth has been very dynamic at time scale that speciation occurs

• eventually populations will become so different genetically that they cannot interbreed
• then 2 new species have been formed (or one diverged from central population (P. strobus and P. monticola are good examples)

NOTE-- This is a continuous process-- (not either/or condition) We should expect to find all stages of the process within the real world-- good example is Salix-- believed to be a group that is undergoing speciations with many intermediate forms

1. Ecological separation-- same area, but on different types of sites, so they don't exchange genes
2. Temporal separation-- same area, similar sites, but flower at different times (P. sabiniana and P. coulteri)
3. Mechanical separation-- same area, sites and flowering time, but pollen not transferred effectively from anther to stigma of different species (usually for insect pollinated plants)
4. pollination incompatibility-- pollen can be transferred to stigma of other species, but prevented from germinating because of chemicals produced by 2nd species that prevents pollen germination
5. hybrid inviability-- pollen tube can grow and pollination occurs-- but gene combinations from different parents are not well enough coordinated to make a viable (functional) individual and the embryo dies
6. hybrid sterility-- hybrid (cross between different species) is viable, but formation of sex cells is impossible (chromosomes not similar to line up in pairs during meiosis)-- e.g., mule (donkey X horse), rabbish,

Principles of Nomenclature

a. if the original parents are known, both names are used with the female parent first, e.g., Populus tremuloides x P. alba;
b. if the original parents are not known, a new name is given, e.g., Platanus x acerifolia.

a. subspecies: e.g., Pseudotsuga menziesii subsp. glauca;
b. variety: e.g., Quercus garryana var. breweri
c. form: add the abbreviated f. before the form name, e.g., Pseudotsuga menziesii subsp. glauca f. pendula.

a. Chamaecyparis lawsoniana ‘Glauca’
b. Chamaecyparis lawsoniana cv. Glauca

Level of Classification Name (key features)

KINGDOM Plantae (many cells, photosynthesis)

DIVISION Spermatophyta (seeds plants)

SUBDIVISION Gymnospermae (naked seeds)

ORDER Coniferales (cone bearing)

FAMILY Pinaceae

GENUS Pseudotsuga

SPECIES Pseudotsuga menziesii (Mirb.) Franco

SUBSPECIES Pseudotsuga menziesii

subsp. glauca (Beissn)