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ESC
200 - Spring 2003
Trees
in Our Environment
Prof: Linda Brubaker
EVOLUTIONARY MECHANISMS (web, ppt) LECTURE OVERVIEW
Basic statement:
Organisms do not remain constant over generations. New species arise from old ones, while others decline. These genetic changes are the result of organic evolution.
Key ideas:
Organic evolution results from the interaction of several processes: 1) mutation, 2) gene flow between populations, 3) random events, 4) mating preferences, and 5) natural selection--differential survival/reproduction in response to the environment. Natural selection is thought to be the driving force of evolution. It maintains genotypes that are successful in a given environment. If the environment remains stable for many generations, natural selection results in adaptation (having a repertoire of solutions for conditions that can be anticipated, based on past experience). Since many environments are constantly changing, species do not remain stable but undergo change as a function of evolutionary mechanisms. Species that inhabit spatially heterogeneous environments often show pronounced polymorphisms (many forms).
Important terms:
- mutations
- migration
- genetic drift
- natural selection
LECTURE OUTLINE
Introduction
We discussed wide variations within species
Some due to Environment, some due genetic make-up
Today we will examine processes that can give rise to genetic differences between populations (within population variation is due to inheritance)
This process is Evolution-- results in differences between populations we can see today-- results in changes in anygiven population over time
Evolutionary Mechanisms
1. Mutation-- instantaneous change in genetic material (ie change that can be inherited
many types (gene level, chromosome level)
simple change can have major effects on phenotype of whole individual
e.g., columnar, weeping, corkscrew
OR they may have no affect-- depends
Mutations are rare events (1/100,000) cell replications
SIGNIFICANCE-- TOO RARE TO CAUSE MAJOR CHANGES BY THEM SELVE-- BUT ONLY REAL SOURCE OF NEW GENETIC VARIATION
2. Migration of genes in and out of populations
adult plants don't move, but plants well adapted for moving genes around-- e.g., pollen, seeds (Betula, Alnus Salix, Populus-- well adapted for wind pollination)
SIGNIFICANCE-- brings in new genes or gene combinations, also makes neighboring populations similar
3. Chance events--
- Genetic drift in small populations (rare genes disappear because they just aren't passed down)
- Founder affect-- founding population has an unusual gene frequency and decendent populations will have that frequency.
SIGNIFICANCE-- important in small populations, founder effect may result in rapid change in gene frequencies.
4. Natural selection--
Genetic make up of population changes because some genes are passed down at faster rate than others because they improve the growth, survival, reproduction of those that posses them.
We might think of many examples of traits that may convey some advantage to individual to possess them.. e.g., Drooping foliage of Chamaecyparis nootkatensis, and short branches of Abies lasiocarpa; root suckering in Populus tremuloides-- good for regrowing after fire Thick bark of Pinus ponderosa and Pseudotsuga menziesii
SIGNIFICANCE-- thought to be most important evolutionary mechanism. Results in adaptation and ultimately in speciation
Contact Linda Brubaker at: lbru@u.washington.edu
[ University of Washington ] [ College of Forest Resources ]