Variation
 

I.  Mustards. 

    Question:  what do cauliflower, broccoli, cabbage and kohlrabi have in common?  Right, they are all members of the same species; Brassica oleracea.  This brings up two questions: (a) what is a species?; and (b) how did these varieties arise?

A.  Species Concept - How can such different looking plants belong to the same species?  Historically, biologists considered individuals to be in the same species if they looked alike.  However, over time it became apparent that this definition wasn't always sufficient since individuals that looked alike were incapable of reproducing while those that looked rather different, like cabbages and kohlrabi which looked very different, could interbreed.  This, the morphological species concept (look alike = same species) was modified to the biological species concept - indviduals that interbreed are the same species. 

 

B.  Variation.  Even though  we can cross breed broccoli and cabbage and produce fertile offspring, these plants look very different.  Thus, they exhibit lots of genetic variation.

 

II.  Genetic Variation  � genetic differences between individual in a population.

• Advantages � provides raw material for crop improvement and evolution

• Disadvantage � crop not uniform; therefore, may be more difficult to grow, harvest, uniformity of product

• Balance � must strike so that there is a uniform crops but we maintain a source of genetic variability in the event needed in future, to increase yield, select against disease, grow under suboptimal conditions (i.e., saline).

• Variation preserved:  (1) in the wild, (2) maintained by farmers globally, (3) seed banks

III. Sources of genetic variation

 
A.  Sexual recombination � process of shuffling up genetic information during sexual reproduction.

Comparison of Inbreeding and Outcrossing
	Inbreeding	Outcrossing
Seed production (number)	higher	lower
Likelihood of seed production	guaranteed	not guaranteed
variability	less, more uniform	higher
yield	lower	higher
uses	produce parental stocks	produce product (hybrid vigor)

        

 

B.  Mutation � ultimate source of all new genes/genetic material; change in DNA.  Occurs naturally, low frequency; breeders can be induced by chemicals or radiation.

 

C.  Polyploidy � additional sets of chromosomes.  Fail to separate properly during cell  division, common in plants, larger plants, more seeds/fruit.  Many of our common crops are polyploid (i.e., wheat)

 

D.  Geographic variation � natural variation in the wild as a result of plants evolving to natural conditions.

 

 

IV.  Techniques to increase genetic diversity

1. Traditional breeding programs � artificial selection.  Breeders select naturally occurring mutants and polyploids, look for wild plants with new sources of genes)

2. Mutations � induced by chemical mutagens or radiation (esp. in 70�s)

3. Tissue culture � mutations show up in plants grown

4. Haploid plant culture � some natural, can be obtained via tissue culture

5. Genetic Engineering/Transgenic plants - insert new genetic information into plants; create new combinations of genes in the laboratory