Hi folks, I haven't posted here in a while because I got a bit sick of the evolution/creation debate, but I've just installed the
BBCode extension to my firefox browser, and decided to try it out. I also have a blog,
Vapid Rants, where I might talk about evolution/creation once in while, I haven't decided yet.
But back to the thread:
A very good example of evolution in the laboratory (at least I think so) is the story of Chlorella vulgaris:
quote:
Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga, Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic flagellated protist (’flagellate‘). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella first formed globose clusters of tens to hundreds of cells. After about 10—20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form indefinitely in continuous culture and when plated onto agar. These self-replicating, stable colonies were virtually immune to predation by the flagellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.
Niet gevonden - Not found
quote:
Chlorella vulgaris is a common unicellular green alga that is used as a "lab rat" in labs throughout the world. We've grown the same strain of it for thousands of generations on agar and in liquid culture without it losing its unicellular morphology. Dozens to hundreds of labs have done this. Steady-state unicellular C. vulgaris cultures were innoculated with the predator Ochromonas vellesiaca, a phagotrophic flagellate. Within less then 100 generations a multicellular form of the Chlorella became dominanat in the culture. (Boraas 1983b, Boraas et al. 1998). The alga first formed globose clusters of tens to hundreds of cells. After 10-20 generations in the presence of the flagellate, eight-celled colonies predominated. These colonies retained the eight-celled morphology indefinitely in continuous culture and when plated onto agar. The basis of the change appears to be a change in the cell wall. Cell division in normal Chlorella occurs within the cell wall of the maternal cell. The cell undergoes 1-4 divisions to form 2-16 daughter cell. This is followed by a split in the mother cell wall and dispersal of the neonatal cells. In a cuture, empty mother cell walls are interspersed with whole cells at a ratio of about 1:4. Empty mother cell walls are not found in cultures of the multicellular form. The colonies are enclosed in a "membrane" which appears to be modified cell wall material.
As was seen in the bacterial cases, this mutation provided Chlorella with resistance to predation at the cost of growth rate. Neonatal colonies are barely small enough for Ochromonas to engulf. After they have grown slightly they are to big to be eaten. In the presence of the predator, the colonial form of Chlorella displaces the unicellular form and persists. When the predator is not present, the unicellular form displaces the colonial form. This makes sense as the colonial form has less surface area exposed to the environment available for nutrient uptake than the unicellular form has.
http://www.talkorigins.org/faqs/mutations.html
quote:
Boraas (1983) reported the induction of multicellularity in a strain of Chlorella pyrenoidosa (since reclassified as C. vulgaris) by predation. He was growing the unicellular green alga in the first stage of a two stage continuous culture system as for food for a flagellate predator, Ochromonas sp., that was growing in the second stage. Due to the failure of a pump, flagellates washed back into the first stage. Within five days a colonial form of the Chlorella appeared. It rapidly came to dominate the culture. The colony size ranged from 4 cells to 32 cells. Eventually it stabilized at 8 cells. This colonial form has persisted in culture for about a decade. The new form has been keyed out using a number of algal taxonomic keys. They key out now as being in the genus Coelosphaerium, which is in a different family from Chlorella.
http://www.talkorigins.org/faqs/faq-speciation.html
"Knowledge is Power" - Francis Bacon