Saturday, December 20, 2008

Evolution and me: a personal story (part 8)

Part 8: Observing is believing?

In 1870, a shipment of nursery stock from China arrived in San Jose, California, carrying an unsuspecting menace that would cause devastation for farmers across Northern America. The small fruit-eating insect, now known as the San Jose scale, soon spread rapidly across United States and Canada, destroying orchid trees as it went. Carl Zimmer, in his book, Evolution, recounts the fascinating story behind this small pest (pages 241-243).

At first, farmers could totally eradicate outbreaks of the San Jose scale by spraying their crops with a mixture of sulphur and lime. However, by the turn of the century, a number of farmers began to realise that this pesticide was beginning to loose its potency; a few scales would survive the spraying and then breed rapidly. Farmers in Washington state were convinced that manufacturers were supplying poor quality pesticide, so they built their own factory to produce a 'pure' form of sulphur-lime. But even their home-made concoction failed to curb the spread of scales. The question was: why did such an effective pesticide became totally useless in just a few years?

Enter the entomologist A.L Melander. In 1912, after studying the San Jose scale problem, he realised that the scales were developing a resistance to the pesticide. How? His short answer was that evolution was happening.

The long answer is as follows:

  1. In the scale population, there were a few individuals that possessed a mutation that made them resistant to the sulphur-lime pesticide.
  2. Under normal circumstances, the mutation would remain in only a few individuals. However, when farmers started applying the pesticide, most of the non-resistant scales – initially comprising a majority of the population – were killed off. But the resistant scales carrying the mutation survived.
  3. The surviving individuals would then breed with each other and other non-resistant scales to produce resistant offspring. Over time, with consistent application of the pesticide killing off non-resistant scales, the mutation would spread throughout the entire population, eventually making the pesticide ineffective against a growing number of resistant scales.

This is a classic example of Darwin's theory of natural selection, beautifully expressed in an example that was experienced by many people. And this scenario has occurred many times in humankind's struggle against germs and insects. One only has to think about the constant need to change medications in order to combat Malaria, to recent scares over drug resistant strains of Tuberculosis. These and many other cases show that biological evolution does happen.

But there is one very valid objection in response to this. One can argue that all I'm describing here is microevolution, the small changes that occur within species, and what I've argued above does not support macroevolution, the large changes that result in new species.

My response to this? Watch this space . . .

Next post: The unbelievability of change
Return to the table of contents for 'Evolution and Me'


Nikeyo said...

I would look forward to your ideas concerning macroevolution, that was going to be my comments really.

But it is a clear portrayal of microevolution and the fact that it's all around us, even happening in our bodies with all the little microbes. It's a theory still widely disbelieved by fundamental Christians for some reason. Although they'll agree that the body adapts to organisms, they refuse to accept the word microevolution which basically means the same thing.

Harmen said...

A minor criticism: I understand that you're building an argument and probably are playing devil's advocate here, but the 'microevolution only' stance really is not "very valid" by a long stretch. But I'm sure you'll set things straight in your next post :)

PS: "The unbelievably of change"...?

Lui said...

I think that, once people accept microevolution, they have pretty much given the game away. Macroevolution, according to the consensus view, is the bread and butter of macroevolution; the latter is simply the consequence of the former taking place over a long period of time between two (or more) divergent populations of a species. The process whereby new species are formed - speciation - has been confirmed, and if you accept speciation, then you've given the game away even more, for now what would stop the descendants of new species from becoming so different over an even longer period of time that they would no longer be recognised as belonging even to the same family, class, and so on? There is a continuum of similarity that is visible among extant organisms, and this attests to the branching process of evolution. Of course, one shouldn't rely on this logical reasoning to "prove" macroevoultion. But this reasoning does have the virtue of at least showing that there is nothing inherently implausible about macroevolution. One would still be right to demand some actual physical evidence to verify it, but that's been provided in spades. I really look forward to the next installment.

Anonymous said...

Lui said, "The process whereby new species are formed - speciation - has been confirmed."

What do you point to as confirmation of speciation?

Lui said...

Anonymous asked: "What do you point to as confirmation of speciation?"

That's an excellent question. The answer is basically to be found in numerous investigations that have demonstrated it happening while it was happening or have documented phenomena that are consistent with what we would expect to find if it had happened (such as hybridisation zones in which individuals from the two species/sub-species were breeding and their offspring exhibited reduced fitness. This is what would be expected of populations that have recently diverged and where genetic and other differences have accumulated, making any offspring resulting from a cross-mating less viable. Since mating costs time and energy, there will be selection for enhanced discriminatory powers, especially among females since they pay a larger cost per mating. In many species males can more readily afford to mate with many females and a few hybrid offspring might not impose such a large inclusive fitness cost. The basic point is that speciation is a self-reinforcing process).

From talkorigins:

"1. New species have arisen in historical times. For example:

* A new species of mosquito, Culex molestus, isolated in London's Underground, has speciated from Culex pipiens (Byrne and Nichols 1999; Nuttall 1998).

* Helacyton gartleri is the HeLa cell culture, which evolved from a human cervical carcinoma in 1951. The culture grows indefinitely and has become widespread (Van Valen and Mioarana 1991)

* Several new species of plants have arisen via polyploidy (when the chromosome count multiplies by two or more; de Wet 1971). One example is Primula kewensis (Newton and Pellew 1929).

2. Incipient speciation, where two subspecies interbreed rarely or with only little success, is common. Here are just a few examples:

* Rhagoletis pomonella, the apple maggot fly, is undergoing sympatric speciation. Its native host in North America is Hawthorn (Crataegus spp.), but in the mid-1880s, a new population formed on introduced domestic apples (Malus pumila). The two races are kept partially isolated by natural selection (Filchak et al. 2000).

* The mosquito Anopheles gambiae shows incipient speciation between its populations in northwestern and southeastern Africa (Lehmann et al. 2003).

* Silverside fish show incipient speciation between marine and estuarine populations (Beheregaray and Sunnucks 2001).

3. Ring species show the process of speciation in action. In ring species, the species is distributed more or less in a line, such as around the base of a mountain range. Each population is able to breed with its neighbouring population, but the populations at the two ends are not able to interbreed. (In a true ring species, those two end populations are adjacent to each other, completing the ring.) Examples of ring species are

* the salamander Ensatina, with seven different subspecies on the west coast of the United States. They form a ring around California's central valley. At the south end, adjacent subspecies klauberi and eschscholtzi do not interbreed (C.W. Brown n.d.; Wake 1997).

* greenish warblers (Phylloscopus trochiloides), around the Himalayas. Their behavioral and genetic characteristics change gradually, starting from central Siberia, extending around the Himalayas, and back again, so two forms of the songbirds coexist but do not interbreed in that part of their range (Irwin et al. 2001; Whitehouse 2001).

* the deer mouse (Peromyces maniculatus), with over fifty subspecies in North America.

* many species of birds, including Parus major and P. minor, Halcyon chloris, Zosterops, Lalage, Pernis, the Larus argentatus group, and Phylloscopus trochiloides (Mayr 1942, 182-183).

* the American bee, Hoplitis (Alcidamea) producta (Mayr 1963, 510).

* the subterranean mole rat, Spalax ehrenbergi (Nevo 1999).

4 Evidence of speciation occurs in the form of organisms that exist only in environments that did not exist a few hundreds or thousands of years ago. For example:

* In several Canadian lakes, which originated in the last 10,000 years following the last ice age, stickleback fish have diversified into separate species for shallow and deep water (Schilthuizen 2001, 146-151).

* Chichlids in Lake Malawi and Lake Victoria have diversified into hundreds of species. Parts of Lake Malawi that originated in the ninteenth century have species indigenous to just those parts (Schilthuizen 2001, 166-176).

* A Mimulus species adapted for soils high in copper exists only on the tailings of a copper mine that did not exist before 1859 (Macnair 1989)."

References can be found on the site. I had to copy all this from a book because the URL was unavailable.

From more info go here.

Also, this is pretty remarkable. It's about a population of lizards that has undergone quite dramatic changes in a very short period of time even without significant genetic differences to the source population. It's an example of phenotypic plasticity, which some biologists now think has played a more important role in evolution than previously thought.

Kevin Parry said...

Harmen wrote
PS: "The unbelievably of change"...?

Hi Harmen. Thanks for pointing that out. I’ve corrected it in the post.

Hi Lui. Thanks for the information on speciation. It’s good to note that not only has evolution been observed, but also speciation.

All the best