One of the best documented examples of natural selection in modern times is the English Peppered Moth. Typically, this moth is whitish with black speckles and spots all over its wings. During the daytime, Peppered moths are well-camouflaged as they rest on the speckled lichens on tree trunks. Occasionally a very few moths have a genetic mutation which causes them to be all black, so they are said to be melanistic. Black moths resting on light-colored, speckled lichens are not very well camouflaged, and so are easy prey for any moth-eating birds that happen by. Thus, these melanistic moths never get to reproduce and pass on their genes for black color. However, an interesting thing happened to these moths in the 1800s. With the Industrial Revolution, many factories and homes in British cities started burning coal, both for heat and to power all those newly-invented machines. Coal does not burn cleanly, and creates a lot of black soot and pollution. Since lichens are extremely sensitive to air pollution, this caused all the lichens on city trees to die. Also, as the soot settled out everywhere, this turned the tree trunks (and everything else) black. This enabled the occasional black moths to be well-camouflaged so they could live long enough to reproduce, while the “normal” speckled moths were gobbled up. Studies done in the earlier 1900s showed that while in the country, the speckled moths were still the predominant form, in the cities, they were almost non-existant. Nearly all the moths in the cities were the black form. It was evident to the researchers studying these moths that the black city moths were breeding primarily with other black city moths while speckled country moths were breeding primarily with other speckled country moths. Because of this, any new genetic mutations in one or the other of those populations would only be passed on within that population and not throughout the whole moth population. Additionally, because the city and country environments were different, there were different selective pressures on city vs. country moths that could potentially drive the evolution of these two populations of moths in different directions. The researchers pointed out that if this were to continue for a long enough time, the city and country moths could become so genetically different that they could no longer interbreed with each other, and thus would be considered distinct species. In this case, what actually happened is that the people of England decided they didn’t like breathing and living in all that coal pollution, thus found ways to clean things up. As the air became cleaner, lichens started growing on city trees again, thus the direction of the selective pressure (birds) was once again in favor of the speckled moths. By now, English cities, as well as countrysides, all have speckled moths, and all are interbreeding at random, thus were not separated for long enough to develop into separate species.However, an article in the 24 May 1999 issue of The Scientist 13(11) presents and discusses data which may refute this long-held idea.
As another example, back in the mid-1950s, a biopsy of cervical cancer was removed from a woman named Henrietta Lacks and grown in tissue culture. While Ms. Lacks died long ago, HeLa cells are a widely-cultured research “organism” available through a number of biological supply companies. Recently, an interesting issue has arisen regarding these cells: are they still “human?” While HeLa cells currently being grown in tissue culture are descendents of the original human cancer cells, by now they have mutated so much that it’s questionable whether they can still be considered “human” tissue.
One important thing thing to note here is that populations, not individuals, evolve: individuals can have genetic mutations, but unless those mutations are passed along to offspring and thus into the gene pool of the population, no real change has occurred. An individual who has a mutation but does not pass it on (for whatever reason) has no long term effect on the genes present in the population as a whole. Another important point is that natural selection operates on the genetics of a species, including whatever genes affect instinctive behavior patterns as well as those affecting the looks of the individuals. Thus, genetics, looks, behavior, and external environmental factors are all interwoven. Sometimes, there are tradeoffs between various genes and selective pressures. For example, the bright red color of a male Cardinal might be very attractive to a female, such that the brighter a male is, the greater his chances of mating, but that same bright red color, especially against a snowy background, makes him much more visible to predators.
Another factor relating to populations as the unit of evolution is that the various alleles for a species’ genes are not equally present in all populations. For any given gene, the number or percentage of each allele in the population can be calculated, and varies among different populations of organisms. This is different from the genetics and Punnett squares we studied last quarter! You may recall from last quarter that “bad” genetic mutations like sickle-cell, cystic fibrosis, Tay Sachs, etc. are more prevalent in certain ethnic groups. As another example, you may recall last quarter’s discussion of PTC (phenylthiocarbamide) paper. People with the dominant allele for the tasting gene (TT or Tt) are able to taste this substance, and react negatively to its bitter taste, while people with the recessive allele (tt) find this substance to be tasteless (and hopefully you still remember how to do a Punnett square for any two given individuals?). However, for the general population of North America, about 70% of the population are tasters, while about 30% are non-tasters. From this, someone who studies population genetics, could calculate that therefore, in a group of 100 people (thus 200 alleles — remember?), there would be 110 t alleles and 90 T alleles, so 20 people would be TT, 50 people would be Tt, and 30 people would be tt. As another example, those of you who had lab last quarter may recall from the blood-typing handout that about 45% of the population of the U. S. is type O, 42% type A, 10% type B, and 3% type AB. However, these percentages are not the same for all populations. For example:
|Ethnic Group||Percentage of the People|
|Type O||Type A||Type B||Type AB|
| 42.6 ||57.4 || 0.0 || 0.0|
|Berliners || 36.5 ||42.5 ||14.5 || 6.5|
| 30.7 ||25.1 ||34.2 ||10.0|
|Native American |
| 23.5 ||76.5 || 0.0 || 0.0|
|Native American |
|100.0 || 0.0 || 0.0 || 0.0|
|Native American |
| 97.4 || 2.6 || 0.0 || 0.0|
|Siberian Buriats || 32.4 ||20.2 ||39.2 || 8.2|
| 30.6 ||30.3 ||29.1 ||10.0|
There are a number of factors that can influence the percentage of the alleles in a given population, and random change in the genetic make-up of a population can be due to a number of factors:
All of these are subjected to natural selection. Some of the alleles code for certain body features or behaviors which are better suited to the environment than others. Organisms with those alleles are more likely to reproduce and pass on those alleles. Other alleles are not as well suited, and an organism with that genotype dies or is not able to reproduce, so the alleles never get passed on. Over time, this leads to speciation, the division of one species into two or more species. By definition, a species is a reproductively-isolated population of organisms. In other words, members can interbreed with each other, but not with non-members of that species.
While all this has been occurring, another influencing factor has been the fact that the continents have been moving at a rate of about 5 cm per year or so. The movement of the continents is called continental drift, and the study of this movement is called plate tectonics. About 200 to 180 million years ago (mya), all of the continents were united in one large land mass called Pangea, thus the ocean was one big ocean called Panthalassa. This, along with the ubiquitous tropical climate, was significant because any animals that were present then could spread everywhere. This time period has been called the “age of dinosaurs” because of their prevalence. There were also large fern-type plants, conifers (relatives of pine trees), and various insects like roaches. Around this time, the first mammals, the marsupials came into existance, and began to spread everywhere. By about 130 to 120 mya, Pangea broke into two land masses. Laurasia was the northern continent (consisting of current North America, Europe, and Asia), and Gondwana was the southern land mass (consisting of South America, Africa, India, Australia, and Antarctica). Before this split, marsupial-type mammals had spread everywhere, but around the time the continents were drifting apart, the placental mammals evolved in Laurasia, and began to spread, outcompeting the marsupials wherever they became established. Everywhere the placental mammals became established, nearly all of the marsupials died off. For example, here in North America, the opposum is the only marsupial mammal left. As the continents moved, eventually the chunk that is Antarctica and Australia broke off from the rest of Gondwana, taking its marsupials with it, but this split occurred before the placentals ever got that far. Eventually, North America and Eurasia split apart (after placentals had become well-established there), and land bridges formed between South and North America (Central America) and between Africa and Europe, so the placental mammals could spread southward and outcompete the marsupials on the southern continents. However, since Antarctica-Australia had already separated, the placentals couldn’t get there. Eventually, Antarctica and Australia split. Antarctica drifted to the South Pole and got too cold for any mammals to live there (so they all died). Australia drifted northward, closer to the Equator and a warmer climate, providing a wonderful “protected” habitat for all of its marsupials to flourish, diversify, and speciate. Thus today, Australia has a large variety of marsupials that have diversified to fill all the niches occupied by placental animals in the rest of the world: kangaroo are grazers like cattle, koalas eat tree leaves, and there is even a wolf-like, carnivorous marsupial.
We humans like to group these species and organize them into categories. The only category that actually exists in nature is individual species — everything else is groupings we have invented based on similarities among the organisms we decided should be members of that group. For example, Monarch Butterflies are real creatures that actually exist. However if we speak of all butterflies, this is a grouping we humans have made based on those organisms having similarly-shapes wings with brightly-colored scales all over them. We also tend to categorize the categories. For example, we say that butterflies are part of the insects, and that insects are part of the animals. Biologists around the world use an official classification system invented by Linnaeus back in the 1700s. In this system (going from the top, down rather than bottom, up as in the previous example), all living organisms are first divided into Kingdoms.
Currently we recognize five kingdoms (which, hopefully you
recall from first quarter), which are:
In biology, each organism’s official name is its genus and its species names together, for example: Homo sapiens. This system of two-part scientific names was also invented by Linnaeus. These names are Latin or Latinized words, thus are a “foreign” language. It is customary, in any book or publication, to italicize anything that is in a foreign language (relative to the language in which the publication is written), and you have, hopefully, seen examples of this in various books you have read. This means that scientific names should also always be italicized, which is relatively easy to do with today’s computers and word processors. However, if you are writing something on a typewriter or by hand (like on a biology test — hint, hint!) the proper, official thing to do is to underline the scientific name to indicate that it is supposed to be italicized. Also, note that the first letter of the genus name is always capitalized, and the first letter of the species name never is, even if it was derived from a proper noun (Newspapers and magazines are notorius for getting this wrong! Don’t always trust what you see there). For example, there is a butterfly named Dryas julia, and even though the scientist named it after his lady friend, we don’t capitalize her name used as a species. Also, our local chickadees are know as Parus carolinensis, and note that the species is still lower case. Thus, the scientific name for humans, correctly spelled and italicized is Homo sapiens. Note also that, yes, the genus and species are two words, and that, yes, sapiens does, indeed, have an “s” on the end like “scissors,” “pants,” or “gas.”
Copyright © 1997 by J. Stein Carter. All rights reserved.
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