Tuesday, November 30, 2010

Chapters 11 and 12: Geographical Distribution and Geographical Distribution continued

Darwin spends two chapters on geographical distribution, rather than just making one forty page chapter on the subject. Without the knowledge of plate tectonics as mentioned in the last post, Darwin didn't really have a scientific context fro the similarities of species that he was seeing in distant lands other than they must have originated from a common ancestor and by one set of means or another managed to migrate to present locales. It was known based on the geological record that the sea level had changed over Earth's history, but the concept that the continents had undergone massive shifts wasn't widely accepted until the mid 20th century.

Darwin suspected that wide spread species began to divide in to variants specialized to a specific location and climate, thus branching into distinct species.  Therefore, all similarities between species could be accounted for by inheritance and similarities were products of selection. This fits with his theories and he strongly believed that all variants despite their present day location, had stemmed from a common ancestor. Placed into the context of modern understanding, this is possible, but at the time, Darwin faced a difficult task of explaining how such widespread migration across land and sea could have happened. However, despite this difficulty he reasoned that to reject the idea of migration and to accept separate acts of creation would be to accept miracles over his entire theory.

Darwin covers Charles Lyell's (the father of geology) concept of means of dispersal. This idea is that migration has been greatly affected by climate change which corresponds to sea level changes. According to this theory, continents and islands were once connected to each other by land bridges that are now covered by ocean. This is true of some land masses (Eurasia and North America were connected by a land bridge that allowed for human migration some 10,000 years ago) but this theory is limited to the idea that the continents don't move. Darwin question Lyell's assertion that all islands were once connected to mainland, and rightly so, since many have been formed by underwater volcanoes.

Darwin believed that plants were better at dispersing themselves than mammals, citing plants that are found around the world and that no continent shares a common mammal (at least one known at the time). Darwin even details his own personal experiments at seeing how well different seeds and other plant material handled sea-water travel to support the idea that seeds could float from the mainland to an island. Other means include seeds on floating driftwood birds transporting seeds by ingesting the fruits of plants, and then fly to islands to deposit them. Darwin even went so far as to investigating bird droppings on his property for evidence of seeds (hardy seeds were unsurprisingly better preserved than soft ones). Darwin also proposed transport via iceberg during glacial periods.

The concept of climate change is explored further while Darwin analyzes the impact of the ice age on different species in Europe and how it forced adaptation or migration south for many species. Darwin also alludes to the concept of the Bering Strait, "We can further understand the singular fact remarked on by several observers, that the productions of Europe and America during the later tertiary stages were most closely related to each other than they are at the present time; for during these warmer periods the northern parts of the Old and New Worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the inter-migration of their inhabitants," (Darwin, 325). It's very striking that in the 1800's (and even earlier) modern concepts were already being predicted and hypothesized thus laying the ground work for modern science.

Above was chapter 11, detailing acts of creation against dispersal from a common source, with attention to plants. Chapter 12 concerns fresh water species and islands.

Specifically with fresh water fish, Darwin inferred that those isolated in Europe would not find their way to North America due to the distance. However, similar species within closer proximity could have been dispersed through changes in water level such as flooding. Again, Darwin repeats the idea of birds transporting plant matter.

For islands, which Darwin had an intimate working knowledge from his travels on the H.M.S Beagle, Darwin proposed that species found in such isolated areas stemmed from relatives on the mainland. Certain species are more able to get to far islands than others, for example birds and bats are able to travel a greater distance than terrestrial mammals.

Finally, Darwin speaks of his Galapagos studies. Namely, he discusses the variations observed on different islands, mentioning specifically birds and plants and that differences between them and mainland species have been generated through selection.

Thursday, November 18, 2010

Chapter 10: On the Geological Succession of Organic Beings

In this chapter, Darwin explores the known evidence for evolution in the fossil record. Through the fossil record, Darwin notes that the rate of change is not the same for every species, and that over long periods of time some forms seem not to change at all.

Darwin claims that land dwelling creatures seem to have changed more than those in the sea, citing that "higher" creatures change more than "low" creatures. That is to say that Darwin suspected that more complex creatures, due to their complexity and opportunities for variation are more likely to change due to environmental factors than more primitive creatures. This isn't necessarily due to Darwin's qualitative assessment of high and low creatures, but rather that highly complex creatures are often very specialized and would necessarily have to change in response to natural selection. The most basic and primitive bacterial extremophiles have not changed tremendously because they can live just about anywhere without too much selective survival.

One rule that Darwin states is that groups of species cannot simply reappear on the fossil record after a lapse of time. Explanations for this could be poor preservation or geographic migration in and out of a region. Basically once something is totally extinct, it's not going to magically reappear. In this chapter Darwin addresses the concept of extinction in detail. Darwin concluded from fossil evidence that most species gradually disappear over time as opposed to disappearing in large number due to catastrophes. While there have been several mass extinction events in earth's history, the appear to be more the exception than the norm. Most mass extinctions were not sudden, and instead drawn out over a long period of time. The Discovery Channel has an interactive website detailing mass extinctions and their likeliest causes.

Darwin predates the theory of plate tectonics by about a century, however in his time it was recognized that certain rock deposits and formations were located in different, seemingly unconnected areas and having similar fossil groups within them. Darwin saw this as parallelism, because it was probably he (and other geologists of the time) had no concept of the continents being in different positions throughout the ages. Instead of convergent evolution, it is more likely that similar forms do share a common ancestor and the population was geographically isolated from each other during continental drift, causing two new branches in the lineage.

Darwin also summarizes the idea of ancient forms being "lower" than modern forms. However, he notes that naturalists were still arguing the concept of what constituted high and low. Darwin simply decided to define it as modern forms were increasingly complex over their ancient forms.

Tuesday, November 2, 2010

Chapter 9: On the Imperfection of the Geological Record

Darwin dedicates this chapter as a sort of extension of chapter 6 (which discussed possible criticisms of his theory) specifically on the geological record. In context, the knowledge of the geological record in Darwin's time was not as large as it is today.

Darwin's main dilemma was that the fossil record was chock full of transitional forms. He knew that presently, it wouldn't be possible to identify transitional forms since it cannot be clear what they're transitioning into. He also recognized that similar existing species weren't transitioning into each other, but rather had a common ancestor that was different from both of them. "So with natural species, if we look to forms very distinct, for instance the horse and tapir, we have no reason to suppose that links ever existed directly intermediate between them, but between each and an unknown common parent," (Darwin, 270). The reason in which it is unlikely there would be intermediates between two living species is that both species would be subject to natural selection over the same time span and one would have to remain unchanged and the other would have had to undergone extensive change.

While it was understood in Darwin's time, largely due to Charles Lyell's research, that the Earth was much older than the Bishop Usher estimate of 6000 years. Yet it was nowhere near the modern accepted age of 4.5 billion years. Therefore, Darwin had understandable questions of whether the Earth was actually old enough to support his theory. The deepest strata uncovered in England was the Paleozoic strata.

A quick lesson on the geological time scale:
The Paleozoic era represents at it's deepest point 540 million years ago. Older than that is considered Precambrian (Stanford). In the last 50 years, the study of Precambrian age fossils has expanded and the oldest known life (microscopic single cell organisms) currently known comes from the Archean era (Schopf).

Fossilization is not an easy process, and many factors determine whether or not an individual is preserved. The specimen must be buried and petrified over time, however the remains may easily be destroyed or scattered by numerous processes (Stanford). The number of fossils known in Darwin's days were much smaller than what has presently been found. Darwin considered the paleontological record to be in a sorry state. Darwin correctly assumed that very few creatures are actually fossilized.

Another factor Darwin considered was the migration of animals to new areas and that an intermediate form may not be found near present day localities of species. He spends most of the chapter summarizing the known geological record and predicting why certain specimens (such as transitional forms) had not been found.

He also muses that even if a transitional form was found, it may not be recognized as such and variations can be categorized as separate species, leading to redundancy and confusion concerning relationships. This is still a concern today, with certain fossils being mislabeled, or one species given several names.

Darwin's main concern was the sudden appearance of complex fossils in the geological record, because for his theory to work, there would have to be more primitive forms before them. At the time, no fossils older than the Silurian stratum had been found, but as I mentioned earlier, Precambrian fossils have been discovered.

Monday, November 1, 2010

Evolution and the Media

I prefer to get most of my news online rather than from the TV these days, and one thing that I've noticed over the past 2 years since starting to study human evolution is that the media tends to skew evolution stories towards a sensational angle. Every fossil primate is toted as a possible human ancestor. Most recently I've found multiple stories about the same set of fossils found in Libya that indicate that primates likely originated in Asia, not Africa. Apparently this is raising a lot of controversy (although, in a class I had last year, Asia as the cradle of primate origin was taught as the accepted view of the paleoanthropology community). These fossils are almost 40 million years old, and while they're certainly human relatives, to call them important discoveries for the field of human evolution is kind of off base.

Yes, plesiadapiforms and other early primates are part of human evolution and I don't want to diminish the importance of such finds, but the media shouldn't be calling them early humans. Humans and chimpanzees didn't even split until 7 million years ago, so to call anything older than that even remotely human is misleading. The news media also tends to group all hominid fossils as humans. This may be personal bias, but I don't consider Homo erectus human, and I certainly don't consider any of the Australopithecus humans, at least not in the sense that the news media is making them out to be.

I realize that my concept of what is human is kind of limited (specifically to anatomically modern humans) but at least I have a clear definition. When the "Ida" fossil was publicized in 2009 a few news sources hailed it as a missing link in human evolution. Yeah, I guess it's part of human evolution but it's more relevant to early primate evolution and the link between lemurs and monkeys. It's very misleading to present anything that far back on the evolutionary timeline as human. That would be akin to me saying that I'm President Obama's cousin, I mean very distantly it's possible that many many many many generations ago we had a common ancestor but it's not like we're celebrating Thanksgiving together.

The purpose of the news is not to inform but rather to make money. Placing a headline that read something about human ancestors is much more likely to be read than a title about what the article is actually about. Ancient monkeys don't sell as well as ancient "humans". Humans are pretty species-centric, and at least the media is getting people to read about evolution (even though they have to trick them into it). Face it, writing about Ida as a lemur-monkey link is far less entertaining and attention grabbing than hailing it as a crucial find for human evolution.

As a whole, it seems most people just don't care about fossils (unless they're mega fauna) or most science in general. The media can help to make certain fields seem sexier, and controversial, but it can also hurt science by misrepresenting it to the public. It's even more damaging when certain theories aren't implemented into the science curriculum of K-12 schools because it may offend people. Science is impartial. Finding facts offensive doesn't negate their truth. This leads to some people only being exposed to specific ideas through these distorted media outlets. This whole system propagates misinformation.

Another saddening news media trend is that science sections are grouped in with technology. Therefore, most of the stories are about the latest Apple product and newest video games rather than actual science. Or, if they're not lumped together, the science section is hidden near the bottom of the page. Most print newspapers only have a science section once a week. Even the Discovery network is dominated by shows about people with 10+ kids, cupcakes, occupational hazards, and other not exactly scientific shows. As painful as it is to face, here in the 21st century science is getting the shaft.

Here are links to two stories about the ancient primates found in Libya. Very interesting, but not in the context of what makes us human.
NYTimes: New Anthropoid Species Uncovered in Libya
The Daily Mail: Human evolution started in Asia? Ancient ancestor walked Sahara 39m years ago.