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Sunday, February 27, 2011

Got beef with worms?

Photo: {http://news.brown.edu/pressreleases/2009/09/bilateral}, by Eric Rottinger at kahikai.org
Flipping through the current issue of Current Biology, it sounds like someone has some serious beef with acoelomorph flatworms. Apparently these critters have been used as a model for the 'missing link' between simple-bodied cnidarians (like jellyfish) and bilaterians (bilaterally symmetrical animals like you and me and flies and fish, and really a good deal of animal biodiversity); and this may be problematic according to the commentary. The origins of bilaterians is a major development in the evolution of body plans, a topic about which I know nothing. But I'm sold on the title and a line of the summary:

Title: A Soap Opera of Unremarkable Worms.
From the summary: "...acoelomorphs might instead be degenerate deuterostomes..."

Take that, you shifty bastard flatworms.

Acoelomorph roast
Lowe CJ, & Pani AM (2011). Animal Evolution: A Soap Opera of Unremarkable Worms. Current biology : CB, 21 (4) PMID: 21334293

Thursday, February 17, 2011

Evolution: What it is and why humans aren't immune to it

An alternate title for this post could be "BigThink Too Big For Own Britches."

Physicist Michio Kaku (via John Hawks via Pharyngula) has re-brought my attention to the fact that a great deal of people who don't study biology have no idea what evolution is or how it works (smart people like Kaku included). I will no further rebuke Kaku for abusing his power as a respected public figure in big Science and saying things that are outside his purview, not to mention just incorrect. His comments on biology would be like me telling high school students that the invention of the wheel or lubricants have obviated the effects of friction. Rather, I think it might be best to refresh people on what evolution is and how it works.

Quite simply, evolution is change in a gene pool. This pool could be an entire species or a small population within that species.

There are a number of ways evolution can happen. A mutation is a new genetic variant that arises in an individual, which can then be spread to later generations when that individual reproduces. A single strand of human DNA is like a string of some 3 billion letters. When a person replicates their DNA for it to be passed on to their offspring (meiosis), having to reproduce such a long strand ensures that a mistake is made at least once in a while. Hence mutations increase variation in a gene pool.

But the frequencies of genes in a population can change, that is they may become more or less common within the gene pool. This could happen by genetic drift, which is the random loss of genes. If a gene is neither adaptive nor harmful, it could simply be lost over time due to sheer chance. In contrast to mutation, drift reduces genetic variation.

If genes are adaptive or harmful, their frequency in a gene pool becomes subject to natural selection. If a gene (or set of genes) is adaptive, that means the possessor of those genes will be more likely to survive and reproduce than others. This advantage ensures the individual will pass on these genes. Over time, the adaptive genes will increase in frequency in a population. Conversely, genes that lower the likelihood of surviving and reproducing will be culled by selection. Either of these scenarios means selection is reducing genetic variation. But sometimes different forms of a gene can be adaptive in different situations or combinations, so selection will act to maintain both of these in the gene pool. So in contrast to mutation and drift, selection can reduce or maintain genetic variation.

Finally, gene flow refers to genes being introduced into a gene pool from another source. This could occur when someone from one population reproduces with an individual from another population, and so new genes may enter one of the groups. Like mutation, this will increase genetic variation in a gene pool.

Common misconceptions
It may seem counterintuitive, but evolution does not equate with progress. This is a common misconception, probably due to the social ideologies under which evolutionary theory developed. Because of selection, evolution often means that a population becomes better-suited to its environment over time, which seems like progress. But as we've seen above, not all evolution is selection; mutation and drift are fairly random processes of evolution that don't necessarily bear on adaptation. In addition, environments and circumstances change, so that even if something evolved in a place where it was adaptive, it may be harmful in a new context. For example, as the earliest humans lost their body hair, they probably evolved to have darker skin: adaptive in the tropics where humans originated. But later, when early humans moved into more northerly latitudes with less ultraviolet exposure from the sun, the dark skin that was adaptive for a hairless human in a tropical environment came to hinder the body's vitamin D synthesis: maladaptive!

Also contra popular opinion, individuals do not evolve, populations do. Trojan brand condoms recently had an ad campaign in which they encouraged men to "evolve" by using Trojan condoms when having promiscuous sex. This is in line with the incorrect idea above that 'evolving' means 'becoming better' or 'more sophisticated.' Of course, condoms may actually help a population to evolve: those who use condoms to prevent pregnancy are ensuring they do not pass on their genes. And if there's any genetic predisposition to make one more likely to use condoms (and there's not), these genes would certainly become less common in future generations. [I am NOT encouraging people not to use protection, by the way]

So this brings us to a final point: the outrageous thing (well, the main one) Dr. Kaku foolishly leashed upon an unsuspecting world is that humans are not evolving. Technology and urbanization, he tells us, has obviated natural selection on human features (well, the "gross" or visible ones). This is very wrong and shortsighted. In fact, this is one of the bases of the eugenics movement of the early 20th century. Eugenicists thought, 'Nature is no longer ensuring some people don't pass on their genes, so we ought to do it ourselves for the good of humankind.' This first thought, about the insufficiency of Nature, is echoed by Dr. Kaku (surely he does not think the second).

Simply HUMANS ARE STILL EVOLVING. Remember, not all evolution = natural selection. The genetic composition of humankind is still subject to the random forces of mutation and drift. In fact, because the human population size has increased exponentially of late, the fact that there are way more people than ever means that there are more mutations entering the population, and at a faster rate, than ever! But selection is still at work, too. There are still diseases that kill people before they can pass on their genes. There are still environmental situations - even in civilized places! - that prevent people from passing on their genes.

We humans are still evolving because we are still subject to the forces of evolution, and we always will be. Now what physicist could've told you that?!

Friday, February 11, 2011

Neoteny in literature

I'm trying something new: recreational reading, non-academic literature to get my mind of work at the end of the day. My Platonic soulmate recommended, almost a decade ago now, Still Life With Woodpecker by Tom Robbins. I was very surprised, then, to run into this passage:
"Neoteny" is "remaining young," and it may be ironic that it is so little known, because human evolution has been dominated by it. Humans have evolved to their relatively high state by retaining the immature characteristics of their ancestors. Humans are the most advanced of mammals - although a case could be made for the dolphins - because they seldom grow up. Behavioral traits such as curiosity about the world, flexibility of response, and playfulness are common to practically all young mammals but are usually rapidly lost with the onset of maturity in all but humans. Humanity has advanced, when it has advanced, not because it has been sober, responsible, and cautious, but because it has been playful, rebellious, and immature.

Why Lucy, what sweet kicks you had

For decades people have debated whether Australopithecus afarensis was an obligate biped like us, or whether our ancestor was a little less lithe in life on land. They asked, sort of, "Would Lucy have rocked some sweet Air Jordans, or would she have put some flat-foot orthotics in her new kicks?"

Carol Ward and colleagues report on a new fourth metatarsal of Australopithecus afarensis from Hadar in Ethiopia, over 3.2 million years old. The foot bone shows that A. afarensis had the two foot arches that we humans enjoy today.

Metatarsals are the longbones comprising much of the foot right before your silly-looking toes. One exceptional thing about our metatarsals compared to our ape cousins is that they contribute to two arches, one running front-to-back and another side-to-side. The arches provide critical support to our foot for bipedal stance, and a little Fred-Astaire-springiness as our foot hits the ground and then lifts off again when walking and running and sashaying.

The new A. afarensis metatarsal (AL 333-160, right) shows that by 3.2 million years ago, our ancestors had these arches, too. The twisting and angulation of the shaft relative to the base show these arches are similar to humans and our later fossil ancestors, whereas apes' MT4s tend to be less twisted and angled. Such morphology was hinted at by the famous Laetoli footprints from Tanzania, around 3.7 million years ago, also attributed to A. afarensis. Other evidence from the skeleton suggested Lucy was a biped and nothing else, and so this new find from Hadar further solidifies the idea that some of our skeletal adaptations to bipedalism are ancient indeed.

UPDATE: Thinking about this finding in the shower this morning, I recalled that buddies Jerry DeSilva and Zach Throckmorton recently published a study where they concluded, based on the morphology of the end of the tibia, that A. afarensis probably had at least a rear-foot arch. Interestingly, though, they found some hominid specimens probably had "asymptomatic flatfoot." Lucy (AL 288) was among these, so maybe she'd be sporting orthicticized Jordans after all.

AL 333-160 image: Carol Ward and Kimberly Congdon (http://news.sciencemag.org/sciencenow/2011/02/lucy-had-a-spring-in-her-step.html?ref=hp)
ResearchBlogging.org
The Papers
DeSilva JM, & Throckmorton ZJ (2010). Lucy's flat feet: the relationship between the ankle and rearfoot arching in early hominins. PloS one, 5 (12) PMID: 21203433

Ward, C., Kimbel, W., & Johanson, D. (2011). Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis Science, 331 (6018), 750-753 DOI: 10.1126/science.1201463