Category Archives: evolution

biology, evolution, Hemiscience, media, science

On evolution and sequels

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So, there are a lot of things in evolution that seem like they are moving in one direction, when actually they are moving the opposite way. Or maybe it’s the other way around – I forget. For instance, one of the things that we know is that the vast majority of naturally occurring mutations are deleterious. That is, just like your crotchety old grandfather always said, children are, on average, a little bit worse than their parents (and the music they listen to is A LOT worse). Yet, somehow, evolution is able to maintain a level of function in the face of these deleterious mutations, and even to create new adaptations.

The reason is natural selection. Children will be worse than their parents on average, but there will be variation. Some will be a lot worse, and some only a little worse. Some may even be a bit better. The key is that the better children will, on average, produce more grandchildren than the worse children will (so your nagging mother was also right). It’s a bit like walking the wrong way on one of those people-movers at the airport.

Of course, there is also noise in the system. Sometimes a big rock falls on the “fittest” individual in a way that has little to do with that individual’s genotype. And sometimes an individual carrying a lot of deleterious mutations starts a polygamous cult and has about a hundred kids. But on average, the filtering effects of selection seem to counterbalance, or even outweigh the effect of those deleterious mutations.

This got me wondering if there was maybe something similar going on with movie sequels. The conventional wisdom in most quarters is the movie sequels suck. Sure, there is the occasional Godfather II, but for every one of those, it seems like there are a hundred films that are closer to Highlander II. So, I did a little study [1], in which I compared three classes of films: movies that got sequels, movies that are sequels, and random movies. Two scores from Rotten Tomatoes were collected for each movie: the “tomatometer” score, which is the percentage of reviews of the movie that were positive, and the user score, which is the average rating (out of 10) by users of the site.

The average scores are:

Movies with sequels: 59.2% positive 5.92 average (coincidence, or Illuminati plot?)
Movies that are sequels: 44.8% positive 5.16 average
Random movies: 45.7% positive 5.21 average

So, what’s our conclusion here? Well, it seems like sequels are, on average, pretty darn similar in quality to the random sample of movies. The outlier is the set of movies that get sequels made. So, maybe we think that sequels suck because we tend to mentally compare them with the originals, and, like our high-school sports careers, they fail to live up to expectations. Maybe sequels suck because movies suck, and a sequel is no more or less likely to suck than anything else. Or is there something about sequelness in itself?

We can drill a little deeper by dividing our movies into five quintiles (with ten movies each) based on the tomatometer scores of the originals:

Bottom quintile:
Movies with sequels: 17% positive 3.6 average
Sequels of movies: 15% positive 3.6 average

Second quintile:
Movies with sequels: 45% positive 5.2 average
Sequels of movies: 22% positive 4.0 average

Third quintile:
Movies with sequels: 58% positive 5.9 average
Sequels of movies: 49% positive 5.3 average

Fourth quintile:
Movies with sequels: 82% positive 7.0 average
Sequels of movies: 64% positive 6.1 average

Top quintile:
Movies with sequels: 94% positive 7.9 average
Sequels of movies: 74% positive 6.8 average

What this makes it look like is that there really is something about making a sequel that makes your movie suck more than the original. For the most part, you can expect a 15-20% drop in the number of favorable reviews going from the original to the sequel, even if the sequel was only average to begin with. The one exception is the bottom quintile, where you can expect your sequel to suck just about as much as the original did. This may be a boundary effect, as the average number of positive reviews is bounded at zero. This is the great thing about making “Baby Geniuses 2” is that it is virtually impossible to underperform “Baby Geniuses.” On the other hand, with a tomatometer score dropping from 2% to 0%, the baby geniuses somehow managed it.

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[1] Not-very-scientific study methodology:

In order to collect a sample of sequels, I went to Rotten Tomatoes, and searched for “2” and “II,” discarding anything that was obviously not a sequel, or for which there was no rating information available. This yielded a list of 50 movies, including “2 Fast 2 Furious,” but not “Aliens.” For each of these, I got the “tomatometer” score and the average user rating for that movie and for the movie of which it was the sequel.

For the random sample, I went to The Movie Insider, and used their list of January-June 2009 releases. I discarded anything that was a foreign film or documentary that had an initial release date prior to 2009. The rationale here was that if a documentary is shown at film festivals in 2007, and then gets a major theatrical release in 2009, this is not a random movie. It is a movie that has already undergone a fairly intense selection process. In the end, this list had 75 movies in it.

The study was not double-blind or vetted by anyone else, and undoubtedly contains errors in both transcription and judgment. However, hopefully it is close enough for analogic use.

biology, evolution, genomic imprinting, primers, science

Genomic Imprinting I

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So, one of the things that I study is genomic imprinting. What is that, exactly?

Even if you’re not a biologist, you are probably familiar with the fact that, for most of your genes, we carry two different copies, or alleles. You get one of those alleles from your mom, and one from your dad. Those two alleles could be the same (have identical DNA sequences) or different (usually only at a small number of positions within DNA sequence). If they are different, then the consequences of those alleles on your traits, like how tall you are or what color your eyes are, are determined by the dominance relationship between those two alleles. For example, the main allele responsible for red hair (at the MC1R locus) is recessive in relationship to alleles for brown or black hair. So, if you have only one copy of the red-hair allele, you will probably have dark hair. Importantly, in terms of what follows, it does not depend whether the recessive red-hair allele you have came from your mother or father.

If you are a biologist, you already knew all of that, but you may or may not be familiar with imprinted genes. About one percent (or possibly more) of our genes are imprinted. For these genes, it does matter which allele came from your mother and which one came from your father. That’s because imprinted genes retain a chemical memory of which parent they came from, and function differently depending on their parental origin. More specifically, at an imprinted locus, alleles are subjected to epigenetic modifications in the germ lines (ovaries or testes). These epigenetic modifications can be chemical modifications applied directly to the DNA itself, or modifications to proteins that are closely associated with the DNA. These modifications alter how the allele functions, without modifying the DNA sequence itself. The key thing is that, for imprinted genes, the epigenetic modifications that are established in the male germ line are different from those established in the female germ line. So the allele that came from your father will function differently from the allele that came from your mother, even if the DNA sequences are identical.

In the simplest cases, one of the two alleles is inactivated, or turned off. The effect of that gene on a given trait, then, depends only on the active allele. To return to the red-hair example, imagine that the MC1R locus was imprinted (which it is not, as far as we know), and that only the paternally inherited copy was expressed. Now, if you had one copy of the red-hair allele, and one of the more common dark hair allele, you would not necessarily have dark hair. Your hair would be dark if your red-hair allele came from your mom, but if it came from your dad, your hair would be red.

Of course, as with all things in biology, once you start looking at the details, everything becomes a lot messier and more confusing. But, that is the basic gist.

Genomic imprinting was one of the biggest surprises to come out of molecular biology in the past few decades. Both the origins of imprinting of particular genes, and the effect of imprinting on the evolution of those genes, are interesting questions that we will return to in future posts. At some point along the way, we will get deeper into those messy and confusing details.