On Population Genetics

So, why do I get so excited about non-invasive methods of collecting genetic samples? Because they can tell you a ton.

Florida panther, Rodney Cammauf/National Park Service

Florida panthers are a poster child for the risks of low genetic diversity. Photo by Rodney Cammauf/National Park Service

You’re probably used to thinking about genetics in terms of an individual–perhaps genetic tests for susceptibility to certain diseases, the potential for gene therapy, or of course as a convenient scapegoat for one’s height (or lack thereof).

But genetics also happen at the level of a population, of course. If you’ve ever taken a biology class, “population genetics” may give you hazy flashbacks to some guys named Hardy and Weinberg and their equilibrium, which is nice and all except for the bit where you have to assume several things that don’t really happen in real life ever (large populations with only two “flavors” of a gene that has no effect on reproductive success and can’t mutate). Where’s the fun in that?

But there are real reasons to care about a population’s genetics, and if you can get enough data from, say, a scat survey, you can look at some interesting questions, particularly if you’re thinking with species conservation in mind.

The first question, of course, is how many animals are around? More is usually better. It’s exceedingly unlikely you got a sample from every animal, of course, but you can get a minimum headcount at least.

Normal female karyotype, from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157 via Wikimedia Commons

Normal female karyotype, from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371 via Wikimedia Commons

Next, what’s the ratio of males to females? This is also reasonably straightforward: check the sex chromosomes. Just as human sex chromosomes come in X and Y (females have two X chromosomes, males have one of each)* and an individual’s sex can be determined by pairing up all their chromosomes (in what’s called a karyotype), same basic principle in animals. A skewed ratio compared to the species’s norm is a red flag: it might mean that environmental stress is triggering changes in survival rates of males and females or causing the comparative birth rates of males and females to change.

But if you want to know how a population is doing, the single most important thing DNA analysis can tell you is how much genetic diversity there is in the population.

Low genetic diversity is bad news bears. Of course, factors like population size and sex ratio also affect genetic diversity–a large population has more opportunity to be genetically diverse (although it isn’t necessarily so). But if you’re only going to look at one factor, this is it.

Low genetic diversity (which is not the same thing as the more familiar inbreeding) is a problem because genes shape traits, and once a trait gets out into the real world of natural selection, it can be the difference between life and death. If all of a population’s eggs are in one trait’s basket and that trait doesn’t get past what nature throws at it today, the population won’t do too well. Sufficient genetic diversity spreads the population’s eggs among a collection of different baskets–so if you lose one, or even two or three, you still have something to work with.

Interestingly, it’s not just about the bits of DNA that guide the construction of specific proteins (aka “coding DNA”). First of all, what proportion of, say, your genome (the proportion changes from species to species), do you think is coding DNA? Nope, too high. Probably still too high again. Try 1.5%.

Diversity in non-coding DNA regions, often (mistakenly) dissed as “junk” DNA, is also important to a population’s health. There are two key reasons: 1) non-coding DNA is not necessarily non-functional DNA, since it can regulate expression of coding DNA and 2) it can creep into coding DNA. Like a ninja.

There’s one more step: genetic diversity is also a factor at the sub-population level. If groups within the larger population are too far away from each other or are blocked from intermingling, they can start genetically separating. On a grand scale, that’s how you get species; but if subpopulations are too small and have too little genetic diversity, you can run into trouble.

And you didn’t even have to learn who’s who. Not too shabby for scat, eh?


*This is incredibly oversimplified: genetic mishaps can lead to all sorts of non-two chromosome counts, including of sex chromosomes.

PS: More on Florida panthers. Genetic diversity fell so low that individuals from another sub-species were introduced. I find purposefully hybridizing for conservation fascinating, so stay tuned for more in a future post.


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2 thoughts on “On Population Genetics

  1. Pingback: It Takes Toucans for These Trees to Tango (Well) | What's for Lunch?

  2. Pingback: My, What Long Data You Have! | What's for Lunch?

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