Nitrogenous Basics: Population Genetics 101

Population genetics is, as you can probably guess from the name, the study of the genetic composition of populations. How genetically diverse are they? What alleles are present and how frequently do they appear? How do populations differ from one another? How do they change across a landscape? Across time? Those are the kinds of questions a population geneticist would look to answer.

It is worth, very quickly, defining what a population is for a geneticist. Or an ecologist. Or most anyone working in the biological sciences. The generic, textbook definition of a population is a group of interbreeding individuals of the same species. Typically for a geneticist, we add on the idea that these individuals randomly interbreed. They don’t really, there are always factors driving how individuals get it on and who they decide to get it on with. But many of the theories underpinning population genetics assume random interbreeding, and so we always tend to consider natural populations under those conditions, and then use deviance from that to draw other conclusions.

This issue of breeding is pretty important though. If you have groups of rabbits that are ten miles apart, but genetically identical, then chances are good they constitute a single population that spreads across the intervening distance. At the same time, you could have two groups of rabbits that are separated by only fifty feet, but, in this hypothetical scenario, a river runs through that space, cutting the groups off. Rabbits are of course, notoriously terribly swimmers (don’t fact check that, I can’t promise it’s true, I’ve never thrown a rabbit in a river because I’m not a monster), and so as a result, you end up with two groups that don’t interact, and so constitute two separate populations. The moral of that story, aside from the obvious “don’t throw bunnies in rivers,” is that when it comes to identifying separate populations, geographical distance doesn’t necessarily mean much. It’s much more important to look at the biology, how individuals at each location interact with individuals from the other. That isolation is what matters. It can be based on distance, creatively called isolation-by-distance, or be a result of geographical features like rivers or mountains or even roads and cities. But the connections or lack thereof between populations are what define how diverse any population can be.

Working in something like population genetics, the main question you are going to get is: who cares? Why should anybody care how rabbit populations interact and change? Is it just a really expensive form of omphaloskepsis? Okay, admittedly it wouldn’t be gazing at your own navel, it would be more like gazing at a rabbit’s navel or frog navels or whatever, but I like that word, and working in ecology and evolution you will eventually be asked whether what you do is just pointless examination of something or if it really has an impact. Is it the pursuit of knowledge for its own sake or does it matter in the bigger picture? I won’t get into the worth of seeking knowledge for its own sake. That’s more a philosophical argument and people don’t like having those anymore, so I won’t waste space here starting one. Instead I’ll explain why fields like population genetics and evolutionary biology matter.

Shetland_pony_inbred.jpg
A simple genealogy showing inbreeding in shetland ponies. In this case, the aa genotype leads to a dangerous (or deleterious if you want to be genetic about it) trait or phenotype. When related individuals breed, you get a higher chance of that dangerous phenotype showing up. A lack of genetic diversity is synonymous with that, as most individuals will carry the same negative allele. (Image source: en.wikipedia.org)

Genetic diversity is incredibly important. When a population isn’t big enough or diverse enough, you get a lot of problems. If individuals are too closely related, you see an overall drop in the health of a population: dangerous alleles start showing up more often, and genetic disorders become more prevalent. Population geneticists call this inbreeding depression, where, as a result of low diversity, the defective and dangerous genes that are normally latent in a population become more common. “Fitness” is usually defined as the ability of a population to sustain itself and, importantly, reproduce itself. A lot of genetic disorders not only compromise the health of individuals, but also make them sterile, so there is a pretty direct connection between an increase in genetic disorders and a decrease in the number of babies being made.

Genetic diversity is also tied to the adaptive potential of a population. When you have a broader spectrum of alleles and traits in a population, it becomes more resilient to possible disturbances or changes. The population is able to more easily and readily adapt to changes in the environment. When you lack diversity, these changes can destabilize a population, leading to what we call an extinction vortex. Extinction vortex is one of my favorite phrases in biology. Science tends to sterilize its subjects with multisyllabic, clinical-sounding terms. From Acquired Immune Deficiency Syndromes to localized edema to inbreeding depression, the words scientists throw around tend to pull listeners away from the visceral realities of AIDS patients losing the body’s defenses to retroviral invaders, or body parts swelling in response to damage and infection, or the grim physical realities of genetic disorders. Not so with extinction vortex. It’s damn near poetic.

extinction_vortex_diagram.eoearth.org
A nice diagram demonstrating how extinction vortices happen. The only thing I haven’t covered in this is genetic drift. That’s the natural drifting of allele frequencies due to random chance rather than any external factors. So in this case, genetic drift would lead towards lowered genetic diversity as the same allele copies proliferate across generations, as the population declines. (Image source: http://www.eoearth.org, although originally published in a Perason textbook it looks like)

What it refers to is when a declining population hits a threshold in size, health, and diversity from which it cannot return. From there on out, the population will spiral towards extinction. Loss of genetic diversity is a huge step towards an extinction vortex, since it both increases the prevalence of dangerous genetic disorders and creates populations that are more vulnerable to environmental disturbance. Extinction vortices are, more or less, a conservationist’s nightmare. A population or species gets to a point where the only possible way to keep them alive is with intensive intervention through captive breeding, controlled reintroduction, and extensive monitoring, all of which are difficult to manage and and even more difficult to finance. So understanding how populations interact across a landscape, how these interactions shape the genetic diversity of a species, and how both the populations and the species change across time are integral to keeping species way from that extinction vortex precipice.

So to summarize, population genetics and evolutionary biology are incredibly important. Beyond just being interesting and illuminating the roots of life on earth and tracing the complex history of our own species and the myriad brilliant forms of life we find ourselves surrounded by on a daily basis, understanding the micro- and macroevolutionary processes at play in populations can help us to preserve the natural order of the world we live in. Whether its through agriculture and industry or art and culture, we rely a lot more than we like to admit on the natural world. A lack of interest in preserving the world around us is basically the same as a lack of interest in preserving ourselves, and that just seems like a bad idea all around.

[A quick note, I pretty shamelessly lifted the word cloud for the feature image here from phylonetworks.blogspot.com. I can’t add an image source to that, so I figured I’d mention it here, credit where credit is due and all that.]

NEXT TIME ON A MASTER’S CLASS: What I Actually Found

 

 

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