Limits of Adaptationism #3: What your momma gave you

Existing genetic variation

Natural selection can only work on existing genetic variation. There is a gene pool, and out of it survivors are selected to reproduce for the next generation's gene pool, but no one who isn't already in the pool can be invited in.

In other words, on your birthday, your momma gave you some genes - dimples, a rear end the size of Milwaukee, maybe a mustache. Bummer- you really wanted rocket thrusters. But your momma could only give you genes that she already has, likewise your daddy couldn't pull rocket thrusters out of the air to give to you.

Some things, like rocket thrusters, would be totally sweet to have, but let's face it- you're not going to get them anytime soon. Fish might really wish that they had lungs in addition to gills, but they're not going to get them anytime soon. So it's foolish to ask "why don't fish have lungs?" or "why can't people fly?"

Adaptationism would cause us to answer these questions by looking for reasons why it wouldn't be advantageous to have rocket thrusters. But clearly, personal rocket-assisted flight would be advantageous. Going back to the insect antennae- we might expect that having foldable, elbowed antennae would be advantageous to any insect. But not everyone's got the genetic wherewithal to make elbowed antennae. The reason why a certain insect has a certain kind of antennae might not be related to what is most advantageous for that insect, but rather, what that insect's momma gave them.

Limits of Adaptationism #2: Genetic constraints

Linked Genes: Pleitropy and Epistasis

Genetics is seldom a straight path from gene to protein to mustache. At every step, there can be modification and multiple effects. Pleitropy is when a single gene has an effect on multiple traits. For our mustache example, a gene regulating secondary sexual characteristics (the changes in men and women's bodies that occur during the advent of adolescence, such as facial hair) can have an effect on hair produced all over the body. Generally, the more hirsute a person is, the more likely they are to have a mustache. Since there is no actual mustache-gene, and since hairlessness isn't necessarily selected for, we will see mustaches as long as there is positive selection for hair.

Epistasis is when the expression of one gene effects the expression of another gene. For example say there is a gene at the "hipster-ness" locus that has two alleles, "hipster" and "non-hipster." When an individual has the "hipster" allele, the expression of this gene will turn on the expression of other genes such as the genes for mustaches, non-prescription thick-rimmed glasses, androgynous hair styles, and appreciation of terrible indie-rock. Any one of those downstream traits may be driving the selection for the hipster allele. Even if the mustache is selected against, it is regulated by another gene that may be positively selected for some inscrutable reason.

In both of these examples when we ask "what for?" we cannot give a satisfactory answer, because the expression of a mustache isn't the real unit of selection. The mustache is linked to other traits that are undergoing selection and influencing the expression of mustaches, despite their unpopularity.

Limits of Adaptationism #1: Founder Effects

This is the first in our series examining the limits of adaptationism, where we will examine why teleological questions (asking "what for?") don't necessarily get us the right answers in biology.

Take for example a man with an elaborate mustache. What is a mustache for? We could come up with all kinds of wild functional explanations for why mustaches exist, but none of them truly convincing. We know that men without mustaches live very normal, if not happier lives, than men with mustaches. In this era of human biology, mustaches are selected against, and the men who grow mustaches are cursed by the necessity of shaving them if they want women-folk to like them. In localized hipster populations, this trend can be reversed, but let's just say for the sake of argument, that all women feel more or less like I do.

So the reason for mustaches isn't really clear. Adaptationism would assert that since it takes energy to produce and maintain a sweet 'stache, it must serve some purpose for the adult male human, otherwise it would have been lost from the population and men would be largely mustache-less. But what if adaptationism has led us down a rabbit-trail looking for reasons where they may not exist?

An alternative explanation for why mustaches exist may be a genetic constraint on natural selection known as the founder effect or legacy effect. Basically, you can't throw out all your starting material and start over when starting a new population. The population always has founders. Perhaps the founders of the male human population had facial hair on their upper lip. Perhaps this was because they were bigfoot or wookie-like creatures with a lot of fur anyway. Selection against large amounts of body hair has occurred over the evolution of male humanity, but because of the founder effect, it's going to take a lot to get rid of those mustache genes.

Similarly, the legacy effect is a constraint placed on selection now due to selection in the past. A couple centuries ago, mustaches were the hotness. Women felt they were manly, and preferentially mated with men who had mustaches over those who did not. Now we have a gene pool full of mustaches, and we can't really get rid of them due to the legacy of the age of mustache-rage.

So, the "what for?" question doesn't help us understand what mustaches are for, unless we ask what mustaches might have been for back in the evolutionary history of hairy men.

If we go back to our insect antennae, we may ask why certain insect taxa seem to have a particular style of antenna. The question may not have a functional answer, because the reason for different styles of antennae is buried far back in the evolutionary history of these bugs. Perhaps the founders of modern butterflies had knobbed antennae, and the founders of modern moths straight or plumose (mustache-like) antennae. The reason why their antennae diverged back in that day may be as simple as genetic drift or it may have had a functional meaning in those days, but today it seems to be the capricious whim of a God who likes knobby antennae.

Photo by Flickr user a4gpa licensed for re-use by Creative Commons license.

Limits of adaptationism

Yesterday we talked about the "why" of insect antennae. Why ask why?

When biologists ask "why" - they are usually asking for a functional "because." The question could be re-phrased as "what for?" This is a very old approach for understanding how biological things work. It assumes that there's a certain order to things, that organisms are a certain way for a reason. God, in his infinite wisdom, made them that way to be suited to their world.

William Paley talks a lot about this. He's the guy who came up with the watchmaker argument for the existence of God. This argument basically states that a watch is complex and contains a mechanism that works in a intricate way to keep time, so when we see a watch, we get a sense that it has purpose and design and therefore a designer. Living things like animals and plants are likewise complex and work in an intricate way to perform their tasks of survival and reproduction. Ergo, living things have design and have a designer. And it follows that the traits expressed by these living things should have a purpose in that design.

But what about traits of living things that seem to have no function? Are they just fanciful inventions of a whimsical God? Whimsy is not something I typically attribute to God. So perhaps there are other explanations. For the next several days we'll examine the limits of adaptationism - why we cannot always ask why and get a straight answer in biology.

Antennation

Insect antennae are sophisticated organs that can perform many functions for insects:

• Olfaction (smell)
• Gustation (taste)
• Mechanoreception (feeling)
• Hygroreception (humidity detection)
• Thermoreception (temperature)

Basically, antennae can do everything but see. They play powerfully in orientation of the insect in space; detecting wind speed and direction, the smell of pheromones in the air leading them to a mate, or the subtle vibration of prey below the bark of a tree. What is interesting to me is that antennae have so many different forms. Termites have simple moniliform antennae like tiny strings of tiny beads. Silkmoth and mosquito males have elaborate bipectinate plumose antennae. My small hive beetles have adorable club-shaped antennae which make them look like Mickey Mouse when they hold their antennae up. The antennae of house flies are two fat dangly bulbs with a single feather mounted at the top of each. The antennae of scarab beetles terminate in a fan-like array of delicate fingers called lamellae that can be spread open or closed tightly like a fist and tucked away into cavities under the insect's head. Dragonflies have nothing but two short bristles for antennae. Dizzying variety is the rule when it comes to antennal form. So why so many types? Does each of them correspond to some special life-style like insect leg types?


Question of the day:

Why are there so many types of insect antennae?

Answer:

Dunno.

No, seriously- we don't know why there are so many types of insect antennae. Generally, where olfaction is important, we find more elaborate or specialized antennae, such as those of male moths. Dragonflies, on the other hand, hunt primarily by sight, and thus may be forgiven for having simple and uninteresting antennae. Evolution seems to have favored divergence in most cases and convergence in a few like the elbowed antennae of weevils and ants. Beyond that, antennae are as diverse as the insects themselves. Family resemblance in the antennae is quite useful for classifying insects, but why scarab beetles have fancy lamellate fingers and small hive beetles small club-shaped antennae is rather a mystery.

Some questions in biology will always be easier to answer with "It's for decoration" or "Because that's how God made him." And perhaps in a sense, this is true- that God has seen fit to elevate diversity over uniformity, and style sometimes seems to trump function. But as we scientists study and search for a function for strange traits and find that they do, indeed, have a function, are we disappointed? On the contrary, when fascinating form and amazing function come together we get a new sort of joy- beyond the joy of beauty and the joy of a well-made machine. It is the joy of something that is, on all accounts, very good - just as the creator said it was.

Anthocyanin Crimson

I have been looking for words to express the red color of anthocyanins in fall leaves. The thesaurus gives me many delightful words for red such as the color of blood, bittersweet, bloodshot, blooming, blush, brick, burgundy, cardinal, carmine, cerise, cherry, chestnut, claret, copper, coral, crimson, dahlia, flaming, florid, flushed, fuchsia, garnet, geranium, glowing, healthy, inflamed, infrared, magenta, maroon, pink, puce, rose, roseate, rosy, rubicund, ruby, ruddy, rufescent, russet, rust, salmon, sanguine, scarlet, titian, vermilion, and wine - some of which (anyone heard of cerise?) are a bit obscure. Whatever you call it, red is definitely one of God's favorite colors, especially around fall.

Anthocyanins (an-though-SIGH-un-ins) are what makes fall leaves red. These pigments are produced in the leaves of deciduous trees as they transition to winter. The leaves will soon be lost, so the trees withdraw their nutrients and store them away as the leaves die off or senesce. Unlike carotenoids, which are present in the leaves throughout the year and are revealed by fading chlorophyll, anthocyanins are produced in the leaves as the leaves are dying.

But it seems illogical to use energy produce new compounds in a leaf that is already on its way out, right? This question has stumped scientists for a long while. They have proposed all kinds of interesting theories for why leaves produce anthocyanins in the fall. One theory that has recently stood out is that anthocyanins are protective compounds that keep the leaves on as long as possible to make sure the tree can suck all the nitrogen out before they fall. Anthocyanins, like carotenoids, are powerful antioxidants and could be protecting the leaves from oxidative stress. In addition, anthocyanins absorb UV and can act like a sunscreen to protect the leaf from light stress (and here I thought that trees couldn't get sunburned).

So the anthocyanin is like a shield detachment sent to cover the retreat of nutrients from the leaves. With anthocyanins, the leaves stay on longer and the tree can store away more nitrogen. But the anthocyanins still cost energy to produce, so trees that grow in nitrogen-rich conditions will be less red because the extra nitrogen that they get isn't worth the investment in the sunscreening anthocyanins. Trees that grow in nitrogen-poor conditions, on the other hand, will glow like red-hot embers as they try to protect as much of their nitrogen as they can.

Fall Colors

Fall is closing in on us rapidly. The past two days have been less sunny, more rainy, and the trees have just begun tinge yellow and red. My friend Katie texted me today and asked "What makes the leaves change colors in the fall?" I pondered how I would answer this in less than 160 characters, then wrote this text:

"The trees are withdrawing nutrients from the leaves before they shed them. When green chlorophyll is broken down you can see other colored pigments!"*

*yes, I am one of those obnoxious people who refuse to abbreviate in text messages. I have a keyboard and goshdarnit, I'm going to use it.

There's a little bit more to it than that, as you might imagine, but not much more. When the day length begins to shorten, the light to weaken, and the temperatures to drop, deciduous trees (trees that shed their leaves in the fall) pass through an important transition to survive the upcoming winter. In the warmer months, leaves are the solar collectors and energy factories of the trees, but in winter, the trees will retreat into a quiescent state of tightly clenched buds and bare stems, and their leaves will be unnecessary to their long winter sleep. Leaves will only make the trees more susceptible to damage in the coming snowfall anyway (see photo here of results of early October snowfall in central Pennsylvania last year). But the leaves represent a lot of energetic investment on the part of the plant, particularly nitrogen, which the tree will need to survive the winter ahead. So as the trees bundle up for winter they pull nutrients out of the leaves to store up for the winter.



Here's how it happens. In the beginning of the fall, trees will gradually shut off the production of chlorophyll, their green pigment and secret to their photosynthetic magic. The remaining chlorophyll and its accompanying photosystems in the leaf tissue will slowly be digested and the nitrogen-rich pieces sucked back into the trunk and roots of the tree. (Imagine the roots of a tree are a basement for sealing up canned goods to be eaten over the winter.) The vibrant green of chlorophyll fades from the leaf to reveal the bright yellows and oranges of carotenoid pigments, accessory pigments that harvest light much like chlorophyll but at a different wavelength. Carotenoids also protect the delicate chlorophyll from UV and oxidative damage and thus are really good for you too in things like carrots (something your mom always told you but you never really wanted to listen to).

Carotenoids are also highly attractive and earn sleepy little wooded hillsides seasonal fame for their spectacular colors every autumn. But now I really digress. Anyway, the bright carotenoids have been there in the leaf all along, helping with photosynthesis and protecting chlorophyll. These pigments are just more stable than chlorophyll, and will persist longer in the leaf after resources have been withdrawn in the fall.

Now, that's yellow and orange leaves explained. Check. But what about red? If you were anything like me in my first fall in the Northeast, you probably searched the forest floor for the brightest, most cheery red-colored leaf you could find, held it tightly in your little gloved hand and marveled at just how very red that leaf shined against the grey days ahead.

Red leaves are more complicated. Maybe we can talk about them tomorrow.

Photo by Flickr user mmwm, licensed for non-commercial use by Creative Commons License.

Water Activity

Today I measured water activity in my pollen substrates. I spent about half an hour with the AquaLab CX-2 in a food science lab near the Creamery, putting stuff into little plastic cups, loading them into the smooth black steel drawer where they fit snugly into a round opening, sliding the drawer closed and turning the instrument to "read," then waiting approximately 4 minutes per sample for the insistent beeping to notify me that my water activity measurement was finally ready.

"As you can see, taking the measurement is a trivial process," said the food scientist in a white lab coat. But the measurement of water activity is quite far from trivial when you consider its implications.

Water activity is a measurement of how much water is "available" in a substance. In technical terms, it's the partial pressure of water vapor in the headspace of a given substance- but let's talk about water "availability." What would this water be available for exactly?

I like to think of water activity a measurement of grooviness. Imagine your material is having a rave. There are lots of substances in attendance: sugars, lipids, proteins, inorganic compounds, organic compounds, water. Naturally, water is the life of the party and everyone wants to dance with it. In some materials, all the water is already locked into dancing with someone else at the party- some sugar or some random alcohol group or what have you. In other materials, someone forgot to invite the water so the party is lame and dry and no one is dancing with anyone. In materials like my pollen dough, the water is there, but only some of it is dancing with other attendees. So the water activity is measuring how much water present in the material is hanging around looking for someone to dance with. This water has a tendency to vaporize and hang out in the headspace or the air around the party, where it can be measured by the AquaLab CX-2 if you have 4 minutes to wait.

Water activity increases with water content, but also varies depending on the composition of the medium (whoever else it has to dance with), so the relationship between water activity and water content is seldom linear. Microbes like my yeast love water and will gladly cut a rug with that unoccupied water and have a grand old time. If there isn't enough water ready to dance, they'll leave the party. In fact, each microbial species has a threshold for water activity, below which they cannot grow. So this is why we measure water activity; it measures how much water is available -or ready to dance with microbes- while water content will only tell you how much water is there at the party.

My yeast seems to need a really groovy party to be happy. So far I have failed to see growth on pollen substrates with an A_w (fancy abbreviation for water activity) as much as 0.77, which should be close to the same as that of bee bread in the hive.

American Goldfinch

Today I have seen a handsome American Goldfinch (Carduelis tristis (L)) on my balcony feeder for the first time. He surprised me when I first saw him, small and unassuming in shape but bright big-bird yellow with black wings crossing his back and clashing with his yellow like crime scene tape. He's been visiting it frequently all day as I watch from the couch, alternately falling asleep, working, and wasting my afternoon. My Audubon guide says that American Goldfinches wait to pair up until late in the summer when tasty seeds tend to be more plentiful. Other species with broader diets are already busy parents, laying eggs and rearing multiple broods of chicks while the Goldfinches are still flocking around in big irresponsible groups.

This makes me smile. The goldfinches have their appointed time for relationship and parenthood, but it's later than the other birds' time. They flock together like anxious graduate students waiting to see how the seeds will shake out before they settle down and start that family. I wonder if my goldfinch has transitioned into family life or if he's still hanging out with all his yellow friends.

Photo by Flickr user Jason Means licensed for non-commercial use by Creative Commons.

Dissertation dump

I am an articulate person. This is a fact that I believe like I believe that I have brown curly hair and stand 5 feet 9 inches tall. I have always loved writing. I won a prize for a science essay in high school and collected cherished praises from every writing professor. In college, I was an English major before I was wooed away from words by the breathtaking beauty of the insect world. Before bugs, I used to stay up all night working on painfully beautiful critiques of medieval poets. Now I only pull all-nighters for insect collections, posing the shiny exoskeletons of dead insects on tiny black-enameled pins and snipping out labels printed in 2pt font on which you will find only the barest details, unpoetic epitaphs to be read by the tearless eyes of scientists behind magnifying glasses.

I am rapidly approaching the culmination of my PhD-seeking career in Entomology. It's called a dissertation. It's a document containing the justification for my existence for the past 4.5 years. If there were anything which really ought to be written well, this is it.

So why is it that when I mean to write something glowing with competence, clarity, and grace- I get something that sounds like an angry grasshopper bouncing on my keyboard? Choppy, unattractive prose flows from the cursor like a malicious force is confounding any ability to communicate that I once possessed.

Part of the problem is that there's too much there- too much going on at once and no good narrative structure to make it spread out on the page. Every sentence carries the burden every minute of the past 4.5 years like a freight train pulling a thousand cars at full speed into the station. There's no parsing it. It flashes by in a blur. Can I make it slow down just for a minute to write a few sentences? Let's hope so, friends. 'Cause if it's not the light at the end of the tunnel, it might be the train.