How strange to call this planet 'Earth' when quite clearly it is ocean. Arthur C. Clarke

Monday, October 21, 2013

How Do You Measure Evolution?


Theodosius Dobzhansky, a famous Russian geneticist and evolutionary biologist, once said, "Nothing in biology makes sense except in the light of evolution." 

Fig. 1. Theodosius Dobzhansky, c. 1966. Photo 
from Wikipedia. No photographer specified.

So, what is evolution? How do we know it is happening? Evolution seems pretty qualitative to me. It's the gradual change of a group of organisms, and generally it is pretty hard for us to detect. It's sort of analogous to watching a tree grow or your skin tanning in the sun. You don't notice it happening, but you notice a difference later. To a human, these things are only detectable with long-term monitoring. 

Evolution is like this, too. It is not sudden and it is not creative. Characteristics a species already has over time slowly change. So if you can measure traits, you can measure evolution.

Over the years, evolutionary ecologists have tried to create standards for measuring trait change. This is hard considering all the different traits possible in all of nature, but it's better than saying, "these trees look shorter than twenty years ago. They must have evolved." We needed a way to quantify trait change, so ecologists came up with units of evolution. They are generally summarized below.


~~~~~~~~~~~~~~~Units of Evolution~~~~~~~~~~~~~~~

The darwin examines the average value of a trait and compares it to the average value of that trait in the past. For example, if average tree height in a population today is 40 m, and 20 years ago it was 50 m, in darwins this is expressed as ln(40) - ln(50) divided by 20 years. 

The haldane examines proportional trait change over time. It takes into account the variation in trait change, which standardizes change for easier comparisons between different populations. Basically you take the difference in the trait means, divide by a standard deviation calculation, and then divide by time, often expressed in generations.
(The information on haldanes is from a University of Michigan page on Gingerich for which there is a link above.)

~~~~~~~~~~~~~~~**********~~~~~~~~~~~~~~~


Traditionally, evolutionary change was seen as taking millennia or eons to occur and being irreversible. However, now that evolution is widely accepted as being able to occur in just a handful of generations (even one generation), there is no reason for it to be irreversible. This is the idea captured in the concept of fitness landscapes. I won't explain fitness landscapes now, but in a nutshell they teach that evolution is ongoing and in a variety of directions. 




Fig. 2. An example of an adaptive landscape (also called a fitness landscape) from a TradeStation forum. 
I think it would be cool if someone developed a fitness landscape with a time dimension!


This idea of irreversible genetic change is likely why the term "plastic" arose to describe non-genetic changes in traits. However, this is a misleading name because it implies that (a) genetic changes are irreversible and (b) non-genetic changes are more likely to be reversible, both of which I'm not entirely convinced are necessarily true. 

Finally, I think it is easy to forget among all the measurement-taking and mathematical analyses what evolution really is. It is one thing to say that nothing in biology makes sense except in the light of evolution; it is another thing to say that nothing in biology makes sense except in the light of regressional correlations between quantitative measures of natural characteristics.

Lastly, I want to add that I am still a student and I in no way claim to be an expert on any of this.

Conclusions:
It is important to understand that evolution is another way of saying change. Nothing in biology makes sense except if you acknowledge that nature is constantly interacting and adapting to change. Evolution is ecology over time.


Thursday, October 10, 2013

Tricky Magic Traits

I was "thumbing" through my favorite eco-evolution blog and read the post on magic traits. "Magic" traits? What on Earth (Ocean) would provoke someone to call a trait magic? thought I. I read on.

Magic traits, which is now a published term, are traits under environmental divergent and sexual selection. For example, if nature is "pressuring" fish to evolve wider mouths, and the females are also really diggin' on the wide-mouthed males, not only do the males grow bigger and stronger because they can fit more in their pie holes but they get to reproduce more because the females find them more attractive. This causes them to rapidly evolve, just like magic! Recently, X. Thibert-Plante and S. Gavrilets published something about how this occurrence, once thought rare, is perhaps actually the norm:

"...certain traits that are under direct natural selection are more likely to be co-opted as mating cues, leading to the appearance of magic traits (i.e. phenotypic traits involved in both local adaptation and mating decisions)."


"Multiple mechanisms of non-random mating can interact so that trait co-evolution enables the evolution of non-random mating mechanisms that would not evolve alone."

(If you understand this last statement, please enlighten me. I put it in here so someone can explain what s/he thinks it means.)

If the traits change quickly, soon enough the divergent trait will become the normal trait. How do females know when selection is no longer divergent? Do they guess based on the number of males they've encountered recently? Obviously I should read the paper, and when I do, this post will likely change.

Another thing: Are these fish evolving the ability to evolve faster? 

These sorts of findings are what make the study of evolution so attractive to me. Nature is so dynamic. It has so many hidden devices that make it resilient, make it adaptable, make it work. Humans will never come close to recreating such compatible complexities.




One of the more interesting images from a Google image search 
on "magic traits." Maybe white Siberian tigers are evolving magically?
Maybe they are magically evolving into zebras. 
Photo from fubiz.net. Art by Andrei Clompos.




Sunday, October 6, 2013

Adaptive Assumptions

I recently read a thought-provoking Eco-Evo Evo-Eco blog post based on the idea that adaptation and constraint can be equally good at explaining change depending on how "deep" within a population you search (i.e. within or among species):

Thesis of the post: "...for functional traits, null hypotheses for variation among species should be adaptive ones (such that the non-adaptive hypothesis bears the onus of proof), whereas null hypotheses for variation within populations should be non-adaptive ones (such that the adaptive hypothesis bears the onus of proof)"


I can see how this would make sense. I suppose it is more likely that differences between two separate populations are caused when each adapts to its separate environment. It seems this would only apply for allopatric populations. On the other hand, if there are differences within a population, you could argue it is more likely that this is just due to random variation or phenotypic plasticity. So the null hypothesis for differences between populations is that they adapted to different environments, and the null for differences within populations is that they have plastic traits. 


I feel this conclusion largely relies on the assumption that allopatric speciation is simpler and much more likely to occur. Which may not be the case. As one commenter put it: "Simplicity always has to depend on the context and is therefore not general."



Why couldn't some Mytilus californianus in a group adapt 
and change more than others?



 Why couldn't there be speciation within a tight cluster 
of Pollicipes polymerus?


Yesterday, on the beach, I was discussing possibilities of evolution between tidepool populations with a friend and how dispersal is basically infinite when the tide comes in. All species can disperse gametes and larvae into the ocean and boom!, essentially infinite dispersal. (This depends on the currents that are depositing the gamestes/larvae, of course. The ocean is not just one big toilet bowl, swirling everything together.) With infinite gene flow, it is hard to develop reproductive barriers that could lead to evolution. After I mentioned this to him, my friend commented, "ah, so they could only speciate sympatrically, then." I had not thought of this. Might all tidepool organisms speciate within populations? This would certainly be a counter example to the above hypotheses. 
This idea is not fully developed in my head yet, but it is something to think about! Thanks, Joseph.


Mytilus californianus 
covered in Balanus glandula




Anthopleura xanthogrammica



Thursday, October 3, 2013

Tidepools by Natural Bridges


Less than a mile from the marine lab there is a state park called Natural Bridges. It seems like a strange name until you go there and see the natural bridges!


Source: Wikimedia Commons. But really, I could have taken this photo.

I went with another new ecology and evolutionary biology (EEB) grad student, and we explored the intertidal environment under this very rock and the pools along the shore for hours. I wish I could have explored more under the natural bridge, but the tide wasn't low enough and I didn't feel like swimming or being thrust against mussel beds. 

The tidepools there are part of a Marine Protected Area where one of my advisors, Pete Raimondi, bases some of his work. He heads a long-term project called the Partnership for Interdisciplinary Studies of Coastal Oceans (called PISCO for short, prounced like "peace-co"). In this project, lots of teams of researchers take biodiversity and geography data on intertidal areas from Alaska to Mexico. 

One of highlights of my trip yesterday to the Natural Bridges tidepools was testing the stinging cells of a green anemone. When you touch the tentacles of an anemone with your fingers, you don't feel a sting because the skin on your hands is too thick. However, a thinner, more membranous epithelial surface, like the lips or tongue, would be affected. So I reached over, stuck out my tongue... 

Credit: Carla S.

...and licked an anemone's green tentacles. My tongue stung (not too seriously) for four to five hours after that. It was a great experience and I encourage everyone to try. 

Ideally, I will study evolution (or lack thereof) in tidepool organisms. I am thinking the intertidal might be a good model system because there are lots of environmental constraints and extremely fluctuating conditions which cause organisms to become very specialized. That way, if one part of the environment changes that an organism was well adapted to, those well-adapted (fit) organisms are no longer so well-adapted (their fitness decreases), and they might reproduce less. This changes the types of organisms that remain in the environment and the population is said to have evolved. What I have just explained here is natural selection.

However, sometimes individual organisms are really good at adjusting to change (this is called plasticity--think of plastic and how it can be melted and reformed), which would mean they would tend not to evolve. Instead, they'd just get used to the new environment and continue living like normal. 

So far I pretty much have no clue how to study this sort of phenomenon in tidepools (i.e. what organisms to use or if there are measurable environmental changes taking place in tidepools near me). Hopefully I will figure that out soon, or at least come up with a good idea or two! I'm not worried. Things will come together. Now to end on one of my favorite biology quotes:

"The true biologist deals with life, with teeming boisterous life, and learns something from it, learns that the first rule of life is living."
J. Steinbeck, The Log from the Sea of Cortez