Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

There have been demonstrations of ecomorphs in different taxa, where each morphology type is associated to a single environment. I know that it happens because special morphological characters increase the fitness and therefore will be positive selected.

My question is; if we find similar places (talking about natural pressures and climatic conditions) on earth and given enough time, can we expect to see more or less the same ecomorphs living in different places? Also, assuming that, could evolution be a regarded a non-stochastic process?

share|improve this question
add comment

2 Answers

The phenomenon you describe, where the same result comes about in more than one time/location via evolution, is called "convergent evolution." It is when the same character, or trait, evolve independently of one another. One example is the wing, it has evolved (at least) several times independently, in bats, birds, and insects. Parallels can also be drawn with other traits like dispersion methods of seeds, or the evolution of fins in fish, dolphins, and whales. Here is a classic paper by Ernest Williams from 1972 which describes one of the earliest examples given of ecomorphs, the anolis lizard, also talked about on this site.

This is from a good article on the new scientist website that would be worth reading:

"Evolutionary convergence occurs at every level, from proteins to societies. An unusual antibody once thought to be unique to camels has a close equivalent in sharks, for instance, while naked mole rats form social colonies like those of ants and bees.

"What this means is that if we could wind the clock back and let life evolve all over again, life might take very different paths but still produce organisms that, in some ways, resemble the organisms alive today.

"There would almost certainly be streamlined swimmers in the oceans and winged creatures in the skies. In fact, some argue that the evolution of intelligence is also virtually inevitable, though intelligent organisms could be very different from us."

The answer to your question is yes, with enough time and the same selection effects we would not be surprised if there was a similar pair of independently evolved characters. However, selection acts on random mutations which is not a directed process, therefore for the same characters to evolve you would need mutations to occur that had similar effects. This is why given enough time we can see the same responses. Despite the non-random effect of selection, we could consider evolution to be a stochastic process, it only arrives at the same converged point by chance. This is because selection (the main driver of evolution) acts on randomly generated genetic variation thus the direction of the evolutionary trajectory is random.

"..often used to represent the evolution of some random value, or system, over time. This is the probabilistic counterpart to a deterministic process (or deterministic system). Instead of describing a process which can only evolve in one way (as in the case, for example, of solutions of an ordinary differential equation), in a stochastic or random process there is some indeterminacy: even if the initial condition (or starting point) is known, there are several (often infinitely many) directions in which the process may evolve." - wikipedia entry on Stochastic Processes



Following the quote above it may seem that evolution is a directed process, i.e. selection is aiming for a specific target. It is not, selection can only weed out the least fit genotypes and increase the frequency of the better ones, selection has no goal or pre-existing aim. Thus, when discussing stochasticity in evolutionary terms it is perhaps better to use evolutionary trajectories, to describe the randomly created paths they move along, instead of "directions."



As an example of convergent evolution we could use the peppered moth and a little imagination because this is purely a hypothetical illustration I am making up on the spot.

The peppered moth is famous for once being a white moth that turned dark during the industrial revolution, when towns and cities grew and pollution killed the light coloured lichens that helped the light morph hide. This meant, rather suddenly by evolutionary time-scales, the white morph of the moth was not very well camouflaged against the dark bark of the trees. This created a positive selection for darker morphs, as such, alleles which made for dark moths would have spread rapidly in the population.

Using a little imagination we can think about two different parts of the world where this same event occurred independently - e.g. Glasgow and Plymouth in Britain which are quite far apart but both grew expansively during the industrial revolution. In both of these locations we would have seen a simultaneous rapid loss of lichens and increase in the dark moth ecomorph. This would have been convergent evolution because it is likely that they were independent of each other, i.e. the dark moth probably did not evolve in Glasgow and then fly itself down to Plymouth.

share|improve this answer
    
Evolution by natural selection is not a stochastic process. Only the generation of genetic variation is stochastic. The question is how much evolution is governed by natural selection versus genetic drift, which is stochastic, and it has been shown that natural selection is dominant. Since convergent evolution is a selection-mediated process, it also would not be stochastic. –  Brandon Invergo Jul 24 '13 at 8:08
    
@BrandonInvergo In my opinion evolution is stochastic because "there are several (often infinitely many) directions in which the process may evolve" because evolution can occur through drift or selection. Convergent evolution is when one identical or similar direction of evolution is achieved. It relies on random mutations, a stochastic process, which selection, a deterministic process, then acts upon, or the more unlikely possibility of genetic drift to the same result. –  GriffinEvo Jul 24 '13 at 8:37
    
You are conflating the result (evolution) with the process (natural selection or genetic drift). Even the quote you gave indicates evolution as the result. Also, watch your wording: evolution is not directional. (I'm not sure why I can't add your name with an @; it's not working. Sorry) –  Brandon Invergo Jul 24 '13 at 9:11
    
@BrandonInvergo I'm not sure I say evolution is directional, maybe I missed that, but I definitely agree, it is not a directed process. Where do you mean (I'd like to change it to make it clear)? Perhaps you mean "directions in which the process may evolve" - this is where evolution proceeds along a trajectory, it's motion along the trajectory is random (therefore not directed) but it is still a direction (away from the origin) nonetheless. –  GriffinEvo Jul 24 '13 at 9:18
1  
@CristianRomán No, the journey to two ecomorphs is not deterministic, it is merely the result of similar mutational effects occurring in scenarios with similar selection. Using the moths example, the selection is created when the camouflage is less effective and thus leaving them more prone to predation. Group A might develop mutations which make them darker which selection would then favour, whereas Group B might instead evolve mutations to fly quicker or see better, making them better at avoiding predators. In convergence they arrive at the same result by chance mutations, not determinism. –  GriffinEvo Jul 25 '13 at 8:00
show 4 more comments

If you have two similar environments and you allow the organisms to evolve indefinitely, then you would, indeed, expect to see convergent evolution, in which different species evolve to have very similar adaptations. However, because the species in the two environments have different genetic backgrounds, they would most likely arrive at these these traits through distinct genetic adaptations (see: analogous traits). Thus, they might look similar but not exactly the same (think: bird wings vs. bat wings vs. insect wings).

Note that this should still be the case even if you take a population of some species, split it in two and put one half in the one environment and the other half in the other environment. Over time, there will begin to be novel traits popping up only in one of the linneages, resulting in the accumulation of genetic differences despite their initial shared history. It's probably that, in the evolution of some new trait, these two populations will have attained it through distinct genetic routes: either through mutations on distinct genes (or their regulatory mechanisms), or through distinct mutations within the same genes (not to mention other mechanisms such as gene duplication).

As for your other question, it's important to remember that evolution by natural selection is a deterministic (non-stochastic) process! All else being equal, the most fit individuals will survive. However, the "raw material" that determines the fitness of individuals, such as mutations and recombination, are stochastic. There is also a stochastic evulotionary process, namely genetic drift, however the actual process of evolution by natural selection is entirely deterministic. So, that's why you would see convergent evolution: species will develop similar adaptations because they deterministically give advantages under certain environmental conditions, but the adaptations will not be exactly the same owing to the stochastic manner in which mutations arose to give rise to those adaptations.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.