It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism, but as life fine tuned itself through evolution, those mutations would be less and less likely to be beneficial.
So two questions: Is this logic valid, and is this happening?
The environment and conditions on Earth are and always have been constantly changing, and life has been changing with it. Countless mutations ultimately prove to be non-beneficial, while those that are get passed on and further refined as conditions change; these mutations happen on a daily, hourly, momentary basis on every level big and small. Evolution is an ongoing, never-ending process; there is never a “final product,” despite man’s egotism. Specific mutations that are beneficial don’t become less beneficial over time or they would not survive.
Life first started out about 3.5 billion years ago. It spent 3 billion years as single celled bacteria. Even today, the vast majority of life is single celled bacteria. Us multi celled weirdos are just irrelevant freaks.
Organisms are still (in recent geological history) evolving in very useful directions… just because they’re not re-inventing the gill or the spinal cord or the opposable thumb doesn’t mean that evolution isn’t occurring in very interesting ways… maybe resistance to a bacteria, or ability to exploit some new plant food, things like that.
Also, regarding the pace of evolution, it has been theorized that it actually speeds up or slows down for particular species in particular environments. It has nothing to do with life ‘fine tuning’ itself but on the selective pressure on the organisms in that niche in time.
Why do you think this?
I think what you’re talking about is best represented in the scientific community by the concept of fitness landscapes. Think of a two dimensional plane representing all possible genotypes (in reality, this would have to be many, many dimensions, but for simplicity’s sake, stick with two). Now, let’s say that for each point on the plane, we raise that point to a height representing the relative fitness of that genotype - if it’s really, really good, it’ll be a high peak, while if it’s only mediocre, it’ll be a small mound. We’re left with a landscape of peaks and valleys.
Now, one important thing about evolution is that it’s only able to move in small steps, which means it’s going to be difficult (though not always completely impossible) to cross big valleys. Once you’re on the slope of a peak, you tend to stay there, and move only to the top of that particular peak, even if there’s a much higher peak not very far away. In technical terms, you tend toward the local maximum.
So given a static fitness landscape, yes, organisms would evolve to their local peak and stay there. In other words, they’d adapt what they have as best they can and then stop evolving. However, this isn’t the case. Fitness landscapes are never static. They’re constantly changing and evolving themselves, as the environment changes. We see this a lot with human activity. Organisms that haven’t needed to change in millions of years suddenly find themselves horribly maladapted when humans show up. Look at the dodo or any of thousands of other examples of island species for an illustration.
Why do you think this? It’s more reasonable to assume that, since mutations are random, they can just as easily be harmful as beneficial.
This may be supposition, but now that there is a species that can somewhat direct its own development, might we consider that directed evolution will augment the usual genomic flow?
Even humans have evolved traits fairly recently. Eurpoeans just became able to produce lactase, so that we can digest the lactose in cow milk. This happened when cows were introduced to Europe. The entire continent evolved in (I’m quoting from memory, so I’m a little off) 3 or 4 generations. It’s easy to see how that “landscape” changed when cows showed up.
Indeed, now and then, 99% of mutations would disappear within a generation or two in complex organisms.
The logic is more-or-less valid, in fact it’s the germ of the idea of ‘punctuated equilibrium’, which is part of modern biology (the arguments are really about how much punctuated equilibrium happens, compared to slow-and-steady changes), but there are a couple points to add:
First, you want to think about the starting point being not just the first instance of life, but any time the environment changes drastically. This means that even an organism that has evolved for a long time to be highly fit in the old environment is now far away from the optimum. An analogy might be taking a modern production car, which is pretty well suited for being a civilian passenger car, and putting it into a racing situation, where it’s not that well suited anymore. So we tend to see bursts of evolution when an organisms surroundings change drastically (an ice age, or even a competitor arriving in the area, say from a new land bridge), and then relative stability until something else changes.
And second, it’s not so much that any change is likely to be positive, as the fact that the positive changes are going to be really valuable. Again, use the analogy of a production car. Most random changes to the car are going to make it worse for both regular use and racing use (since any random change is most likely going to make the car not run). But it’s hard to make it a much better street car, because it’s already a good street car, so improvements are going to be small. But it’s easier to make it a much better racer (e.g. drop the passenger seats), since it’s not a great racer right now.
Directed human evolution is a) difficult to practically bring into effect and b) fraught with ethical conundrums. Eugenics is basically an attempt to direct human evolution, and since its heyday in the late 19th/early 20th centuries, it’s gone out of favor.
If you look at history of various species, generally they start out as smaller, less specialized types… The example, perhaps, is the Kangaroo. I saw a small rodent in the Sydney zoo which they think was related to the precursor of all kangaroos. Without different competition, this thing - that looks like a mouse that hops as much as it walks - diverged into various other species, including much larger kangaroos. Why? Think of the fitness landscape mentioned above as “opportunity”, or food and safety. Does what the animal have fit neatly into what’s available? Is there some improvement that makes a better fit - gets them more food, protects them from predators better? Bigger size, faster jumps, different coloration? Every step in that direction means you and your offspring are more likely to survive. However, if someone else is already there, you have to be better than them to eat ther lunch. Perhaps the Kangaroos were lucky that they had limited competition in isolated Australia.
Size works in some situations - but… if you are too big, what happens if there’s a drought or other food shortage? The smaller ones survive, usually unless the big ones can push them aside. A bigger animal needs more range meaning more exposure. A bigger monkey, for example, may be too heavy to reach the outer branches of a tree.
Ditto for smart or fast; smart uses up a lot of calories. That brain better be good for something, like figuring out how to get away from predators, or how to steal the chimp’s lunch. Extra fast means extra lean - not an advantage if food is periodically scarce. Extra fast means no wasted matter - if that brain doesn’t need to be that big, it’s a disadvantage to carry it around; so you’re either slow and smart, or fast and dumb. Evolution is a trade-off.
Once some species reaches that “peak”, they are optimized for their conditions. As pointed out, those conditions could change.
Imagine the first pair of lemurs washed up on Madagascar - an island full of food with nothing except maybe some insects eating it; instant population explosion! As the population exceeds the food, now some mutations allow you to eat something nobody else does - you and others like you enjoy a food and population boom, but only if you mate with others who can eat more easily that food. You nutcrackers are your way to being a separate species from the fruit eaters.
To steal someone else’s lunch, and push them out of an ecological niche? You need an edge. For example, warm blooded animals are able to keep moving while the cold-blooded ones slow down; so a rat has an advantage over a lizard in cooler climates; once again, it comes with a price - to stay warm it has to eat; rats wolf down everything in sight, and reptiles eat very little. So reptiles can hold their own in deserts where food is scarce, or tropical climates where temperture is not an issue.
Similarly with humans - there are obvious adaptions - skin color for sun exposure, for example. There are more subtle ones - there are some Andean Indians well-adapted for high altitudes. Eskimos have the body shape and fat deposits well-adapted for cold weather. Europeans males have the rampant facial hair needed to protect hunters from bad climates.
However, humans game the system. We make our food with tricks like irrigation or imports, so are less susceptible to food pressures.We have things like sunscreen and clothing, vaccines and antibiotics, and vitamin supplements to offset environmental or body shortcomings. We use eyeglasses, teeth braces, and plastic surgery to enhance our abilities and reproduction appeal despite the fact we will pass those bad genes on to our children. The most successful sue birth control more often, thus ensuring that in fact the least capable are more likely to reproduce. Perhaps in the case of the most successful humans, evolution is running backward.
So the short answer is - evolution happens when there is pressure to change. When an animal is well-adapted to a stable environment, they have probably reached their peak.
Do you mean that ecological niches might be generally underexploited and even where occupied, maybe not by a vast diversity of competition? That could be correct, but in the big picture, it probably only lasted a comparatively short time.
I’d like to take the quantitative approach here-- how can we measure the rate of evolution, and what does it look like?
In principle, this is extremely simple. If you have an evolutionary tree based on DNA sequences, it will give you a measure of how many nucleotide changes took place along a given branch. That gives you some sort of distance measure. Then, if you know how much time each branch represented, you can use the formula distance = rate x time to find the rate of evolution.
But this is problematic for a number of reasons.
First, the distances will be an underestimate for the actual amount of evolution that took place. If one nucleotide changes to another nucleotide, then back to the original, then you would really have two changes, but you would measure zero changes.
Second, determining the amount of time a branch represents is tricky. One of the sources of calibration points is the fossil record, which of course suffers from preservation bias.
Third, there is the assumption that the rate of evolution is the same across all stretches of DNA. This is clearly wrong. Genes coding for the crucial functional sites of enzymes change more slowly than the not as crucial parts of the enzyme, or junk DNA.
So, to give a quantitative answer to your question, we would have to factor in a certain amount of error in the distances and have a complete and precisely dated fossil record of early life on Earth, which we don’t have and never will. We would also have to figure out some way to answer your question that accounts for the differing rate of evolution within the genome of a single organism.
Still, quantitative assessment of rates of evolutionary change is a very hot topic in phylogenetics right now, and a lot of interesting work is going into it both on the biology and the CS/math sides of things.
Good job as usual, Dopers. You fine tuned my question and covered it and more. Thanks.
Measuring “evolution” isn’t done by just counting nucleotide changes – the vast majority of these are irrelevant, neither harmful nor helpful. Instead one looks for long stretches of DNA that are nearly constant throughout a population (of humans for example), but different from a separate related group (e.g. a different human or primate population): that’s the signature of a useful mutation.
I read recently that when this is done, the evolutionary distance between two human groups (e.g., Australian and Amerindian), that is the changes in 60,000 years or so, is much greater than would be expected if evolution had continued at the same rate as it did over the millions of years separating humans from a common ape ancestor. I think a big part of the reason for this is that evolution proceeds much more readily with a large population. (When there’s a survival bottleneck of a few hundred individuals you’re “stuck with what you’ve got;” when there’s many thousands, you’ve got “room to experiment.”)
Forget eugenics. How far do you think we are from directed human evolution by way of practical and effective genetic manipulation rather than selective breeding?
How are you measuring? Nearly all of the biomass on the planet comes from multi-celled organisms.
Probably by absolute number of organisms. In terms of biomass plants alone would vastly outweigh all single-celled organisms.