Well, you know, it can't be observed. So how can you prove that it does happen?
Actually, it can and has been observed. We have seen organisms evolve in the laboratory (fruit flies, bacteria, mice) and in the field (tilapia, lizards).
Denier follow-up: Oh, but these are relatively minor, what I call micro-evolution events. I have no problem with that. What about major evolutionary changes, like the formation of a new species (speciation). Where have we seen that?
It's simple. There are many examples of speciation that have been observed in our lifetime. We've seen it in things ranging from wildflowers to moths to fish.
http://www.talkorigins.org/faqs/speciation.html
Also, past speciations can be inferred as well, from the fossil record. Transitional fossils provide extremely plausible links between species -- archaeopteryx, for example, between birds and dinosaurs.
Denier follow-up: Oh, but shouldn't the world be swarming with these "transitional fossils"? Why aren't there more?
This might have been more true in Darwin's time. Since then, biologists have found scores of transitional fossils. What's more, the relative shortage of transitional fossils can be explained by a principle called "punctuated equilibrium" -- that organisms generally stayed the same except for the occasional major speciations.
All this is still insufficient. How the hell can you go from fishes and bacteria to elephants and humans?
People who generally ask this question don't understand the power of numbers. Millions and billions of years is a heck of a long time. Scientists think Homo erectus, the first hominid or human-like species that used fire, first appeared 2 million years ago. That in itself is a thousand times longer than the time between today and when Jesus existed. It's about 70,000 modern human generations, and given that people reproduced earlier and died earlier in the past -- that number could easily be a 100,000 or more. That's an unbelievably long time -- and the only major thing different between us and homo erectus is brain size (which of course leads to a lot of other important things like language, better tool use, religion, agriculture and so on).
Let's think about this more: life is thought to have started on earth around 3.5 billion years ago in the form of proto-cells and proto-bionts. That is almost 2000 times longer than the time between us and homo erectus. Also, the fastest growing bacteria reproduce every 15-20 minutes. Can you imagine how many generations these bacteria would have had in their arsenal in the beginning to evolve into more complex beings?
These numbers are staggering -- and it's understandable why humans find them hard to contemplate. The point is that it's entirely conceivable and plausible that minor changes and speciations occurring over such a long time led to the diversity and complexity of life we see today.
Genetics and genomics come to our rescue in this argument as well: DNA sequences of organisms that are very close together (like humans are chimpanzees) are also very similar. Meaning that chimps and humans have diverged relatively recently -- as in about 7 million years ago. Whereas the DNA sequences of a human and a fish are very different. Based on these variations, it's easy and intuitive to build a "tree of life".
Yeah, but we're talking about random changes. How the hell can random changes lead to a complex system?
The word "random" is the most misused when talking about evolution. These changes aren't random -- sure, the underlying mutations may be random. But the point is that natural selection plays a huge role, and it sure isn't a random process. A mutation that leads to a favorable outcome for that species is likely to make the species survive and reproduce, passing that mutation on to the next generation. The eye didn't just "randomly" evolve. Organisms living in water started from a few cells that had sensitivity to light. These gradually evolved into sheets of photosensitive cells, because they allowed these organisms to tell the difference between light and dark. These sheets evolved curvatures, because a curved sheet allows an organism a field of vision -- it can tell what direction the light is coming from. This went on until the sheets became so curved that they became circular in shape, with a slit or hole on top. This allowed for a pinhole camera-like eye, which gives a rudimentary image. That's again a huge advantage. This was further enhanced by the evolution of lens tissue -- and so on until we got to where we are today.
Denier follow-up: Yeah, but then why haven't species without eyes died out, if it's so favorable to have an eye?
It's not necessary for a species to die out just because it didn't evolve something that other species have. Species can eventually find their "niche" -- an environment where they are comfortable, food is plentiful, and there are no predators around. They don't necessarily stop evolving -- it's just that they will evolve other things that are more relevant to their environment. A species that lives in the dark, deep ocean doesn't need to have an eye. It has other, more important things to worry about -- the extreme high pressure, for example. A random mutation that allows a species to mitigate this high pressure would obviously get propagated. What's more, in the deep ocean, eyes are probably a hindrance because of the same high pressure.
Feel free to add your own thoughts.
No comments:
Post a Comment