Neck Pouch
Is the necessity of a species to carry out some specific function, the cause of its evolution?
Is this the reason, why the giraffe has a long neck and legs, camel has a flat broad hoof, elephant has a trunk, kangaroo has a pouch on stomach, and most of the animals have tails?
Who feels the necessity and who causes the evolutionary changes in that direction?
There’s no conscious driving force (at least that we can figure out using the scientific method) behind evolution. So in this case, you have it reversed. It’s not the need for a function that leads to evolution, but rather evolution that leads to a beneficial function.
First, evolution occurs to a population, not an individual (or necessarily a whole species. Say that a river’s course is diverted through a forest, dividing it in two. Since squirrels don’t swim, the squirrels in the forest are divided up into two populations that don’t interbreed with each other (since they can’t cross the river). These two populations, now separate are free to take independent evolutionary paths.
Next, we need to start with the premise that there will always be natural variation within a population. This isn’t a stretch at all… all it means is that offspring aren’t exactly like their parents (or even a perfect 50/50 blending of their traits). Some might be shorter, taller, weaker, stronger, furrier, longer claws, etc.
So, let’s say that there is a hypothetical population of 4-legged animals in an area made up mostly of grasslands. Over time, natural climate shift starts turning the area into a sandy desert. There are still enough plants for the animals to feed off of, but the sand makes movement difficult. The animals don’t think “I need a way to deal with the sand” because, well, animals don’t think like that. Instead, natural variation and selection drives the process.
Remember how I said that there is natural variation within a population? One of those traits, in this case, would be the size and shape of the hooves. On normal, solid ground, variation in hoof size wouldn’t really matter (just like the size of a human foot doesn’t give any real advantage or disadvantage). In a sandy environment, however, wider hooves would be beneficial – the animals could move easier (using less energy) and faster (to escape predators).
Natural selection would give larger-hoofed animals a slightly higher chance of survival. This would mean that there would be a slightly higher number of large-hoofed animals surviving to the point where they could reproduce and pass on their large-hoofed genes. This wouldn’t guarantee that their offspring would have large hooves… it would just increase the likelihood. If you applied this selective pressure for many, many generation (thousands of years), you would see an overall increase in hoof size.
These changes can also occur through spontaneous mutations. DNA replication is a very complex and efficient process, but cells still make mistakes. Throw in external factors like UV and chemicals in the environment (both natural and artificial), and the rate skyrockets. Normally, you never notice them since cells have their own ways of dealing with mutations. Sometimes, though, they can lead to a new trait. Sometimes, this is harmful, but sometimes beneficial.
Say a mouse is born with a mutation that decreases the function of hemoglobin. The mouse will be less able to distribute oxygen around its body and will probably die (and won’t have offspring to pass the mutation on to). But let’s say that the mutation *increases* the function of hemoglobin… the mouse will be more fit, and will have a greater chance of passing on the mutation to its offspring (like the wide-hoofed camels above). Gene duplication is another type of mutation, but in this case, it results in two copies of a gene, allowing one to remain the same, and the other to be “tweaked” by evolution. You’ll need to study genetics before you get a true grasp of the finer points, though.
Let’s go back to the example that I gave about the two populations of squirrels. If the two populations were unable to interbreed, there would be nothing tying the populations together. Think of evolution as a game of “Telephone,” where a sentence is repeated and changed slightly each time, until, many iterations later, winds up a completely different sentence. When you break one population into two, it becomes like a game of “Telephone,” only where the sentence goes off in two different independent directions. In the end, you’ll have two sentences, neither of which is the same as the original… and by the same token, are completely different from *each other.* This is how speciation occurs. A population is split into two independent groups, and genetic differences accumulate to the point where, many thousands of years later, they become two separate species.
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