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Real World Physics Problems Newsletter - Natural Selection, Issue #20
June 25, 2015
Natural selection is a process by which the traits of an organism best suited for survival and reproduction become the dominant traits in the population. The theory of natural selection was first published by Charles Darwin in his 1859 book The Origin of Species. Since that time the evidence supporting his theory has become overwhelming. Natural selection is truly the main driving force for how a species evolves.
Traits that help an organism survive in a particular environment are selected for, and traits that help an organism reproduce are also selected for. The ability to survive and reproduce must both be present because if either one, or both, are not then an organism cannot usually pass on its genes to the next generation. It would do no good for an animal to have great survival ability but be physically unattractive to the opposite sex, just like it would do no good to be physically attractive but not be able to avoid predators. An animal needs to be around long enough to have mating opportunities become available and when those opportunities do become available it needs to be attractive enough to mate. And the genes that create both of these abilities will then be passed on.
Traits that benefit survival and those that benefit reproduction can sometimes work in opposition to each other. For example, the striking colors and large plumage of a male peacock, although attractive to females, also make it more visible to predators. But the additional mating opportunities available as a result of that overtake any decreased survival ability due to easier detection by predators.
On the other hand, traits that benefit survival and those that benefit reproduction can sometimes work in tandem with each other. For example, the alpha male leader among chimpanzees has considerably more mating opportunities than the other males. Being the leader often requires physical strength, fighting skills, and intelligence which are all good for survival, and also aids in creating more mating opportunities. So once again we have natural selection at work. Females that mate with the strongest, fittest males themselves produce offspring that are among the strongest and fittest, and which are more likely to survive. These offspring therefore exist in greater numbers in the population, and in turn also produce more female offspring who desire to mate with the strongest and fittest males, which can protect them, thereby ensuring their survival (and that of their offspring) more than females who produce offspring with weaker males.
But the colorful and large plumage of a male peacock, which makes it more prone to predation, has a diminished survival advantage as a result of it, so it seems that the only real advantage of it is that the females like it so much, and no other real benefit exists. Clearly it would be better, for survival of the species, if the peahen was less choosy and chose males that were less colorful and therefore less likely to be predated, keeping all else equal, since this would increase the ability of the species to survive. But that's not how natural selection works. Natural selection is all about what works best given whatever constraints and limitations are in place.
An animal simply having the ability to survive and reproduce is not enough to guarantee that its traits will become the dominant traits in the population. It is the animals that can best survive and reproduce that will have their genes become the dominant genes in the population. This is known as "survival of the fittest". It is the fittest animals that will most successfully pass on their genes, since it is these animals that will win out over their competition and will have the most mating opportunities to produce the most offspring. They will have some combination of lifespan (survivability) and attractiveness that will result in the most mating opportunities relative to the other animals in the population. These animals will consequently produce the most offspring which will, over time, result in future generations having more of their traits than the other animals. And eventually, these traits become the dominant traits in the population, until such time that a new trait, in some random animal, comes about which gives it an even greater survival and reproduction advantage than its predecessors, and that will eventually become the new dominant trait in the population.
Once you get used to the idea of natural selection you realize that it explains almost everything about how species evolve. It's like a mathematical game of winner-take-all, where the best traits eventually dominate the population, leaving little room for anything else.
The key element in all this is "survival of the fittest". The genes that produce the fittest animal are the genes that will become dominant in a population. Every species has traits in common and it is these traits that are the best suited for survival and reproduction inside a particular environment. If those common traits do not correspond to the "fittest state" among all the members of the population, then those traits will change over time and the new traits will correspond to that particular animal (or animals) that is fitter than the average.
Random genetic mutations naturally exist among members of any given species. These genetic mutations cause differences in physical appearance, behaviour, and physical ability. If these genetic mutations result in an organism being fitter than other members of the population, then it will have a good chance of passing on its genes to the next generation, and over time infuse the population with its superior traits. It is this way that a species is said to "evolve".
Changes in the environment can also steer the course of evolution. A change in climate and/or geography acts like a new genetic filter on a population, causing a different set of traits to become best suited for survival and reproduction. A species will either contain organisms with the right genetic variant allowing the species to survive in the new environment, or the species will go extinct, meaning that no members of the species had the characteristics enabling it to survive in the new environment. Extinction will happen if the environment changes too quickly, such as is the case with the dinosaurs. When a big asteroid struck earth 66 million years ago, the resulting world wide environmental changes were too drastic and too quick for any dinosaurs to survive. Big environmental changes that happen very slowly however, are much more likely to be adapted to by a species, since it provides a lot of time for the evolution of adaptive genetic changes to occur in the species' population. A species is much more able to play genetic "catch-up" if environmental changes happen very slowly.
For example, the sword-billed hummingbird of South America evolved a very long beak over a very long time because the flower that it feeds on for nectar grew longer corollas over a very long time. The only way the hummingbird can reach the nectar is with a longer beak. The picture below shows this hummingbird.
Source: Wikimedia via Francesco Veronesi
Until next time.
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