In plants it is particularly obvious that many more potential offspring (seeds) are produced than can survive. To a very large extent it is a matter of chance as to which are the survivors. Some are eaten, others overlooked or stored away and forgotten. Those that survive to germinate might be on unsuitable soil, too dry or too wet, so that they shrivel or rot. The successful seedling could be in poor soil, deficient in minerals, or there may be many other plants that are already established and blocking out the light, so preventing the seedlings from carrying out the vital process of photosynthesis. (This is the mechanism by which 'plants, fuelled by sunshine, can combine water, carbon dioxide and a few nutrients and so produce sugars and starches' [p. 61].) So survival is not always down to chance. Survival is sometimes achieved at the expense of others, or by being able to exploit a situation that others cannot. For example, a seedling with a physiological modification that makes it resistant to drought, or a seed with a slightly tougher, more waterproof coat that does not rot in the wet, may survive in conditions where others will die. Some nuts are not eaten because they are too tough and although the TV programme showed the agouti effortlessly chiselling through a large nut, personal experience with nutcrackers reveals that some nuts are harder than others and we, like the capuchin monkey, seen in the programme at 00.44, sometimes abandon the attempt to extract the kernel. A nut that survives this particular challenge, in theory, gets a chance to reach maturity. On the other side of the fence, the seed eater with slightly sharper teeth could use the nut that was discarded by its fellow. Having the edge over other individuals of your species makes a difference when resources are in short supply. The rodent with slightly sharper teeth or stronger jaw muscles might get through a hard winter because it could open all its stored nuts, whilst other less well-equipped individuals perish. In these circumstances we would draw the conclusion that the sharper teeth had given the animal a selective advantage. We could also reasonably claim that possession of these sharp teeth had been an adaptive feature.
The example above paints an extreme scenario. Much more subtle differences can confer advantage and the advantage may not make an immediately obvious difference to life expectancy. If one squirrel has a characteristic that enables it to make use of food that others discard, it could be better nourished itself and might also be able to rear more offspring. In the search for food, rodents compete with the farmer and they also compete with one another.
If you have worked through the other units in this series so far, you'll have seen several examples of competition between individuals of the same species. Some of the most spectacular have been over access to females rather than food. Give some examples of competition between individuals of the same species.
Kangaroo (S182_1), shrews (S182_2), and marmots and capybara from this course. These were the examples that came to my mind, but you may well have noticed others.
All these examples show direct competition. There is direct physical interaction with a competitor. Other examples would include blocking access to food or water, as when a male lion chases younger members of the pride from a kill. Equally as important is indirect competition. Here there is no physical contact; instead, the successful competitor is one that is better able to utilise the resource that is limited, such as the nut discarded by others because it was too tough to crack. Many of these competitive characteristics are unseen. Examples include: being more resistant to infections; having a more efficient digestive tract or metabolism, so that you don't need as much food; or temperament - being bold or being nervous/timid - one can argue the case for either one being the more adaptive! So survival can be a result of successfully outcompeting the opposition. Yet what we judge as failure can be mistaken. In the programme 'Plant Predators' male topi antelopes fight one another and fall exhausted to the ground. In its last moments one individual is brought down and killed by hyenas. A fairly natural conclusion would be that here was an animal that had lost the struggle for existence, outclassed by other individuals.
In the commentary, DA says that contrary to what this premature end might suggest, this topi's life had been 'a triumph'. By what measure was it deemed a success?
DA says the topi's life was a success because that season it had mated with a number of females in the previous few days.
Breeding is a biological measure of success; that one survives long enough to produce offspring that can themselves survive and reproduce. (Of course, we don't actually know about the abundance and fate of our topi's young, so it is a bit premature to declare his life a success!) The reason for this rather exacting measure of success is that many characteristics are passed down to subsequent generations by inheritance. If the topi was an exceptional beast, more efficient at obtaining and digesting food and more fearsome in fights than his rivals, but he spent all his time fighting and never got around to mating, then there would be no opportunity for these characteristics to be passed to succeeding generations. But if these characteristics have a heritable component and he breeds, and because of his superior powers he gains access to more females and breeds more frequently than his rivals, then his offspring will be numerous as well as being similarly advantaged relative to less efficient, less ferocious topi. Thus his offspring, in turn should have the opportunity to breed and leave relatively more progeny who also have inherited these desirable characteristics. Hence these desirable characteristics would come to be more widespread within the population. Of course, over a period of time the particular characteristics that advantaged the topi relative to others could become so widespread that they were possessed by all topi. When we can identify that such a change has occurred over time (e.g. from the fossil record), we demonstrate that evolution has occurred but we can only speculate on the role of natural selection as the mechanism.