In many mammals, such as lions and gorillas, the first act of a new dominant male following his ascent to power is to kill the younger offspring sired by the previous dominant male. This has the double effect of removing offspring from the group that are not his, and bringing their mothers into heat so he can mate with them to produce his own offspring. This is selfish natural selection at work. The struggle for existence among living organisms today is a result of sin entering a perfect creation and is not the method of bringing that creation into existence. Nature is in turmoil.
Organisms do struggle for survival.
Competition is often fierce. While there are many examples of cooperation in nature, it can always be explained in terms of selfish gain and cooperation is the easiest way to obtain the desired end. Organisms do act selfishly. Some Christians debate the effects of the fall and how far back into earth history the effects can be realized. But the point is that something happened at the fall.