Final answer:
In Darwin's theory of natural selection, individuals that survive tend to have variations best suited to their environment, which enable them to compete effectively for resources, survive, and reproduce, thus passing on their advantageous traits.
Step-by-step explanation:
According to Darwin's theory of natural selection, the individuals that tend to survive are those that have variations best suited to the environment. This concept stems from the principles that most characteristics of organisms are inherited, more offspring are produced than can survive due to limited resources, and there is variability among offspring in terms of their inherited traits. Individuals with inherited characteristics that improve their ability to compete for scarce resources are more likely to survive and reproduce, passing these advantageous traits to future generations. This process results in populations that are increasingly well adapted to their local environments over time, a process Darwin described as descent with modification. Conversely, individuals with less favorable variations may not survive long enough to reproduce, and thus their traits are less likely to be passed on.
It is crucial to note that fitness in the context of natural selection does not only relate to physical strength or speed, but rather to an organism's overall ability to survive and produce fertile offspring in its particular environment. Therefore, survival of the fittest refers to the survival and reproduction of those organisms that are best suited or most 'fit' for their environment. This form of selection, where nature effortlessly determines which variations are beneficial, is why we refer to it as natural selection as opposed to artificial selection, which is driven by human choice in breeding domestic species.