That evolution is a blind, purposeless process is difficult to grasp, yet it is a fundamental part of understanding biology. The concept is explicitly mentioned in college level textbooks, although some introductory biology textbooks place less emphasis on it than you will find in more advanced courses.
Here's how Douglas Futuyma describes purpose in Evolution (2005) (p. 12).
Above all, Darwin's theory of random, purposeless variation acted on by blind, purposeless natural selection provided a revolutionary new kind of answer to almost all questions that begin with "Why?" Before Darwin, both philosophers and people in general answered "Why?" questions by citing purpose. Since only an intelligent mind, with the capacity for forethought, can have purpose, questions such as "Why do plants have flowers?" or "Why are there apple trees?"—or diseases, or earthquakes—were answered by imagining the possible purpose that God could have had in creating them. This kind of explanation was made completely superfluous by Darwin's theory of natural selection. The adaptations of organisms—long cited as the most conspicuous evidence of intelligent design in the universe—could be explained by purely mechanistic causes. For evolutionary biologists, the flower of the magnolia has a function but not a purpose. It was not designed in order to propagate the species, much less to delight us with its beauty, but instead came into existence because magnolias with brightly colored flowers reproduced more prolifically than magnolias with less brightly colored flowers. The unsettling implication of this purely material explanation is that, except in the case of human behavior, we need not invoke, nor can we find any evidence for, any design, goal, or purpose anywhere in the natural world.Richard Dawkins made the same point in his book River Out of Eden (1995) (pp. 96-98). Here he is reading those pages.
It must be emphasized that all of science has come to adopt the way of thought that Darwin applied to biology. Astronomers do not seek the purpose of comets or supernovas, not chemists the purpose of hydrogen bonds. The concept of purpose plays no part in scientific explanations.
The concept of purposeless, or accidental, evolution comes naturally to those evolutionary biologists who are used to thinking about random genetic drift. Those biologists tend to be comfortable with the idea that the tape of life will never be replayed.
But, as Richard Dawkins notes at the end of his reading, there are other biologists for whom "the illusion of purpose is so powerful that [they] use the assumption of good design as a working tool." Many of these biologists are not completely comfortable with the idea that the tape of life may not play out the same. They tend to see convergence, and other things, as evidence of some sort of inevitable purpose (design) in the history of life. This is, of course, materialistic design, not supernatural design.
This point of view crops up in terms such as "the evolution of evolvability," "facilitated variation," and even "self-organization."
In addition to this subtle form of "purpose" we see clear evidence of true purpose in animals with sophisticated brains. Those animals clearly develop goal-oriented behaviors.
Dawkins addresses these points in his current lecture tour by making a distinction between different definitions of purpose.
The Purpose of PurposeWesley Elsberry was at the Dawkins lecture in Michigan on March 2nd and he has posted a lengthy summary on The Panda's Thumb: Richard Dawkins and “The Purpose of Purpose”. It's very helpful for those of us who couldn't attend one of these lectures.
"We humans are obsessed with purpose. The question, “What is it for?” comes naturally to a species surrounded by tools, utensils and machines. For such artifacts it is appropriate, but then we go too far. We apply the “What is it for?” question to rocks, mountains, stars or the universe, where it has no place.
How about living things? Unlike rocks and mountains, animals and plants, wings and eyes, webbed feet and leaves, all present a powerful illusion of design. Since Darwin, we have understood that this, too, is an illusion. Nevertheless, it is such a powerful illusion that the language of purpose is almost irresistible. Huge numbers of people are seriously misled by it, and biologists in practice use it as a shorthand.I shall develop two meanings of “purpose”. Archi-purpose is the ancient illusion of purpose, a pseudo-purpose fashioned by natural selection over billions of years. Neo-purpose is true, deliberate, intentional purpose, which is a product of brains. My thesis is that neo-purpose, or the capacity to set up deliberate purposes or goals, is itself a Darwinian adaptation with an archi-purpose.
Neo-purpose really comes into its own in the human brain, but brains capable of neo-purposes have been evolving for a long time. Rudiments of neo-purpose can even be seen in insects. In humans, the capacity to set up neo-purposes has evolved to such an extent that the original archi-purpose can be eclipsed and even reversed. The subversion of purpose can be a curse, but there is some reason to hope that it might become a blessing."