From the theist perspective, this design can be explained as teleology.
Teleology, a concept first studied in depth by Aristotle, is the directedness of natural change. It constitutes what we think of as the regularity of nature. Things fall down, not up. Sparks cause fire, not ice. The heart beats, it doesn't make bile.
Aristotle understood teleology to be essential to nature-- part of its most basic fabric. In fact, he called it the "cause of causes"-- the property of nature without which nothing makes sense. Nothing in nature makes sense except in light of teleology.
Most people who think about teleology immediately think of God, Who directs nature. Interestingly, Aristotle did not think that teleology was evidence for God, although Aristotle did believe that God exists and that His existence can be proven (the Prime Mover argument).
Aquinas and many others do believe that teleology implies God's existence. This was Aquinas' Fifth Way of demonstrating that God exists. Aquinas put it this way: "an arrow directed to a target implies an archer."
It's important to note that the classical philosophers did not associate teleology with complexity. Any directed change in nature was teleological, even the simplest, was teleological. In this view, an electron circling an atom is no less teleological than the genetic code in DNA.
Up to modern times, people have tended to understand teleology in one of two ways: intrinsic teleology and extrinisc teleology.
Intrinsic teleology is the classic Aristotelian/Thomist teleology, an inherent tendency for directedness in each thing.
Extrinsic teleology is the view that directedness is imposed on a thing from without. Extrinsic teleology often invokes complexity as evidence. Paley's watchmaker argument is a classic extrinsic teleology argument.
A very interesting debate has arisen in ID circles about the relative merit of these views of teleology. As one might expect, the genuinely interesting debate is on the ID side.
So design in life is obvious, and there exists several thousand years of the deepest thinking applied to understanding the nature of this design.
If you're a Darwinist, what do you do?
You have to deny the reality of design, because you're an atheist and a materialist, and your whole worldview depends on denial. Ideology is what drives your science. How do you accomodate apparent design and the massive literature explaining it?
Your answer: make up a theory of directness in nature that precludes agent intelligence, and give it a name that makes it sound plausible.
Teleonomy is Darwinian teleology. It doesn't explain design. It explains it away.
Here's a nice summary of evolutionary pioneer Ernst Mayr's views on teleonomy by Allen MacNeill, a biology professor at Cornell:
• The use of so-called teleological language by biologists is legitimate; it neither implies a rejection of physico-chemical explanation nor does it imply non-causal explanation
The use of teleological language in biology isn't 'legitimate'. It's indispensible. The heart pumps. Flagella are for locomotion. Kinases bind. Since Darwin's theory is supposed to eliminate teleology from biology, the continuing indispensiblity of teleology to understand biology is a glaring embarassment to Darwinists. They had two choices:
1) Purge biology of all references to teleology, which would make it incomprehensible and lead to the casting-aside of the Darwinian paradigm.
2) Make up a new word that sounded like teleology, but that could be claimed to do away with it. Since most people won't bother to figure out the philosophical subtilties, the fact that it's gibberish wouldn't matter.
Teleonomy got a foothold.
• At the same time, it is illegitimate to describe evolutionary processes or trends as goal-directed (teleological).Most natural change manifests directedness, as understood classically. Recognition of this fact of nature is not "illegitimate". It's merely honest.
The only reason not to admit the obvious directedness of natural change, including evolutionary processes, is if one is protecting an ideology from scrutiny.
Selection [reifies] past phenomena (mutation, recombination, etc.), but does NOT plan for the future, at least not in any specific way [as far as we can tell]
"Selection" doesn't do anything , because selection isn't a mechanism. 'Survivors survive' is a tautology, and tautologies don't 'do' anything.
The actual means by which living things change over time are varied and complex, imperfectly understood, and mostly teleological.
There are no doubt non-teleological events in evolution-- the coincidences that Aristotle called chance (a rock blown by the wind falls on an animal), but most of biological change is teleological, in accordance with natural laws.
• Processes (behavior) whose goal-directedness is controlled by a program may be referred to as teleonomicA program is a process by which an input is transfrmed into an output in a consistent way. That is a manifestly teleological process. There is no reason to invoke "teleonomic", unless you are just making up a word to avoid the philosophical implications of teleology.
• Processes which reach an end state caused by natural laws (e.g. gravity, second law of thermodynamics) but not by a program may be designated as teleomatic
Natural laws are teleological. There's no reason to make up another word-- teleomatic-- unless you're afraid that people will see the motive behind your previous neologism.
• Programs [of the type described above] are in part or entirely the product of natural selection
Tautologies (e.g. natural selection) don't produce anything. Natural change produces things, and natural change is teleological.
• Teleonomic (i.e. programmed) behavior occurs only in organisms (and man-made machines) and constitutes a clear-cut difference between the levels of complexity in living and in inanimate nature [i.e. they are “emergent properties” of living systems, not present in the non-living materials of which living organisms or their artifacts are composed]Even Darwinists in the midst of a torrent of neologisms can't deny the inference to design ("occurs only in organisms (and man-made machines" ). It's funny to watch.
• Teleonomic explanations are strictly causal and mechanistic. They give no comfort to adherents of vitalistic concepts [including supporters of “intelligent design,” if such supporters believe that the kinds of programs desctibed above come into existence as the result of a purposeful process]
The distinction between teleology and purpose is subtle, and has been the subject of much debate. Biology is teleological, and obvious manifests purpose. Parts of living things are for something (circulation, locomotion, excretion, etc). Whether that obvious purpose and teleology proves intelligent agency is a matter of debate. Darwinists elide the real debate by wordplay.
• The heuristic value of the teleological Fragestellung makes it a powerful tool in biological analysis, from the study of the structural configuration of macromolecules up to the study of cooperative behavior in social systems.
Teleological understanding isn't merely a powerful tool in biology.
Nothing in biology mades sense except in light of
Ed Feser is a philosopher who writes on teleology, and many other philosophical topics, with exceptional clarity. I highly recommend his books, and his blog.