What I am thinking of here is a situation like this: A lineage enters a new habitat or acquires a mutation that allows it to exploit a hitherto unavailable resource. At this stage, the organisms in question are not particularly good at making use of the new environment but since there is nobody better around they can nonetheless flourish. Crucially, among the members of this lineage the race is now on to get better at making use of the new environment, and that is what gives this situation a directionality.
As an example from the plant kingdom, we may consider the first green plants that made it onto dry land and moved into what we might term the 'big land plant niche'* hundreds of millions of years ago. The ancestors of our contemporary vascular plants were, as far as we can infer, basically green blobs bound to the moistest conditions. What happened then?
- The first step was to evolve some protections against water loss. This came in the form of a waxy cuticle covering the surface of the plant body. However, that also makes gas exchange with the environment more difficult. A conundrum: you need to breathe but breathing loses you water. The solution: stomata, that is pores in the surface that can be opened or closed depending on circumstances.
- Now often it is an advantage to grow bigger, and in fact we are talking about the big land plant niche here. But then arises another problem because it is difficult to distribute water, nutrients and assimilates throughout a big body. So the next crucial innovation were vascular bundles, strands of tissue that act as the transport system inside a plant body. They contain two types of tissue: the xylem transports water and nutrients from the base to the apex, the phloem transports sugar and amino acids to wherever they are needed. As an added bonus, lignified xylem can also provide structural support to the plant.
- By now the plants are growing into longer and longer branch systems but they are still only anchored and acquire water and nutrients with a few ridiculously small cells called rhizoids. That won't do. How about sticking a few branches into the ground? Sure, they will not be able to photosynthesise but they can anchor the plant safely in place. Even better, they can reach for deeper water sources, allowing it to grow in superficially dry places. Congrats, we have evolved roots!
- We also notice that there is a limit to how big we can grow if we only rely on branching stems. If they are too thin, there is not enough structural support. If they are too big, there is too little surface for gas exchange and photosynthesis to feed the massive stems. Perhaps it is time for another round of specialisation. How about we have a fat stem for support but make the terminal branches flat so that they have a lot of surface? We need a different word for that kind of branch though, what do you think of "leaf"?
- Now everybody can grow quite big already but that only means that it is important to grow even bigger. After all, we need to over-top our neighbours to get more of that delicious sunlight. Sadly, we have hit another obstacle. There is a limit to how thick we can reasonably grow our stems and roots in one go before they need to reach for the light and water, respectively. Perhaps if we put a continually growing layer of cells into the stem and roots, to make more xylem and phloem as need, we could grow even thicker and thus support even more height. Achievement unlocked: secondary growth. You are now a tree!
- Somewhere along the line, we might wonder whether it is really such a good idea to have motile, flagellated sperm cells. That worked well when the ancestors of the land plants were aquatic, but now that they are on dry land it seems a bit silly that one still needs a film of water to connect male and female reproductive organs to have sex. Any ideas?
It is important to understand that I simplified a lot and that the steps described above did not necessarily occur in that order. Indeed different lineages of land plants evolved some of these innovations in varying order, or not at all. Obviously also individual sub-lineages of the land plant lineage went back into the water: in the case of duckweed or sea grass, the race is on to become more algae-like because that is what makes them better at surviving in the new habitat they have now entered.
The point is simply that the first land plants were poorly adapted to the new environment whereas their descendants have had a long time to get better at dealing with it. Contemporary plants can deal with the problems of living on dry land in a way that makes their ancestors look primitive.
But that is just it: their ancestors, not other contemporary land plants. Even those that have not yet evolved secondary growth, for example, or even secondarily lost it, are apparently good enough to thrive. They aren't primitive, otherwise they would not have survived. But still we might say that something like Aglaophyton, without leaves and roots, represents a first primitive solution to the adaptive problem of living on dry land compared with later, more advanced solutions represented by its own descendants. In other words, it does not make sense to talk of primitive and advanced in one time slice but it can sometimes be justified when looking across time.
Similar situations abound, especially at small scales: after a pollinator shift, in plants colonizing a newly arisen mountain or island, in a parasite changing hosts, etc. Thus evolutionary biologists distinguish stabilizing or purifying selection, in which a population is already well adapted to its environment and all deviations are selected against, directional selection, in which a population is over time becoming better adapted, and disruptive selection, in which a population is slowly split in two directions. In the second case the concept of directionality is already included in the name.
Of course, none of this implies that there is any plan or purpose behind evolution. The directionality comes simply from the involuntary movement of a population onto a new peak on the fitness landscape and as far as we can tell not from some cosmic intelligence pushing things in that direction.
*) A keen observer will notice that I am neglecting the bryophytes here. Theirs is a second viable strategy for living on dry land but it is completely different in that they do not attempt to keep their water behind a cuticle when it gets drier, they simply dry out and wait for new water to arrive. That also means that they cannot get very big. Still, a contemporary moss is not any more primitive than an oak tree, it has merely found a second adaptive strategy to the problem of living on dry land. Among all the plants trying the same adaptive strategy, however, we could distinguish better or worse solutions.