twinsen wrote:The main problem I see here is if you have like 10 parts of a system.
part 1 is harmful, becouse you lose energy, required to survive
part 2 is harmful, becouse you lose energy, required to survive
part 3 is harmful, becouse you lose energy, required to survive
part 10 is harmful, becouse you lose energy, required to survive
If you have the 10 parts, you gain 100 times the invested energy, ensuring your survival.
It is obvious , that N better than N-1 doesnt work, becouse you need to activate thouse parts all together, to make any sense.
The simple answer is that that sort of system doesn't get produced by evolution. Anything which requires
multiple steps which are significantly disadvantageous will not get off the ground. I say significantly disadvantageous because selection is slow to remove slightly disadvantageous mutations, especially in a large population, so some of these may persist long enough for other synergistic changes to occur before they are selected out of existence.
The key is that there are a surprising number of ways to get to complex systems which are neutral-to-advantageous at every stage. The argument you are making is called irreducible complexity
and IMO it's mainly an illusion that comes from not having a lot of knowledge about the detailed anatomy/pysiology/biochemisty etc. that's being discussed.
To take your example of carnivorous plants, firstly there is no
plant which receives its energy or carbon from eating animals. All carnivorous plants are also photosynthetic and get their energy from sunlight and their carbon from the atmosphere. What carnivorous plants get is some of their nutrients
(and not usually all of their nutrients) from animals. That's things like nitrogen, potassium, phosphorus, iron etc. The one that is particularly important is nitrogen, and increasing acquisition of nitrogen in nitrogen-poor soils is the principal selective driver for the evolution of plant carnivory.
This reduces the problem you pose somewhat, as any modification which leads to more insects/other animals dying close to the plant will increase the local availability of nitrogen and be selected for. Most carnivourous structures in plants are thought to evolve from hairy leaves. Hairy leaves exist for multiple other reasons, including to discourage predators, to trap dew for water and to protect the plant from photobleaching caused by excess sunlight. I think we can agree that making a leaf more or less hairy is not irriducibly complex.
Hairy leaves trap water droplets, and insects get stuck to water droplets due to surface tension (you will have seen this if you have ever had a fly land in your drink). These trapped insects die and are decomposed by bacteria. This releases nutrients onto the leaf surface, which plants are able to take up. Lots of plants can do this to some degree, so lots of "non-carnivorous" plants are already very inefficiently carnivorous and all natural selection has done is increase the efficiency of this process. This can be achieved in multiple ways.
So to look at your points:
twinsen wrote:For this, it needs to have some cells that produce nectars to lure insects.
Some cells that produce something that makes insect into "plant food".
Some leaves that are shaped as containers.
Some leaves (or part of other ones), that can contract and capture them.
1) Not really - insects already land on plants all the time, maybe to eat the plant, maybe just to have a place to rest or a good vantage point to observe predators or prey. Sure, if a plant produces attractive substances they can get more insects to land on them, and at every stage this is selected for because any increase in insects increases the nitrogen availability.
2) Not really - insects that die on or around plants already get broken down, by bacteria. If you secrete digestive enzymes then the process becomes more efficient and this can be selected for. Note that all plants produce various degredative enzymes used for internal metabolic processes, defence against pathogens etc., so it's not as if the plant has to evolve an entirely new class of enzyme.
3) Not really - insects are already trapped by water droplets on hairy leaves. Modifications can again make this process more efficient, for example by not fully unrolling a furled leaf during its development so that it forms a closed tube as in a pitcher plant, or secreting sticky substances as in a honeydew.
4) Not really - this is not a mechanism used by all carnivorous plants, and it's definitely not required for efficient carnivory. Plants can have contractile structures for other purposes e.g. for tracking sunlight or hiding leaves from predators, so again evolution is gradually able to modify existing mechanisms that evolved for other, simpler, purposes.
I think the key is that the system is not as interdependent as it first appears, and improving individual components of the system generally does actually produce individual selective advantages. Whether those advantages are sufficient to overcome any disadvantages (e.g. requirement for greater energy) depends on the degree of selection pressure. In the case of carnivourous plants the selection pressure imposed by nitrogen limitation can be massive
. These plants generally evolved in places where finding nitrogen is like finding water in the sahara, places where most plants can barely grow. Anything which gives one plant even slightly more nitrogen than its competitors is hugely advantageous. Energy isn't necessarily scarce in these environments - really you're trading a common resource (energy) for a rare resource (nitrogen), so the amount of energy that it makes evolutionary sense to spend on acquiring nitrogen can be quite large.
If you are interested in more on this a lot of this information came from the wikipedia article on carnivorous plants
, which seems to actually be rather good and contains a large section on evolution.