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It's uncertain that the parasite really knows anything. It could just be acting out of instinct.
Studies into epigenetics have shown that certain experiences can cause hereditary behavioral changes. For example, when mice are taught to associate a specific smell with an electric shock, they will produce offspring that instinctively avoid that smell. This can explain why parasites may appear to instinctively 'know' things about their host.
Evolution theory can explain how symbiosis and parasitic relationships come about. Here's one scenario:
You have two species that don't interact with each other, but eventually they are put into an environment where they interact. Let's say there's a larva which burrows into fruit, but for some reason, it burrows into a snail that is on a piece of fruit. It turns out that living in the snail is safer and easier than burrowing into fruit, so this larva survives, produces offspring that are in proximity to other snails, and they do the same thing.
These parasites live for generations inside snails and they stop burrowing into fruit all together. Eventually, they lose their ability to live in fruit, and they become dependent on the snail.
Now, natural selection favors the parasites that are best at surviving in snails. Beneficial changes in behavior or morphology increase the parasite's chances of survival, allowing it to reproduce and pass this change onto the next generation. A negative change will decrease the parasite's chance of survival, making this change less likely to pass to the next generation.
The species adapts as these beneficial changes slowly stack up over generations, until eventually you get an extremely sophisticated, complex method of survival.
Quoting: Anonymous Coward 75814481 Yes, but your example explains the interaction between a parasite and one type of host.
This parasite (and others like it) needs two entirely different species to complete its complex life cycle.
Evolution theory says adapting to the environment and passing on the adapted behaviour/genes to the offspring is how it works. And it works by linking this to survival.
"Learning" how to jump from snail to bird would take countless trials and errors and many failures for those trying. Two problems here:
1) Nature doesn't work like this. Instinct is to keep it simple and ensure survival and reproduction. If the parasite got inside the snail why would it risk anything and not keep its life cycle limited to the snail.
2) The heredity aspect is questionable. If you have a large population of parasite successful in survival by infecting snails, a few who somehow manage to cross to birds too through a risky previously untested process - how would they pass that on to the whole population?
Leucochloridium infect snails, birds merely eat the snail. That doesn't require any trial and error or learning on the part of the parasite. You're assuming this larva somehow learned to get eaten by a bird.
Because birds eat the infected snails, and their droppings spread the eggs of the parasite to other snails, then their is a survival advantage for any parasite that can increase the chances of a bird eating the snail. This could be done by simply slowing the snail down, or making it more visible.
We don't know exactly how this relationship came about, but it's not a problem for evolution.
The bird is essential for their reproductive cycle.
No bird, no reproduction. It's not about survival advantage, it's about survival period.
Just because the bird is essential now, that doesn't mean it always was. Humans have domesticated plenty of species, and many of them are now incapable of living without human assistance. But in the past, their ancestors could survive in the wild.
It's possible the parasite's ancestors were capable of living without one or both of these hosts, but as the parasite adapted, it eventually became dependent on them. Just as domesticated animals are now dependent on us.
Also, the parasite makes sure the infected snail "signals" to the bird by making its tentacles look like caterpillars.
How and why would the parasite develop such a complicated reproduction process?
Quoting: RomanianGuy Again, Evolution can explain it. If getting eaten by a bird increases your chances of reproducing, then natural selection will favor the individuals that are best at attracting birds. It's like this:
First, You have a parasite that lives in the snail but does not modify the snail's behavior. Then, some parasites acquires the ability to make the snail stay in the open, where it can be spotted. Birds target the new parasites over the old ones, therefor the new parasites reproduce more often, until they outnumber the old parasites and eventually replace them.
Now all the parasites cause snails to stay in the open. Next, some acquire the ability to swell up the snails body, making it more visible. These new parasites out-compete the old ones, until they become the norm.
Next, some parasites begin to swell the snail's eye stalk, and they out-compete the old ones.
Next, some parasite's cause the eye stalk to pulsate, which makes them stand out more.
Next, some start to resemble caterpillars, and so on. Every step increases the parasite's chances of reproduction.
The end result is a parasite that is perfectly adapted to attracting birds.
It only makes sense if it was already "programmed" to do so, with the behaviour fitting perfectly in the larger picture of the biotope.
The parasite has no clue what a bird is, let alone what a bird likes to eat and is attracted by. In a normal evolutionary logic there would be no point for the parasite to use an intermediary species in a complicated and risky reproduction cycle.
Quoting: RomanianGuy How do you know that this life cycle couldn't come about through gradual changes? Just because you can't imagine how this could evolve, that doesn't mean it didn't.
Leucochloridium is a flat worm. Evidence suggests that flatworms existed before birds, and different flat worm species share a common ancestor. Your spontaneous creation hypothesis is not supported by the data.
