Why are some tarantulas larger than others?

Sure okay some tarantulas developed a dwarfism trait and some developed a trait in their DNA that allowed them to grow abnormally large. But how did these species survive? What factor (maybe like environment space or something) allowed them survive instead of just dying off?
Example, lets just talk about Darwin's findings on the Galapagos islands. Some finches developed a mutation where they got a different shaped beak. This benefited them because a thinner beak allowed some finches to feed from pollen and such, and the bigger, wider, stronger beaks allowed some finches to crush nuts.
But why was a dwarf tarantula, or a large tarantula, beneficial? How did these species thrive?

I hope you understand what I'm asking.

I'm doing an essay, actually. This is why I'm asking. What better example to use than tarantulas? Or maybe tarantulas aren't a good example... I'm just trying to compare how two towns in a book (The Chrysalids in case you're wondering) resembles evolution.
If you can, can you perhaps recommend me to someone who might know, or a website, or something? I need a secondary source to back up my points.

BUT YEAH. Why or how did these tarantulas survive?

My theory is that bigger tarantulas -> scarce food, largeness allows them to easily take down larger prey like rodents rather than just insects. Possibly also because they had to travel at a greater distance to find food or etc. Smaller ones -> smaller prey. Able to catch and eat smaller prey. Also able to fit into smaller areas.
Also H gigas can swim... maybe because their species was more towards a lake or ocean, and in that period of time there was little land-food, so they went to go and fish?

Damn I'd love to study the genetic code of tarantulas...





Thanks for any help.

Seeing as you refer to Darwin, you partially answered your own question, each specie evolved to be best suited for its environment. A lot of dwarfs live in places like Peru where there are a lot of mountains, perhaps because there was little threat to them, it was not disadvantageous to be small but owning dwarf species myself, I can tell you that they are very flighty and can be outright defensive, perhaps to make up for their small size. You would think the spiders of the Mesothelae family would be dead by now as they are slower and most have no venom glands unlike modern spiders, but they are still here because they still thrive in their environment. Old world spiders have no hairs they can flick and thus can be very defensive and fast, most of them inhabit harsh places like Africa,India,Austrialia and Asia (including pacific islands). If you look at those places they have a multitude of fierce predators and the spider must have a no prisoners attitude and many are quite large. Why is big good? well you can tackle a greater variety of food then say a dwarf can, species evolve and fill niches. Perhaps the ancestors of T.stirmi were able to take advantage of more prey then the smaller members of their species, giving them a better survival rate and the population eventually changed enough to give us the T.stirmi we know today. Also the spider tree of life is a total mess, constantly new families are being created and destroyed all the time. The big question in true spiders is if cribellate silk evolved once or twice in spider history, we have no idea and extensive genetic analysis of spiders is something very underfunded. I could perhaps send you over to my spider professor but finals are coming up so I'm not sure how willing she would be to talk.

This is easy to answer, but more difficult to conceptualize. Tarantula size (as with any
other physical trait) started out as a random mutation...nature was rolling the dice.
However, in a particular environment, a particular mutation will provide an organism with asurvival advantage. In some environments, large tarantulas had a higher frequency of
survival...in others, smaller tarantulas fared better. That's it.

I don't really think your theory on 'bigger tarantulas mean less food' type thing. If you look at all the Theraphosa (largest tarantula) they come from Guyana, in the Amazon Rainforest. Which is the exact opposite of scarce food.


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In terms of sizes it may have something to do with altitude levels and the amount oxygen in the air...

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It's pretty much been covered, but I need to add that not all traits are "mutations" randomly appearing that add a survival advantage. Large (or small) size can come from generations of the largest specimens in a species successfully mating and surviving into adulthood until their size is vastly different from thousands of generations before. There are many traits we don't understand so it's hard to say why they exist at all. Why does most of genus Ceratogyrus have a horn? Did it confer a survival or reproductive advantage or is it simply a mutation that never disappeared because it didn't give them a disadvantage?

It's nice to think of evolution as nice and neat and orderly, and for the most part it is, but sometimes it doesn't seem to be.

Anyway, short answer to your question is: because that's how it worked out for each individual species. ("Specie" is not a word)