Why are certain animals much much bigger than other ones?

Question

If you look at the following image:

The Argentinosaurus was the largest dinosaur that ever existed. If dinosaurs came through the process of evolution, and evolution is small changes in DNA and/or mutations, how can certain species by immensely bigger then others?

Even consider the blue whale. Why is it so much bigger then other whales. None of the others even come close to it, and same as the Argentinosaurus.

I understand if some are "a bit" bigger, but this dinosaur was the length of 3, and really the question that I want to ask is that, can evolution really make someone/something that much bigger then it's kind?

Answer 1

There is much more size variation than you seem to acknowledge!

The blue whale is actually much heavier than the Argentinosaurus was (180 tons against 90 tons).

If you are impressed by size different between humans and some dinosaurs, then you will really be amazed to appreciate of how much size variation exist.

You currently spend one orders of magnitude (2 meters for humans and 30 meters for the Argentinosaurus). If you include all of life, including bacteria, you will spend 9 orders of magnitude from ~$10^{-7}$ meters for the smallest bacteria (I excluded viruses, viroids and prions) to ~$10^2$ meters for the blue whale and the sequoia trees.

Of course, size does not mean much. Do you mean, the longest, the heaviest, the tallest, etc... A fungi might actually well deserved to be recognized as the biggest living thing on earth (see The biggest underground living animal)

You might also want to have a look at How did the huge dinosaurs cope with gravity and loads on bones?.

Evolution misrepresentation

You said

evolution is small changes in DNA and/or mutations

Evolution has brought different lineages to have DNA sequences that differ drastically. Compare an angiosperm (say a dandelion) and a bacteria (say and E. coli), you will hardly be able to align any of their DNA sequence. They are genetically completely different.

The standard saying from layman people is to say that evolution occurs through small gradual changes. Over a long period of time, such small and gradual changes end up making organism that differ a lot. The most recent common ancestor between the dandelion and the E. coli lived about 3 billions years ago (maybe more).

Now, even the saying that evolution is necessarily very slow and go through small gradual changes is quite wrong. Specific mutational event can be extremely drastic and there are period of extremely high speciation rate in a given lineage. But all of this is a story for another time!

Coming back to size, you might want to have a look at the post Do large animals often evolve into smaller animals?.

Answer 2

Why there are so many big dinosaurs

In evolution there is something called preadaptation or exaptation that is adaptations that have benefit in one way that happen to have another benefit after another adaptation occurs. Consider the avian breathing system which evolved in small ground running animals turns out to also make their breathing work at high altitude, this was not part of the conditions the adaptation evolved in/for but is a side benefit that is exploited later. Because there are multiple solutions to an adaptive problem and evolution has no foresight, some groups will just happen start in a better position anatomically to evolve large size.

Compared to mammals dinosaurs start out with better preadaptations to large size. mammals have a lumbar region in which all the weight is supported only by the spine while dinosaurs have ribs from the neck all the way to the pelvis, this means the torso is far more rigid, if you grow very large this helps distribute weight more efficiently. The avian breathing system (in theropods and sauropods) scales better and doesn't loose efficiency as fast as the mammalian system, air sacs allow them to better compensate for the large dead air spaces caused by large size. Air sacs also allow bones to be larger for the same weight, sauropod bones are mostly hollow where as mammal bones are either solid (or filled with something just as heavy) so if they were the same size the mammal bones will be significantly heavier. Now this is not to say dinosaurs are completely superior, live births is a big advantage for large size, but the simple anatomical advantage dinosaurs start with just make achieving large size far easier.

Why size and fossilization vary

As for why animals vary in size it is the same reason they vary in so many other respects, different solutions to the same problem, how to get your genes into the next generation, the same factors that favor different diets or anatomies can favor different sizes. A rat and an elephant are drastically diffrent is size but both are successful body plans. You are mistaken that nothing comes cclose to these animals it is just that the ones that do are dead (in the case of whales) or didn't leave fossils, keep in mind only the tiniest sliver of a fraction of organisms ever fossilize. Only one Argentinosaurus fossil is known to exist, that does not mean that 95 million years ago only one Argentinosaurus existed. Fossilization is so rare that in a population of literally billions you might only leave one or even no fossil remains. The fossil record is like taking one random photograph in paris and trying to reconstruct france we know we are missing massive amounts of what was there but it is still incredibly informative compared to not having it.

Why does size vary.

This one is simple, because selective pressures vary, different pressures favor different things, and different condition create different costs. In whales for instance the fact Blue whales have to fast for part of the year encourages large stores and thus large size, that is just one factor favoring large size. the largest whales and dinosaurs are not significantly bigger than their ancestors, the changes in each generation is minor and usually due to changes in different developmental triggers/regulators. Argentinosaurus is big nut it is comparable to other large sauropods.