That’s a great question! The key to thinking about evolution is to recognize that there’s no ultimate goal. Evolution is just changes in how common certain genes are in populations. As you know, if certain genes are helpful, individual that have those genes are more likely to reproduce. So their genes will usually become more common. This is natural selection. Ask yourself when it is better to be big or small. Being big can mean that you are too big for predators to capture easily, but it also means that you have to eat a lot more. This means moving around a lot to get food. It also becomes very hard to hide. So being a certain size could be good or bad depending on your environment.

Think about horses. Their distant ancestor lived in forests, and ate leaves from trees, bushes, and other plants. It did not eat grass because grass did not exist yet. These animals were only about the size of a dog and could hide from predators. As grass evolved, some species evolved to eat it. Grass is very tough and contains silica, which is what glass and sand are made from. Some descendants of the horse ancestor moved onto the grassy areas. There was no benefit to being small since you can’t hid in the grass once you are taller than the grass. Also, it takes a long time to digest grass, so a big “storage tank” in the digestive system is useful. Your gut can’t get big unless your body does. So a gene that made the grass-eater larger would be useful. Individuals that were larger were more successful, so the genes for being big were passed on more than the genes for being small.

So what changed that caused a change in size? The environment. The costs and benefits of size changed when the world around the animal changed. If you look around at the horse family today, you will see small burros, medium-sized zebras, and big horses. Of course, domestic horses were bred by humans to be different sizes. That’s artificial selection. The little burros do well in very dry areas where plants are fewer and scattered. Bigger zebras need more food and live in big grasslands with more big predators.

There’s no rule that says species will get bigger, smaller, or stay the same. It all has to do with what works best in a particular environment at a particular time. What’s the biggest land animal you can think of? Did you say “elephant?” Fossils show that elephants on islands became way smaller over evolutionary time. Why would it be better to be small if you lived on an island?

Why body size changes in animals is a good question and we still don't know the answer. I'm not aware of spiders once having been bigger, though; from what I understand, the range of size in spiders is more or less as it always has been (from giant tarantulas the size of your hand to tiny things you can barely see).

A number of patterns have been identified, and that is that animal size varies a great deal on islands relative to mainlands, but the direction is not well-understood. Lots of animals on islands become giants, while others become dwarfs. Again, I don't think we understand the details at this time.

Good questions, but "grow" and "shrink" probably aren't the words you meant. A puppy grows into a dog, but over time horses evolved from small ancestors into the fairly large animals of today. At various points in Earth's past the atmosphere was richer in oxygen than it is today--this allowed some animals, including spiders, which have relatively poor respiratory systems, to attain larger sizes.

There are various evolutionary advantages to both large and small body size. For example, large body size helps to ward off predators. But at the same time, there are many more spaces for small organisms to hide.

Being large or small isn't all good or all bad--depending on the kind of animal and the kind of environment, both can be advantageous or disadvantageous--it all depends. In some ways this is lucky: if one specific body size were clearly superior to all others, one would expect organisms to have converged on this one size, and would have looked boringly similar.

As it is, organisms across a huge size spectrum are suited to various of the multitude of environments that exist on our planet, contributing to the tremendous diversity of living things we see around us.

Best of luck with your studies,

This is an interesting question. However, we need to be careful here: I believe there are probably many confounding variables associated with the trend your teacher suggested.

"Confounding variables" are hidden variables that correlate with both the dependent and independent variables we are interested in. In your teacher's statement, size is the independent variable (how big or small the animal started out), and the tendency to grow or shrink is the dependent variable (whether it grows or shrinks depends on how big the animal was to begin with).

So examples of confounding variables could be something like the type of food the animal ate, or the type of animals that would eat our animal of interest. Thus, we cannot necessarily say that the animal's "original" size caused it to grow or shrink.

However, to answer your example about spiders: spiders, as well as insects were very large early in their history. However, as time passed, they shrank in order to better avoid being eaten by the large birds that were starting to appear and behave as predators toward them. I hope this helps!