|What causes small animals to grow, while big
animals tend to shrink? (over long periods of
time) My teacher was talking about how for example
spiders used to be bigger? But how can that be?
Yes I learned some things about evolution and how
over long periods of time things tend to change,
but why shrink? Why grow?|
|Question Date: 2013-05-24|
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
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?
Thanks for asking,
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
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
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
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
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
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
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