Understanding Evolution - Balancing Selection Pressures, Or Why All Features Are Tradeoffs
To celebrate Darwin Day, I'm going to recycle a recent Quora answer about evolution. Somebody had asked, Why would a gene that makes a gazelle slightly faster, but still much slower than a cheetah be favored by evolution?. Here's my answer.
Because everything in life is a trade-off, and cheetah attacks aren't a gazelle's only concern.
Running faster comes at a cost. In particular, it means bigger or stronger muscles to be able to propel yourself faster. Bigger muscles take more food to grow, and more food to maintain. So, the fastest gazelle is also the most likely to starve in times of scarcity. And it's also putting more of it's food resources into those muscles instead of reproduction and/or nurturing young, and may end up not having as many offspring / surviving offspring as a slightly slower gazelle.
And gazelles have other predators besides cheetahs. One in particular is so efficient that it doesn't really matter how fast a gazelle runs - our bullets are faster. And which animals do trophy hunters target? The biggest and most impressive. It's already been documented that trophy hunting has led to bighorn sheep having horns that aren't so big (Phys.org - Intense trophy hunting leads to artificial evolution in horn size in bighorn sheep), and that size limits in fishing has led to smaller fish (Phys.org - Intensive fishing leads to smaller fish). I don't know if gazelles have been studied in this manner, but I wouldn't be surprised at all if human hunting had strong selection pressures on their sizes.
There's this whole complex network of selection pressures acting on gazelles (and all other organisms). Evolution has to balance (metaphorically since evolution isn't conscious) an organism's strategies to dealing with these pressures, and can't focus on optimizing completely for one selection pressure if it means compromising too much on other ones. So, cheetah attacks are one pressure on gazelles, and this particular pressure pushes gazelles to be faster. So, evolution pushes them to be fast enough to greatly lower their likelihood of being caught by a cheetah. But going even faster would only reduce that risk slightly, and at the cost of hurting the gazelles chances of survival/reproduction in other ways. So, gazelles are fast enough, and there's no reason to waste their limited food resources on even bigger muscles, when they could be using those resources for other activities, or even just being smaller so that they don't need as much food.
If you want to look at it another way, it's like wondering why everybody doesn't have a Ferrari. Sure, Ferraris are fast, but they're also expensive, use a lot of gas, and have many compromises that make them less than practical everyday drivers. Evolution could make gazelles faster, but only by compromising them in other ways.
To add one more thing - the reason gazelles just have to be reasonably fast, but not as fast as or faster than a cheetah, has to do with the way attacks actually play out in real life. As Brian Dean pointed out in his answer, it's not like a track race, where the fastest organism is the winner. Cheetah's are only sprinters, with limited stamina. Gazelles are keeping a lookout for cheetahs already, trying to make sure the cheetahs don't get too close. The usual result is that the cheetahs can only get so close before starting their sprint, meaning the gazelles have a head start. The gazelle only needs to be fast enough that it can avoid the cheetah until the cheetah gives up, which is still pretty fast, but a good deal less fast than a cheetah. And an extra few miles an hour on the gazelle's top speed is a sizable percentage difference in how much more time it has to evade the cheetah.
Image Source: Wikimedia Commons