There is a joke that goes something like this:
A scientist at the local university found himself one Summer without any grant money. So he asked around town if anyone would be willing to hire him to apply his skills and knowledge to any problems they had. A local dairy farmer decided it might be worthwhile to hire the scientist to work on the issue of increasing milk production.
A deal was signed. The scientist got money and some space in the barn to work on the issue of increasing milk production.
After about two months, the farmer was wondering if he was getting his monies worth. After all, he never saw the scientist feeding the cows, or milking them or doing any farm chores. All he ever did was spend his day in the barn with his chalkboard. The farmer asked the scientist if he had made any progress on the issue of milk production and the scientist replied that he had made tremendous progress and he would love to share his findings with the farmer. The next morning, the farmer goes to the barn to see what the scientist has accomplished.
The scientist goes to the blackboard, draws a large circle and begins, "Assume a spherical cow of uniform density."
Cows are not spherical, nor are they uniformly dense. There is certainly nothing wrong with the general abstractions or simplified models scientists make. They help to focus on the central issue without getting lost in details. However, sometimes, these abstractions and models are taken a little too far and assumed to be suitable replacements for the real thing. After all, who wants to deal with a myriad of messy details?
Pharmaceutical testing is one such spherical cow. Drugs are tested using a model that only considers the effect of the drug, dealing with drug interactions is messy. As well, to ease testing of the drug, it is typically done only on men - since women have hormonal cycles that complicate things. When pharmaceuticals are tested on women, they usually require women to be using some sort of hormonal contraceptive - again, to get rid of all those messy details that complicate research. There are a lot of spherical cows in cloning, genetic research, climate change and pretty much any other scientific issue you want to consider.
When Zebra mussels (driessena polymorpha), an invasive species, first arrived in Canada, much was said about how destructive they are - how they are displacing native mollusks, how they have no natural predators. Both facts are true. Calling them destructive is a judgment - since they really only do what they normally do - change is a mainstay of biological systems and nothing ever remains the same. We certainly don't pine for 2 km thick ice sheets over Canada (which was the natural state 10,000 years ago) – the remnants of these ice continental sheets remain over Greenland and Antarctica). That they displace native mollusks is true, but that is part of evolution*, either an organism adapts and survives in its environment or it dies out or gets displaced. It is also true they have no natural predators, but, in 2004 it was discovered that indigenous fish (yellow perch) now prey on them (technically, the perch prey on the Quagga Mussel, dreissena bugensis, a relative of the Zebra Mussel, and another invasive mollusk).
Spherical Cow image courtesy of teh Space Telescope Science Institute, which can be found here.
Zebra Mussel photograph courtesy of the U.S. Geological Survey. That image, along with others, can be found here.
For those who might object that evolution doesn't work that way, that it is slow and gradual and does not include invasive species introduced by humans, erm ... I don't agree with you. Invasive species can be brought or gain a foothold many ways - on driftwood, by ocean currents, migrating birds / animals, the wind, forest fire. Humans are just one transmission vector of many.