Field mustard is an annual plant that grows wild in many regions of the world. It has small yellow flowers and is pollinated by insects. Field mustard has to flower and produce seeds to give rise to the next generation. Southern California experienced a severe drought between 2000 and 2004. Plants that flowered earlier reproduced. Those that flowered later died without producing seeds. This led to more early-flowering field mustard plants. This change in population characteristics took only a few generations. How do you think scientists discovered that the flowering time of the field mustard had changed?

The kidney filters potentially toxic substances in the blood, and thus “clears” the blood of those substances. this clearance function is dependent upon and proportional to the diffusion gradient of the substance across filtering capillaries, i. e. if the concentration of the substance is doubled, twice as much will be cleared from each ml of blood that is filtered. suppose that the body produces a constant amount of a substance x per unit of time. the kidneys eliminate substance x at a rate directly proportional to the concentration of the substance and the volume of blood cleared each minute (c): elimination = c × [x], where [x] is the steady-state concentration of substance x. imagine an individual with an initial concentration of x equal to [x]0 who develops kidney disease. her baseline clearance c0 drops to one half of the original (½c0). what is the new steady state concentration of x? (for simplicity, assume that substance x is 100% filtered by the kidney).