Brodie et al. (1992) studied how bird predation exerts natural selection on garter snake traits (some details here are highly simplified compared to the true, natural system). Cryptic color patterns (broken-up pattern, similar to camouflage on military uniforms) help the snakes to hide from birds, but interestingly, the benefit of cryptic coloration depends on the snakes’ behavior. If a snake tends stay put when threatened by a predator (bird), cryptic coloration will prevent the bird from spotting and eating the snake. But if a snake moves when threatened by a predator, it will be spotted and eaten if it has the cryptic color pattern. Whereas, a moving snake with an alternate color pattern (longitudinal stripes) is hard to visually track when moving, and will thus escape without being eaten. To summarize, there are two important traits, each with two possible variants: a behavioral trait (we’ll call the variants ‘stayer’ or ‘mover’) and a coloration trait (we’ll call the variants ‘broken’ and ‘striped’). A snake can only be a stayer OR a mover (not both), and only broken OR striped. But being a stayer is not mutually exclusive from having broken coloration. Finally, the stayer/mover trait (event) is independent of the broken/striped trait (event). Imagine that 25% of a population of garter snakes are ‘stayers’, 10% of that same population are ‘broken’ (coloration), and that birds encounter snakes (i.e., get within visual range) at random (analogous to reaching into a jar of blue and green balls and selecting a ball at random). A)What is the probability of a bird randomly encountering a snake that is striped AND a stayer, resulting in the snake being eaten? 0.225, 0.15, 0.75, 0.98, 0, or 1 B) What is the probability of a bird randomly encountering a snake that is striped OR a stayer?
Brodie et al. (1992) studied how bird predation exerts natural selection on garter snake traits (some details here are highly simplified compared to the true, natural system). Cryptic color patterns (broken-up pattern, similar to camouflage on military uniforms) help the snakes to hide from birds, but interestingly, the benefit of cryptic coloration depends on the snakes’ behavior. If a snake tends stay put when threatened by a predator (bird), cryptic coloration will prevent the bird from spotting and eating the snake. But if a snake moves when threatened by a predator, it will be spotted and eaten if it has the cryptic color pattern. Whereas, a moving snake with an alternate color pattern (longitudinal stripes) is hard to visually track when moving, and will thus escape without being eaten. To summarize, there are two important traits, each with two possible variants: a behavioral trait (we’ll call the variants ‘stayer’ or ‘mover’) and a coloration trait (we’ll call the variants ‘broken’ and ‘striped’). A snake can only be a stayer OR a mover (not both), and only broken OR striped. But being a stayer is not mutually exclusive from having broken coloration. Finally, the stayer/mover trait (
Imagine that 25% of a population of garter snakes are ‘stayers’, 10% of that same population are ‘broken’ (coloration), and that birds encounter snakes (i.e., get within visual
A)What is the probability of a bird randomly encountering a snake that is striped AND a stayer, resulting in the snake being eaten?
0.225, 0.15, 0.75, 0.98, 0, or 1
B) What is the probability of a bird randomly encountering a snake that is striped OR a stayer?
1 ,0.925, 0.25, 0.15, or 0
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