Grasshoppers Change Their Tune
The male Chorthippus biguttus uses a combination of high pitched and low pitched sounds (a harmony) to attract females to mate with. The females find that combination of the two tones the most attractive, so a C. biguttus lacking one of the sounds will be less likely to get a mate. Unfortunately for the ones that live next to a highway there are some complications. Due to the near constant noise coming from the road, the male C. biguttus’ low pitched part of his mating call is getting neutralized and drowned out. This of course is less attractive to the females. However, the males who’s low pitched tone is a bit higher and doesn’t get neutralized by the highway now have an advantage (they are exceptions/variation in the species). Now they are the only ones whose two pitches can be heard, making them the ideal mates (because these are the traits the females are attracted to).
This trait thus became more advantageous. Now the males with the higher low pitch are the ones having offspring and passing this trait on to the next generation. The next generation will already have a higher percentage of males with that specific trait and over time this will become the ‘average’. This causes directional selection in the evolution of the C. biguttus. After a couple generations, if you compare the C. biguttus from the louder habitat to the one from the quieter one you will see that the latter won’t have changed their lower pitch much because they wouldn’t have had a reason to (no highway that drowns out the sound) while the ones from the roadside will have a slightly higher pitched harmony (the high pitches in the harmonies won’t be as different, if at all). This is further supported by the fact that the C. biguttus collected near the highways that have been there for a longer time have adapted more than the ones from near newer highways. The older ones have had more time (generations) to adjust to the new environment.
So, if you look at just the grasshoppers that live near the highway, their evolution will be directional. However, if you look at the species in general the evolution will be disruptive causing speciationdisruptive, (causing on) because different traits will be advantageous depending on their environment, thus making them evolve differently.
Evolution of the Central European Blackcap Warbler
When the blackcaps first migrated (accidentally) to the UK they found themselves in a completely different environment than what they were used to. For example, their diet changed drastically. Instead of eating fruits in Spain they found themselves eating seeds from bird feeders. Suddenly, their short and wide beaks weren’t as useful anymore. Now, having long and narrow beaks was more advantageous. It made picking up the seeds (and potentially prying them open) easier. Thus, the birds with the longer beaks could eat easier and faster. Now that the advantageous trait shifted, a different group, that would otherwise have struggled more to get food in Spain, were prospering. They were now the ones surviving the winter and being able to fly back to their breeding grounds to mate. Because the ones with narrow, long beaks, mostly mated with birds with the same advantageous trait (also because they were the ones who survived) their offspring also inherited this trait. After many generations, this trait will be the ‘average’ and most blackcaps migrating to the UK will have it. Because this specific trait had such a huge impact on their survival (either they eat and live, or are not able to eat the seeds and starve) soon, in only fifty years, a slight change could already be seen between the population of blackcaps migrating to Spain and the ones migrating to the UK.
This is because if you have a short, wide beak in Spain you will survive easily, while if you migrated to the UK you would have a harder time finding suitable food. And since the two groups rarely interbreed (because they don’t have the same migrating route) the species would eventually be split up (by a disruptive selection) into two different species of blackcaps.