E.1.4. Explain how animal responses can be affected by natural selection, using two examples.

Heinemann HL Pg. 462-464

Effects Of Natural Selection

Animal behaviour is more than a single reflex but a complicated series of responses to the environment. Some populations of organisms have changed their behaviour in response to a change in the environment. 

Variations in behaviour can occur in populations in the same way as variation in the characteristic, such as colour, of the animals. The characteristic of an animal is determined by genes just as behaviour can be determined by genes. 

Variations in behaviour can be selected by the environment. Since a genetically programmed behaviour can have variations, one behaviour can work better than another in a changing environment. The variation will allow one group of organisms to survive and reproduce better in the new environment. 

The theory of natural selection states that the organism best fitted for the environment is more likely to survive to reproduce. 

European Blackcaps

European blackcaps are small warblers which usually migrate between Spain and Germany. They breed in Germany in the spring and summer and spend the winter in Spain. 

About 50 years ago, bird lovers noticed that some blackcap warblers were coming to the UK instead of Spain for the winter. Ornithologists began studying their behaviour. It was noticed that the UK blackcaps left to go back to Germany 10 days earlier than the Spanish blackcaps. It was also noticed that the earlier the birds arrived in Germany, the more choice of territory they had, and the more eggs they laid. The UK blackcaps had a distinct advantage over the Spanish blackcaps. 

In order to determine if this behaviour had a genetic basis, an experiment was carried out. Eggs were collected from parents who had been in the UK the previous winter and other eggs collected from the Spanish birds. The young were raised and direction of migration was recorded. The absence of their parents meant that they weren't able to teach the young in what direction to fly. All of the birds in the study, no matter where they had been reared, tended to migrate in the same direction that their parents had gone. This supports the hypothesis that blackcaps are genetically programmed to fly in a certain direction.

Something in the blackcaps genetic variation causes the birds to leave for German earlier than those in Spain. Arriving in German early is an advantage. Warm winters in the UK also increased the survival rate of the birds. This change in migration patterns may eventually result in a new species - this is likely if small changes in courtship behaviour occur which will separate the species even more.

Sockeye Salmon

Some of the sokeye salmon in lake Washington migrate to the Cedar River, which flows into the lake. The river flows quickly, but the lake is deep and quiet. These are two different types of aquatic environment which are connected to each other. 

DNA evidence has shown that river salmon and lake salmon have stopped interbreeding. 

The lake salmon have one breeding method and the river salmon have another. The lake salmon spawn on the beaches; females lay their eggs in the sand. The males have heavy bodies, perfect for hiding in the deep waters of the lake. The large males, if put in the river, are not efficient in navigating fast currents. 

The males of the Cedar River population have traits naturally selected to be successful in fast-moving rivers. Their bodies are thinner and narrow for better maneuverability in the current. Females of the river group bury their eggs deep in the sandy river bottom so that they won't be washed away. Genetic studies show that fish hatched in the river had little success trying to spawn on the beach of Lake Washington.

Variations in the original salmon population were selected for by the two different environments. The original population diverged into two different breeding populations. The lake conditions favour one set of traits and the river conditions favour another set of traits. Sockeye salmon are now split into two genetically distinct populations: beach-spawning and river-spawning.