Answer: natural selection
The bones in a bird are hollow, reducing its weight for flight, this is an example of ADAPTATION. The adaptive responses emerge as a consequence of different environmental pressures.
<h3>Adapatation and environment</h3>
Adaptation is an evolutionary process by which organisms adjust to the environment in which they live.
For example, the bones in a bird are hollow because this adaptive feature enables them to fly.
Adaptations are a consequence of the process of natural selection, i.e., the differential survival and reproduction of the fittest organisms.
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Tentacles of an octopus as well limbs of a lizard are analogous structures.
Analogous structures or organs perform the same function in different organisms that bear no resemblance to each other anatomically. Analogous structures are formed as a result of convergent evolution, i.e. different structures evolving for the similar function and thus having similarity. Tentacles of an octopus as well as limbs of a lizard are used for the similar function, i.e. locomotion in this case.
On the other hand, homologous structures result from divergent evolution. Homologous organs contain a similar basic structure but perform distinct functions in different organisms.
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