Answer:
The phrase "disease is an agent of natural selection" means that those individuals which carry specific genes to resist the disease will be selected by the nature, whereas those individuals which that don't have or not capable of developing resistance genes will be excluded.
It is a well-established fact that diseases (mainly outbreaks) are best action of nature to select only those individuals which can modify there genetic material and thrive against the changing environmental conditions. Let's take the example of Coronavirus outbreak (COVID-19) this year. From the statistics we know that old people that develop the disease are most likely killed within a few days. However, young people can develop immunity against the virus easily. Hence, nature selects young people and exclude older people. This natural phenomenon is however driven by the disease.
They both use <span>light to energize the creation of glucose also
</span> <span>Chlorophyll is a green pigment found in most plants, algae, and
cyanobacteria that as you probably know attracts light, chloroplasts
absorb light and turn it in to a freeform of energy << this is
called photosynthesis. </span>
The source is the allele for beak size being different, or more specifically the DNA sequences being different.
Scientists will probably look at the birds' beaks, feathers, skeletal structure to see if they are closely related.
