-A population of birds with various size beaks eats seeds. Small seeds can be eaten by birds with small beaks. Larger,
thicker seeds can only be eaten by birds with larger thicker beaks. During a drought, only large thick seeds exist so only
the large, thick-beaked birds survive.
is it directional, disruptive or stabilizing selection

Directional selection

Explanation:

The conditions of the environment change over time. These changes bring about the adaptation or not of species, hence, bringing about SELECTION. Selection is of different types but based on the example given in the question, DIRECTIONAL selection suits the description best. Directional selection is a kind of selection that occurs when an environmental change causes species with a particular phenotypic characteristics to be favored over the contrasting phenotype. Directional selection eventually results in the one-directional increase of species with a particular phenotype.

This is the case in this example involving a population of birds with different sizes of beaks (phenotype). The size of beaks determines the size of seed eaten by that bird i.e. small-beaked bird eats small seeds while birds with large and thick beaks eat large and thick seeds. A change in environmental conditions, which is drought, causes only large and thick seeds to be readily available. This means that only the birds with large and thick beaks can feed. Hence, the environment has selected or favored the birds with a large and thick beak over the ones with smaller beaks. This is an example of DIRECTIONAL selection because the selection goes in one phenotypic direction (birds with large, thick beaks).

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