Geospiza fortis Skull or Medium Ground Finch is museum quality polyurethane cast. Medium Ground Finch Skull is made in USA. One of Darwin’s Finches

The Geospiza fortis or Medium ground finch is a species of bird in the family Thraupidae.

It is endemic to the Galapagos Islands. Its primary natural habitat is tropical shrubland. One of Darwin’s finches, the species was the first which scientists have observed evolving in real-time.

The Geospiza fortis or Medium ground finch population of medium ground finches has been experiencing inbreeding depression due to small population numbers.

Inbreeding depression occurs when there is a decrease in fitness due to individuals mating with genetic relatives. Typically, this leads to a loss of genetic diversity and a reduction in heterozygosity.

Like the other members of its genus, the medium ground finch is strongly sexually dimorphic. The female’s plumage is brown and streaky, while male’s is solid black, with white tips to the undertail coverts.

The bird measures 4.9 in. in length, which falls between the lengths of the small and large ground finches.

The bill of this species is quite variable in size, though the length of the upper mandible is always greater than the depth of the bill at its base.

The wing shape, on average, seems to change with ecological shifts. Different selective pressures act on the wing shape of the finches, such as natural and sexual selection.

The males have shorter, rounder wings, which help with maneuvering around a female during sexual displays.

In 1977, a severe drought reduced the supply of seeds in the Galapágos. The finch, which normally preferred small and soft seeds, was forced to turn to harder, larger seeds.

This strong selective pressure favoring larger beaks, coupled with the high heritability of traits relating to beak size in finches, caused the Geospiza fortis or Medium ground finch population to experience evolution by natural selection, leading to an increase in average beak size in the subsequent generation.

Evidence of evolution through character displacement has been found in a population of medium ground finches on the Galápagos island of Daphne Major.

During a drought in 2004, overlap in the diets of the Geospiza fortis or Medium ground finch population and a recently settled population of large ground finches led to competition for a limited supply of seeds on which the medium ground finch population normally fed.

Because the large ground finches were able to out-compete the medium ground finches for these seeds due to both a larger beak and body size, the medium ground finch population experienced a strong selective pressure against large beaks to avoid competition, ultimately leading to dramatic evolutionary change favoring smaller beaks in the subsequent generation.