What genetic abnormality causes down syndrome? how does this usually happen?

An extra chromosome, also known as trisomy can lead to down syndrome.

The most common form of Down syndrome is known as trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. Trisomy 21 is caused by an error in cell division called nondisjunction. This leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.

2, 3, 5

Explanation:

Darwin's theory of evolution is most often associated with his study of finches.

He noticed that finches present on different islands had different types of beak suited to that particular environment. On one island, the beaks were better suited for cracking nuts. On another, they were long and thin for searching for bugs.

These finches came from a common ancestor, and migrated to different islands. Due to natural selection and lack of intermixing between the islands, the finches developed different traits over time.

He did this work with other species as well, noting that different species had developed distinct features that suited them to their environment.

Darwin also studied fossils, which provided evidence that animal form (anf function) changed over the course of thousands of years. This supported his idea of evolution that resulted from natural selection.