<u>Answer:</u>
Recent evidence suggest that feathers evolved from scales and suggest that 'feathers and pycnofibers' could be homologous.
<u>Explanation:</u>
- One of the major difficult issue related to bird evolution is the evolution of feathers.
- Feathers are considered as the most 'complex integumentary structures' which are found in vertebrates.
- Evolutionary developmental biology suggests that the 'planar scale structure' is been modified for developing into feathers by 'splitting' to form web like structures.
- Scales and Feathers consist of 'two distinct form of keratin' so it was thought that 'each type was exclusive skin structure' but recent study suggests that they are developmental expressions of same skin structures.
The correct answer is D. species.
The definition of a species is termed as a group of interbreeding individuals cannot be easily applied to organisms that they reproduce only or mainly by asexual methods.
Species is termed as the basic unit of classification and taxonomic rank and also unit of biodiversity. It is the largest group of organisms whereby two individuals produce fertile offspring.
They are typically by sexual reproduction. Boundary which is between closely related species they becomes unclear.
