Homologous vs Analogous
Analogous characters and homologous characters are characters used in phylogenetic analysis.
When a group of organisms has a homologous structure, which is specialized to perform a variety of different functions, it shows a principle known as adaptive radiation. For an example, all the insects share the same basic plant for the structure of the mouth parts. A labrum, a pair of mandibles, a hypopharynx, a pair of maxillae and a labium together form the basic plan of the mouth parts structure. In certain insects, certain mouth parts are enlarged and modified, and others are reduced and lost. Due to this they can utilize a maximum range of food material. This gives rise to a variety of feeding structures. Insects show a relatively high degree of adaptive radiation. This shows the adaptability of the basic features of the group. This can also be called the evolutionary plasticity. This has enabled them to occupy a wide range of ecological niches. A structure present in an ancestral organism becomes greatly modified and specialized. This can be called a process of descent by modification. The significance of adaptive radiation is that it indicated the existence of divergent evolution, which is based on the modification of homologous structures over time.
Structures and physiological processes can be similar in organisms that are not closely phylogenetically related and they may show similar adaptations to perform the same function. These are referred to as analogous. Some examples for analogous structures are eyes of vertebrates and cephalopods, wings of insects and birds, jointed legs of vertebrates and insects, thorns on plants and spines on animals etc. Similarities found in analogous structures are only superficial. For example, insect wings and wings of bats and birds are analogous structures, but the wings of the insects are supports by veins composed of cuticle and the wings of birds and bats are supported by bones. Also, vertebrate eyes and cephalopod eyes are analogous structures, but the embryological development of the two is different. Cephalopods have erect retina and photoreceptors facing the incoming light. In contrast, in vertebrates the retina is inverted and the photoreceptors are separated from the incoming light by the connecting neurons. Therefore, the vertebrates have a blind spot and the cephalopods do not have a blind spot. Convergent evolution is supported by the presence of analogous structures.
What is the difference between Homologous and Analogous Characters?
• Characters that are similar in function but have different evolutionary origins are known as analogous characters, whereas characters that have the same evolutionary origin are known as homologous characters.
• Analogous characters cannot be used to infer evolutionary relationships between taxa whereas homologous characters are used to construct evolutionary relationships and phylogeny of taxa.