Question

With the help of examples, explain the process of biological evolution on the basis of morphological evidence.

Answer 1

Evolution is a process that results in heritable changes in the population spread over many generations leading to the diversity of organisms on earth. Evidence of biological evolution comes from- Palaeontology, comparative anatomy and morphology, biochemical/ physiology, biogeography and embryology.

Comparative anatomy and morphological evidence show the similarities and differences among the organisms of today and those that existed years ago. These evidences come from the comparative study of external and internal structure.

1) If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous. Examples include the forelimbs of whales, humans, birds, and dogs which look different on the exterior. That is because they are adapted to function in different environments. However, in the bone structure of the forelimbs, the pattern of bones is very similar across species. It is unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, dogs, and birds. Sometimes, organisms have structures that serve no apparent function but are homologous to useful structures in other organisms. These reduced or nonfunctional structures, which appear to be evolutionary "leftovers" are called vestigial structures. Examples of vestigial structures include the tailbone of humans, the hind leg bones of whales, and the underdeveloped legs found in some snakes.

2) Organs which are anatomically different but functionally similar are called analogous organs. They evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures. Analogous organs are the result of convergent evolution. It is the evolution in which different structures evolve for the same function and hence, have similarity. For example, two distantly related species that live in the Arctic, the arctic fox and the ptarmigan (a bird), both undergo seasonal changes of colour from dark to snowy white. This shared feature doesn't reflect common ancestry i.e., it's unlikely that the last common ancestor of the fox and ptarmigan changed colour with the seasons. Instead, this feature was favoured separately in both species due to similar selective pressures.