Convergent and Divergent Evolution

Analyse asks you to identify components and show the relationships between them. Here the components are ancestry, selective pressure and structure, and the relationship between them is the whole answer.

6 marks. Establish the distance between the two lineages, then the shared pressure, then what the resemblance is and is not, then name the process and what it implies for classification.

The thylacine was a marsupial and the grey wolf is a placental mammal, and their last common ancestor lived more than 100 million years ago. The thylacine's closest living relatives are the quolls and the Tasmanian devil, not any dog. The two are about as distantly related as two living mammals can be, yet they are so similar in skull and body form that they are routinely confused in photographs.

Both lineages were selected as medium-sized, pursuit-hunting terrestrial carnivores taking similar prey in similar open and wooded habitats. That role imposes specific and closely comparable demands: a long narrow skull with jaws that can grip struggling prey, teeth specialised for slicing flesh, a deep chest, and a body built for sustained running rather than short bursts or climbing.

Given similar demands, natural selection acting independently on two lineages favours similar solutions, because the range of workable body plans for a given way of life is limited by physics and by the materials available. Each lineage arrived at the same design from a different starting point and by independent changes, so the resemblance is the result of two separate evolutionary paths converging on one functional answer, not of shared inheritance.

The resemblance is superficial in a specific sense: the similar structures are analogous, meaning similar in function and appearance but of different evolutionary origin, rather than homologous, meaning inherited from a common ancestor. Underneath the similar exterior the two retain their ancestral differences, and the thylacine kept the marsupial features of its lineage, including a pouch, which no wolf has. Detailed anatomy and molecular data place it firmly with the dasyurid marsupials.

This is convergent evolution: unrelated or distantly related lineages independently evolving similar traits under similar selective pressures. It contrasts with divergent evolution, in which one ancestral lineage splits and its descendants become progressively less alike, producing homologous structures such as the forelimbs of a whale, a bat and a human. The consequence for classification is important: overall resemblance is an unreliable guide to relatedness, which is why biologists classify using homologous structures, developmental evidence and molecular data rather than appearance.

Explain means give the reason. The reason here is about what each kind of similarity can and cannot be produced by, so define both kinds precisely before comparing their evidential weight.

5 marks. Define homologous and analogous, show that appearance has two possible causes, show that homology has effectively one, then give an example and a limit.

A homologous structure is one shared by different species because it was inherited from a common ancestor, and it is recognised by a shared underlying structure and pattern of development even where the function differs. An analogous structure is one that is similar in function and outward appearance but arose independently in separate lineages, and it does not share that underlying structure.

Overall similarity in appearance is weak evidence because it can be produced in two quite different ways: by shared inheritance, or by convergent evolution, in which unrelated lineages face similar selective pressures and independently arrive at similar solutions. Observing that two species look alike therefore does not distinguish between those explanations, and the thylacine and the grey wolf show how far the resemblance can go between lineages separated by more than 100 million years.

Shared underlying structure is different, because selection acts on function rather than on hidden architecture. A bat wing, a whale flipper and a human hand perform flight, swimming and manipulation, yet all three are built from the same bones in the same arrangement, a single upper bone, a pair of lower bones, then wrist bones and five digits. Convergence explains why unrelated animals that fly have wings; it does not explain why animals doing entirely different jobs should share one inherited skeletal plan. Inheritance from a common ancestor does explain it.

Homology is stronger still because it is testable against independent evidence. Structures identified as homologous by adult anatomy generally also develop from the same embryonic tissues and in the same sequence, and the pattern of homologies generally agrees with the pattern of relationships derived from DNA and protein sequences. Agreement between independent methods is what turns a plausible reading into strong evidence.

The distinction requires care rather than being automatic, since some structures are genuinely hard to classify and a structure can be homologous at one level and analogous at another, as with the wings of a bat and a bird, which are homologous as forelimbs but analogous as wings. The conclusion stands nonetheless: appearance is consistent with both common ancestry and convergence, while shared underlying structure across different functions, corroborated by development and molecular data, is best explained by common ancestry alone.