Mosaic evolution

In **mosaic evolution** some characters in a transitional form are basal, while others are remarkably advanced.

Apparently, evolutionary change takes place rapidly in some body parts or systems without simultaneous changes in other parts. Another definition is the "evolution of characters at various rates both within and between species".<sup>408</sup> Its place in evolutionary theory comes under long-term trends or macroevolution.

Evolution from a basal (early) form to a derived (later) form takes place in stages. Modules (groups of characters) change semi-independently of each other. They change at different times, so producing a mosaic of primitive and derived traits.

These changes play a leading role in major evolutionary transitions. It may involve speciations producing a series of species, only a few of which would be found as fossils.

By its very nature, the evidence for this idea comes mainly from palaeontology. It is not claimed that this pattern is universal, but it is common. There are now a wide range of examples from many different taxa.

## Examples

- Human evolution. The early evolution of bipedalism in Australopithecines, and its modification of the pelvic girdle took place long before there was any significant change in the skull, or brain size.
- Evolution of the brain. The different parts of the mammalian brain evolved at different rates.
- _Archaeopteryx_. Nearly 150 years ago Thomas Henry Huxley compared _Archaeopteryx_ with a small theropod dinosaur, _Compsognathus_. These two fossils came from the Solnhofen limestone in Bavaria. He showed that the two were very similar, except for the front limbs and feathers of _Archaeopteryx_. Huxley's interest was in the basic affinity of birds and reptiles, which he united as the _Sauropsida_. The interest here is that the rest of the skeleton had not changed.
- Meadow voles during the last 500,000 years.
- The pterosaur _Darwinopterus_. The type species, _D. modularis_ was the first known pterosaur to display features of both long-tailed (rhamphorhynchoid) and short-tailed (pterodactyloid) pterosaurs.
- Evolution of the horse. The major changes took place at different times, not all simultaneously.
- Mammalian evolution during the Mesozoic is another good example.

### A famous case re-examined

Huxley had pointed out that _Archaeopteryx_ was a mixture of reptile and bird features. Without the feathers and arms, its skeleton looked just like that of _Compsognathus_. We now know its bone growth physiology was much slower than modern birds, and more like that of its dinosaur ancestors. This means it would take longer after hatching before it could fly. A modern precocial bird takes for 3–6 weeks from hatching to flying. In _Archaeopteryx_ this milestone might have taken about 18 weeks. It might have taken two to three years to reach its final adult size. The evolution of the physiology of modern forms occurred later in the group's history. They have had over 140 million years to evolve since _Archaeopteryx_.

## Background

It has long been known that changes in the genes which control development cause changes in the final adult animal.

Recent work shows how master control systems in development ('homeoboxes') can organise selective changes in different parts of an organism. This is what underlies mosaic evolution.

## Related pages

- Gene regulation
- Heterochrony
- Neoteny
- Evo-devo
- Parallel evolution
- Convergent evolution
- Macroevolution
- Transitional fossil