Evo-Devo

Atavisms: talk about old school

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This month’s Current Biology has a “Quick Guide” segment by Brian Hall on atavisms: the occasional and random appearance of ancestral traits in individuals of species that no longer have that trait. Examples Hall provides are vestigial hindlimbs (legs or fins) occasionally found on dolphins or snakes, which evolved from animals that did have limbs.

This is wild, because it implies that part of the ancestor’s developmental program has been furtively retained in its descendants, but this program generally never gets carried out. But every now and again a mutation may arise that causes the ancestor’s developmental program come alive all Franken-style. Nuts!
Here’s a crazy hypothetical example: the axolotl is an evolutionary abomination, a salamander in a state of arrested development. It’s basically a salamander that terminates development in what would otherwise be the larval stage of any other salamander. This is a nice real-life example of heterochrony (changes in the timing and rates of developmental events). Here, it’s a adult descendant that resembles the juvenile form of the ancestor (“neoteny”). (photo credit: John Clare, axolotl.org)
Wouldn’t it be wild, then, if the there was an axolotl in whom the ancestral full-salamander developmental plan was completed, resulting in an accidental salamander?! And then you could try to select for this atavism, possibly breeding peramorphic-atavistic-salamander axolotls (“salamander axolotls” for short)! If grad school doesn’t work out, this’ll be my Plan B.
Poll: If you could have any atavism, what would it be?
Reference
Hall BK. 2010. Quick guide: Atavisms. Current Biology 20: R871.

Bridging the gap: Australopithecus from Woranso

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Recently discovered Australopithecus fossils from the Ethiopian site of Woranso-Mille help fill a gap between parts of the early hominin fossil record (Haile-Selassie et al, in press). The fossils date to between 3.8-3.6 million years ago (Ma), and consist of several teeth and a jaw fragment. These specimens show a number of features that are intermediate in morphology between the earlier Au. anamensis (4.2-3.9 Ma) and later Au. afarensis from Laetoli (~3.7-3.5 Ma). As a result, the Woranso fossils lend support to the hypothesis that Au. anamensis and Au. afarensis represent a single evolving species (i.e. Kimbel et al. 2006).



I think this is exciting for two reasons. First, the fossils bridge the morphological gap between the older anamensis and younger afarensis fossils. As a result, we get to ‘see’ anagenetic evolution—changes within a single lineage. One topic in evolutionary biology is about the mode and tempo of evolution: are species fairly constant, then evolve into multiple ‘daughter’ species (“punctuated equilibrium”); or does evolutionary change tend to occur more within individual lineages (“anagenesis”)? Obviously neither is mutually exclusive, rather evolution is probably best characterized variously by both processes. Still, in the world of paleoanthropology, where many researchers argue for rapid and constant species turnover within the human lineage, it is cool to see a convincing argument for anagenesis. However, this ignores the meager (but intriguing) K. platyops material (Leakey et al. 2001), dating to around 3.5 Ma, possibly indicating the proliferation of at least two hominin species shortly after 4 Ma.



Second, the morphological intermediacy of the Woranso fossils allow a look at the patterns of evolutionary change within the anamensis–afarensis lineage. The authors note that the teeth of the Woranso hominins are generally more similar to anamensis, but have some derived characters of the later afarensis teeth. If we truly have a glimpse of dental evolution within a single lineage, we can ask questions about the evolution and development (“Evo-Devo”) of teeth. Are changes in these teeth correlated in a way that could be predicted by certain developmental models? Or is selection acting independently on various tooth traits?



References

Haile-Selassie Y, Saylor BZ, Deino A, Alene M, and Latimer BM. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and Taxonomy of Early Australopithecus. American Journal of Physical Anthropology, in press.

Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, and Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of Human Evolution 51: 134-152.

Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, and McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410: 433-440.