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Evolution After Darwin

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Evolution After Darwin BIOLOGY & MEDICINE_Evolutionary Biology Evolution after Darwin Darwin’s pioneering work On the Origin of Species, in which he first formulated the principles of his theory of evolution, was published 150 years ago. This theory changed the thinking in biology significantly and influenced almost all areas of biological research. What could be more appropriate, then, than to stage a Max Planck Symposium on Evolutionary Biology in the Darwin Year 2009. TEXT CHRISTINA BECK volution goes on all the time.” Lenski’s studies began with twelve It was only after 33,000 generations This is the credo of Richard E. identical populations of an E. coli that a variant (Cit+) capable of exploit- Lenski of Michigan State Uni- strain. His objective was to find out ing citrate emerged – a fitness advan- versity. The American scientist whether random mutations that oc- tage that resulted in a clear increase in tracks evolution in the labora- curred in the past facilitate the evolu- the size of the population. So was this E tory with his team: some 21 years ago tion of key innovations. an extremely rare mutation, which he embarked on a long-term experi- would explain why its arrival was so ment with E. coli. Bacteria provide BILLIONS OF MUTATIONS delayed? Or was it a simple mutation good conditions for such experi- that had to be preceded by other muta- ments, as a bacterial population grows “One of the big advantages is that you tions in order for it to be phenotypical- to billions of cells within a very short can freeze bacteria and in this way ob- ly effective? “We tested these hypothe- time. Studying evolution in action tain fossil images,” explains the biolo- ses in experiments in which we calls for large numbers – it’s like play- gist. The bacteria grew on a medium repeated the evolution from different ing dice: if you roll the dice a hundred with a limited supply of glucose, starting points in the population his- times, you will almost certainly get a which also contained citrate. Howev- tory,” explained Lenski. six. Lenski hopes to overcome coinci- er, E. coli cannot use this citrate as a Not a single Cit+ mutant occurred dence through the sheer volume of source of carbon under oxygen condi- among the ten trillion ancestor cells cells and generations alone (“Our lab tions. “In the course of 30,000 gener- in the repeat experiments, either. All has produced 45,000 generations of ations, not a single generation devel- 19 Cit+ mutants obtained by the sci- bacteria so far”). His ultimate aim is to oped the ability to exploit citrates, entists originated from later clones. A identify the events that result in the even though each population had bil- sequence comparison of Cit+ and Cit advent of something new and original lions of mutations,” said Lenski. mutants showed that the necessary ci- in evolution. trate transporter was generated only 66 MaxPlanckResearch 3 | 09 The research studies presented at the Max Planck Symposium on Evolutionary Biology examined very different animal species including the beach mouse, which is a native of the US, cichlids from east Africa, the European great tit, and primates, including chimpanzees and us humans (l-r). when the corresponding gene was Hopi Hoekstra from Harvard Univer- to their conspecifics inland, they have able to recruit a promoter – the start sity in her lecture “From Mice to Mol- lighter-colored and significantly re- sequence that made it possible to read ecules.” Convergence is a fascinating duced coat pigmentation on their fac- it in the first place. And this, in turn, phenomenon in evolution: unrelated es, flanks and tails. The fact that the had become possible only as a result species develop similar characteristics lighter-colored coat provides better of the duplication of another gene. or a similar appearance in response to camouflage in the sand dunes was The evolution of this phenotype was similar selection pressures. But does demonstrated by a simple experiment thus dependent on the population’s this phenotypic convergence imply in which the researchers placed a fake “genetic history.” genetic convergence? In other words: dark mouse coat in the dunes: it was Are the same genes actually responsi- immediately carried off and then later HOW MICE ACQUIRE A ble for the development of similar abandoned by a predator (probably an DIFFERENT COAT COLOR forms and patterns? To find out, owl or a hawk) when it realized it had Hoekstra and her team studied coat been tricked. Genetic research provides the strong- color variants in natural mouse popu- “At least three genes – Mc1r, Agou- est proof of evolutionary theory to- lations. ti and Corin – influence coat color,” re- day. Charles Darwin could not have The mouse species Peromyscus po- ported Hoekstra. The Melanocortin-1 even guessed at such things. Indeed, lionotus populates fallow fields in the receptor, Mc1r for short, plays a key the door to a more profound under- southeastern US. However, popula- role here in that it controls whether standing of evolution opened only tions are also found in the bright sand the dark pigment eumelanin or the when James Watson and Francis Crick dunes along the Gulf coast and more light pigment pheomelanin is pro- succeeded in decoding DNA in 1953. than 300 kilometers away on the At- duced. The researcher succeeded in “What can genes tell us about evolu- lantic coast near Florida. They are demonstrating that the exchange of a Photos: Joel Sartore – Joelsartore.com/Vincent Musi/iStockphoto (2) tionary adaptation processes?” asked known as beach mice and, compared single nucleotide (a T instead of a C) in 3 | 09 MaxPlanckResearch 67 BIOLOGIE & MEDIZIN_Evolutionsbiologie left The coffee breaks provided an opportunity for lively conversation and exchange, as seen here between Dieter Ebert from the University of Basle (center) and David G. Heckel from the Max Planck Institute for Chemical Ecology. right In her lecture “From Mice to Molecules,” Hopi Hoekstra explained the genetic mechanisms that control adaptations in the coat color of beach mice. the sequence of the receptor gene in Using the Galapagos finches, Darwin been thought. Natural selection, that the mouse population originating had already clearly demonstrated the apparently endlessly cumbersome and from the Gulf coast altered the effec- fact that patterns, colors and forms slow mechanism of evolution, can tiveness of the receptor and hence can change as a result of evolution- clearly create new conditions within a changed the coat color. Arginine is ary adaptation processes. His draw- single generation. now inserted in position 65 in the ami- ings on this topic can still be found In another exciting finding, the re- no acid chain instead of cystein, and in every school science textbook to- searcher couple discovered that a new- this alters both the bonding of ligands day, and show how the finches devel- ly migrated larger species of bird could and the potential of the receptor for oped a thick beak for biting seeds, a make better use of the thicker seeds signal transmission. “Such Mc1r muta- longer and sharper one for flowers, than the long-established birds. The tions have also been observed in mam- and a short, pointed beak to access established residents responded to the moths,” added Hoekstra. tiny insects in rock crevices. As the arrival of the new competition through famous naturalist wrote in his travel adaptation – in precisely the opposite WHY DARWIN’S FINCHES APPEAR report of 1839: “Seeing this grada- direction as before, in that they now IN EVERY TEXTBOOK tion and diversity of structure in one developed smaller beaks. This meant small, intimately related group of they could access food sources in rock Interestingly, the lighter-colored coat birds, one might really fancy that crevices, for example, which their in the mice from the Atlantic coast from an original paucity of birds in thick-beaked rivals could not reach. could not be explained by the same this archipelago, one species had This adaptation, too, was quantifiable allele – the latter did not occur in been taken and modified for differ- in one generation of the birds. The re- them at all, nor could the researchers ent ends.” searchers call this phenomenon mi- find any new mutations in the Mc1r Rosemary and Peter Grant from croevolution. gene that would have influenced the Princeton succeeded in demonstrat- activity of the receptor. “Our findings ing the effect of natural selection di- HOW GREAT TITS FOLLOW show that a pigmentation pattern rectly. They studied the beaks of CLIMATE CHANGE that arose convergently in different ground finches on the island of mice populations that actually devel- Daphne Major and, at the same time, Other mechanisms also exist that oped under very similar conditions is recorded their food sources. In years could be the result of adaptation proc- clearly the result of completely differ- of extreme drought, a large part of the esses. Based on a long-term study car- ent genetic mechanisms,” said Hoek- population died and only the finches ried out in the UK, Ben Sheldon from stra. So there are different molecular with larger beaks survived because the University of Oxford demonstrat- solutions for attaining the same phe- they could crack thicker and harder ed that great tits are so plastic, in oth- notype under comparable environ- seeds when the seeds they usually fed er words adaptable, in their behavior mental conditions. The combination on became scarce. The beak shape is that the population as a whole is suc- of different alleles of the aforemen- not only inherited, as the Grants dis- cessfully adapting to the rapidly tioned genes Mc1r, Agouti and Corin covered. It also becomes established emerging changes in the climate.
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