Cuckoo finch egg in zitting cisticola nest.
There has been some serious forgery going on in the world of evolution!
And, despite any hopes the thought of that might raise in the deluded minds of creationists, I'm not talking about human forgeries such as the famous Piltdown skull hoax, but natural forgeries produced by the natural force of evolution by natural selection.
The forgeries in question are the way brood parasite birds' eggs mimic those of the hosts.
I wrote about this very phenomenon in my popular, illustrated book, The Malevolent Designer: Why Nature's God is not Good, in which I said:
Cuckoo Finches and Arms Races.Now one of the zoologists I cited in that section of my book, Professor Claire Spottiswoode from the Department of Zoology, Cambridge University, England, UK and the FitzPatrick Institute of African Ornithology, University of Cape Town, South Africa, along with Professor Michael Sorenson of the Department of Biology, Boston University, Boston, MA, USA, and others, has published an open access paper in PNAS which addresses the following questions:
The cuckoo finch or parasitic weaver, Anomalospiza imberbis of East Africa is an obligate brood parasite on other birds of the Cisticolas and Prinias families. As we have seen with the earlier brood parasites, this has resulted in the inevitable evolutionary arms race as the parasitized hosts evolve strategies to reduce the depredation and the cuckoo finch evolves ways to circumvent the defence strategies of its intended hosts.
The strategy adopted by the tawny-flanked prinia, Prinia subflava, is to change the colours of its eggs more quickly than the cuckoo finch can and to produce eggs in an array of colours with markings that the prinia recognises as its own, almost like a signature.
The red-faced cisticola, Cisticola erythrops, on the other hand, while not varying the colour of its eggs has evolved to be better at spotting the cuckoo finch’s eggs and removing them. Another species, the rattling cisticola, Cisticola chiniana which might be expected to be an ideal host for the cuckoo finch appears to have won the arms race and is not parasitized at all (89).
- How can a single brood-parasitic bird species simultaneously mimic the eggs of several different bird species to trick them into raising their young?
- How do these parasitic forgers pass this ability on to their young despite interbreeding between birds raised by different hosts?
In the case of A. imberis the genes that determine the colour pattern of the eggs the female lays are carried on the W chromosome, and females lay their eggs in the nests of their own adopted parent species, so females fall into several different evolved strains, and it matters not which males she mates with.
The Cambridge University news item, by Tom Almeroth-Williams, explains:
The team's findings are published, open access in PNAS. The statement of significance and the abstract contain more detail:Such ‘maternal inheritance’ allows cuckoo finches to side-step the risk of inheriting the wrong mimicry genes from a father raised by a different host, and so has allowed distinct lineages of cuckoo finch females to evolve specialised egg mimicry of several different host species. Such mimicry dupes host parents into accepting a parasitic egg as their own rather than throwing it out of the nest, and so has been crucial to the success of these African birds.In this particular coevolutionary arms race between species, natural selection has created a double-edged sword.
While maternal inheritance has allowed cuckoo finches to exploit multiple host species, it’s likely to slow their ability to evolve counter-adaptations as their hosts evolve new defences.
In particular, parasites face a daunting challenge because some host species have in return evolved an astonishing diversity of egg colour and pattern ‘signatures’, that help hosts to distinguish their own eggs from parasitic mimics.
Professor Claire Spottiswoode
Department of Zoology
Cambridge University, England, UK
And FitzPatrick Institute of African Ornithology
Department of Science and Technology–National Research Foundation Centre of Excellence
University of Cape Town, Rondebosch, South Africa
But the researchers believe that this long-established ‘genetic architecture’ of maternal inheritance may come back to haunt the cuckoo finches.
Cuckoo finch mimicry of tawny-flanked prinia eggs
This photograph shows eggs of the cuckoo finch (middle circle) and one of its common host species, the tawny-flanked prinia (outer circle), revealing the diversity of host egg colour and pattern “signatures” (an anti-parasite adaptation that aids host parents in recognizing their own eggs) that are mimicked by cuckoo finch egg “forgeries".
However, cuckoo finches do not mimic the rich olive-green eggs (top left) laid by some tawny-flanked prinia females. The new study suggests that a genetic constraint in cuckoo finches may account for their apparent inability to mimic this host egg type.Image: Professor Claire N. SpottiswoodeIn their fightback against the forgers, grass-warblers have become skilled quality controllers, rejecting eggs that differ from their own in colour and pattern, and all four species have evolved the ability to deposit unique ‘signatures’ onto their own eggs to enhance their detection of intruders. Tawny-flanked prinias, for example, lay eggs with blue, white, red, or olive-green backgrounds overlaid with a variety of patterns.Cuckoo finch and host chicks. A cuckoo finch chick (top) begs for food very vigorously as soon as it hatches, outcompeting the host parents’ own chicks - here zitting cisticolas - which typically soon die of starvation.
Image: Professor Claire N. Spottiswoode
Cuckoo finches have responded not only by evolving mimicry of the eggs of their several host species, but have also further diversified to mimic at least some of the signature-like variation seen in the eggs of different females within each host species.Cuckoo finches are missing out on a powerful source of evolutionary novelty and that could prove costly in this ongoing arms race.
The way they inherit their ability to mimic host eggs has a downside by likely making the grass-warblers’ defences more effective, and constraining the parasite’s ability to respond.
We may see the emergence of unforgeable egg signatures which could force cuckoo finches to switch to other naïve host species. Or the parasitic birds might become increasingly dependent on young host individuals that haven’t yet learned their own signatures and are bad at spotting mismatched eggs.
Professor Claire N. Spottiswoode
The researchers believe that the cuckoo finches now face an uphill struggle because they cannot recombine the different forgery traits evolved by their separate family lines.
For example, two different lineages of cuckoo finch mothers have evolved eggs with either blue or red backgrounds, as an evolutionary response to similar diversity in their tawny-flanked prinia hosts, but there is no evidence that they can create the precise mixture of pigments needed to produce the olive-green eggs that some host females can produce.
In a previous study, Professor Spottiswoode found that a growing proportion of eggs laid by tawny-flanked prinia hosts are olive-green, suggesting this is part of an accelerating evolutionary fightback. As expected, the team found that these host birds are passing down their anti-fraud ‘egg signature’ abilities through a different genetic process (bi-parental inheritance) to that used by the cuckoo finches.
SignificanceWhat we appear to have here, with the ability of some tawny-flanked prinia to lay olive-green colored eggs which the cuckoo finch seems unable to mimic is an example of evolution in progress. Unless the cuckoo finch evolves the ability to mimic those eggs, the ability to lay them will be a very significant advantage to the prinias and, with their bi-parental inheritance of this trait, they will be in a strong evolutionary position to pass it on to their descendants. The cuckoo finch, on the other hand, will be slow to adapt and may even never get there, so that maternal line is at risk of extinction.
Validating an almost century-old hypothesis, we show that a critical host-specific adaptation in a brood-parasitic bird, mimicry of host egg coloration, is maternally inherited, allowing mothers to transmit specialized mimicry to their daughters irrespective of the father’s host species. This genetic architecture, however, is a double-edged sword for parasites: the loss of recombination and heterozygosity as sources of evolutionary novelty likely constrains the parasite from mimicking the full range of color polymorphisms that hosts have evolved as an escalated defense against parasitism. This important tradeoff of asexual inheritance may have relevance for understanding coevolution in other host–parasite systems.
Abstract
In coevolutionary arms races, interacting species impose selection on each other, generating reciprocal adaptations and counter adaptations. This process is typically enhanced by genetic recombination and heterozygosity, but these sources of evolutionary novelty may be secondarily lost when uniparental inheritance evolves to ensure the integrity of sex-linked adaptations. We demonstrate that host-specific egg mimicry in the African cuckoo finch Anomalospiza imberbis is maternally inherited, confirming the validity of an almost century-old hypothesis. We further show that maternal inheritance not only underpins the mimicry of different host species but also additional mimetic diversification that approximates the range of polymorphic egg “signatures” that have evolved within host species as an escalated defense against parasitism. Thus, maternal inheritance has enabled the evolution and maintenance of nested levels of mimetic specialization in a single parasitic species. However, maternal inheritance and the lack of sexual recombination likely disadvantage cuckoo finches by stifling further adaptation in the ongoing arms races with their individual hosts, which we show have retained biparental inheritance of egg phenotypes. The inability to generate novel genetic combinations likely prevents cuckoo finches from mimicking certain host phenotypes that are currently favored by selection (e.g., the olive-green colored eggs laid by some tawny-flanked prinia, Prinia subflava, females). This illustrates an important cost of coding coevolved adaptations on the nonrecombining sex chromosome, which may impede further coevolutionary change by effectively reversing the advantages of sexual reproduction in antagonistic coevolution proposed by the Red Queen hypothesis.
Spottiswoode, Claire N.; Tong, Wenfei; Jamie, Gabriel A.; Stryjewski, Katherine F.; DaCosta, Jeffrey M.; Kuras, Evan R.; Green, Ailsa; Hamama, Silky; Taylor, Ian G.; Moya, Collins; Sorenson, Michael D.
Genetic architecture facilitates then constrains adaptation in a host–parasite coevolutionary arms race
Proceedings of the National Academy of Sciences (PNAS) 119(17); e2121752119; DOI: 10.1073/pnas.2121752119
Copyright: © 2022 The authors. Published by National Academy of Sciences of the Unites States of America
Open access
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license (CC BY-NC-ND 4.0)
The team believe it was selection pressure by host resistance that led to the evolution of maternal inheritance for the egg mimicry genes about 2 million years ago. This sort of short-term advantage at the expense of long-term adaptability is, of course, exactly what those who understand the utilitarian, unplanned and undirected nature of evolution would expect.
However, what is not at all clear is why creationists imagine this could be produced by an intelligent entity who knows exactly what the consequences of its designs will be and exactly what the future holds. For such an entity, there could be no concept such as short-term gain at the expense of long-term costs, and yet we see these sorts of things all the time in the detail of the natural world, and that's before we even consider the stupidity of arms races such as we see all the time in almost any host-parasite relationship.
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