What are the odds that two genes would independently converge on a similar DNA sequence? Well, they’re so low that molecular systematists make one of their ground rules the supposition that any two genes that have molecular homology MUST have shared a common ancestor. The odds of two genes independently arriving at the same sequence are simply too low.
Yet sometimes that’s exactly the conclusion that evolutionists are forced to draw. In the past here on ENV, we’ve covered this problem for Darwinian biology — called convergent genetic evolution. Now, a new paper from Science, titled “Parallel Molecular Evolution in an Herbivore Community,” finds many parallel, convergent genetic similarities between various insects that are resistant to the effects of cardenolides, a toxic compound produced by plants to prevent other organisms from feeding upon them:
Numerous insects have independently evolved the ability to feed on plants that produce toxic secondary compounds called cardenolides and can sequester these compounds for use in their defense. We surveyed the protein target for cardenolides, the alpha subunit of the sodium pump, Na+,K+-ATPase (ATP?), in 14 species that feed on cardenolide-producing plants and 15 outgroups spanning three insect orders. Despite the large number of potential targets for modulating cardenolide sensitivity, amino acid substitutions associated with host-plant specialization are highly clustered, with many parallel substitutions.
(Ying Zhen, Matthew L. Aardema, Edgar M. Medina, Molly Schumer, Peter Andolfatto, “Parallel Molecular Evolution in an Herbivore Community,” Science, Vol. 337:1634-1637 (September 28, 2012).)
These systems were so similiar that they were forced to conclude that the same genes experienced duplications and then experienced the same amino acid changes independently in widely different organisms: “Thus, in addition to parallelisms in gene copy number and at the level of individual amino acid substitutions, we also observe parallel evolution of expression patterns for ATP?1 duplicates arising in distantly related taxa.”
An article on this paper at Science Daily, Far from Random, Evolution Follows a Predictable Genetic Pattern, reports widespread convergent genetic evolution in very diverse groups of insects:
The researchers carried out a survey of DNA sequences from 29 distantly related insect species, the largest sample of organisms yet examined for a single evolutionary trait. Fourteen of these species have evolved a nearly identical characteristic due to one external influence — they feed on plants that produce cardenolides, a class of steroid-like cardiotoxins that are a natural defense for plants such as milkweed and dogbane.
Though separated by 300 million years of evolution, these diverse insects — which include beetles, butterflies and aphids — experienced changes to a key protein called sodium-potassium adenosine triphosphatase, or the sodium-potassium pump, which regulates a cell’s crucial sodium-to-potassium ratio. The protein in these insects eventually evolved a resistance to cardenolides, which usually cripple the protein’s ability to “pump” potassium into cells and excess sodium out.
The gene sequences in these “distantly related insect species” independently evolved highly similar sequences:
“The finding of parallel evolution in not two, but numerous herbivorous insects increases the significance of the study because such frequent parallelism is extremely unlikely to have happened simply by chance,” said Zhang, who is familiar with the study but had no role in it.
“It shows that a common molecular mechanism is used by many different insects to defend themselves against the toxins in their food, suggesting that perhaps the number of potential mechanisms for achieving this goal is very limited,” he said. “That many different insects independently evolved the same molecular tricks to defend themselves against the same toxin suggests that studying a small number of well-chosen model organisms can teach us a lot about other species. Yes, evolution is predictable to a certain degree.”
Sure, if they assume that Darwinian evolution is correct then the necessary conclusion is that it must be “predictable.” Is this what they expected, however? Not at all. As one scientist quoted in the same article said: “Is evolution predictable? To a surprising extent the answer is yes.”