The phrase "survival of the fittest" makes the principle of evolution by natural selection easy to understand -- individuals with a trait that adapts them well to their circumstances are more likely to pass that trait along. But as a new study explains, multiple factors make predicting the fate of a trait fiendishly difficult.
Not only would improved predictive models help scientists better model how evolution works, but also they could aid in efforts to prevent infectious diseases. Every year, for example, vaccine makers, epidemiologists and physicians strive to predict where diseases such as influenza, Zika, HIV and Ebola may be headed next.
The fitness of a genetic trait (an allele) may vary over time, rather than remain constant. In this simple model, populations with two different alleles (black or yellow) see-saw between advantage and disadvantage as their relative fitness changes over time (blue line below).
Credit: Weinreich et. al.
Fundamentally, the problem is that a trait conveyed by a gene variant, or allele, may be advantageous for one or a few generations, but provide no advantage or become a liability when circumstances change, said senior author Daniel Weinreich, a professor of ecology and evolutionary biology at Brown University. But most theoretical models of population genetics assume that fitness remains constant.
"We are articulating a number of different biological contexts in which the fitness of an allele might change over its 'lifetime' or lineage" in a population," Weinreich said. "We are convinced that the other contexts, where it is constant, are the exceptions, not the rule."
The new study in the Annual Review of Ecology, Evolution and Systematics provides an overview of what complicates predictive models and how scientists are trying to make progress, for the benefit of public health, among other areas.
"Infectious diseases experience constantly varying selective pressures as they spread within and between hosts and encounter drugs and host immune responses," said lead author Christopher Graves, who earned his Ph.D. from Brown and is now a researcher at Bayer. "Understanding how evolution proceeds in scenarios of highly variable selective pressures will increase our ability to predict drug resistance and disease outbreaks and ultimately lead to the creation and deployment of more clever drug and vaccine strategies."
Fitness can be fickle
Perhaps the most obvious way that the fitness of a trait can vary is that the environment can change, not only over time but also over space. Consider the population of a species of weed in a vacant lot. Some might carry an allele that helps them thrive in a hot sun and others might have an allele that conveys a relative advantage in cool shade. Not only could weather patterns change dramatically over timescales ranging from days to years, but also new buildings might go up or get torn down around the lot, creating new patches of shade or sun. A model projecting the fate of each allele becomes much more complicated along multiple dimensions.
Another dimension that can vary is the "social" life of alleles. Alleles that result in "cheating" are abundant in nature, but they are most effective when they are rare. Once everyone is cheating, it might no longer be an advantage, so the trait over time can become a victim of its own success. Moreover, genetic predispositions to cooperation doesn't just roll over. The paper cites cases in which "policing" behaviors have evolved, such as insects that preserve the supremacy of the queen by destroying the "selfishly" laid eggs of mere workers, or genes that produce a tumor-suppressing immune capacity to destroy cancer cells because they are growing too fast.
Conditions can even vary within a lineage because one allele might emerge that affects another. Weinreich has studied this in the emergence of antibiotic resistance in bacteria. He found that four mutations of a particular enzyme sometimes increased drug resistance and sometimes didn't, depending on what other mutations were present or absent.
Even more complications
That any of these circumstances can change over time adds yet another layer of complexity, Weinreich said, because the rate at which circumstances change matters. When circumstances change faster than the organism's rate of reproduction -- for instance sunny or cloudy weather patterns that come and go over a few days -- the sun- or shade-loving weeds each experience only minor influences on their reproductive success. But if circumstances vary more slowly -- for instance a large new building shades the entire lot for decades -- the sunny allele carriers could vanish from the lot and the shady allele will fully displace the other type. In this case, the sun-loving weeds may have gone extinct by the time the building is torn down again.
Indeed, Weinreich said, many models for predicting the fate of alleles have overlooked the possibility that traits can go completely extinct.
Meanwhile, the rate of environmental change is very similar to the rate at which natural selection acts, the math becomes especially tricky.
Pressing for progress
In their search for solutions, population geneticists have employed new approaches, Weinreich and Graves wrote. Among the most exciting, Weinreich said, are those in which they join forces with and borrow techniques from ecology and epidemiology -- two fields in which modeling dynamic and complex change is central. This summer, for example, has featured a workshop, "Eco-Evolutionary Dynamics in Nature and the Lab" at the University of California at Santa Barbara's Kavli Institute for Theoretical Physics, that is dedicated to exploring such intersections.
Weinreich said he plans to delve deeper into the complexities of changes in fitness deriving from varying rates of change in social (e.g. cheaters), genetic (e.g. competing alleles) or environmental (e.g., weather) parameters.
"The overlap between ecological and evolutionary processes -- that those two things speak to each other very intimately in a way that's been overlooked in many models -- is the way forward," Weinreich said. "That's what's needed to make critical improvements to models."
The National Institutes of Health and the National Science Foundation supported the researchers in their work.
David J. Orenstein | EurekAlert!
Internal mechanism found to be responsible for the limitless growth potential of epithelial tumors
11.09.2017 | Institute for Research in Biomedicine (IRB Barcelona)
Cilia: 'The bouncer' of bacteria
08.09.2017 | University of Southern California
Physicists from the University of Basel have developed a memory that can store photons. These quantum particles travel at the speed of light and are thus suitable for high-speed data transfer. The researchers were able to store them in an atomic vapor and read them out again later without altering their quantum mechanical properties too much. This memory technology is simple and fast and it could find application in a future quantum Internet. The journal Physical Review Letters has published the results.
Even today, fast data transfer in telecommunication networks employs short light pulses. Ultra broadband technology uses optical fiber links through which...
Fifty years after Jocelyn Bell discovered the first pulsar, students are no longer going through reams of paper from pen chart recorders but instead search through 1,000s of terabytes of data to find these enigmatic pulsating radio stars. The most extreme binary pulsar system so far, with accelerations of up to 70 g has been discovered by researchers at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. At their closest approach the orbit of the pulsar and its companion neutron star would easily fit inside the radius of the Sun.
Although most of the more than 2,500 pulsars known are solitary objects, a few are found in tight binary systems. The discovery of the first of these, the...
G protein-coupled receptors are the key target of a large number of drugs. Würzburg scientists have now been able to show more precisely how these receptors act in the cell interior.
The human genome encodes hundreds of G protein-coupled receptors (GPCRs). These form the largest group of receptors through which hormones and...
Nuclear pore complexes are tiny channels where the exchange of substances between the cell nucleus and the cytoplasm takes place. Scientists at the University of Basel report on startling new research that might overturn established models of nuclear transport regulation. Their study published in the Journal of Cell Biology reveals how shuttling proteins known as importins control the function of nuclear pores – as opposed to the view that nuclear pores control the shuttling of importins.
Genetic information is protected in the cell nucleus by a membrane that contains numerous nuclear pores. These pores facilitate the traffic of proteins known...
Today’s world, rapidly changing because of “big data”, is encapsulated in trillions of tiny magnetic objects – magnetic bits – each of which stores one bit of data in magnetic disk drives. A group of scientists from the Max Planck Institutes in Halle and Dresden have discovered a new kind of magnetic nano-object in a novel material that could serve as a magnetic bit with cloaking properties to make a magnetic disk drive with no moving parts – a Racetrack Memory – a reality in the near future.
Most digital data is stored in the cloud as magnetic bits within massive numbers of magnetic disk drives. Over the past several decades these magnetic bits...
06.09.2017 | Event News
06.09.2017 | Event News
31.08.2017 | Event News
11.09.2017 | Physics and Astronomy
11.09.2017 | Life Sciences
11.09.2017 | Life Sciences