Tag Archives: population genetics

Hybridization of the New Zealand kaki- good, bad, or simply unnecessary?

Hybridization of closely related species to due land use and climate changes is accelerating. While hybridization often results in offspring sterility and thus reduces the fitness of parent species, in some cases viable hybrids are produced and the benefit or cost of this to the original species must be judged on a case by case basis. One of the world’s rarest birds, the New Zealand black stilt or kaki, has recently begun breeding with a self-introduced cogener, the pied stilt or poaka.

One of the world's rarest birds, the kaki is an iconic New Zealand native
The poaka, a close relative of the kaki, extends from the Philippenes and Indonesia across Australia and now into New Zealand

A recent molecular analysis of hybrid DNA confirmed that birds classified as hybrids on the basis of adult plumage are indeed genetically related to both parent species. Hybridization is both extensive and bidirectional- both sexes of both species will breed with the opposite sex of the other species.

An important question for scientists studying hybridization is whether introgression is occurring. Introgression, the introduction of genes from one distinct species into another, occurs via the backcrossing of hybrids to individuals of the parent species.  To make this concept clearer, imagine two farmers from two different towns. One farmer specializes in growing cucumbers, just like everyone else in his town. The other farmer, in the tradition of his town, grows only pumpkins. Now, if the children of these two farmers meet in college and decide to get married, they may well decide to start their own farm and choose to raise both cucumbers and pumpkins. Some years later, one of the children from this “hybrid” farm decides to start her own farm, but realizes that she’s really only interested in growing pumpkins (they make such tasty pies!). However, the cucumber-growing town has a much better craft beer selection, so of course she must live there. This third generation child has, in a sense, caused an introgression event to occur- she has brought a skill only available in one town into another due to the fortuitous marriage of her parents.

Why does introgression matter? In species that engage in hybridization, it can be an important mechanism for the transfer of new genes into either original species, thus maintaining a healthy level of genetic variation. In fact, “genetic rescue”  of an endangered species through intentional hybridization has become a population conservation strategy for preventing imminent extinction. Hybridization between individuals in a small population with reduced genetic diversity and a population of unrelated individuals increases the “genetic load” of the small, low diversity population.

Now, back to the mysterious kaki bird. Despite widespread evidence of hybridization, there is no evidence of introgression between hybrids and either parent species. This may be the first study to document a general lack of introgression, despite a history of extensive hybridization.

Okay, so a few weird hybrids aren’t backcrossing to the parent kaki species, so what?
Some conservationists think that introgression may be just what is needed to keep this rare species alive. Once abundant and widely distributed across New Zealand, the kaki is now critically endangered and currently restricted to the Upper Waitaki River Basin of the South Island. Intensive efforts to conserve the iconic bird have caused the population to nearly quadruple, from a scant 23 adults in 1981 to 98 adults in 2010. This population size is, however, far from ideal, and introduced mammalian predators are now representing a major threat to further species recovery.

Given that introgression may provide the fresh genetic load needed to keep the kaki species alive, researchers studying the kaki began searching for an explanation for this unusual lack of backcrossing. It turns out that the survival rate of fledgings born from hybrid female x kaki male pairs is significantly lower than the survival rate of either the normal kaki x kaki pairing, or hybrid x hybrid crosses. Coupled with an extremely small hybrid population size, which leads to less predictability in survival rates, the reduced fitness of hybrid female x kaki male pairings is not terribly surprising.

If genetic rescue will not offer relief to the kaki, are we nearing the end of the road for this marginalized species? The authors of this study don’t think so. The flip side to intentional hybridization is reduced breeding opportunities between kaki birds themselves (maybe they don’t need our help after all!). Although inbreeding and reduced genetic diversity are expected due to the low kaki population size, there is little genetic evidence for high inbreeding among kaki birds. Kaki exhibit relatively high fitness when compared to other New Zealand endemics that have suffered comparable reductions in population size. Rather than encourage further hybridization, the authors advocate active conservation strategies that promote the formation of “pure pairs” and maintain a balanced adult sex ratio in order to keep survival chances high.

Steeves et al. 2010. Genetic analysis reveals hybridization but no hybrid swarm in one of the world’s rarest birds. Molecular Ecology 19: 5090-5100

Human Genographic project samples Ithaca, NY

This past weekend, over 200 Cornell students participated in the Human Genographic Project by submitting saliva  samples for DNA testing. The Human Genographic project, sponsored by National Geographic and IBM, aims to map the prehistoric migration of human populations by developing a database of the genetic relatedness of extant human populations. The project has currently analyzed the genomes of 72,000 indigenous people and another ~400,000 people who have bought a $99.00 kit to discover the “deep ancestry” hidden in their DNA.

Charles Aquadro, professor of population genetics at Cornell, organized the event as part of an effort to foster discussion of the legal and social implications of genetic testing. Aquadro is interested in seeing if the Cornell group is as genetically diverse as recent volunteers from a street fair in Queens, who turn out to be wel representative of all major migration routes from Africa.

If you’re interested in learning more about the Human Genographic Project, or getting your own ancestry traced, check out the main website here (The interactive atlas of human migration is particularly cool):

https://genographic.nationalgeographic.com/genographic/index.html

Researchers identify “impulsivity gene” in Finnish criminals

Impulsivity, the tendency to act without thinking, is one of those complex behavioral traits that neuroscientists have been recently been attempting to link to functional genes. No small task- many behavioral traits are thought to be controlled by numerous genes, gene expression pathways, and environmental variables. A recent study in Nature, however, has found a single gene mutation in a Finnish population that may be at least partially responsible for violent impulsive behavior.

The Finnish people have become very intersting to geneticists over recent years. The founder population of Finland is believed to derive from two distinct migration events 4,000 and 2,000 years ago. Until very recently,this small population has lived in relative isolation. Because of their isolation and small population size, the Finns have probably undergone “genetic bottlenecking” , the process of losing genetic diversity due to generations of breeding amongst a relatively homogenous population. One consequence of bottlenecking can be a high frequency of genetic muations that would be extremely rare in a larger, more outbred population.

In their search for genes that may link to impulsive behavior, researchers Bevilacqua et al. focused on a group of individuals with “extreme manifestations of impulsiivity” – violent criminal offenders in a foresnsic psychiatric unit. Using high-throughput DNA sequencing technology to examine regions of the genetic code that may offer clues, they found a previously unknown mutation in the HTR2B gene. This gene is involed in modulating both serotonin and dopamine release in the nucleus accumbens, a brain region involved with impulsive behavior.

Bevilacqua et al.1 find that, in a Finnish subpopulation, a mutation in the serotonin receptor HTR2B is linked to severe impulsivity. In the nucleus accumbens region (green) of the brain, projections of neurons that secrete serotonin (red) interact with those that secrete dopamine (blue). This region has been repeatedly shown to play a crucial part in choice and impulsivity. Mutations in HTR2B, which modulates the release of dopamine and serotonin in the nucleus accumbens, may reduce the release of these neurotransmitters, leading to increased impulsive behaviour. ---Image and caption courtesy of John Kelsoe's article in Nature Magazine, December 2010

Over-expression of HTR2B has already been linked to abnormal behavior. The drug ectasy has been shown to increase serotonin and dopamine levels in the brain by directly activating HTR2B. Bevilacqua et al. report a single base-pair mutation in HTR2B that stops RNA from translating the gene into its normal protein product, rendering it entirely non-functional. Individuals with this mutation are likely to have low unusually low levels of serotonin and dopamine, two important mood-enhancing neurotrasmitters. Incidence of this mutation was found to be three times higher among Finnish criminals than a control population.

Though this particular mutaion is unique among Finns (according to current research), other mutations in HTR2B could exist in different populations. Linking abnormal HTR2B genes to impulsive behavior in other populations would strengthen this gene’s candidacy for being a primary driver of impulsivity.

wyrd evolution

A population essentially evolves through the accumulation of random changes in its genetic makeup over time. These genetic changes modify organisms’ phenotypes, and over time change the distribution of traits in a population. Many traits which become prevalent in a population do so because they make the population more “evolutionarily fit”- better able survive and reproduce in its environment. Darwin coined the famous term “natural selection”to describe this phenomenon, though he wasn’t aware of the complex genetic mechanisms underlying it.

Evolutionary theory is anchored on the principle that the biology of the past has shaped the diversity we see today. Though countless examples in nature substantiate the important role of natural selection in evolution, it is important to understand that natural selection itself is not a conscious force. Rather, it is it is a pattern that produces predictable outcomes. Stochastic probability tells us that, over a long enough time and with large enough populations, traits that allow organisms to produce more offspring will come to dominate a population, simply because the individuals possessing these traits will pass along more of their genes into the next generation.

Because evolution by natural selection is not a conscious force, and because it must work to improve upon what already exists in nature, evolution cannot rapidly produce superanimals that are perfectly adapted to their environments. As the French biochemist Francois Jacob once eloquently described it, evolution is a tinkerer that works to improve upon what is already there, but its creative freedom is heavily constrained by existing body plans and biochemical pathways. Moreover, natural selection works to optimize organisms, not isolated systems. A trait that may seem advantageous, such as a genetic mutation producing enhanced night vision, may be helpful for a large predator on the Savannah, but useless for a cave fish that is rarely exposed to any sunlight and must use other sensory systems to perceive its environment. The cave fish would not develop improved eyesight because the selective advantage conferred by this ability would not outweigh its energetic costs.

As this last example begins to illustrate, natural selection is often working in concert with another force known as selective constraint. When a gene, biochemical pathway, or phenotypic trait is under selective constraint, it is maintained over evolutionary time. There are many reasons that selective constraint could operate. A biochemical pathway could be so fundamental to an organisms ability to survive that any small alterations to that pathway would be lethal. A limb or sensory organ could already be well suited for its environment, or the benefits of  making any changes to it may not outweigh the costs. A single mutation event in a gene encoding an essential protein could alter the protein’s structure and make it useless.

Natural selection and selective constraint are two important paradigms for understanding evolution. They are not the entire story, but they do help us to understand how evolution produces produces change but also propagates sameness. An alien visiting earth 3 billion years ago could not have imagined that the simple life he discovered would lead to the overwhelming diversity we see today. And yet in spite of all the novelty and innovation that has appeared over evolutionary time, this diversity has drawn upon itself, reaching outwards without  breaking its ties to the past.