Why populations can’t be saved by a single breeding pair

3 04 2018


© Reuters/Thomas Mukoya

I published this last week on The Conversation, and now reproducing it here for CB.com readers.


Two days ago, the last male northern white rhino (Ceratotherium simum cottoni) died. His passing leaves two surviving members of his subspecies: both females who are unable to bear calves.

Even though it might not be quite the end of the northern white rhino because of the possibility of implanting frozen embryos in their southern cousins (C. simum simum), in practical terms, it nevertheless represents the end of a long decline for the subspecies. It also raises the question: how many individuals does a species need to persist?

Fiction writers have enthusiastically embraced this question, most often in the post-apocalypse genre. It’s a notion with a long past; the Adam and Eve myth is of course based on a single breeding pair populating the entire world, as is the case described in the Ragnarok, the final battle of the gods in Norse mythology.

This idea dovetails neatly with the image of Noah’s animals marching “two by two” into the Ark. But the science of “minimum viable populations” tells us a different story.

No inbreeding, please

The global gold standard used to assess the extinction risk of any species is the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Read the rest of this entry »

Avoiding genetic rescue not justified on genetic grounds

12 03 2015
Genetics to the rescue!

Genetics to the rescue!

I had the pleasure today of reading a new paper by one of the greatest living conservation geneticists, Dick Frankham. As some of CB readers might remember, I’ve also published some papers with Dick over the last few years, with the most recent challenging the very basis for the IUCN Red List category thresholds (i.e., in general, they’re too small).

Dick’s latest paper in Molecular Ecology is a meta-analysis designed to test whether there are any genetic grounds for NOT attempting genetic rescue for inbreeding-depressed populations. I suppose a few definitions are in order here. Genetic rescue is the process, either natural or facilitated, where inbred populations (i.e., in a conservation sense, those comprising too many individuals bonking their close relatives because the population in question is small) receive genes from another population such that their overall genetic diversity increases. In the context of conservation genetics, ‘inbreeding depression‘ simply means reduced biological fitness (fertility, survival, longevity, etc.) resulting from parents being too closely related.

Seems like an important thing to avoid, so why not attempt to facilitate gene flow among populations such that those with inbreeding depression can be ‘rescued’? In applied conservation, there are many reasons given for not attempting genetic rescue: Read the rest of this entry »

50/500 or 100/1000 debate not about time frame

26 06 2014

Not enough individualsAs you might recall, Dick Frankham, Barry Brook and I recently wrote a review in Biological Conservation challenging the status quo regarding the famous 50/500 ‘rule’ in conservation management (effective population size [Ne] = 50 to avoid inbreeding depression in the short-term, and Ne = 500 to retain the ability to evolve in perpetuity). Well, it inevitably led to some comments arising in the same journal, but we were only permitted by Biological Conservation to respond to one of them. In our opinion, the other comment was just as problematic, and only further muddied the waters, so it too required a response. In a first for me, we have therefore decided to publish our response on the arXiv pre-print server as well as here on ConservationBytes.com.

50/500 or 100/1000 debate is not about the time frame – Reply to Rosenfeld

cite as: Frankham, R, Bradshaw CJA, Brook BW. 2014. 50/500 or 100/1000 debate is not about the time frame – Reply to Rosenfeld. arXiv: 1406.6424 [q-bio.PE] 25 June 2014.

The Letter from Rosenfeld (2014) in response to Jamieson and Allendorf (2012) and Frankham et al. (2014) and related papers is misleading in places and requires clarification and correction, as follows: Read the rest of this entry »

We’re sorry, but 50/500 is still too few

28 01 2014

too fewSome of you who are familiar with my colleagues’ and my work will know that we have been investigating the minimum viable population size concept for years (see references at the end of this post). Little did I know when I started this line of scientific inquiry that it would end up creating more than a few adversaries.

It might be a philosophical perspective that people adopt when refusing to believe that there is any such thing as a ‘minimum’ number of individuals in a population required to guarantee a high (i.e., almost assured) probability of persistence. I’m not sure. For whatever reason though, there have been some fierce opponents to the concept, or any application of it.

Yet a sizeable chunk of quantitative conservation ecology develops – in various forms – population viability analyses to estimate the probability that a population (or entire species) will go extinct. When the probability is unacceptably high, then various management approaches can be employed (and modelled) to improve the population’s fate. The flip side of such an analysis is, of course, seeing at what population size the probability of extinction becomes negligible.

‘Negligible’ is a subjective term in itself, just like the word ‘very‘ can mean different things to different people. This is why we looked into standardising the criteria for ‘negligible’ for minimum viable population sizes, almost exactly what the near universally accepted IUCN Red List attempts to do with its various (categorical) extinction risk categories.

But most reasonable people are likely to agree that < 1 % chance of going extinct over many generations (40, in the case of our suggestion) is an acceptable target. I’d feel pretty safe personally if my own family’s probability of surviving was > 99 % over the next 40 generations.

Some people, however, baulk at the notion of making generalisations in ecology (funny – I was always under the impression that was exactly what we were supposed to be doing as scientists – finding how things worked in most situations, such that the mechanisms become clearer and clearer – call me a dreamer).

So when we were attacked in several high-profile journals, it came as something of a surprise. The latest lashing came in the form of a Trends in Ecology and Evolution article. We wrote a (necessarily short) response to that article, identifying its inaccuracies and contradictions, but we were unable to expand completely on the inadequacies of that article. However, I’m happy to say that now we have, and we have expanded our commentary on that paper into a broader review. Read the rest of this entry »

De-extinction is about as sensible as de-death

15 03 2013

Published simultaneously in The Conversation.

On Friday, March 15 in Washington DC, National Geographic and TEDx are hosting a day-long conference on species-revival science and ethics. In other words, they will be debating whether we can, and should, attempt to bring extinct animals back to life – a concept some call “de-extinction”.

The debate has an interesting line-up of ecologists, geneticists, palaeontologists (including Australia’s own Mike Archer), developmental biologists, journalists, lawyers, ethicists and even artists. I have no doubt it will be very entertaining.

But let’s not mistake entertainment for reality. It disappoints me, a conservation scientist, that this tired fantasy still manages to generate serious interest. I have little doubt what the ecologists at the debate will conclude.

Once again, it’s important to discuss the principal flaws in such proposals.

Put aside for the moment the astounding inefficiency, the lack of success to date and the welfare issues of bringing something into existence only to suffer a short and likely painful life. The principal reason we should not even consider the technology from a conservation perspective is that it does not address the real problem – mainly, the reason for extinction in the first place.

Even if we could solve all the other problems, if there is no place to put these new individuals, the effort and money expended is a complete waste. Habitat loss is the principal driver of species extinction and endangerment. If we don’t stop and reverse this now, all other avenues are effectively closed. Cloning will not create new forests or coral reefs, for example. Read the rest of this entry »

Translocations: the genetic rescue paradox

14 01 2013

helphindranceHarvesting and habitat alteration reduce many populations to just a few individuals, and then often extinction. A widely recommended conservation action is to supplement those populations with new individuals translocated from other regions. However, crossing local and foreign genes can worsen the prospects of recovery.

We are all hybrids or combinations of other people, experiences and things. Let’s think of teams (e.g., engineers, athletes, mushroom collectors). In team work, isolation from other team members might limit the appearance of innovative ideas, but the arrival of new (conflictive) individuals might in fact destroy group dynamics altogether. Chromosomes work much like this – too little or too much genetic variability among parents can break down the fitness of their descendants. These pernicious effects are known as ‘inbreeding depression‘ when they result from reproduction among related individuals, and ‘outbreeding depression‘ when parents are too genetically distant.

CB_OutbreedingDepression Photo
Location of the two USA sites providing spawners of largemouth bass for the experiments by Goldberg et al. (3): the Kaskaskia River (Mississipi Basin, Illinois) and the Big Cedar Lake (Great Lakes Basin, Wisconsin). Next to the map is shown an array of three of the 72-litre aquaria in an indoor environment under constant ambient temperature (25 ◦C), humidity (60%), and photoperiod (alternate 12 hours of light and darkness). Photo courtesy of T. Goldberg.

Recent studies have revised outbreeding depression in a variety of plants, invertebrates and vertebrates (1, 2). An example is Tony Goldberg’s experiments on largemouth bass (Micropterus salmoides), a freshwater fish native to North America. Since the 1990s, the USA populations have been hit by disease from a Ranavirus. Goldberg et al. (3) sampled healthy individuals from two freshwater bodies: the Mississipi River and the Great Lakes, and created two genetic lineages by having both populations isolated and reproducing in experimental ponds. Then, they inoculated the Ranavirus in a group of parents from each freshwater basin (generation P), and in the first (G1) and second (G2) generations of hybrids crossed from both basins. After 3 weeks in experimental aquaria, the proportion of survivors declined to nearly 30% in G2, and exceeded 80% in G1 and P. Clearly, crossing of different genetic lineages increased the susceptibility of this species to a pathogen, and the impact was most deleterious in G2. This investigation indicates that translocation of foreign individuals into a self-reproducing population can not only import diseases, but also weaken its descendants’ resistance to future epidemics.

A mechanism causing outbreeding depression occurs when hybridisation alters a gene that is only functional in combination with other genes. Immune systems are often regulated by these complexes of co-adapted genes (‘supergenes’) and their disruption is a potential candidate for the outbreeding depression reported by Goldberg et al. (3). Along with accentuating susceptibility to disease, outbreeding depression in animals and plants can cause a variety of deleterious effects such as dwarfism, low fertility, or shortened life span. Dick Frankham (one of our collaborators) has quantified that the probability of outbreeding depression increases when mixing takes place between (i) different species, (ii) conspecifics adapted to different habitats, (iii) conspecifics with fixed chromosomal differences, and (iv) populations free of genetic flow with other populations for more than 500 years (2).

A striking example supporting (some of) those criteria is the pink salmon (Oncorhynchus gorbuscha) from Auke Creek near Juneau (Alaska). The adults migrate from the Pacific to their native river where they spawn two years after birth, with the particularity that there are two strict broodlines that spawn in either even or odd year – that is, the same species in the same river, but with a lack of genetic flow between populations. In vitro mixture of the two broodlines and later release of hybrids in the wild have shown that the second generation of hybrids had nearly 50% higher mortality rates (i.e., failure to return to spawn following release) when born from crossings of parents from different broodlines than when broodlines were not mixed (4).

Read the rest of this entry »

Hot inbreeding

22 07 2009

© R. Ballen

Sounds really disgusting a little rude, doesn’t it? Well, if you think losing species because of successive bottlenecks from harvesting, habitat loss and genetic deterioration is rude, then the title of this post is appropriate.

I’m highlighting today a paper recently published in Conservation Biology by Kristensen and colleagues entitled Linking inbreeding effects in captive populations with fitness in the wild: release of replicated Drosophila melanogaster lines under different temperatures.

The debate has been around for years – do inbred populations have lower fitness (e.g., reproductive success, survival, dispersal, etc.) than their ‘outbred’ counterparts? Is one of the reasons small populations (below their minimum viable population size) have a high risk of extinction because genetic deterioration erodes fitness?

While there are many species that seem to defy this assumption, the increasing prevalence of Allee effects, and the demonstration that threatened species have lower genetic diversity than non-threatened species, all seem to support the idea. Kristensen & colleagues’ paper uses that cornerstone of genetic guinea pigs, the Drosophila fruit fly, not only to demonstrate inbreeding depression in the lab, but also the subsequent fate of inbred individuals released into the wild.

What they found was quite amazing. Released inbred flies only did poorly (i.e., weren’t caught as frequently meaning that they probably were less successful in finding food and perished) relative to outbred flies when the temperature was warm (daytime). Cold (i.e., night) releases failed to show any difference between inbred and outbred flies.

Basically this means that the environment interacts strongly with the genetic code that signals for particularly performances. When the going is tough (and if you’re an ectothermic fly, extreme heat can be the killer), then genetically compromised individuals do badly. Another reasons to be worried about runaway global climate warming.

Another important point was that the indices of performance didn’t translate universally to the field conditions, so lab-only results might very well give us some incorrect predictions of animal performance when populations reach small sizes and become inbred.

CJA Bradshaw