I know I have been going on about range shifts for quite
some time now, but I ask that you just humour me for just one more week, because I think I’ve
found something you’ll find interesting.
During my weekly dive into the literature, I
stumbled across a relatively new paper by Cobben et al. (2012) that revealed to me a side of range shifts which I’d never really thought about before. What Cobben et al. discuss is the genetic impacts of changing range shifts
and how these can massively disadvantage a species. What I'd like to talk about today then is quite theoretical, but nonetheless is hugely interesting.
The genetic composition of a species is
largely dependent upon interactions within the population and the species’
adaptations to their environmental conditions. This means that the genetic
make-up of a species is rarely uniform; in fact, it can see large variations
across its range, specifically so between the central and marginal regions.
Those living in the central region are at the optimum environmental conditions
for their survival, and it is here that you generally find members of the
species that are more specialised – that is, they have specialist gene alleles
that provide them with greater fitness in their environment. Move to the
outskirts, however, and you find more challenging environments, with more
diverse and generalist species members who are designed genetically to deal
with it; here there is a strong trend towards animals with more generalist
alleles that allow them to take on a greater variability of environments,
although with reduced fitness across the board.
I’m just going to take a second to explain some basic genetics to assure we are all on the same page. An allele can be defined as a variant form of a gene; that is, many
different alleles do essentially the same job, but in different ways. An easy
example is eye colour in humans – this is controlled by a single gene, which
can be made of up many different types of alleles which determine whether you
have blue eyes (like me) or brown eyes (like my brother). It is the same for
other species; a single allele can determine whether you are a specialist,
adapted specifically for certain environmental conditions, or a generalist, a
jack of all trades but master of none. Different combinations of alleles create
different genotypes (the part of the DNA sequence that is responsible for a
certain characteristic), and so, we can have generalist genotypes, or
specialist genotypes.
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| Here you can see different allles making up the genes for a flower; note how different alleles carry with them different characteristics. |
What
Cobben et al. argue is that range
shifts can have massive impacts on this genetic make-up, and, following the
rather depressing theme of this blog, this is usually for the worst. As
the potential range increases with temperature, you have what are called “founder
events” – these are merely colonisations at the expanding margin. These are
conducted mostly by generalists, who show better fitness than specialists in
less than optimum habitats and hence have greater success, meaning new populations generally tend to have a
significant majority of members with generalist genotypes. These “generalist-polarised”
founder events can therefore affect the local evolutionary process and cause
dramatically reduced allelic variation at the margins of populations, leading
to a genetic bias towards generalists as the generalist allele becomes more dominant/prominent.
As ranges continue to shift polewards, this polarisation
tends to increase, and the locations of generalists, originally at the range
margins shift closer towards the centre of the distribution. Specialists are
also observed to move towards the lower margins of the distribution. This tends
to happen in times of rapid warming (such as that we are experiencing at the
moment) and can be augmented by habitat fragmentation preventing successful “migration”
of the species to track their optimum conditions. This leads to general maladaptation
of the species to their environs; generalists at the centre of the distribution
are significantly less fit than other species adapted to said niche, and the
specialists, who find themselves pushed towards the margins as range shifts
overtake them are removed from their niche and put at a distinct disadvantage.
Cobben et al.
model this process using METAPHOR, a simulation model designed to test changes
to metapopulation demography. Specifically they modelled the Middle Spotted
Woodpecker (known to his friends as Dendrocopos
medius), as within our wood-pecking friend a single gene determines adaptation
to local conditions, making it the ideal test subject. Throughout the model,
various climate change scenarios (matching predicted change) and weather variability
were applied.
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| This the little guy himself, doing some modelling of his own for us |
They found that the original distribution of genotypes within
the distribution was changed dramatically by climate change induced range
shifts. Generalists were seen to increase in relative number and size of area –
an increase that occurred at the cost of specialist populations. After a
relative period of temperature increase, the positions of both specialists and
generalists seemed to be at odds to the position of their respective optimum
environments; generalists occurred at the temperature optimum, where
specialists had a selective advantage, and specialists were seen at the range
margin, where the generalists tended to have a greater fitness. Eventually,
after a long period, the specialist allele was observed to go extinct. Put simply, the range shifts resulted in the centre of the distribution moving to areas where generalists were originally on the margins, whilst specialists, originally at the centre of the distribution, found themselves now at the edge. Due to reduced genetic variation within the generalist populations, there was little room for specialists to retain dominance, and hence generalists relatively grew in number.
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| Here you can see the results of Cobben et al.'s (2012) modelling in graph form. As you can see, across the years, the specialsits (light grey bars) tend to migrate (or more accurately, are forced) towards the lower bound of the distribution, whilst generalists (black bars) move to occupy both the centre and upper bounds of the distribution. By year 600, the specialist genotypes are all but gone. |
This transformation in the genetic make-up of species has
numerous knock-on effects, most noticeably on the size and persistence of
populations. The occurrence of genotypes in areas where they had less than
optimum fitness will generally result in a decreased metapopulation size, as
species are far less successful in surviving to maturity and producing
offspring. The persistence of a species is also threatened; not only are they
disadvantaged, putting them at risk of invasive species or extreme events
dramatically reducing population sizes, but also the decreased genetic
diversity reduces their ability to adapt to future changes. This genetic
polarisation hence makes species extremely vulnerable.
It is shocking just how deep the threats from range shifts
run for the species of our planet. Not only are external factors reducing their
ability to survive, but threats are also coming from within the species itself.
Contemporary range shifts truly are one of the biggest challenges to the flora
and fauna of our planet that they have seen in thousands, if not millions, of years, and their
futures at the moment hang in the utmost uncertainty. Humans really have to turn things
around soon, and truly devote themselves to helping our fellow residents,
before it is too late.
If today's blog hasn't upset you enough, then you may want to read this article that explores the reasons behind the mystery of 85,000 Saiga antelope dying in just one day (later revealed to be 211,000, or 70% of the entire species). It is an interesting, yet very morbid read - find it here
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