[Post 5] Ferret-polecat hybrids

Ferret-polecat hybrids

The two specimens to the far left are hybrids, the three to the right are pure polecats.

Ferrets (Mustela putorius furo) and European polecats (Mustela putorius) are both in the Genus mustela, which also contains weasels and mink.  Ferrets are domesticated animals (domesticated circa. 2,000 years ago) and are often kept as pets. Often species escape captivity and some can survive and establish self-sustaining populations in the wild, independent of their previous owners. This is likely to have happened with ferrets. It has been noted that feral ferrets are breeding with wild polecats. 

In the 19^th century, polecats declined in number due to persecution from gamekeepers. Since this time, polecat populations have been expanding, however introgressive hybridisation with ferrets has become a significant conservation issue. The wild polecats have certain traits that assist their survival, which are determined by their genes. When ferrets mate with polecats, the polecat’s genes are diluted, therefore affecting their well-adapted traits. By genetically ‘polluting’ the polecat population with ferret genes, the fitness of the hybrids can be affected. However, it is has been suggested that hybridisation with ferrets may actually have played a part in the recovery and increase in polecat population size, but it is still not fully understood.

Hybridisation may be beneficial for the population size, however, conservation policy can be confused over who to conserve, as genetic integrity of the polecat is compromised by ferrets. Conservation is now focused in the regions home to the purest polecats in the UK and further work aims to minimise the escape of captive ferrets. By focusing efforts on the purebreds, conservationists hope that the species will expand their range and begin to reverse the effect of introgressive hybridisation.

Specimens to the left are ferret-polecat hybrids, to the right are pure polecats. Markings are different between the two, the hybrids generally have a larger body size with more white fur along the back. Pure pole cats have a more white underside to the head compared to ferrets. These taxidermy specimens are located in the World Museum, Liverpool. 

[Post 4] European wildcat and domestic cat hybrids

European wildcat and domestic cat hybrids

Introgressive hybridization is an issue for European wildcat conservation. Hybrids between the European wildcat (Felis silvestris silvestris) and domestic cat (Felis catus) are commonly found in Scotland and Hungary. Domestic cats were historically spread across the globe by humans and are found in most regions that humans inhabit. Therefore, wildcat-domestic cat hybridisation is likely to have been facilitated by humans. 

When two species hybridise, the offspring has a mixture of genes from each species. When fertile hybrid offspring are produced, the more that they breed with each other and the common species, the more likely it is for the rarer species population to reduce in its genetic diversity. The amount of the original genes in a species is called the ‘genetic integrity’. For example, a purebred wildcat would have 100% genetic integrity. The genetic integrity of the wildcat is declining due to ‘genetic pollution’ from domestic cats. The dilution of the original wildcat genes can and has led to the extinction of some wildcat populations. 

The conservation of the wildcat, particularly in Scotland, is an issue. Domestic cats are almost ubiquitous, therefore some cross breeding between the two species will inevitably occur. Conservation management within the UK must decide whether to conserve hybrids or only wildcat purebreds. With a dwindling population size, due to habitat loss, conservation of wild hybrids perhaps should be considered. Current conservation measures are based around captive breeding programs using zoo populations and the reintroduction of these individuals into the wild. However, even some of these captive populations are ‘polluted’ with domestic cat genes. The conservation management of this hybridization is proving very difficult. 

A method of identifying a European wildcat markings on the back and tail.  Fur around the mouth should always be brown in this species. Spotted markings are usually an indication of hybridisation with the domestic cat. The above images are of a taxidermy mounted European wildcat from the World Museum, Liverpool.

[Post 3] Hybrid species

Hybrid species

Hybrid species are caused by two separate species interbreeding, and this is fairly common within nature (this is known as introgressive hybridisation). Up to 10% of animals, and 25% of plants are involved in some form of hybridisation (classic or introgressive). Hybrids somewhat confuse the current definition of a species, as sometimes the offspring of two species are born fertile.  Often in law, the conservation status of these hybrids are unclear, for example, in the 1973 Endangered Species Act in the United States.  Humans are believed to have exacerbated hybridization through activities such as aquaculture, agriculture, habitat alteration and the accidental introduction of invasive species, for example, via ballast water from boats . Climate change is also likely to have enhanced hybridisation, due to species shifting their home ranges. Usually the conservation issue with hybridisation occurs when a rare or endangered species can form viable offspring with a very common, or non-threatened species. Introgressive hybridisation is the term used to describe when genes flow between the two mating species backwards and forwards, in other words, hybrids are breeding with an individual from one of its parental species. Introgressive hybridisation is different from ‘classic’ hybridisation as it contains a different mix of genes eg. 25% one species, 75% another species, by mating more often with a certain species. Whereas classic hybridisation, that occurs between two different species will contain roughly 50% of each species genes, when there is no preference to mate with a certain species. Hybridisation is a very important part of speciation and maintains genetic diversity by mixing genes. 

The above image is a hybrid individual between a mistle thrush and a blackbird, both species are in the same genus (Turdus). Due to sharing the same genus, the two species are closely related, which is why this individual will have developed well within the egg and survived into adulthood. It is likely that this specimen was infertile. This taxidermy specimen is located in the World Museum, Liverpool. 

[Post 2] How does one species become another?

How does one species become another?

Within a species, each individual slightly varies in its genes, environment, behaviour and cognitive ability – the same as humans. Each slight variation can either benefit, have a neutral effect or cause a cost to the individual. The variation amongst individuals means that some survive better or reproduce better than others. The individuals who have the most beneficial variation usually are fitter and produce more offspring. The genes are then passed onto the individuals young. These offspring are fitter, so produce fitter offspring when they reach adulthood. Meanwhile, poorly adapted individuals, who have these costly variations/traits, suffer morbidity or mortality resulting in their genes not being passed on to future generations. Therefore, traits that increase fitness are ‘favoured’ by natural selection. This is where the well-known phrase “the survival of the fittest” originates.

Speciation is the name of the evolutionary process of forming a new, discrete species. There are two forms of speciation; sympatric and allopatric.

Allopatric speciation occurs when a new species arises as the result of a physical barrier from its original species (usually a geographical feature, such as a river or a mountain range). The barrier stops the flow of genes from one population to another, and coupled with environmental factors (that lead to different selection pressures), the species diverge from one another. This form of speciation has occurred in some species of monkeys within Africa, partly due to the Congo and Sanaga rivers.

Sympatric speciation is when a new, distinct species arises without being spatially isolated from its original species but due to a non-physical barrier. For example, producing infertile young or differences in courtship behaviours. These barriers prevent successful mating/reproduction. Differences in mating behaviour have stopped the interbreeding of two chiffchaff species, the common and Iberian, in Spain and France.

The image below illustrates these forms of speciation:

[Post 1] What is a species?

 What is a species?

A species is defined as a group of similar animals that can breed together to produce fertile offspring. Taxonomy is the scientific name for the classification of living things by humans. A species is the ‘narrowest’ form of classification, despite its common use among the non-scientific community. Carl Linnaeus first developed the taxonomy hierarchy in the 18^th century.

As you can see below, there are seven ‘layers’ to classification. The example given is for the domestic cat. A helpful way to remember this is the acronym Keep Ponds Clean Or Frogs Get Sick.

- (Kunz, 2013)

In the past, species were categorised according to their morphology (body shape) and common features. However, this method is not accurate to today's standard. We now know that evolution has resulted in animals evolving structures with similar functions separately, multiple times (known as analogous features). For example, wings in birds and bats. Simply sharing this feature, does not mean that bats and birds closely related.

Species are constantly being re-named and re-categorised, largely due to the new technology available to taxonomists today. DNA analysis can be used to work out the tree of life of modern species and their ancestors (the phylogeny). This technology has led to many species being re-classified, such as the mammal the pangolin. These creatures were once thought to be closely related to anteaters and sloths (Order: Xenarthra) to form the toothless group of mammals named the Edentata. However, DNA analysis has resulted in pangolins being re-classified into their own Order, the Pholidota . The analysis of their genes has determined pangolin's closest living relatives are actually carnivores! 

DNA technology has discovered that many animals, previously thought of as a species, are actually two or more sub-species meaning that they are distinctly different from each other. This is the case in the tuatara reptiles (genus Sphenodon) on different islands near New Zealand. This knowledge strongly affects the conservation of these reptiles, as they must be managed as different species. Management aims to avoid the dilution of their rare and unique genes, amongst the more common sub-species caused when two sub-species mate.

A species is actually just name created to classify nature into categories. There is a debate regarding how to define a discrete ‘species’ and whether individual species actually exist. Darwin and many others argue that species are just a belief by humans, and that a species is not a 'natural category', but more of a continuum. This debate has been occurring for the last 150 years. Living organisms are constantly under the influence of evolution and selection influecnes the population. Under intense natural selection or over long time periods species change and become new species. The issue is, how do we declare one species has become another? And when do we decide this?