WOLF - DOG HYBRIDISATION
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Although the wolf and the dog look differently, especially when you compare the wolf with various modern dog breeds, they can mate and give fertile offspring. Studies from various countries showed that this process is not very frequent, but happened also in the past. If a female wolf mates with a male dog, the pups stay with the mother, as male dogs usually don’t participate in raising the offspring.
What are the reasons for such cross-breeding? Hybridization may happen in case of the lack of the wolf mating partner due to, for example, hunting pressure, which changes the wolf pack composition. Hybridization is also more frequent at the edge of the wolf distribution range. This means that currently expanding European wolves may be more prone to the cross-breeding behaviour, when inhabiting areas not occupied by their conspecifics.
Possible consequences of hybridization may include the process called introgression, which is the transfer of variants of genes from one species to another. Those new variants can have an effect on appearance or behaviour. An example of such introgression is a gene variant transferred from dogs to wolves, which is associated with the black coat colour.
What are the reasons for such cross-breeding? Hybridization may happen in case of the lack of the wolf mating partner due to, for example, hunting pressure, which changes the wolf pack composition. Hybridization is also more frequent at the edge of the wolf distribution range. This means that currently expanding European wolves may be more prone to the cross-breeding behaviour, when inhabiting areas not occupied by their conspecifics.
Possible consequences of hybridization may include the process called introgression, which is the transfer of variants of genes from one species to another. Those new variants can have an effect on appearance or behaviour. An example of such introgression is a gene variant transferred from dogs to wolves, which is associated with the black coat colour.
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BIG BAD WOLF AND MEN'S BEST FRIEND - IS IT TRUE?
In Poland and many other European countries wolf is seen as a negative character in many tales and phrases. For example 'wolf in sheep’s clothing' meaning someone dangerous, false but pretending to be kind and friendly.
In Poland and many other European countries wolf is seen as a negative character in many tales and phrases. For example 'wolf in sheep’s clothing' meaning someone dangerous, false but pretending to be kind and friendly.
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Did you know that in other than European cultures people are seeing wolves differently? In Kirghiz if you call someone a wolf you are indicating that this person is brave, fearless and with great physical health. In the Caucasus, the Dargwa people use a phrase Betsivan duraukhun (Came out like a wolf) to describe someone bold and ready to enter battle.
Is it true?
In fact, opinions about big bad wolf and dog as a man’s best friend are not fully true. Wolves are generally peaceful and are afraid of people. When they are injured, sick or habituated (e.g. by feeding) they may lose their fear for humans and become dangerous. However, situations in which wolves attack humans are very rare. Actually, more frequent are attacks by dogs.
Is it true?
In fact, opinions about big bad wolf and dog as a man’s best friend are not fully true. Wolves are generally peaceful and are afraid of people. When they are injured, sick or habituated (e.g. by feeding) they may lose their fear for humans and become dangerous. However, situations in which wolves attack humans are very rare. Actually, more frequent are attacks by dogs.
GENETIC DIVERSITY AND EVOLUTIONARY INFERENCE IN KILLER WHALES
The text below is the result of a collaboration with Hiroya Minakuchi, an extremely talented Japanese wildlife photographer. This text is part of a book showing a series of stunning photographs of Orcas from around the world, and accompanies information about their known ecological diversity. You can find the book here, and learn more about Hiroya's work here. The book is entirely written in Japanese, but you can read below the English version. Many thanks to Hiroya for allowing me to reproduce it on this website, and for the opportunity to participate in this wonderful initiative.
Orcas were one of the earliest cetacean species to be studied using genetic tools. As a consequence, Orcas are one of the better studied cetaceans in terms of their genetic composition. Earlier studies were mostly focused on identifying the presence of distinct Orca populations around the world, with particular attention to the well known ecotypes. Through the years, studies on Orca genetics have built up on this approach, and gradually provided a clearer idea of how many ecotypes exist around the world, how they relate to each other, when and where they originated from, and what evolutionary processes led to the current state of genetic variation. However, because of their widespread distribution and some specifics of their genetic composition, there is still a fair degree of uncertainty and much left to discover. The development of technologies analysing genetic variation at the whole genome level has provided significant breakthroughs, and we are likely to see a broadening of research topics addressed through genetic analyses in the future.
Where do modern Orcas come from?
Studies on genetics all show that Orcas are characterized by relatively low levels of genetic diversity. Wild animals with large populations and high ecological diversity are usually characterized by high genetic diversity, and so this low diversity is somewhat unexpected given Orcas global distribution and various well known ecotypes. Because of this, studies on Orca genetics often discuss the possibility of some type of genetic bottleneck occurring recently in the species past. Genetic bottleneck is a generic term used by population geneticists to describe a relatively quick loss of genetic diversity, due to some unusual ecological or demographic event. These events can produce characteristic patterns in the genetic composition of a group, which modern researchers can use to reconstruct past demographic changes of that group. Although often interpreted as being synonymous with a strong reduction in population size, this is not necessarily the case, as other more complex demographic events are known to create the characteristic patterns of genetic bottlenecks. Such patterns are found in many Orca groups, however details of the demographic events that might have caused them are still not fully known.
To this end, much progress has been made recently with the development of genomic tools and analyses. Studies using genomic data, all suggest that modern Orca ecotypes originated from a global expansion in a relatively quick timeframe. Furthermore, it appears likely that this expansion involved the dispersal of small groups of individuals over large distances across the world, as opposed to a single population that grew increasingly larger. These small groups would have experienced what is called a founder event, which can lead to the occurrence of genetic bottlenecks, and could thus partly explain the low genetic diversity that characterizes Orcas worldwide. This expansion is at present thought to have originated somewhere in the Southern Ocean, although the exact location has not yet been identified with precision. One of the earliest genomic studies found that orcas sampled in waters off South Africa had the highest genetic diversity of all worldwide locations analysed. This would suggest that this population might have been stable for longer, because wild populations accumulate genetic diversity by mutation, and the longer a population exists the higher the chance of new mutations occurring.
This interpretation is also consistent with our current understanding of how certain oceanographic features have changed in the last 1-2 million years. Around the world, there are regions where marine productivity is elevated due to the combined action of ocean currents, prevailing winds, and the coastline profile. These regions are characterised by a phenomena called upwelling, where organic matter from the sea floor gets brought to the surface, creating rich productive food webs. Such systems provide an abundance of prey that top predators, such as Orcas, can rely on for their long term survival. The upwelling systems around South Africa are thought to have been more stable during the past than upwelling systems elsewhere in the world, because of the relatively stability of the Agulhas (in the Atlantic Ocean side) and Benguela (on the Indian Ocean side) current systems. However, this type of evidence is purely circumstantial, and although it is relatively credible, it’s difficult to make a strong casual inference between the environmental phenomena and the genetic patterns of specific animal species.
Furthermore, as more locations around the world have been included, Orca genetic composition is proving to be slightly more complex. Studies also consistently show that Antarctic ecotypes (called Types A, B, C and D) have relatively high genetic diversity, and are likely one of the “oldest” modern lineages. In this context, the term “old” is useful but not entirely correct, as from an evolutionary perspective, all modern animals belong to lineages that are equally old. What evolutionary biologists really mean when they refer to a lineage as “old”, is that its separation from other known lineages in a group appears to have occurred at an earlier time in the past (therefore older). In the context of Orcas, referring to Antarctic ecotypes as older thus mean that one of the oldest separations is between Antarctic and most other ecotypes worldwide. In this context, Orca samples from other regions such as Indian Ocean, South Pacific and South Atlantic also appear to be part of relatively “old” lineages, although these regions are not as extensively sampled as the Antarctic. It is therefore possible (even likely) that the modern Orca expansion did not start from one specific location, but rather from one population which was distributed more widely over the Southern hemisphere Oceans.
This expansion is thought to be associated with the most recent glacial cycle (called the Weichselian), which had its coldest period roughly 12 000 years ago. Most studies suggest that one of the first ecotypes to separate from the Southern Ocean was the North Pacific ‘Transient’. This was followed by the formation of the North Atlantic ecotypes (‘Type 1’ and ‘Type 2’) and the other well described North Pacific ecotypes (‘Offshore’ and ‘Resident’), but the exact route has not been determined precisely. These could all have diverged from Southern Ocean populations very closely in time, or could have instead originated more locally in the North Pacific. If this was the case, then the Atlantic ecotypes likely originated from animals crossing the Northern passage in periods of low Arctic ice coverage. This would be consistent with data from other marine animals in the region, which also show a pattern consistent with colonization of the Atlantic originating from the Pacific since the last glacial cycle. More recently, as the Arctic ice caps have receded to record levels, cases of North Pacific cetaceans crossing into the North Atlantic have been identified, namely Orcas and Grey whales, so this is a well known dispersal route for cetaceans in periods of low ice coverage. The implication of these findings is again that evolution in Orcas might be strongly influenced by oceanographic changes associated with global climate cycles, although the exact biological mechanism is harder to unravel.
One difficulty is that although it appears clear that the last glacial cycle is associated with the appearance of modern killer whale ecotypes, their exact timing of origin is still being determined. Estimating the time of evolutionary events from genetic data alone is extremely challenging and imprecise, made particularly difficult by the low genetic variation characteristic of Orcas. Genetic studies often rely on very small number of genetic differences between ecotypes, and therefore lack resolution when trying to infer parameters such as the timing of differentiation between them. This would be akin to try and describe a complex urban landscape such as Tokyo, while looking through a black canvas containing only a few small holes through which light can go through. Not impossible, but different people standing in different places of the canvas would get very different perspectives of what Tokyo looks like. Similarly, when unravelling the complex history of Orcas from genetic data, it can be expected that estimates will vary slightly between studies using different datasets.
Why so many ecotypes?
The complexity of Orcas recent range expansion has created an interesting and somewhat unusual pattern, where large geographic distances between different ecotypes do not necessarily correspond with strong levels of genetic differentiation between those same ecotypes. The most notable example is the genetic differentiation between the ‘Resident’ and ‘Transient’ ecotypes in the North Pacific. Although they are often recorded in relatively proximate locations in the coasts of North America, they are some of the most genetically differentiated ecotypes. The ‘Offshore’ ecotype distribution is geographically similar in relation to the distribution of both ’Transients’ and ‘Residents’, but ’Offshores’ are genetically closer to ‘Residents’. The North Atlantic ecotypes are genetically closer to the North Pacific ‘Offshore’ ecotype, while Antarctic Orcas are more closely related to the North Pacific ‘Transients’ (although Antarctic Orcas form a distinctive genetic group in itself). Other regions of the world have been relatively less represented in genetic studies, but a similar pattern is apparent. Orcas from the Indian Ocean are more closely related to North Pacific ‘Transients’, while Orcas from South Pacific and Atlantic are more closely related to the North Atlantic ecotypes.
The biological reasons for this pattern provide fertile ground for scientific studies on the formation of new species, and many projects on Orca genetics have addressed this issue over the years. Although it is agreed that only one species of Orca exist at present, there is an emerging consensus from genetic studies that some of the Orca ecotypes might be on the path to becoming different species (notably ‘Residents’ and ‘Transients’). There is a classic concept in evolutionary biology, which predicts that geographically isolated groups will eventually become different species after sufficient time in isolation. However, as we discussed just now, groups of Orcas which are not geographically isolated appear to be on the path to becoming different species. This raises the question of what mechanisms other than geographic isolation might be promoting speciation in these animals. The scientific literature on this topic is too vast to cover in detail here, and has been considered in the context of many different wildlife species. In the case of Orcas, the possibility that environmental differences might have a role in the process is again relevant here, and one biological mechanism that is often mentioned is their specialisation on different types of prey.
When looking at patterns of genetic variation, there is a tendency for ecotypes with well known feeding specialisations to be more differentiated at the genetic level as well. For example, the ‘Residents’ and ‘Transients’ which (as we’ve seen before) are genetically distinct, also have very different and narrow diets, with ‘Residents’ feeding mostly on salmon and ‘Transients’ mostly on seals. In contrast, the North Atlantic ‘Type 1’ and ‘Type 2’ ecotypes are not as strongly differentiated genetically. While ‘Type 1’ seems to target mainly Herring and ‘Type 2’ other cetaceans, indirect measures of diet (based on a technique called stable isotopes) does not show such discrete and specialized feeding ecologies as observed in the North Pacific. However, a group of Orcas which follows the tuna migration into the strait of Gibraltar (let’s call it ‘Type T’), is genetically well differentiated from the other North Atlantic ecotypes (‘Type 1’ and ‘Type 2’).
The idea of population differentiation being driven by different feeding specializations is related to a concept called diversifying or disruptive selection. This is when adaptation to different environments through natural selection limits connectivity between two groups, thus accelerating genetic divergence between them. Some studies on Orca genetics support this might be the case with feeding specializations. These studies have scanned Orca genomes for genes that might show patterns of diversity typical of disruptive selection between ecotypes with different feeding habits, and revealed that genes involved in digestion and protein metabolism show the strongest signal of being under natural selection. This suggests that some ecotypes would not survive as well if they fed on a different prey item due to their specific physiology, and that these prey specialisations can thus be considered as adaptive. However, these types of studies are still rare, and their results should be considered preliminary at this stage. We can call it a prototype idea which, although interesting, still needs more testing before we can certainly say it reflects the biological reality of these animals.
Other possible explanations involve time segregation, where although Orcas occupy the same geographical space they do so at different times and are therefore isolated. This temporal isolation could be caused by differences in behavioural routines, but also be a by-product of the different feeding specialisations. For example, ‘Residents’ follow the salmon migrations closely, which occurs at specific times of the year, while ‘Transients’ follow the movement of seals which might not occupy the same areas at the same time as salmon. This temporal segregation can create what is effectively a type of geographic isolation, which if maintained for long enough, could lead to separation into different species.
Another seductive hypothesis is that behavioural differences (passed from parents to offspring), acts as an isolating barrier limiting interactions between ecotypes. Ecotypes showing behavioural differences are often also genetically different, and it is suggested that the behavioural differences appeared first, with the genetic differentiation resulting from those differences. This is known as “cultural-hitchhiking”, as it suggests that species isolation hitchhikes on cultural isolation. This is credible for Orcas, given that many ecotypes show tight family structures where offspring’s learn much of their behaviour for their closest kin. This can include vocalisation patterns, hunting strategies, and even reproductive rituals. For example, ‘Residents’ and ‘Transients’ are known to behave very differently in preparation for mating, and these differences can thus limit how much these two groups would interbreed, even if they were to occur in the same location at the same time. Simulation studies have shown that cultural-hitchiking, if present, can effectively accelerate the rate of differentiation between different groups. However, testing this hypothesis using genetic data is difficult, as behaviours tend to be malleable and have a complex genetic basis. Furthermore, it’s almost impossible to determine the behaviour on ancient populations, and we are thus faced with a chicken-and-egg type of question. Furthermore, genetic studies have shown that mating between different ecotypes still sometimes occur (more details on this in the next section).
It should be noted however, that these different hypothesis are not necessarily mutually exclusive, and that all could, to some extent, reflect the biological reality of Orcas. It is entirely possible that the relatively large number of Orca ecotypes results precisely because all those elements are present in nature, acting in combination to accelerate the rate of diversification between certain ecotypes.
What is still not known?
Although some ecotypes are relatively well represented in genetic studies (and therefore, well known), there are places in the world from where information is scarce or altogether absent. This imbalance is expected when we consider that Orcas are found across a very extensive range worldwide, but it could (and probably does) affect the accuracy of inference regarding Orca genetic structure and evolutionary past.
In waters around Mexico (including the semi enclosed Gulf of California) Orcas are often included into a single group called ‘Eastern Tropical Pacific’ (often shortened to ETP), as the same individual Orcas are re-sighted in the area with some regularity. Genetic studies also show the occurrence of a separate population in that region that differentiates well from other Pacific Orcas, although some individuals sampled have genetic lineages more closely related to ‘Transients’, ‘Offshores’, and even Antarctica and North Atlantic ecotypes. There are different explanations for this pattern, but at present it’s not entirely possible to identify which one is correct. One possibility is that animals from the ‘Transient’ and ‘Offshore’ ecotypes occasionally travel to the region, and could have been sampled as being ‘Eastern Tropical Pacific’ erroneously. Another explanation is that there is still some residual mating between the different ecotypes, or that not enough time has passed since their separation for those shared lineages to be lost (let’s remember that Orca ecotypes result from a recent expansion). Or it could be that studies so far have not had the genetic resolution to accurately identify their relationships to other ecotypes. Orcas are also seen throughout the Pacific coastal waters of other Central and South American countries (e.g. Ecuador, Colombia), but not enough detail is available to provide a clear picture of their genetic distinctiveness and relationships to other well known ecotypes.
Orcas found along the North Pacific Russian coastlines are genetically differentiated from the other North Pacific ecotypes (‘Transient’, ‘Resident’ and ‘Offshore’), but their relationships to other worldwide Orcas are not yet known. In Japan, Orcas are seen regularly in the Northern Island of Hokkaido, and although these are likely related to Orcas seen in Russia, no genetic data is available at present. Orcas are also seen more sporadically in Northern Honshu (Main Island) and around the island of Shikoku (in southwest Japan) but there is almost no genetic data from these animals, with one sample being more similar to samples from the Indian Ocean and Australasia region. Very few samples have been analysed genetically for those region where Orcas are commonly seen, consisting mostly of individual samples obtained in Australia, Thailand, Maldives and Seychelles. There is, however, not enough genetic resolution to make firm conclusions regarding potential ecotype assignment in the region, nor how they might relate to other well known ecotypes. Orcas are also seen in the South East Asia coasts, but very little information is available for those locations.
Orcas from Argentina (famous for hunting seals by lunging past the waterline into dry land) are genetically more similar to the North Atlantic ecotypes (‘Type 1’, ‘Type 2’ and ‘Type T’), but appear well differentiated from them. However, genetic studies analysing this group have typically used very small sample sizes, and our understanding of their genetic relationships is thus liable to change as more samples are incorporated in future genetic studies. Within Antarctica, genetic data shows that ‘Type D’ is well differentiated from all other Antarctic ecotypes, but ‘Type B’ and ‘Type C’ do not clearly separate from each other as they share a substantial amount of genetic variation. No data exists at present for ‘Type A’, and inference regarding its genetic relations to other ecotypes will only be possible once samples from these animals are collected and analysed.
Our inference of expansion routes and timing is strongly dependent on the genetic relationships inferred between animals sampled in different locations. As an example, although we currently think that ‘Transients’ originated from an expansion starting in the Southern Ocean, inclusion of samples from Southeast Asia might change this inference if they revealed to be more closely related to Antarctic ecotypes than ‘Transients’. These types of inference are further complicated by various studies showing some ongoing mating between Orcas in distant locations. In the North Pacific, genetic studies on parentage have identified several cases of mating between individuals from different ecotypes (e.g. between a ‘Transient’ female and an ‘Offshore’ male). Possible mating was also identified between Orcas sampled in the North Atlantic and others sampled in the North Pacific. However, the exact details of how often these type of events occur, and how much they confuse the relationships between ecotypes inferred from genetic data is still being determined. Studies identifying the kinship between individual animals are very different in concept, from studies analysing how much genetic variation is shared between populations. Populations can share genetic variation even if individual animals never currently mate between the populations. Similarly, individual mating between populations might not lead to a detectable sharing of genetic variation in population level studies. Therefore, this is likely to be an area of active research in the future, and could potentially change our understanding of Orca evolutionary past.
We should therefore expect that in the coming years, a more comprehensive and accurate image of Orcas history to emerge, likely with some surprising results sprouting up occasionally. Particular aspects to watch out for are more comprehensive analyses of natural selection from genetic data, better incorporation of the potential for cross-ecotype mating when inferring genetic relationships, and of course the inclusion of samples from yet unsampled regions (e.g. Asian coastlines). Referring back to the black canvas analogy, as more holes are punched through the canvas, more light will pierce through and the more accurate our perception of the landscape behind this canvas will be.
Orcas were one of the earliest cetacean species to be studied using genetic tools. As a consequence, Orcas are one of the better studied cetaceans in terms of their genetic composition. Earlier studies were mostly focused on identifying the presence of distinct Orca populations around the world, with particular attention to the well known ecotypes. Through the years, studies on Orca genetics have built up on this approach, and gradually provided a clearer idea of how many ecotypes exist around the world, how they relate to each other, when and where they originated from, and what evolutionary processes led to the current state of genetic variation. However, because of their widespread distribution and some specifics of their genetic composition, there is still a fair degree of uncertainty and much left to discover. The development of technologies analysing genetic variation at the whole genome level has provided significant breakthroughs, and we are likely to see a broadening of research topics addressed through genetic analyses in the future.
Where do modern Orcas come from?
Studies on genetics all show that Orcas are characterized by relatively low levels of genetic diversity. Wild animals with large populations and high ecological diversity are usually characterized by high genetic diversity, and so this low diversity is somewhat unexpected given Orcas global distribution and various well known ecotypes. Because of this, studies on Orca genetics often discuss the possibility of some type of genetic bottleneck occurring recently in the species past. Genetic bottleneck is a generic term used by population geneticists to describe a relatively quick loss of genetic diversity, due to some unusual ecological or demographic event. These events can produce characteristic patterns in the genetic composition of a group, which modern researchers can use to reconstruct past demographic changes of that group. Although often interpreted as being synonymous with a strong reduction in population size, this is not necessarily the case, as other more complex demographic events are known to create the characteristic patterns of genetic bottlenecks. Such patterns are found in many Orca groups, however details of the demographic events that might have caused them are still not fully known.
To this end, much progress has been made recently with the development of genomic tools and analyses. Studies using genomic data, all suggest that modern Orca ecotypes originated from a global expansion in a relatively quick timeframe. Furthermore, it appears likely that this expansion involved the dispersal of small groups of individuals over large distances across the world, as opposed to a single population that grew increasingly larger. These small groups would have experienced what is called a founder event, which can lead to the occurrence of genetic bottlenecks, and could thus partly explain the low genetic diversity that characterizes Orcas worldwide. This expansion is at present thought to have originated somewhere in the Southern Ocean, although the exact location has not yet been identified with precision. One of the earliest genomic studies found that orcas sampled in waters off South Africa had the highest genetic diversity of all worldwide locations analysed. This would suggest that this population might have been stable for longer, because wild populations accumulate genetic diversity by mutation, and the longer a population exists the higher the chance of new mutations occurring.
This interpretation is also consistent with our current understanding of how certain oceanographic features have changed in the last 1-2 million years. Around the world, there are regions where marine productivity is elevated due to the combined action of ocean currents, prevailing winds, and the coastline profile. These regions are characterised by a phenomena called upwelling, where organic matter from the sea floor gets brought to the surface, creating rich productive food webs. Such systems provide an abundance of prey that top predators, such as Orcas, can rely on for their long term survival. The upwelling systems around South Africa are thought to have been more stable during the past than upwelling systems elsewhere in the world, because of the relatively stability of the Agulhas (in the Atlantic Ocean side) and Benguela (on the Indian Ocean side) current systems. However, this type of evidence is purely circumstantial, and although it is relatively credible, it’s difficult to make a strong casual inference between the environmental phenomena and the genetic patterns of specific animal species.
Furthermore, as more locations around the world have been included, Orca genetic composition is proving to be slightly more complex. Studies also consistently show that Antarctic ecotypes (called Types A, B, C and D) have relatively high genetic diversity, and are likely one of the “oldest” modern lineages. In this context, the term “old” is useful but not entirely correct, as from an evolutionary perspective, all modern animals belong to lineages that are equally old. What evolutionary biologists really mean when they refer to a lineage as “old”, is that its separation from other known lineages in a group appears to have occurred at an earlier time in the past (therefore older). In the context of Orcas, referring to Antarctic ecotypes as older thus mean that one of the oldest separations is between Antarctic and most other ecotypes worldwide. In this context, Orca samples from other regions such as Indian Ocean, South Pacific and South Atlantic also appear to be part of relatively “old” lineages, although these regions are not as extensively sampled as the Antarctic. It is therefore possible (even likely) that the modern Orca expansion did not start from one specific location, but rather from one population which was distributed more widely over the Southern hemisphere Oceans.
This expansion is thought to be associated with the most recent glacial cycle (called the Weichselian), which had its coldest period roughly 12 000 years ago. Most studies suggest that one of the first ecotypes to separate from the Southern Ocean was the North Pacific ‘Transient’. This was followed by the formation of the North Atlantic ecotypes (‘Type 1’ and ‘Type 2’) and the other well described North Pacific ecotypes (‘Offshore’ and ‘Resident’), but the exact route has not been determined precisely. These could all have diverged from Southern Ocean populations very closely in time, or could have instead originated more locally in the North Pacific. If this was the case, then the Atlantic ecotypes likely originated from animals crossing the Northern passage in periods of low Arctic ice coverage. This would be consistent with data from other marine animals in the region, which also show a pattern consistent with colonization of the Atlantic originating from the Pacific since the last glacial cycle. More recently, as the Arctic ice caps have receded to record levels, cases of North Pacific cetaceans crossing into the North Atlantic have been identified, namely Orcas and Grey whales, so this is a well known dispersal route for cetaceans in periods of low ice coverage. The implication of these findings is again that evolution in Orcas might be strongly influenced by oceanographic changes associated with global climate cycles, although the exact biological mechanism is harder to unravel.
One difficulty is that although it appears clear that the last glacial cycle is associated with the appearance of modern killer whale ecotypes, their exact timing of origin is still being determined. Estimating the time of evolutionary events from genetic data alone is extremely challenging and imprecise, made particularly difficult by the low genetic variation characteristic of Orcas. Genetic studies often rely on very small number of genetic differences between ecotypes, and therefore lack resolution when trying to infer parameters such as the timing of differentiation between them. This would be akin to try and describe a complex urban landscape such as Tokyo, while looking through a black canvas containing only a few small holes through which light can go through. Not impossible, but different people standing in different places of the canvas would get very different perspectives of what Tokyo looks like. Similarly, when unravelling the complex history of Orcas from genetic data, it can be expected that estimates will vary slightly between studies using different datasets.
Why so many ecotypes?
The complexity of Orcas recent range expansion has created an interesting and somewhat unusual pattern, where large geographic distances between different ecotypes do not necessarily correspond with strong levels of genetic differentiation between those same ecotypes. The most notable example is the genetic differentiation between the ‘Resident’ and ‘Transient’ ecotypes in the North Pacific. Although they are often recorded in relatively proximate locations in the coasts of North America, they are some of the most genetically differentiated ecotypes. The ‘Offshore’ ecotype distribution is geographically similar in relation to the distribution of both ’Transients’ and ‘Residents’, but ’Offshores’ are genetically closer to ‘Residents’. The North Atlantic ecotypes are genetically closer to the North Pacific ‘Offshore’ ecotype, while Antarctic Orcas are more closely related to the North Pacific ‘Transients’ (although Antarctic Orcas form a distinctive genetic group in itself). Other regions of the world have been relatively less represented in genetic studies, but a similar pattern is apparent. Orcas from the Indian Ocean are more closely related to North Pacific ‘Transients’, while Orcas from South Pacific and Atlantic are more closely related to the North Atlantic ecotypes.
The biological reasons for this pattern provide fertile ground for scientific studies on the formation of new species, and many projects on Orca genetics have addressed this issue over the years. Although it is agreed that only one species of Orca exist at present, there is an emerging consensus from genetic studies that some of the Orca ecotypes might be on the path to becoming different species (notably ‘Residents’ and ‘Transients’). There is a classic concept in evolutionary biology, which predicts that geographically isolated groups will eventually become different species after sufficient time in isolation. However, as we discussed just now, groups of Orcas which are not geographically isolated appear to be on the path to becoming different species. This raises the question of what mechanisms other than geographic isolation might be promoting speciation in these animals. The scientific literature on this topic is too vast to cover in detail here, and has been considered in the context of many different wildlife species. In the case of Orcas, the possibility that environmental differences might have a role in the process is again relevant here, and one biological mechanism that is often mentioned is their specialisation on different types of prey.
When looking at patterns of genetic variation, there is a tendency for ecotypes with well known feeding specialisations to be more differentiated at the genetic level as well. For example, the ‘Residents’ and ‘Transients’ which (as we’ve seen before) are genetically distinct, also have very different and narrow diets, with ‘Residents’ feeding mostly on salmon and ‘Transients’ mostly on seals. In contrast, the North Atlantic ‘Type 1’ and ‘Type 2’ ecotypes are not as strongly differentiated genetically. While ‘Type 1’ seems to target mainly Herring and ‘Type 2’ other cetaceans, indirect measures of diet (based on a technique called stable isotopes) does not show such discrete and specialized feeding ecologies as observed in the North Pacific. However, a group of Orcas which follows the tuna migration into the strait of Gibraltar (let’s call it ‘Type T’), is genetically well differentiated from the other North Atlantic ecotypes (‘Type 1’ and ‘Type 2’).
The idea of population differentiation being driven by different feeding specializations is related to a concept called diversifying or disruptive selection. This is when adaptation to different environments through natural selection limits connectivity between two groups, thus accelerating genetic divergence between them. Some studies on Orca genetics support this might be the case with feeding specializations. These studies have scanned Orca genomes for genes that might show patterns of diversity typical of disruptive selection between ecotypes with different feeding habits, and revealed that genes involved in digestion and protein metabolism show the strongest signal of being under natural selection. This suggests that some ecotypes would not survive as well if they fed on a different prey item due to their specific physiology, and that these prey specialisations can thus be considered as adaptive. However, these types of studies are still rare, and their results should be considered preliminary at this stage. We can call it a prototype idea which, although interesting, still needs more testing before we can certainly say it reflects the biological reality of these animals.
Other possible explanations involve time segregation, where although Orcas occupy the same geographical space they do so at different times and are therefore isolated. This temporal isolation could be caused by differences in behavioural routines, but also be a by-product of the different feeding specialisations. For example, ‘Residents’ follow the salmon migrations closely, which occurs at specific times of the year, while ‘Transients’ follow the movement of seals which might not occupy the same areas at the same time as salmon. This temporal segregation can create what is effectively a type of geographic isolation, which if maintained for long enough, could lead to separation into different species.
Another seductive hypothesis is that behavioural differences (passed from parents to offspring), acts as an isolating barrier limiting interactions between ecotypes. Ecotypes showing behavioural differences are often also genetically different, and it is suggested that the behavioural differences appeared first, with the genetic differentiation resulting from those differences. This is known as “cultural-hitchhiking”, as it suggests that species isolation hitchhikes on cultural isolation. This is credible for Orcas, given that many ecotypes show tight family structures where offspring’s learn much of their behaviour for their closest kin. This can include vocalisation patterns, hunting strategies, and even reproductive rituals. For example, ‘Residents’ and ‘Transients’ are known to behave very differently in preparation for mating, and these differences can thus limit how much these two groups would interbreed, even if they were to occur in the same location at the same time. Simulation studies have shown that cultural-hitchiking, if present, can effectively accelerate the rate of differentiation between different groups. However, testing this hypothesis using genetic data is difficult, as behaviours tend to be malleable and have a complex genetic basis. Furthermore, it’s almost impossible to determine the behaviour on ancient populations, and we are thus faced with a chicken-and-egg type of question. Furthermore, genetic studies have shown that mating between different ecotypes still sometimes occur (more details on this in the next section).
It should be noted however, that these different hypothesis are not necessarily mutually exclusive, and that all could, to some extent, reflect the biological reality of Orcas. It is entirely possible that the relatively large number of Orca ecotypes results precisely because all those elements are present in nature, acting in combination to accelerate the rate of diversification between certain ecotypes.
What is still not known?
Although some ecotypes are relatively well represented in genetic studies (and therefore, well known), there are places in the world from where information is scarce or altogether absent. This imbalance is expected when we consider that Orcas are found across a very extensive range worldwide, but it could (and probably does) affect the accuracy of inference regarding Orca genetic structure and evolutionary past.
In waters around Mexico (including the semi enclosed Gulf of California) Orcas are often included into a single group called ‘Eastern Tropical Pacific’ (often shortened to ETP), as the same individual Orcas are re-sighted in the area with some regularity. Genetic studies also show the occurrence of a separate population in that region that differentiates well from other Pacific Orcas, although some individuals sampled have genetic lineages more closely related to ‘Transients’, ‘Offshores’, and even Antarctica and North Atlantic ecotypes. There are different explanations for this pattern, but at present it’s not entirely possible to identify which one is correct. One possibility is that animals from the ‘Transient’ and ‘Offshore’ ecotypes occasionally travel to the region, and could have been sampled as being ‘Eastern Tropical Pacific’ erroneously. Another explanation is that there is still some residual mating between the different ecotypes, or that not enough time has passed since their separation for those shared lineages to be lost (let’s remember that Orca ecotypes result from a recent expansion). Or it could be that studies so far have not had the genetic resolution to accurately identify their relationships to other ecotypes. Orcas are also seen throughout the Pacific coastal waters of other Central and South American countries (e.g. Ecuador, Colombia), but not enough detail is available to provide a clear picture of their genetic distinctiveness and relationships to other well known ecotypes.
Orcas found along the North Pacific Russian coastlines are genetically differentiated from the other North Pacific ecotypes (‘Transient’, ‘Resident’ and ‘Offshore’), but their relationships to other worldwide Orcas are not yet known. In Japan, Orcas are seen regularly in the Northern Island of Hokkaido, and although these are likely related to Orcas seen in Russia, no genetic data is available at present. Orcas are also seen more sporadically in Northern Honshu (Main Island) and around the island of Shikoku (in southwest Japan) but there is almost no genetic data from these animals, with one sample being more similar to samples from the Indian Ocean and Australasia region. Very few samples have been analysed genetically for those region where Orcas are commonly seen, consisting mostly of individual samples obtained in Australia, Thailand, Maldives and Seychelles. There is, however, not enough genetic resolution to make firm conclusions regarding potential ecotype assignment in the region, nor how they might relate to other well known ecotypes. Orcas are also seen in the South East Asia coasts, but very little information is available for those locations.
Orcas from Argentina (famous for hunting seals by lunging past the waterline into dry land) are genetically more similar to the North Atlantic ecotypes (‘Type 1’, ‘Type 2’ and ‘Type T’), but appear well differentiated from them. However, genetic studies analysing this group have typically used very small sample sizes, and our understanding of their genetic relationships is thus liable to change as more samples are incorporated in future genetic studies. Within Antarctica, genetic data shows that ‘Type D’ is well differentiated from all other Antarctic ecotypes, but ‘Type B’ and ‘Type C’ do not clearly separate from each other as they share a substantial amount of genetic variation. No data exists at present for ‘Type A’, and inference regarding its genetic relations to other ecotypes will only be possible once samples from these animals are collected and analysed.
Our inference of expansion routes and timing is strongly dependent on the genetic relationships inferred between animals sampled in different locations. As an example, although we currently think that ‘Transients’ originated from an expansion starting in the Southern Ocean, inclusion of samples from Southeast Asia might change this inference if they revealed to be more closely related to Antarctic ecotypes than ‘Transients’. These types of inference are further complicated by various studies showing some ongoing mating between Orcas in distant locations. In the North Pacific, genetic studies on parentage have identified several cases of mating between individuals from different ecotypes (e.g. between a ‘Transient’ female and an ‘Offshore’ male). Possible mating was also identified between Orcas sampled in the North Atlantic and others sampled in the North Pacific. However, the exact details of how often these type of events occur, and how much they confuse the relationships between ecotypes inferred from genetic data is still being determined. Studies identifying the kinship between individual animals are very different in concept, from studies analysing how much genetic variation is shared between populations. Populations can share genetic variation even if individual animals never currently mate between the populations. Similarly, individual mating between populations might not lead to a detectable sharing of genetic variation in population level studies. Therefore, this is likely to be an area of active research in the future, and could potentially change our understanding of Orca evolutionary past.
We should therefore expect that in the coming years, a more comprehensive and accurate image of Orcas history to emerge, likely with some surprising results sprouting up occasionally. Particular aspects to watch out for are more comprehensive analyses of natural selection from genetic data, better incorporation of the potential for cross-ecotype mating when inferring genetic relationships, and of course the inclusion of samples from yet unsampled regions (e.g. Asian coastlines). Referring back to the black canvas analogy, as more holes are punched through the canvas, more light will pierce through and the more accurate our perception of the landscape behind this canvas will be.
