Do species become more different from one another to avoid breeding?
Keith S. Taber
A tamarin monkeyA different tamarin
They say "opposites attract". True perhaps for magnetic poles and electrical charges, but the aphorism is usually applied to romantic couples. It seems like one of those sayings that survives due to the 'confirmation bias' in human cognition. That is, as long as from time to time seemingly unlikely couplings occur, the explanation that 'opposites attract' seems to have some merit, even in it only applies to a minority of cases.
Apparently, in the area of overlap the red-handed tamarins seemed to have adapted one of their calls so it sounds very similar to that of the pied tamarins. (N.b. The images above represent two contrasting species, just as an illustration.) The suggested explanation was that this modification made it more likely that the monkeys of different types would recognise each other's calls – in particular that "…they are trying to be understood, so they don't end up in a fight…".
Anthropomorphism?
I wondered if these monkeys were really "trying" to achieve this, or whether this might be an anthropomorphism. That is, were the red-handed tamarins deliberately changing their call in this way in order to ensure they could be understood – or was this actually natural selection in operation – where, because there was an advantage to cross-species communication (and there will be a spread of call characteristics in any population), over time calls that could be understood by monkeys of both species would be selected for in a shared niche.
Then again, primates are fairly intelligent creatures, so perhaps Dr Dunn (who, unlike me is an evolutionary biologist) means this literally, and this is something deliberate. Certainly, if the individual monkeys are shifting their calls over time in response to environmental cues, rather than the shift just occurring across generations, then that would seem to suggest this is learning rather than evolution. (Of course, it could be implicit learning based on feedback from the responses to their behavior, and still may not be the monkeys consciously adopting a strategy to be better understood.)
Becoming more distinct
Dr Dunn's explanation of the wider issue of how similar animals will compete for scarce resources intrigued me:
"When you have species that are closely related to one another and live in sort of overlapping areas there's quite a lot of pressure because they're likely to be competing for key resources. So, sometimes we see that these species actually diverge in their traits, they become more different from one another. Examples of that are sort of coloration and the way that animals look. Quite often they become more distinct than you would expect them to, to avoid breeding [sic] with one another."
My initial reaction to this was to wonder why the two species of monkeys needed to avoid breeding with each other. 'Breeding' normally refers to producing offspring, reproduction, but usually breeding is not possible across species (except sometimes to produce infertile hybrids).
Presumably, all tamarins descended from a common ancestor species. Speciation may have occurred when different populations become physically separated and so were no longer able to inter-breed (although still initially sexually compatible) simply because members of the two groups never encountered each other. Over time (i.e., many generations) the two populations might then diverge in various traits because of different selection pressures in the two different locations, or simply by chance effects* which would lead to the two gene pools drifting in different ways.
Two groups that had formed separate species such that members of the two different species are no longer able to mate to produce fertile offspring, might subsequently come to encounter each other again (e.g., members of one species migrating into to the territory of the other) but inter-breeding would no longer be possible. A further mechanism to avoid breeding (by further "diverge[nce] in their traits") would not seem to make any difference.
If they actually cannot breed, there is no need to avoid breeding.
A breeding euphemism?
However, perhaps 'breeding' was being used by Dr Dunn as a euphemism (this was after all a family-friendly radio programme broadcast in the afternoon), as a polite way of saying this might avoid the moneys copulating with genetically incompatible partners – tamarins of another species. As tamarins presumably do not themselves have a formal biological species concept, they will not avoid coupling with an animal from a different species on the grounds that they cannot breed and so it would be ineffective. They indulge in sexual activity in response to instinctive drives, rather than in response to deliberate family planning decisions. That is, we might safely assume these couplings are about sexual attraction rather than a desire to have children.
I think that was what Jürgen Habermas may have meant when he wrote that:
"…the reproduction of every individual organism seems to warrant the assumption of purposiveness without purposeful activity…"
In terms of fitness, an animal is clearly more likely to have offspring if it is attracted to a sexually comparable partner than a non-compatible one. Breeding is clearly important for the survival of the species, and uses precious resources. Matings that could not lead to pregnancy (or, perhaps worse from a resource perspective, might lead to infertile hybrids that need to be nurtured but then fail to produce 'grandchildren'), would reduce breeding success overall in the populations. Assuming that a tamarin is more likely to be attracted to a member of a different species when it does not look so different from its own kind, it is those monkeys in the two groups that look most alike who are likely to be inadvertently sharing intimate moments with biologically incompatible partners.
A teleological explanation
Dr Dunn's suggestion that "quite often [the two species] become more distinct than you would expect them to, to avoid breeding with one another" sounds like teleology. That is, it seems to imply that there is a purpose (to avoid inter-breeding) and the "species actually diverge in their traits" in order to bring about this goal. This would be a teleological explanation.
I suspect the actual explanation is not that the two species "come more distinct…to avoid breeding with one another" but rather than they come more distinct because they cannot breed with each other, and so there is a selection advantage favouring the most distinct members of the two different species (if they are indeed less likely than their less distinguishable conspecifics to couple with allospecific mates).
I also suspect that Dr Dunn does not actually subscribe to the teleological argument, but is using a common way of talking that biologists often adopt as a kind of abbreviated argument: biologists know that when they refer to evolution having a purpose (e.g., to avoid cross-breeding), that is only a figure of speech.
Comprehension versus accuracy?
However, I am not sure that is always so obvious to non-specialists listening to them. Learners often find natural selection a challenging topic, and many would be quite happy with accepting that adaptations may have a purpose (rather than just a consequence). This reflects a common challenge of communicating science – either in formal teaching or supporting public understanding.
The teacher or science communicator simplifies accounts and uses everyday ways of expressing ideas that an audience without specialist knowledge can readily engage with to help 'make the unfamiliar familiar'. However, the simplifications and approximations and short-cuts we use to make sure what is said can be understood (i.e., made sense of) by non-specialists also risks us being misunderstood.
It would seem (rightly) indecent for your great great grandfather to have procreated with your sister – but if you could go back far enough in your family tree you would surely find even more extreme examples of intergenerational couplings!
Skulls images by Parker_West from Pixabay
Some approaches to conceptualising speciation may by definition impose sharp distinctions: in one version of cladistics it is assumed that at any speciation event the ancestor species ceases to be extant, and the new species comes into existence at one moment in time – if members of (what was) the ancestral species happily carry on living their lives despite this conventional extinction, as a new species branches off from the ancestral line, they are judged now to be members of another new species. That is, in this system a species is never considered to give rise to a new species and also continue, but rather transitions into two new species, even if one contains individuals indistinguishable from those in the ancestral line (LaPorte 2004). However, I am going to take the position here that if experts in the field cannot distinguish specimens as being from different species then it is reasonable to consider those specimens as conspecific.
To take an example close to home, consider the species Homo sapiens. Every human alive today had parents who were, like themselves, specimens of the species Homo sapiens. These parents also had parents who were specimens of the species Homo sapiens. So did their parents – and (to avoid this text becoming extremely tedious) so on, through a large number of generations. However, modern humans are understood to have evolved from earlier hominids (who in turn evolved from non-human primates, who evolved from non-primate mammals, who evolved from non-mammalian chordates, and so forth.)
A thought experiment about ancestry
So, consider a thought experiment where scientists had physical evidence of the full ancestry of someone, some specimen of Homo sapiens, alive today – bones, DNA, whatever. It is only a thought experiment, so it only has to be possible in principle, not feasible in practice. And forensic science today achieves things that might have seemed fantastic just a few decades ago – so who can say what might become feasible in time?
Experts in anatomy or genetics would agree that the generation of the parents of our human friend were Homo sapiens, as were the previous generation, and the generation before that, and… However, at some point many, many generations back, the experts would agree that the scientific evidence showed these more distant ancestors were not Homo sapiens, but something else – perhaps Homo heidelbergensis.
We would be going back something of the order of tens of thousands of generations. Perhaps (for the sake of this thought experiment – the actual numbers are not critical to the argument) all the experts agree that the ancestors in generation n-14000 (n minus fourteen thousand, where n is the current generation, our living person) were members of our species, Homo sapiens, and perhaps these experts also all agree that the ancestors in generation n-17000 were a different species, not Homo sapiens: but where does this transition occur?
It seems unlikely that the experts would be able to agree, based on clear distinctions in the material evidence (even if we assumed the evidence available, as this is a thought experiment), that ancestors in generation n-15777 (for example) were the earliest ancestors who were members of Homo sapiens and that the ancestors in generation n-15778 were members of a different species.
Gradual change
This is not simply unlikely because the experts would not agree as some would be more expert than others, and so be more likely to get things right: it is simply that the distinctions between species are not sudden and abrupt, but occur over time. Those transitions may often appear rapid when looking at the geological record, in terms of what is sometimes called 'deep time', but even allowing that evolution may not be as gradual and even as was once widely considered (Gould and Eldredge 1993/2000), the shift between distinct species is gradual in terms of our experience of the natural world. Our lives occupy a tiny period in the vastness of the history of the biota on Earth, so we experience the living things in our environment as if a single cross-section of a cone of biological development.
We are in effect living upon one cross-section, one microtome slice as it were, of deep-time – and so species appear as discrete kinds (Figure from Taber, 2013/2017.)
A compromised geometric progression?
Before moving on, it is worth highlighting the absurdity of extrapolating what seems commonplace on a 'local' (temporal) scale to a geological scale. Most people have 2 (21) parents, who were probably alive at the same time (i.e., their lives must have overlapped for them to be parents, unless there was some cryogenic storage of sperm or eggs – something that is now possible and means a very small proportion of people alive today have been conceived at a time when only one biological parent was alive), and 4 (22) grandparents whose lives nearly always overlapped in time, and 8 (23) great-grandparents whose lives probably overlapped in time… We might be tempted to generalise to having 2n ancestors if we go back n generations.
This pattern does not necessarily repeat indefinitely however. So, the British Head of State, at the time of writing, is Queen Elizabeth II. Two of her great-great grandparents were Queen Victoria and her escort Prince Albert. Elizabeth is married to Prince Philip. Two of his great-great grandparents were (also) Queen Victoria and Prince Albert. The Children of the current Queen (Charles, Anne, Andrew and Edward) therefore do not have a full, unique set of great-great-great grandparents, as Victoria and Albert each occupy positions on their family tree that could in principle have been filled by two different people (although that of course, would not have given rise to the existence of the particular individuals Charles, Anne, Andrew and Edward who are alive today).
It is a common view that the degree of inbreeding among the royal houses of Europe was responsible for the instances of certain medical conditions among the royals. Indeed, haemophilia was referred to as 'the royal disease'
Finding a mate
Although marriage and breeding within the extended family has been particularly noted among royalty, it was by no means their exclusive practice. In highly stratified societies where marrying above or below one's supposed rank was not acceptable, the range of potential mates in one's social circle might be very limited (as reflected in novels of the likes of Jane Austen).
Marrying relatives who were not immediate family was common and often productive. Charles Darwin married a cousin, Emma Wedgewood, which led to a very happy marriage, and some highly achieving offspring. Charles and Emma shared a grandfather – Josiah Wedgwood (the famous potter) – and grandmother. Social circle and extended family could overlap considerably.
A trivia quiz question might be:
How was John Allen Wedgwood able to legally marry two of his cousins on the same day? **
For much of human pre-history people lived in small groups where the range of potential mates would have been severely limited, leaving aside questions of social status. Indeed it is possible that the common taboo on sexual relations with very close relatives, i.e. incest, developed in a context where the number of feasible candidates for a mate was often very small.
A paradox? You have more human ancestors than the number of people who have ever lived
Returning, then, to our thought experiment. If each of their theoretical possible ancestors in generation n-15777 were discrete, individual, specimens (of whatever species) then our contemporary subject would have 215777 ancestors in that generation. That is a number vastly greater that the number of people living today (which is less than 233) or indeed who have ever existed – and is even vastly greater than estimates of the number of particles in the whole universe! (One estimate for the total number of quarks plus electrons is 'only' around 2268.) Some estimates for the size of the early Homo sapiens population are around 214 – rather less than 215777!
The vast discrepancy here then comes from assuming that the number of ancestors doubles in each generation. Most people have two parents, four grandparents, and eight great grandparents – but if one goes back a large number of generations there must have been considerable redundancy in the sense of individual ancestors taking up a number of positions on one's personal family tree. And we cannot even assume these multiple roles fall within the same generation.
The notion that anyone alive today would have all their ancestors from generation n-15777 alive at the same time is unreasonable.
If we assume that through most of human history the time lapse between generations was largely in a range 15-25 years (and clearly there will have been plenty of children born to parents younger than 15 and older than 25, so this is a conservative range) then it becomes obvious that at the time when one of our ancestors in generation n-15777 was alive, so were many of our ancestors in a wide range of other generations.
If the mean gap between generations was 20 years, then 15 777 generations ago was about 315 540 years ago. At the same time a line of descent with an average gap between generations of 15 years would be a little more than 21 036 generations ago, and a line of descent with an average gap between generations of 25 years would be 12 622 generations ago.
A schematic representation of the distribution of a person's ancestors living c.316 000 years ago in terms of how many generations separate them from that person. Many (most) ancestors will be represented many times (by different lines of descent) across a spread of points in the distribution.
It may seem strange to think that some of the ancestral pairings that led to us were between individuals that from our (temporally reversed) perspective were in generations that were hundreds or perhaps even thousands apart***: but of course the point is they were alive at the same time.
A highly simplified scheme showing descent along only two lines. Using the simplified example that people are born when their mother is 18 but their father is 24 (clearly there will normally be much variation in any 'branch' of any 'tree') it does not take many generations before ancestors alive (and of reproductive age) at the same time can be considered to be from different generations.*** Bearing in mind that we all have far fewer direct ancestors than potentially unique places on the 'tree', we could in principle trace many of our ancestors through multiple routes relating to different generations. In this simplified scheme the person's father's father's father is also their mother's mother's mother's father. So the same person could be your great-grandfather and also your great-great-grandfather. M = mother; FF = father's father; MMMF = mother's mother's mother's father, etc.
You are a member of the 15 778th generation of Homo sapiens, and you are a member of the 15 779th generation of Homo sapiens, and you are a member of the 15 777th generation of Homo sapiens, and…
By the same (or, if you prefer, the reverse) logic, even if we were (adopting a cladistic approach) able to pinpoint a precise moment in time when Homo sapiens appeared, generation 'Homo sapiens 1', then a person alive today would not by comparison be unequivocally in generation 'Homo sapiens 15778' (or whatever), at least, not unless we adopted a convention to count down through a particular line (e.g., always the mother). Rather, they would be in a hybrid generation with a wide range, say generation 'Homo sapiens 11 246-to-19 975', or whatever.
As a final observation, a common definition of species refers to breeding populations that can produce viable (fertile) offspring. If the distinction between Homo sapiens and, say Homo heidelbergensis, is a gradual shift and not a sharp cut off, then the question of interbreeding between co-existing species is somewhat avoided: but there is much evidence that our ancestors interbred with Neanderthals, even though they are traditionally considered to be a distinct species (Homo neanderthalensis).
Waking up a different species
So the biological species concept, whilst being extremely useful in science, would seem to either be somewhat arbitrary (if we adopt a cladistics perspective, and just define by fiat specific speciation events at which point old species become extinct and new ones are said to come into existence), or to have rather fuzzy edges.
The cladistic perspective keeps things rather nice and tidy but it would seem a bit like living in Europe during the restoration, when a person could go to bed an orthodox believer and wake up the next day a heretic because the sovereign had decided to switch the National faith from Catholicism to Protestantism (or vice versa). The person had not changed, but the definitions had. A helpful perspective, perhaps, is to treat the notion of biological species as a scientific hypothesis (Knapp 2017), in that when a scientist proposes a species this is a hypothesis about a certain regularity in the natural world: a hypothesis which is then the basis for further investigation.
** The answer does not relate to a tragic wedding-reception death followed by an indecently short whirlwind romance, but rather that the Rev. Wedgwood officiated at the wedding of his cousin Emma to his cousin Charles.
*** Of course, by definition the couple were in the same generation back along the line of descent they shared, but possibly in very different generations back along alternative lines of descent. So, the individual highlighted with the pink circle in the preceding figure has children with two different partners in the ancestral 'tree' (really, a network) as MMMMMMMM mating with MMMMMMMF, and as FFFFFM mating with FFFFFF. In both cases she is the same number of generations back as her partner in terms of their child on the particular ancestral line, BUT she is both a great-great-great-great grandmother and a great-great-great-great-great-great grandmother of the same individual. So in that sense, she belongs to two different generations. That is only considering 'fruitful' couplings that led to an offspring in the direct ancestory of some individual. There will clearly be many couplings that did not lead to offspring among someone's ancestors (or indeed no offspring at all) where the couple concerned only appear in the ancestral 'tree' of some individual in different generations.
Sources cited:
Gould, S. J., & Eldredge, N. (1993/2000). Punctuated equilibrium comes of age. In H. Gee (Ed.), Shaking the Tree: Readings form Nature in the history of life (pp. 17-31). Chicago: University of Chicago Press.
* When writing 'The Nature of the Chemical Concept' I was discussing the idea of natural kinds in chemistry (for example, 'potassium' has a better claim to refer to a natural kind than 'acid'), and the limitations of the notion of a natural kind. An example that I assumed would be familiar to readers was that of species. Species used to be considered different natural kinds each with their own essence, that were, largely at least, found distinct in nature:
Species as Natural Kinds? A Warning from Biology
"People, not just scientists, tend to naturally (sic, automatically) notice kinds in nature: for example, kinds of mineral, kinds of meteorological conditions (e.g., types of clouds), and perhaps most obviously, kinds of living thing…. When children, we all readily notice and learn that the world contains different kind of living things. There are birds and horses and dogs and fish and so forth. We come to recognise levels of classification without difficulty: this animal is a dog, and also a Labrador; this creature is both a sparrow and a bird. Later, when we study science in school we find that such distinctions are made formally by scientists, although not always in ways that entirely fit with informal everyday use… so mushrooms should not be considered plants, for example.
More advanced study might lead us to realise that the recognition of species and other higher-level taxa is not so straightforward. When I was at school, it was considered that the dinosaurs, the 'terrible lizards', as a group became extinct around 66 million years ago at the time of the formation of the Cretaceous-Paleogene (aka KT, Cretaceous-Tertiary) boundary, but since then 'lizard' has become a questionable category of natural kind, whilst many biologists now claim that birds are technically extant (rather than extinct) dinosaurs.
Having learnt that the main orders of vertebrates were fish, amphibians, reptiles, birds, and mammals, it appears that not only might birds be considered reptiles, but that reptiles are by some biological criteria not actually an essential kind (that is a kind with a particular essence). Even fish are not exempt. Leaving aside the tendency of the term fish to sometimes be used in a vernacular sense of sea creature (to include whales and 'shell-fish' for example), it seems that by some criteria fish do not share a particular essence as a group, as some fish are more closely related to members of other groups than they are to some other fish…. Guinea pigs are no longer seen as members of the mammalian group of rodents. In addition, these are just some examples from the vertebrates, among the most familiar groups of animals to most people….
Modern scientific thinking, post-Darwin, suggests that there are no absolute distinctions between species. Darwin himself thought he had done biology a service in offering the perspective on the biota suggested by his theory of natural selection. Descent of different groups from common ancestors, should (Darwin thought) have brought an end the interminable wrangling about whether particular groups were 'really' different species or actually varieties of the same species. For Darwin, understanding the origin of species suggested there could be no absolute distinct essence of any particular biological grouping such that there would always be an absolute distinction between specimens of one species and another"
Taber, 2019: 121-123
In writing about how the shift between species was a gradual process I went into the ideas about how over a long period of time the number of generations separating two individuals becomes ambiguous and how most of our ancestors must appear multiple times on our 'tree' of descent (which also means that if you go back far enough, most of those alive then, who have offspring alive today, are probably shared ancestors of most of us). However, this was getting somewhat peripheral to my key point about species and natural kinds. So I excised that material, thinking I might find another use for it. That text is reproduced above.
