adaptation - Science-Education-Research

Opposites avoid attracting

Do species become more different from one another to avoid breeding?

Keith S. Taber

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.

Trying to avoid a fight

What got me thinking about this was an interview (on BBC's Inside Science radio programme/podcast) with Dr Jacob Dunn, Associate Professor in Evolutionary Biology at Anglia Ruskin University, who studies primate vocal communication. He was discussing his research into the calls of tamarin monkeys in the Amazon rainforest, and in particular the calls of two different species where their ranges overlap.

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.

(* Read about 'Intergenerational couplings in the family: A thought experiment about ancestry')

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.

(Read about 'Teleology')

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.

Many generations later it's just naturally always having fur

Keith S. Taber

Bert was a participant in the Understanding Science Project. In Y11 he reported that he had been studying about the environment in biology, and done some work on adaptation. he gave a number of examples of how animals were adapted to their environment. One of these examples was the polar bear.

…our homework we did about adapting, like how polar bears adapt to their environments, and camels….
And so a polar bear has adapted to the environment?
Yeah.
So how has a polar bear adapted to the environment?
Erm, things like it has white fur for camouflage so the prey don't see it coming up. Large feet to spread out its weight when it's going over like ice. Yeah, thick fur to keep the body heat insulated.

Bert gave a number of other examples, including dogs that were bred with particular characteristics, although he explained this in terms of inheritance of acquired characteristics: suggesting that dogs that have been taught over and over to retrieve have puppies that automatically have already got that sense. Bert realised that his example was due to the work of human breeders, and took the polar bear as an example of a creature that had adapted to its environment.

Yeah, so how does adaption take place then? …
I don't know. It may have something to do with negative feedback.…Like you have like, you always get like, you always get feedback, like in the body to release less insulin and stuff like that. So in time … organisms, learn to adapt to that. Because if it happens a lot that makes a feedback then it comes, yeah then they just learn to do that.
Okay. Give me an example of that. I'm trying to link it up in my head.
Okay, like the polar bear, like I don't know. It may have started off just like every other bear, but because it was put in that environment, like all the time the body was telling it to grow more fur and things like that, because it was so cold. So after a while it just adapted to, you know, always having fur instead of, you know, like dogs shed hair in the summer and stuff. But like if it was always then they'd just learn to keep shedding that hair.
So if it was an ordinary bear, not a polar bear, and you stuck it in the Arctic, it would get cold?
Yeah.
But you say the body tells it to grow more fur?
Erm, yeah.
How does that work?
I'm not sure, it just … I don't know. Like, erm, like the body senses that it's cold, it goes to the brain, and the brain thinks, well how is it going to go against that, you know, make the body warmer. And so it kind of, you know, it gives these things.

So Bert seemed to have notion of (it not the term) homoeostasis, that allowed control of such things as levels of insulin. He recognised thus was based on negative feedback – when some problematic condition was recognised (e.g. being too cold) this would trigger a response (e.g., more insulation) to bring about a countering change.

However, in Bert's model, the mechanism was not initially automatic. Bert thought that this process which initially was based on deliberation became automatic over many generations…

I see. So the bear has already got a mechanism which would enable it to have more fur, but it's turned on to some extent by being put into the cold?
Yeah.
And then over a period of time, what happens then?
Erm I guess it just it doesn't really need that impulse of being cold, it's just naturally there now, to tell it to do it more.
So how does that happen? Is this the same bear or is this many generations later?
I would probably think many generations later.
Right, so if it was just one particular bear, it wouldn't eventually just produce more hair automatically itself, but its offspring eventually might?
Yeah.
So how does that happen then?
I don't know. Probably from DNA or something. We haven't gone over that yet.

So for Bert, the individual bear could change its characteristics through activating a potential mechanism (in this case for keeping year-round thick fur) through a process of sensing and responding to environmental conditions. Over many generations this changed characteristic could become an automatic response by eventually being coded into the genetic material. As with his explanation of selective breeding, Bert invoked a model of evolution through the inheritance of acquired characteristics, rather than the operation of natural selection on the natural range of characteristics within a breeding population.

Like many students learning about evolution, Darwin's model of variation offering the basis for natural selection was not as intuitively appealing as a more Lamarckian idea that individuals managed to change their characteristics during their lives and pass on the changes to their offspring. This is an example of where student thinking reflects a historical scientific theory that has been discarded rather than the currently canonical scientific ideas taught in schools.

The brain thinks: grow more fur

The body senses that it's cold, and the brain thinks how is it going to make the body warmer?

Keith S. Taber

…our homework we did about adapting, like how polar bears adapt to their environments, and camels….
And so a polar bear has adapted to the environment?
Yeah.
So how has a polar bear adapted to the environment?
Erm, things like it has white fur for camouflage so the prey don't see it coming up. Large feet to spread out its weight when it's going over like ice. Yeah, thick fur to keep the body heat insulated.

Bert gave a number of other examples, including dogs that were bred with particular characteristics, although he explained this in terms of inheritance of acquired characteristics: suggesting that dogs that have been taught over and over to retrieve have puppies that automatically have already got that sense. Bert realised that this example was due to the work of human breeders, and took the polar bear as an example of a creature that had adapted to its environment.

Yeah, so how does adaption take place then? You've got a number of examples there, bears and dogs and camels and people. So how does adaption take place?
I don't know. It may have something to do with negative feedback.
That's impressive.
Like you have like, you always get like, you always get feedback, like in the body to release less insulin and stuff like that. So in time people like or whatever, organisms, learn to adapt to that. Because if it happens a lot that makes a feedback then it comes, yeah then they just learn to do that.
Okay. Give me an example of that. I'm trying to link it up in my head.
Okay, like the polar bear, like I don't know. It may have started off just like every other bear, but because it was put in that environment, like all the time the body was telling it to grow more fur and things like that, because it was so cold. So after a while it just adapted to, you know, always having fur instead of, you know, like dogs shed hair in the summer and stuff. But like if it was always then they'd just learn to keep shedding that hair.
So if it was an ordinary bear, not a polar bear, and you stuck it in the Arctic, it would get cold?
Yeah.
But you say the body tells it to grow more fur?
Erm, yeah.
How does that work?
I'm not sure, it just … I don't know. Like, erm, like the body senses that it's cold, it goes to the brain, and the brain thinks, well how is it going to go against that, you know, make the body warmer. And so it kind of, you know, it gives these things.
Is that an example of feedback?
Yes.

However, in Bert's model, the mechanism was not automatic. Rather it depended upon conscious deliberation: "the brain thinks, well how is it going to …make the body warmer". Bert thought that this process which initially was based on deliberation then became automatic over many generations.

This seems to assume that bears think in similar terms to humans, that they identify a problem and reason a way through. This might be considered an example of anthropomorphism, something that is very common in student (indeed human) thinking. To what extent it may be reasonable to assign this kind of conscious reasoning to bears is an open question.

However there was a flaws in the process described by Bert that he might have spotted himself. This model suggested that once the bear had become aware of the issue, and the needs to address, it would be able to grow its fur accordingly. That is, as a matter of will. Bert would have been aware that he is able to control some aspects of his body voluntarily (e.g., to raise his arm), but he cannot will his hair to grow at a different rate.

Of course, it may be countered that I am guilty of a kind of anthropomorphism-in-reverse: Bert is not a bear, but rather a human who does not need to control hair growth according to environment. So, just because Bert cannot consciously control his own hair growth, this need not imply the same is true for a bear. However, Bert also used the example of insulin levels, very relevant to humans, and he would presumably be aware that insulin release is controlled in his own body without his conscious intervention.

As often happens in interviewing students (or human conversations more generally) time to reflect on the exchange raises ideas one did not consider at the time, that one would like to be able to to text out by asking further questions. If things that were once deliberate become instinctive over time, then it is not unreasonable in principle to suggest things that are automatic now (adjusting insulin levels to control blood glucose levels) may have once been deliberate.

After all, people can control insulin levels to some extent by choosing to eat a different diet. And indeed people can learn biofeedback relaxation techniques that can have an effect on such variables as blood pressure, and some diabetics have used such techniques to reduce their need for medical insulin. So, did Bert think that people had once consciously controlled insulin levels, but over generations this has become automatic?

In some ways this does not seem a very likely or promising idea – but that is a judgement made from a reasonably high level of science knowledge. It is important to encourage students to use their imaginations and suggest ideas as that is an important aspect of how science woks. Most scientific conjectures are ultimately wrong, but they may still be useful tools for moving science on. In the same way, learners' flawed ideas, if explored carefully, may often be useful tools for learning. At the time of the interview, I felt Bert had not really thought his scheme through. That may well have been so, but there may have been more coherence and reflection behind his comments than I realised at the time.

Puppies that automatically retrieve your stick

Dogs that have been taught over and over to retrieve have puppies that automatically have already got that sense

Keith S. Taber

…our homework we did about adapting, like how polar bears adapt to their environments, and camels….
And so a polar bear has adapted to the environment?
Yeah.
So how has a polar bear adapted to the environment?
Erm things like it has white fur for camouflage so the prey don't see it coming up. Large feet to spread out its weight when it's going over like ice. Yeah, thick fur to keep the body heat insulated.
What about a camel then?
Well it has long eyelashes to keep the sand out of it. It has pretty much all its fat stored in its hump so that it can erm, so all the body, so that not much body heat is produced from everywhere else. It doesn't have hair on its belly to increase heat loss. And yeah, oh yeah, they're quite big so it has quite a lot of grip on the sand.
…
No, okay. So do you have any other examples of adaption?…
Oh well, well there's humans isn't there. Because like they started off like with an arched back and they went on all-fours and everything. And well their minds obviously have adapted and evolved, yeah. Erm (pause) and dogs, they have different … because people who are actually breeders, they, when they breed dogs they breed them to be like, like Retrievers. Because they've like been taught over and over to retrieve. And so when they have puppies then they automatically have already got that sense. That's not really adapting though is it?
So somebody has trained these dogs to go and, when they shoot birds or something, they're trained to go and get the birds they've shot and bring them back?
Yes.
Okay. And if you do that enough, baby puppies bred from those dogs will just know to do that?
Well they won't know to do that, but they'll already have that kind of sense. And like, well my dog that I have, it's a Chocolate Labrador, and I said look, she had webbed feet which is adapted for swimming, for retrieving, I don't know, retrieving birds from water or something.

Although Bert was aware of how traits could be passed on to offspring he was thinking in terms of the inheritance of acquired characteristics – a Lamarkian model of evolution – rather than the selection of qualities that vary across a population. For some pupils the notion of evolution makes sense, but in terms of changes that occur in an individual in response to environmental challenges being somehow passed on to their offspring. The inheritance of acquired characteristics is a scientific concept, that is a historical (scientific) concept, but not a canonical (current scientific) concept, so Bert's understanding of evolution would be considered an alternative conception.

(Bert then went on to consider an example of a naturally occurring adaptation, the polar bear's fur, however he again considered this in terms of an acquired characteristic being passed on to future generations.)