Evidence undermining neo-Darwinism and supporting Lamarckism
Jonty Haywood
Thursday, 29 January 2009 17:03 UTC
I have recently written a paper explaining the significance of sex ratio biasing to evolutionary biology. I would be very grateful for any comments, suggestions or criticisms. I will copy/paste the introduction here. The full paper can be found at www.losethegame.com/neolamarckism.htm
This paper makes three key proposals:
1. Sex ratio biasing explicitly undermines neo-Darwinian theory.
2. Genotype biasing will not be limited to offspring sex determination.
3. Genotype biasing will involve environmentally-modified genotype-linked segregation distorters.
There is now significant evidence from a variety of mammalian species that parents can bias the sex ratio of their offspring depending on environmental factors (Trivers-Willard theory and the local-resource competition model). The premise of this paper is that sex ratio biasing explicitly undermines neo-Darwinian theory, and that there is no reason to assume that such genotype biasing is limited to offspring sex determination. Parents will use equivalent mechanisms to bias any offspring genotypes depending on their environment. Just as a well-fed hamster or high-ranking deer is more likely to have male offspring, a parent living in a darker than usual environment will be more likely to have offspring with darker than expected fur, a parent living in a colder environment will be more likely to have offspring with thicker fur and so on. Such adaptations will occur not only over many generations through natural selection, but in the very next generation.
Genotype biasing must involve segregation distorter genes, and this paper proposes a mechanism involving segregation distorters which are both indirectly modified by the parent environment and linked to genes for appropriate phenotypes. Such a mechanism could be used to benefit the entire genome, not just those genes linked to the segregation distorter, and as such would be widespread in sexually-reproducing organisms.
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Replies
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All I see is hand-waving. When you write
The mechanism by which sex ratios are biased must involve sex-linked segregation distorters which are activated by environmental factors such as parent condition or social rank.
I immediately want to know (a) what evidence is there that segregation distorters are involved in sex ratio determination, and (b) what evidence is there that they can be affected by the relevant environmental cues?
If you are to make a case, you have to review what is known about the mechanisms of sex ratio determination, to show us what is known and how it fits in with your theory.
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(a) Normal gametogenesis results in an equal probablility for alleles from either parent to be present in a gamete. This is true of all alleles including those which determine sex, hence a normal sex ratio of 50:50. Segregation distorters are genes which bias the chance of themselves and any linked genes from being present in a gamete. Sex ratio bias (also known as sex ratio distortion) increases the chance of the allele/chromosome determining one sex to be present in a gamete over the allele/chromosome determining the other sex. Therefore sex ratio distortion must involve segregation distorters by definition.
(b) Evidence for sex ratio distortion, and the relevent environmental cues, is discussed in the “Evidence of offspring genotype ratio biasing by parent environment” section of my paper which I will paste here for your convenience:
Trivers-Willard theory:
Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females. (Trivers, R.L., & Willard, D.E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179, 90-92.)
This has been documented in Venezuelan opossums (Austad and Sunquist 1986) and hamsters (Clutton-Brock and Iason 1986; Clutton-Brock 1991; Huck, Labov and Lisk 1986).
A similar phenomenon, known as the local-resource competition model, has shown sex ratio bias determined by the social rank of the parent and the rank inheritance pattern for the species. High-ranking females of species in which rank is inherited by male offspring (female exogamous species), such as red deer (Clutton-Brock, Albon and Guinness 1984) and spider-monkeys (Symington 1987), are more likely to have male offspring. High-ranking females of species in which rank is inherited by female offspring (male exogamous species), such as baboons (Altmann 1980), macaques (Silk 1983; Simpson and Simpson 1982; Small and Hrdy 1986) and howler monkeys (K. Glander), are more likely to have female offspring.
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Jonty – yes I know that segregation distorters distort segregation. But that doesn’t mean that they’re the only thing that does. There can also be other mechanisms that bias survival of male and female sperm, for example. Your statement
Therefore sex ratio distortion must involve segregation distorters by definition.
assumes that distorters are the only method. But if you’re going to establish this, you first must establish that there is no other way of distorting sex ratios other than genetic distorters. You also have to show that sex-linked distorters do exist (shouldn’t be too difficult, as I think the sequences are well known. You might have a bigger problem if the regulation is sex-linked, though).
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I am assuming two things to make such a statement. Firstly, the mechanisms which bias egg/sperm/zygote survival are ultimately caused by genes in the parent genome. Secondly, that these genes must be linked to sex determining genes. Therefore these genes are, by definition, segregation distorters. I have posted this in another forum where I was told that “sex ratio distortion” is already very well-documented. I can not think of any other way this mechanisms could exist without segregation distorters but if you can suggest an possible alternative I would be very interested.
Other discussion has made me realise that my points in the paper I linked to here are not very well explained and I have misused some terms. I shall paste my latest post from that discussion here so that you can see a (hopefully) better explaination of what I’m trying to get across:
Firstly, regarding an “environment-dependent selective pressure on certain genes in early stage zygotes”. Again I don’t think I’ve got my point across clearly. I understand that the zygote is a phenotype and it is under normal selection pressures as much as at any time in an organism’s life. My point is about the nature of the selection. If a zygote had a “sticky” phenotype, which meant it kept sticking to the sides of the Fallopian tube and reduced its chances of reaching the uterus, then this would obviously reduce fitness and be selected against, which would be an example of natural selection. In sex ratio distortion, the zygote has a phenotype which, in certain parent environments, gives it a “kill me” tag. Again this reduces fitness, is selected against and is an example of natural selection. The point I am trying to make is focusing on the what links the parent environment to the “kill me” tag. In sex ratio distortion, the “kill me” tag signals a genotype which will have reduced fitness in the future, according to the parent environment. A mechanism has evolved to select against this genotype (signalled by the current phenotype), but the ultimate reason that it’s current phenotype is being selected against is because of what its phenotype will become, because of what its genotype signals about what its future phenotype will be. As you can probably tell I’m finding it quite difficult to express myself here, but I’m hoping you will get the gist of what I’m saying may be able to explain it better than I have done so, as you have already done with a number of my previous points.
A brief additional point I should make, which is probably obvious, is that distortion could also occur prior to fertilisation by the killing of certain eggs or sperm (which means it could be imposed by the male too).
So to summarise my first major point, I believe that sex ratio distortion, although caused by natural selection, is an example of an adaptive process fundamentally different to natural selection. The selection of the genotype has to be based on zygotic/gametic phenotype, but the reason this selection is being made is due to phenotypes that the genotype does not currently express but will express in the furture.
My second point is about distortion being used to bias genotypes other than sex. As you say this is entirely a supposition without any evidence, but I believe it should be be taken seriously for the following reasons. I understand that the costs of distortion mean that any evolved mechanisms that use distortion must bring very significant benefits. I also understand the significant benefits of sex ratio distortion. But if you look at this from another angle, one could say that when studying genotype distortion, biased sex ratios are the most obvious things to notice. Everyone knows that sex ratios should be 50%, and most organisms, especially mammals, are very easy to sex. As such, instead of arguing that we would expect to find genotype distortion in sex ratios because of the obvious benefits, we could reverse the argument and say that we would find genotype distortion in sex ratios because distortion of sex ratios is so easily noticable.
To summarise my second major point, if sex ratio biasing occurs, then due to the variety of nature, it is almost inevitable that it occurs for genotype ratios other than sex in some organisms. It is hard to judge how common it would actually be, like you say, it would have to bring very significant benefits. But I believe certain mechanisms, such as choosing the best MHCs for your offspring (see my hypothetical examples), could easily bring massive fitness benefits.
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My second point is about distortion being used to bias genotypes other than sex. As you say this is entirely a supposition without any evidence..
What Bob said, Jonty.
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Or, if it is not clear, your description certainly does not bear out the title of your post!
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Therefore these genes are, by definition, segregation distorters.
No they’re not. They could be selectively killing one sex post-zygotically, i.e. the sex ratio at segregation is 1:1, with a subsequent biasing.
I can not think of any other way this mechanisms could exist without segregation distorters but if you can suggest an possible alternative I would be very interested.
See above. I would suggest you google around. I found an example (which I can’t find now) where a spider could manipulate the sex ratio by placing the sperm packet from the male in a different place. As you write, a lot is known about this, and a lot is available on the web. You should perhaps do the research first.
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