jackson@utzoo.uucp (Don Jackson) (03/14/88)
Richard Harter (25442@cca.CCA.COM) writes: >NOTE THAT IT IS THE FEMALE THAT SELECTS. Females select, fundamentally, >because they make the big investment in offspring. This is true as a generalization. However, there are countless numbers of species where the investment by the male is equal to or exceeds the female. Numerous species of fish reproduce in nests formed by the male and guarded by males after spawning. The greater costs (e.g. energy) incurred by the male (in terms of reduced growth and increased mortality) appear to counter your argument that females have the greater investment. There may be natural selection against distinctive females through increased risk of predation. An example of this is the Guppy (Poecilia reticulata) where coloration increases predation. Therefore, dull females have greater fitness than colorful females. However, sexual selection may favour brightly colored males provided the benefits derived from their coloration are greater than the increased costs associated with predation. Trying to distinguish between different types of selection (e.g. natural versus sexual) is a matter of semantics. The process implies that, on average, some trait (e.g. color, larger tail, etc.) results in a greater fitness (i.e. more progeny) than a similar organism lacking that trait. -- Name: Don Jackson Mail: Dept. Zoology, Univ. Toronto Toronto, Ontario, Canada M5S 1A1 UUCP: {allegra,ihnp4,linus,decvax}!utzoo!jackson
g-rh@cca.CCA.COM (Richard Harter) (03/14/88)
In article <1988Mar13.160941.22096@utzoo.uucp> jackson@utzoo.uucp (Don Jackson) writes: > >Richard Harter (25442@cca.CCA.COM) writes: > >>NOTE THAT IT IS THE FEMALE THAT SELECTS. Females select, fundamentally, >>because they make the big investment in offspring. > >This is true as a generalization. However, there are countless numbers of >species where the investment by the male is equal to or exceeds the female. >Numerous species of fish reproduce in nests formed by the male and guarded >by males after spawning. The greater costs (e.g. energy) incurred by the male >(in terms of reduced growth and increased mortality) appear to counter >your argument that females have the greater investment. There are two investments that one has to take into account; one is the investment in creating the offspring, and the other is the investment in caring for them. The thing that distinguishes females from males is the females make the larger investment in creating offspring. The two genders have an intrinsic conflict of interest -- it is to the benefit of each that the other care for the young, and it is to the benefit of both that the young prosper. A general rule is that the last gender to make contact with the young is more likely to get stuck with caring for them. In most higher animals this is the female, since the egg is developed after fertilization. In a number of fish and amphibians, the females lays unfertilized egg masses and the male fertilizes them after they are laid. The male, being the last in contact, gets to make the big investment in caring for them. In the more usual case where fertilization occurs a priori rather than a posteriori, the female has a number of strategies available, the principle one being to refuse to mate unless the male makes a suitable investment in caring for the offspring. How well this works depends very much on the species. In some species the female abandons the effort to get the male to contribute to the rearing of the offspring; instead they settle for selecting the 'best' possible father. These species tend to have strong dimorphism and a few males that do most of the breeding. In species with strong pair bonding the leverage of the female is much greater -- instead of picking the 'best' father along with many other females making the same pick, she picks a single 'more useful' one. Species with pair bonding tend to less dimorphism and more equality in rearing the young. There are no hard and fast rules, though, it is very much a case by case thing. Human beings, are, as one might expect, very much a special case. Dimorphism in human beings is quite marked. From this one would expect that humans would have sexual patterns similar to other dimorphic primates, with an alpha male having primary access to a number of females, and other males having very little access. In actual fact, humans are a strong pair bonding species (albeit with surprising variations) and human males make a large investment in rearing the young. >There may be natural selection against distinctive females through >increased risk of predation. An example of this is the Guppy (Poecilia >reticulata) where coloration increases predation. Therefore, dull >females have greater fitness than colorful females. However, sexual >selection may favour brightly colored males provided the benefits derived >from their coloration are greater than the increased costs associated with >predation. This is rather general; in most species the female is less conspicuous than the male. Inconspicuous is rather generally selected for. >Trying to distinguish between different types of selection (e.g. natural >versus sexual) is a matter of semantics. The process implies that, >on average, some trait (e.g. color, larger tail, etc.) results in a greater >fitness (i.e. more progeny) than a similar organism lacking that trait. I disagree that it is simply a matter of semantics. The final criterion of fitness is, as you say, a matter of more progeny. However this is a matter of contingent probabilites. The individual must survive to breeding age. Given that the individual has survived to breeding age, the individual must breed successfully. Given that the individual has bred successfully, its offspring must do the same. Selection pressures apply equally to the genders when it comes to simple survival; however they apply differently when it comes to successful breeding. -- In the fields of Hell where the grass grows high Are the graves of dreams allowed to die. Richard Harter, SMDS Inc.
kevin@chromo.ucsc.edu (Kevin McLoughlin) (03/17/88)
In article <25527@cca.CCA.COM> g-rh@CCA.CCA.COM.UUCP (Richard Harter) writes: >Human beings, are, as one might expect, very much a special case. Dimorphism >in human beings is quite marked. From this one would expect that humans >would have sexual patterns similar to other dimorphic primates, with an >alpha male having primary access to a number of females, and other males >having very little access. In actual fact, humans are a strong pair >bonding species (albeit with surprising variations) and human males make >a large investment in rearing the young. Actually, humans are only moderately dimorphic. Examples of species with a strong degree of dimorphism are gorillas and baboons--in these species males may be up to twice as massive as females. This doesn't happen among humans (of course what's scientifically relevant here is the degree of dimorphism within the sort of genetically homogenous, small interbreeding populations that were the venue for evolution. Thus we don't compare size differences between Swedish men and Vietnamese women, for example, but between Vietnamese men and Vietnamese women. Under these constraints dimorphism is found to be moderate.) ----------- Susan Nordmark Internet: kevin@chromo.UCSC.edu UUCP: ...ucbvax!ucscc!chromo.kevin Santa Cruz, CA
g-rh@cca.CCA.COM (Richard Harter) (03/18/88)
In article <2403@saturn.ucsc.edu> kevin@chromo.UUCP (Susan Nordmark) writes: ... Re my comment that humans are markedly dimorphic >Actually, humans are only moderately dimorphic. Examples of species >with a strong degree of dimorphism are gorillas and baboons--in >these species males may be up to twice as massive as females. >This doesn't happen among humans (of course what's scientifically >relevant here is the degree of dimorphism within the sort of genetically >homogenous, small interbreeding populations that were the venue >for evolution. Terms like markedly and moderately depend very much on which apples and oranges you are comparing. If your context is the great apes, humans are moderate. If you context is mammals in general, humans are well up there. If your context is pair-bonding mammals with paternal sharing in the support of the young, humans are highly dimorphic. And if your context is anglerfish, the dimorphism is marginal [Natural science joke -- if you don't know, don't ask.] -- In the fields of Hell where the grass grows high Are the graves of dreams allowed to die. Richard Harter, SMDS Inc.
michaelm@vax.3Com.Com (Michael McNeil) (03/24/88)
In article <25669@cca.CCA.COM> g-rh@CCA.CCA.COM.UUCP (Richard Harter) writes: >In article <2403@saturn.ucsc.edu> kevin@chromo.UUCP (Susan Nordmark) writes: > > ... Re my comment that humans are markedly dimorphic > >>Actually, humans are only moderately dimorphic. Examples of species >>with a strong degree of dimorphism are gorillas and baboons--in >>these species males may be up to twice as massive as females. >>This doesn't happen among humans (of course what's scientifically >>relevant here is the degree of dimorphism within the sort of genetically >>homogenous, small interbreeding populations that were the venue >>for evolution. > >Terms like markedly and moderately depend very much on which apples >and oranges you are comparing. If your context is the great apes, >humans are moderate. If you context is mammals in general, humans >are well up there. If your context is pair-bonding mammals with >paternal sharing in the support of the young, humans are highly >dimorphic. And if your context is anglerfish, the dimorphism is >marginal [Natural science joke -- if you don't know, don't ask.] To quote Jacob Bronowski in this regard: [...] in general there is much less difference between men and women (in the biological sense and in sexual behavior) than there is in other species. That may seem a strange thing to say. But to the gorilla and chimpanzee, where there are enormous differences between male and female, it would be obvious. In the language of biology, dimorphism is small in the human species. Jacob Bronowski, *The Ascent of Man*, 1973 I might also add that since our closest living relatives are the gorillas and chimpanzees, which are much more markedly dimorphic, humans most probably lost much dimorphism during our evolution. Michael McNeil 3Com Corporation Santa Clara, California {hplabs|fortune|idi|ihnp4|tolerant|allegra|glacier|olhqma} !oliveb!3comvax!michaelm William James used to preach the "will to believe." For my part, I should wish to preach the "will to doubt." ... What is wanted is not the will to believe, but the will to find out, which is the exact opposite. Bertrand Russell, *Sceptical Essays*, 1928
g-rh@cca.CCA.COM (Richard Harter) (03/24/88)
In article <1125@3comvax.3Com.Com> michaelm@3comvax.UUCP (Michael McNeil) writes: >To quote Jacob Bronowski in this regard: [...] in general there is much less difference between men and women (in the biological sense and in sexual behavior) than there is in other species. That may seem a strange thing to say. But to the gorilla and chimpanzee, where there are enormous differences between male and female, it would be obvious. In the language of biology, dimorphism is small in the human species. Jacob Bronowski, *The Ascent of Man*, 1973 >I might also add that since our closest living relatives are the >gorillas and chimpanzees, which are much more markedly dimorphic, >humans most probably lost much dimorphism during our evolution. If you take all species of animals for your standard of comparison, then the norm is for the female to be substantially larger than the male. If you take all species of vertebrates for you standard of comparison, then I suspect that you will find that males are larger than females, on average, but not to the extent characteristic of humans. If you take large mammals (and humans are large mammals) as your standard of comparison, then I think you will find that find that humans are less dimorphic. Re the loss of dimorphism during our evolution. It is my impression it was about the same as that of modern day humans for Afarensis, H. habilis, H. erectus, and H. Neanderthalis, and the Boseii was more dimorphic than humans. If any one out there knows for sure, I would be interested in hearing about it. -- In the fields of Hell where the grass grows high Are the graves of dreams allowed to die. Richard Harter, SMDS Inc.
firth@sei.cmu.edu (Robert Firth) (03/25/88)
In article <1125@3comvax.3Com.Com> michaelm@3comvax.UUCP (Michael McNeil) writes: >I might also add that since our closest living relatives are the >gorillas and chimpanzees, which are much more markedly dimorphic, >humans most probably lost much dimorphism during our evolution. Well, since both pongos and chimpanzees are more highly differentiated than humans (and read that as "more evolved" if you wish), it is not impossible that the opposite is the case - that they have gained some dimorphism rather than that we have lost some.
michaelm@vax.3Com.Com (Michael McNeil) (03/26/88)
In article <4736@aw.sei.cmu.edu> firth@bd.sei.cmu.edu.UUCP (Robert Firth) writes: >In article <1125@3comvax.3Com.Com> michaelm@3comvax.UUCP (Michael McNeil) >writes: > >>I might also add that since our closest living relatives are the >>gorillas and chimpanzees, which are much more markedly dimorphic, >>humans most probably lost much dimorphism during our evolution. > >Well, since both pongos and chimpanzees are more highly differentiated >than humans (and read that as "more evolved" if you wish), it is not >impossible that the opposite is the case - that they have gained >some dimorphism rather than that we have lost some. I agree it's *not impossible* that chimpanzees and gorillas gained dimorphism rather than humans having lost it, but it's less probable. The reason is due to the fact that humans, chimpanzees, and gorillas apparently are roughly equally divergent from a common ancestor. In such a three-way branching situation, if two species share a trait -- such as the knuckle-walking behavior of chimpanzees and gorillas -- it is quite likely that our common ancestor possessed that trait as well. The alternative -- that convergent evolution independently invented a trait within different species -- is of course possible, but generally considered somewhat less likely, depending on the trait's complexity. Follow-ups to sci.bio and talk.origins. Michael McNeil 3Com Corporation Santa Clara, California {hplabs|fortune|idi|ihnp4|tolerant|allegra|glacier|olhqma} !oliveb!3comvax!michaelm ... It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed. Charles Darwin, 1871