Banner
    Sexual Selection In The Fossil Record: Survival Of The Prettiest?
    By News Staff | January 29th 2013 10:16 AM | 1 comment | Print | E-mail | Track Comments

    Sexual selection refers to the evolutionary pressures that relate to a species' ability to repel rivals, gather mates and pass on genes. We can observe those processes happening in living animals and, now, detecting sexual selection in the fossil record is also possible, according to researchers. 

    It has been challenging to recognize sexual selection in extinct animals. Many fossil animals have elaborate crests, horns, frills and other structures that look like they were used in sexual display but it can be difficult to distinguish these structures from those that might play a role in feeding behavior, escaping predators, controlling body temperature or not having any important function at all.

    In a new review of studies, a group argues that clues in the fossil record can indeed be used to infer sexual selection.  Modern examples of sexual selection, where species have evolved certain behaviours or ornamentation that repel rivals and attract members of the opposite sex, include the male peacock's display of feathers, and the male moose's antlers for use in clashes during mating season.




    Sexual dimorphism in the pterosaur Darwinopterus. Males had large head crests while females were crestless. This dimorphism is a clear indication that sexual selection pressure was shaping the evolution of these animals. Credit: Mark Witton


     "We see much evidence from the fossil record suggesting that sexual selection played a major role in the evolution of many extinct groups," says  co-author and palaeozoologist Dr. Darren Naish of the University of Southampton . "Using observations of modern animal behaviour we can draw analogies with extinct animals and infer how certain features improve success during courtship and breeding."

    The authors state that the fossil record holds many clues that point to the existence of sexual selection in extinct species, for example weaponry for fighting, bone fractures from duels, and ornamentation for display, such as fan-shaped crests on dinosaurs.

    Distinct differences between males and females of a species,'sexual dimorphism', can also suggest the presence of sexual selection, and features observed in sexually mature adults, where absent from the young, indicate that their purpose might be linked to reproduction.

    Researchers can also make inferences from features that are 'costly' in terms of how much energy they take to maintain, if we assume that the reproductive advantages outweighed the costs.

    Whilst these features might have had multiple uses, the authors conclude that sexual selection should not be ruled out.

    "Some scientists argue that many of the elaborate features on dinosaurs were not sexually selected at all," adds Naish. "But as observations show that sexual selection is the most common process shaping evolutionary traits in modern animals, there is every reason to assume that things were exactly the same in the distant geological past."


    Published in Trends in Ecology and Evolution


    Comments

    Gerhard Adam
    ...ornamentation that repel rivals and attract members of the opposite sex, include the male peacock's display of feathers...
    Perpetuating the myth.

    http://www.science20.com/gerhard_adam/peacock_problem-79331

    Researchers can also make inferences from features that are 'costly' in terms of how much energy they take to maintain, if we assume that the reproductive advantages outweighed the costs.
    This is always a suspect claim.  Cost is extremely difficult, if not impossible to assess in many cases, and often is simply subjective.  Whether something is metabolically "costly", or increases risk of predation is only a valid consideration if in fact it can be demonstrated that it actually has an effect on fitness.  Without that, it's an irrelevant point.

    Moreover, what does "costly" even mean?  The brain may be metabolically "costly" but if there is no deficiency in available food or nutrients, then so what? 

    This is precisely the problem with the peacock's tail, where it is often inferred that it increases the risk of predation, and yet there isn't a single study that quantifies whether these animals are actually killed more frequently and how it relates to their plumage.  After all, since the peacock doesn't retain it's tail all year long, it should be easy enough to see whether there are more killed when they have the tail versus when they don't.
    "But as observations show that sexual selection is the most common process shaping evolutionary traits in modern animals, there is every reason to assume that things were exactly the same in the distant geological past."
    This statement makes no sense.  Since all sexual creatures mate, then by definition, sexual selection is the most common process in evolution.  But the question is whether or not there is actual "selection" taking place.  After all, it doesn't make sense to be choosy, if the result is that no mating occurs at all.  This also has relevance to the number of mates that are available, so to infer "choice" is also something that can't be determined without more exhaustive information.  Since many animals mate somewhat indiscriminately, it would be a stretch to infer that selection [beyond gender] occurred.
    Mundus vult decipi