Meiosis from Tokyo Medical and Dental University
Among the most rapidly evolving parts of the genome are those parts dealing with reproduction and resistance to disease. Thus, sex should be a good thing to look at for real world examples of evolutionary forces at work. But....

Q: What is sex, exactly?

Gametes: Remember meiosis, the process by which gametes are generated. Genetic recombination is facilitated by:

Plasmids: This arrangement didn't appear overnight, but precursors are apparant. Prokaryotes (E. G. bacteria) which lack distinct chromosomes nevertheless recombine genes by swapping plasmids, small loops of DNA. This fact:


Parthenogenic whiptail lizards from Reed College, Biology

Q: What is the evolutionary benefit of sex?

Asexual reproduction: Any gardener knows that many species can reproduce asexually, budding off pieces of themselves that grow into a new organism. That's a good "quick and dirty" method of establishing a population from a single pioneer. Yet almost no species completely forgoes sex. Those very few who do, including complex animals like several "species" of whiptail lizard (right), show why this is a poor approach.

Such parthenogenic species occasionally result from hybridizations and accidents in meiosis in which females end up with more than two sets of homologous chromosomes. These can sometimes reproduce parthenogenically (i.e. asexually), yielding clones of themselves.

Easy enough, but there are two hassles:

Thus, parthenogenic species apparantly last only a few thousand years.

Indeed, the lizards' need to engage in pseudocopulation in order to ovulate reveals their fundamentally sexual nature. Even what looked like textbook examples of parthenogenic species have recently been shown to sex after all. Consider the virginal shrimp Vestalenula.

Bdelloid rotifer from Wikipedia
The current asexual champs are Bdelloid rotifers, which seem to have gone without for the last forty million years. We'll see. Recent research indicates that bdelloids can possibly get away with asexuality because of a unique adaptation for dealing with disease pathogens by physically abandoning the environments in which they live.


An example of the efficacy
of diverse morphotypes from Rutgers University
The advantages of sex:


Gametes from Mark Rothery's Biology Web Site

Q: Why are we male and female?

Complex species with three or more genders remain the domain of SciFi, however there are many unicellular organisms: But for multicellular organisms, there seem to be two optimal strategies for making gametes: I.e. ova and sperm.

Mitochondrion from DKimages

Why this is so is not absolutely clear. It might be to manage the transmission of cytoplasmic components of the genome. (After all, if you got genetically dissimilar versions of the organelles with their own genomes like mitochondria (right) or chloroplasts from both parents, they might waste time fighting rather than doing their jobs.)

Ocellaris clownfish, Amphiprion ocellaris from Wikipedia

Nevertheless, if you make ova, you are female. If you make sperm, you are male. NOTE: These are NOT mutually exclusive catagories. Creatures may be:


Every organism has one evolutionary goal: to maximize the representation of its genes in the next generation.

How one goes about that varies greatly between species and between genders in a species. Despite their great diversity, one great pattern emerges from organismal sex practices:

The battle of the sexes.

You would think that by "putting all their gametes in one basket," males and females would have an incentive for complete cooperation, but this is not so. In fact, males and females each have their own agendas that can, at times, be so divergent that they actively sabotage one another. The paradox of sex (from an evolutionary sense, at least) is that while it requires intimate cooperation with another individual, it is utterly selfish.

Q: What is the female agenda?

  • The limiting gender: Typically, a male who choses his mate badly has lost a few minutes and a little effort. The female is usually the limiting gender - the one whose large investment in reproduction is the limiting factor for success. She must choose more rigorously. The limiting gender usually gets to exercise the most exclusive mate choice. For most animals, this is the female, although there are exceptions.

    With a more limited number of gametes, maximum fitness comes from optimizing the survival chances for each ovum. That typically means:

    Q: What is the male agenda?

    With a lots of gametes to spread around, maximum fitness comes from maximizing the number of ova fertilized. That typically means:

    Other issues: Of course, the "battle of the sexes" is constrained and moderated by:


    Females seeking males

    Maximizing one's female fitness - being the optimal nurturer - requires:

  • Selecting optimal mates:


    These choices can be heavily weighted by peculiarities of the reproductive systems or environment.

    Courting peacock from Wikipedia

    Mate choice: When we think of sexual selection we normally think of females exercising conscious mate choice among males that must accommodate their preferences, however inconvenient; but it can also happen cryptically. E.G.:

    Fig wasp life cycle from The Fig and the Wasp

    Environmental pressure: For the fig wasp, environmental pressure conditions mating systems. These creatures only survive briefly outside of a fig. You can't find a mate unless you're in a fig, so it's worth it to stick with your fig, even at the risk of mating with your siblings who are likely to be the only partners around. In fact, insects like wasps, bees, and ants, practice haplodiploidy, a genetic mating system that minimizes the hazards of in-breeding. In haplodiploid animals, males are haploid, thus, any deleterious recessive gene is automatically phenotypically expressed, resulting in its quick removal from the gene pool. (See below)


    Males seek females

    A typical male dilemna: Ideally, a male would seek to mate with as many females as possible and, hopfully, exclude his rivals from so doing.

    But the problem: Every minute spent guarding your mate from other males is a minute not spent searching for new mates. In most species, the female plumbing is arranged such that the last male to mate with her is the most likely to father her offspring. Strategies to deal with this include:


    From Connexions
    Maximizing coverage and excluding competitors can lead to novel strategies.

    Male bedbugs are very strange:



    Q: Monogamy: Why would anyone limit themselves to a single mate?

    Definitely not the norm no matter how much we, for cultural reasons, want it to be. Why would members of either sex forego access to multiple partners? Indeed, the spread of genetic testing has shattered our illusions about many species formerly thought to be monogamous. Seems that even those that pair up for life as a social arrangement frequently sneak around when it comes to actual mating. But true monogamy happens in some unusual circumstances.

    Anglerfish couple from National Geographic
    Danger: The male dragonfly has it easy in that there are typically plenty of females around to fight for. In some environments, potential mates are so rare, and the environment so hostile that one's chances of ever finding one mate before dying are not good. If you're lucky enough to find a mate, better to stick with him/her rather than risk dying alone. An extreme expression of this is in deep-sea angler fish, in which the male, in effect, becomes part of the female's body. The ultimate must be the Australian red-backed spider, whose male deliberately somersaults himself onto his mate's fangs, allowing her to start eating him while he is still busy copulating. (Male preying mantises, of whom we hear much, at least try to get away.)


    Crowned hornbill couple from Birdinfo
  • Mutual-Assured-Destruction: The combination of demanding offspring, unusual life histories, and extreme environments can lead to very unusual levels of paternal involvement in the rearing of offspring, even to the extent that if either parent doesn't cooperate fully, reproduction will simply fail. Examples include hornbills (right) and the famous emperor penguins.

    Q: Hermaphrodites: Why not give everyone the option of being male or female?

    Mating slugs from
    Maybe the key to ending the war of the sexes is to give everyone the same equipment? Alas, no. Although there are many species of cross-fertilizing hermaphrodites in which each individual simultaneously uses both male and female systems, the matings of hermaphrodites are often downright violent. E.G.:

    Penis-fencing flatworms from Reed College Biology 342
    Among the more extreme expressions of hermaphrodite violence, marine flatworms must fight over who gets to inseminate whom with caustic, ulcer-producing semen. The "loser" has to be the "girl," but at least "she" has made "her" mate pass a qualifying exam - a form of female choice.

    Snail empaled by love-dart New Scientist
    Snails routinely jab one another during mating with darts covered with pheromones that stimulate the recipient's female system. If that's not enough, after mating they may try to chew their partner's penis off.

    Maybe it's easier to be born knowing who gets to be the boy and who gets to be the girl and who's a potential mate and who's a potential rival without having to fight over it.


    Q: Not seeking anyone

    What would make a female evolve to be infertile? That's what happens in a handful of eusocial animals - those that live in complex societies in which most individuals are non-reproductive workers. Examples include:

    Why hymenopterans? They are the ones who practice haplodiploidy, in which fertileized ova develop into diploid females and infertile ones develop into haploid males. (Remember the fig wasps) You need to track where genes are coming from and who is most closely related to whom.

    Human (either gender)

    % of your genes shared (maternal) % of your genes shared (paternal) % of your genes shared (total)
    Mom 50% 0% 50%
    Dad 0% 50% 50%
    Sister approx. 25% approx. 25% approx. 50%
    Brother approx. 25% approx. 25% approx. 50%
    Child - - 50%

    Hymenopteran female

    % of your genes shared (maternal) % of your genes shared (paternal) % of your genes shared (total)
    Mom 50% 0% 50%
    Dad 0% 50% 50%
    Sister approx. 25% 50% approx. 75%
    Brother approx. 25% 0% approx. 25%
    Child - - 50%

    The result: Whereas a female shares 50 percent of her genes with her mother, she shares, on average, 75 percent with her sister. Thus, to maximize the representation of her genes in the next generation, her best strategy is to farm mom for more sisters.

    Other factors must be involved (otherwise why termites, etc.?) Possibilities: