Birds and Bees

SEX IS HARD for the theory of evolution to explain. Why would simple organisms, which can reproduce all by themselves, evolve into creatures that need a mate? What is the advantage that sexual reproduction gives that results in a victory in the struggle for survival? How could male and female varieties evolve simultaneously? Why would the opposite sexes be drawn to each other? How would they know what to do once they found each other? It doesn't make much sense. It makes more sense to write for a free copy of our newsletter. Science Against Evolution, P.O. BOX 923, Ridgecrest, CA 93556. [The post office box is now closed.]

Birds do it, bees do it, people do it--in fact the vast majority of organisms do it--but sexual reproduction often seems more trouble than it's worth. For organisms, sex often means spending huge amounts of energy finding and wooing desirable partners. Within the cell, male and female genomes must recombine without major mistakes. Gametes must promote compatible fusions while upholding barriers between species. And at each step, the conflicting interests of each sex must be delicately negotiated in order to benefit the species as a whole.

Asexual reproduction--a simple "copy and divide" strategy--at first glance seems both less messy and more efficient. Yet sexual reproduction is by far the dominant mode, which leads researchers to conclude that sex offers some evolutionary advantage. Biologists have been spinning theories about just what that advantage might be for quite some time, but so far there is no single clear answer.

The debate continues energetically …. The topic falls into two parts. First, why did sex evolve in the first place and why is it so pervasive in the natural world? And secondly, how do these amazingly complex sexual systems evolve?

… most theories suggest that sex and recombination remove harmful mutations and allow new combinations of genes to come together, providing more opportunities for improved fitness and offering the flexibility to adapt to new environments. Biologists are finding new ways to test these theories, Wuethrich reports, but proving that they are actually operating in the natural world remains a challenge. ¹

Today, across the tree of life, sex reigns--many unicellular and just about all multicellular organisms do it. Yet how sex began and why it thrived remain a mystery. After all, asexual organisms were here first, and new asexual species continue to arise, if only to go extinct in fairly short order. Why did sex overtake asexual reproduction some billion or more years ago, and why does it continue to upstage asexuals? What gives sex its edge?

Biologists have come up with a profusion of theories since first posing these questions a century ago. … But the devil is in the data--or lack thereof. "I emphasize experimental problems, because we have tons of theories, and some are completely crazy," says Alexey Kondrashov, an evolutionary geneticist at Cornell University. ²

If asked why sex is such a widespread phenomenon, most biologists would say that it promotes genetic variability, and hence allows evolution to proceed faster than in its absence. This explanation suffers from several difficulties. First, it is not clear a priori that the heritable variance in fitness (the material for adaptation by natural selection) is significantly increased by sex. Second, the most obvious effect of recombination is to break up favorable sets of genes that have accumulated through selection, leading to a recombination load. Similarly, segregation of genes at a single locus eliminates heterozygotes that may be favored by selection. These forces should cause sex and genetic recombination to be eliminated from a population at equilibrium under selection alone. Third, in organisms with anisogamy (male and female gametes), there is a built-in cost to sex. If there are separate males and females, for example, a mutation that causes females to produce only daughters, but has no other effect, will initially double in frequency in each generation (this is often termed the cost of sex). ³

Fertilization, in which genes from different parents fuse, creates yet more genetic combinations. All this shuffling is more likely to break up combinations of good genes than to create them--yet nature keeps reshuffling the deck.

This paradox is compounded by the cost of sex--which is primarily the cost of producing a male. Imagine 1 million sexually reproducing snails and a single asexual female mutant. Say that she has two daughters, who (on average) have another two daughters, and so on. Meanwhile, the sexually reproducing females would be diligently producing a female and a male--who would not directly produce any progeny. Soon, the few sexual organisms would be lost in a sea of asexuals and find it all but impossible to locate a mate. All else being equal, the asexual clone would entirely replace its sexual counterparts in only about 52 generations, says evolutionary biologist Curtis Lively of Indiana University in Bloomington. Yet this happens rarely, if ever. Despite the cost, sexual species persist, while most asexuals quickly go extinct. ⁴

In elementary school, we're taught that one X and one Y chromosome makes you a man, and two X's make you a woman.

This is (mostly) true in humans. Actually it's (mostly) true in mammals. Move outside mammals, though, and it's a brave old world. Birds are just the opposite: roosters are XX, hens, XY. (In the literature, the sex chromosomes from such “backwards” systems are called ‘W’ and ‘Z,’ so that a rooster would be WW and a hen WZ, but that's just a mnemonic device.)

Sex in butterflies is done just like sex in birds, but fruit flies are like us.

Well, sort of. Girl flies are XX and boy flies are XY. Fly into the odd corners of the Drosophila personal ads, though, and differences emerge. In mammals, having a Y chromosome makes you a male: people with two X's and a Y--Klienfelter's syndrome--are male, people with one X but no Y--Turner's syndrome--female, In flies, it's just the opposite. Two X’s makes you a female: X0’s (flies with one X and no Y) are boys, and XXYs are girls.

Sure, this will win you points in Trivial Pursuit (Sex-Determination Edition), but so what? Think for a second: How could you ever turn one system into another?

(And how in the heck will the two Jeffreys turn this into a software column? Patience. We'll get there. Relax and enjoy the ride.)

This is a wonderful puzzle. Sex is very important; species that don't reproduce successfully suffer instant extinction. Yet though flies and butterflies share a common ancestor (which must have had some sex determination scheme) they've managed to evolve completely opposite ways to decide what sex they are. We're much closer to birds than either of us is to insects, but our sex-determination scheme is the same as the fly's investigating the bottle of homebrew next to our terminal. Both are completely backwards from the system used by the magpie squawking outside our window, and the sphinx moth getting ready to pop out of the pupal case we're keeping on our windowsill.

Oh, but life gets weirder.

Praying mantisses, which we enjoy in our gardens, have one X and several Ys: That is, females have two Xs, while males have one X and a handful of distinct Ys: each Y is different, and each male has one Y of each kind.

Other species turn this on its head, and have several X different chromosomes but only one Y: each female may have, say 3 different pairs of X's, while her mate has 3 X's and a single Y.

Some animals, like the voles getting ready to hibernate as we write, have gotten rid of their Y's altogether. One sex has two X's and the other has one.

Each of the spiders that invade our office, this time of year, has several X chromosomes. Females might have six, while males have three --but no Y.

The ants trying to take up residence in our kitchen are odder still: they have no sex chromosomes at all. Females have two of every chromosome; males only have one of each.

But there's a constant thread running through all of this: Despite this wild diversity, there are always exactly two sexes, males and females. One has one sex-chromosome makeup; the other has a second. Also, one sex makes two kinds of gametes, in equal numbers, and the other only makes one. All human (or fly) eggs have an X chromosome, but each Jeff makes two kinds of sperm: X sperm and Y sperm. If we were birds, we'd only make one kind of sperm, but our mates would make two kinds of eggs.

In spiders with three sets of X chromosomes, spider moms always make eggs with three X's, while spider dads make sperm that have either three X's or no X.

Ants, bees, and wasps take this to an extreme. Effectively, every chromosome is an X: An egg has one full set of chromosomes. Fertilized eggs (ones that get an additional full set from dad), become daughters, unfertilized eggs (ones that get no set from dad--effectively an “empty sperm”) become sons.

(“Stop,” says the one of us trained as an engineer, “I'm missing something. If it's an empty sperm, what triggers the ‘life starts’ magic? Or is that a religious question beyond the scope of this column?” “You're missing nothing,” the geneticist replies. “It's done with mirrors. There are lots of interesting consequences, but they're more than we need to set up the problem.”)

The regularity means that when you mate a male and a female, you get two kinds of offspring: sons and daughters. And the production of equal numbers of two kinds of gametes by one sex means that there are the same number of sons as daughters.

A good rule of thinking about nature is, “When something's constant, look closer: it might be important.”

Why is sex always the same?

The Exception That Proves the Rule

Here's another good rule of thinking about nature: “The exception proves the rule.” If something's odd, look closer, it may help you understand the norm.

During a casual afternoon in the dusty stacks of his local University library, Haemer stumbled on a report that one population of Central American fish--a swordtail, for you tropical fish types--has three different flavors of sex chromosomes: an X, a Y, and a Z. Here's how sex determination works:

Each individual only has two sex chromosomes: XY, YZ, XZ, ... but not XXX, XYZZ, or anything else with more than two. If you carry a Z, you're a female. Otherwise, if you have a Y, you're a male.

This gives us the following possibilities:

    chromosomes     sex
    YY              male
    XY              male
    XX              female
    YZ              female
    XZ              female

What about ZZ? Can't happen. A ZZ would have to get a Z from mom and a Z from dad. But swordtails with Z's are never dads.

This is a very different system with lots of odd pairings. For example, XX x YY gives all sons, while three-quarters of the offspring of XZ x XY will be daughters.

(Yes, this is very strange. It simply adds to our respect for J.B.S. Haldane, the father of theoretical population genetics, who observed that life is not just stranger than we imagine, but stranger than we can imagine.) ⁵ [italics in the original]

There is a bewildering variety of sex determining mechanisms in nature. Some of the major ones are:

1. Genetic sex determination (GSD)
1. male heterogamety
2. female heterogamety
3. polyfactorial
2. Environmental sex determination (ESD)
3. Cytoplasmic sex determination
4. Arrhenotoky (haplodiploidy)

Definitions:

⁶

General points:

1. Male heterogamety and female heterogamety both common
2. But male heterogamety more prevalent
3. Some taxa [biological classification categories] show both forms of heterogamety (e.g., ranid frogs)
4. Other broad taxa show only one form of heterogamety (e.g., birds and mammals)
5. As we shall see later some of these taxa also have other types of sex determination (especially ESD)