evolution – Empirical Zeal

Flies alter their ejaculate to get the best bang for the buck

drosophila mating — Smarter than you think..

Sex is war. It’s a battle for limited resources.

The source of sexual conflict is this: sperm is a relatively cheap resource for males to produce, whereas producing eggs and rearing offspring is a much larger investment on the part of the female. Darwin was the first to realize the implications of this. He reasoned that this imbalance should result in males competing with each other to fight for the limited resource, and females exerting a strong choice on who to mate with. Taken together, male competition and female choice were the two pillars of the theory that he called sexual selection.

The battle of the sexes is not a new idea, but it has changed with the times.

In the 1960s and 70s, the sexual revolution was eroding away conservative ideas about sex. This was the era of promiscuity. And this changing social fabric was being mirrored in science. The ‘free love’ era brought about an equally potent, but more silent, revolution within biology – one that completely shook up our old, prudish notions of reproduction. Research from these decades onwards taught us that in almost all animals, from insects to birds and mammals, females typically copulate with multiple males. We learned that promiscuity is not a freak event, it’s actually the norm. What this implies is that, like any modern war, the battle of the sexes is a messy and involved affair, often leading to surprising and unexpected consequences.

For one thing, it’s being fought on many fronts. In many species, competition between males for the egg doesn’t stop at intercourse. Even after the female is inseminated, the battle rages on inside her reproductive tract. In this alien battlefield, the sperm cells of different males compete with each other to fertilize the eggs. Meanwhile, the reproductive organs of the female can still exert control by choosing between the different sperm.

And just like the Greeks who sneaked into Troy, the soldiers in this battle use every trick at their disposal to gain an upper hand. Some males do the equivalent of taking their ladies out to a fancy restaurant – they present females with a nutritious meal in their sperm, at substantial cost to themselves (delightfully, biologists call this a prenuptial gift). Others resort to date rape – their sperm includes a harmful cocktail of drugs that alter the females’ behavior in their favor. Even more chilling, there are species in which the males engage in traumatic insemination, where they essentially rape the females. Other males are just outright weird. Some leave their penis behind to plug the vagina from use by other males. Others have smelly sperm that repels other males. And others have spiky penises, that scrape the vagina clean of the sperm of competitors.

Not a pleasant lay. That thorny structure is the penis of a bean weevil.

Continue reading Flies alter their ejaculate to get the best bang for the buck

Why have sex? To fight parasites, of course!

This post was selected by Vincent Racaniello as an editor’s selection on ResearchBlogging.org The (revised) title of this post was suggested by Lucas Brouwers. Check out his excellent blog on evolution, Thoughtomics.

: New Zealand mud snails, before and after infection by parasites. These tiny creatures may move slowly, but peering beneath the surface reveals an incredible race for survival.

Why do we have sex? If this question keeps you up at night, you either have really loud neighbors, or you have the makings of an evolutionary biologist. Some of the most brilliant minds in the field – William Hamilton, John Maynard Smith and George Williams – have spent much of their careers wondering about the value of sex. This is not a reflection on the quality of their sex lives. Rather, it has more to do with their creative insight and ability to look at the world with fresh eyes.

A billion years ago, our ancestors inhabited a world without sex. This was the era of the clones. In this strange world, all organisms reproduced by creating identical genetic copies of themselves, somewhat similar to how modern-day bacteria reproduce [1]. But this clonal strategy has a problem. Populations made up of identical twins are more vulnerable to infection. When a disease comes along, it doesn’t just wipe out a few individuals. It can take out the whole lot.

When sex arrived, it introduced a new pace to life. Organisms were mixing and matching genes in combinations never seen before. Imagine a world where you had to dress well to survive. In such a world, the invention of sex is like going from wearing uniforms to having your own wardrobe. You could pick a gene from here, another from there, and put together a novel offspring. And if a particular outfit were deemed ‘unfit’, it’s not a huge tragedy as there are plenty of alternatives.

In this way, sex helps us by innovating new evolutionary solutions and by protecting us from disease. But sex is not without its discontents. For one thing, sexual reproduction implies that you only pass down half your genes to your offspring. The other half come from the other parent, and they combine to make an offspring with a full set of genes. On the other hand, in asexual reproduction, the mother passes on a full set of genes to her offspring. So by adopting sex, your genes are travelling half as far. In evolutionary terms, this is a huge cost, and sex had better have a lot to offer for it.

The Two-Fold Cost of Sex — John Maynard Smith described "the two-fold cost of sex" - Asexual populations (b) grow twice as fast as sexual populations (a).

Do the benefits outweigh the costs? We would certainly like to think so. But when evolutionary biologists did the math, they worked out that the answer is usually no. Your genes typically have more to gain if you reproduced asexually.

So what gives? Why, then, do so many species adopt a sexual lifestyle? Well, here’s a brilliant solution offered by Hamilton and others: if you are under constant attack by rapidly evolving parasites, then sex is a better strategy than cloning yourself. This idea came to be known as the Red Queen hypothesis and can be summarized in one line: it’s harder to hit a moving target.

alice red queen — "Now, here, you see, it takes all the running you can do, to keep in the same place."

Continue reading Why have sex? To fight parasites, of course!

Why moths lost their spots, and cats don’t like milk. Tales of evolution in our time.

In the children’s game of hide-and-seek, it doesn’t matter much whether you win or lose. In the animal kingdom, however, the stakes are significantly higher. If you’re found, you’re food.

And death is not just the end of the individual, it’s the end of a lineage. Organisms that die before they can reproduce deny their genes a road to the next generation. In this simple fact lies the engine of change. For example, genes that make a prey more camouflaged will end up increasing their reproductive success, whereas genes that make them more noticeable are not going to get very far. In this way, natural selection is driving prey to become better hiders, and predators to become better seekers.

Nowhere is this evolutionary race more evident than in the case of the peppered moth. This is a species of moth that is found all across England and Ireland. When people first studied them in the early 1800s, almost all the moths looked something like this:

As you can see (if you’re looking closely), the white and black speckles are effective camouflage. For ages, these moths have hidden on light colored trees and lichens. Over time, they have evolved this distinctive pattern to help them evade the notice of the birds that prey on them.

But just fifty years later, things were beginning to change. By the 1850s, moths of the same species had stumbled upon a new color. These new moths were called carbonaria after their carbon-black color, to distinguish them from their salt-and-pepper colored relatives with the dull name typica.

By the end of the nineteenth century, the change was drastic. In 1895, a study in Manchester showed that 95% of the peppered moths were now of the black type. So what was going here? What could cause such an incredible change in appearance in just a hundred years?

Continue reading Why moths lost their spots, and cats don’t like milk. Tales of evolution in our time.

When nice guys finish first: a lesson from tiny robots

Meet Alice. She is 4 centimeters tall and moves about on wheels. Her goal in life is to look for food. Remarkably,the foraging behavior of this tiny robot has not been programmed by humans. Instead, her creators gave Alice a brain, and let evolution do the job of programming it. And Alice is going to show us why it is that individuals often make sacrifices for each other.

Animals often behave in seemingly selfless ways. The most regimented examples come from the social insects – the ants, termites, wasps and bees. Here selflessness is built in to the fabric of their society, as there are sterile castes of workers who tend to the eggs of the queen. Worker bees will often make the ultimate sacrifice and die protecting the hive from invaders. These are all altruistic acts, as they harm the individual while benefitting someone else.

Honeypot Ant — A sweet deal? That's not a drop of honey, but in fact it's the engorged abdomen of the honeypot ant. These ants are used by the rest of the nest as living storage pots.

Take a moment to think about this behavior from the point of view of evolution. If everyone’s competing to get ahead, why take an unnecessary risk or suffer to help someone else? You really couldn’t do much worse than adopt a sterile lifestyle – it’s an evolutionary dead end.

People used to talk about such altruistic behavior as being ‘for the good of the species’. But this explanation does not work. Natural selection does not operate at the level of species, it is solely concerned with the reproductive success of the individual. Any gene that inclines an individual to be more concerned with the welfare of the species than with their own welfare is not going to get very far.

This type of evolutionary logic paints a picture of a world red in tooth and claw, one where you need to constantly be watching your back. But if everyone is looking out for their own selfish interests, where does selflessness come from? The solution to this puzzle was put forward by J. B. S. Haldane in the 1930s, and made precise by William Hamilton in 1963. Hamilton had the remarkable insight to think of this as an economics problem, and rephrase it in terms of costs and benefits.

Continue reading When nice guys finish first: a lesson from tiny robots