Tag Archives: Reproduction

Sex With Aliens

Science fiction is full of humans interbreeding with intelligent aliens from all over the galaxy.

In the original Star Trek series, Captain Kirk mates with a wide variety of aliens, although he never fathers any children with them that we know of. Spock is half Human, half Vulcan. There are several characters in Star Trek: The Next Generation who are the result of alien-human mixing, as well. Worf is 100% Klingon, but his wife, K’Ehleyr, is half Klingon and half Human. Their son, Alexander, is therefore one quarter Human and three quarters Klingon. Deanna Troi is half Human, half Betazoid. The list goes on and is certainly not limited to Star Trek.

Star Trek’s Spock is half Human and half Vulcan. Image from wikipedia.org

I hope I’m not ruining anyone’s fantasy by pointing out up front that this is not even remotely plausible.

Imagine a human trying to produce offspring with an insect or a tree. Once you’ve stopped laughing, imagine trying to do the same with a bacterium, which is as distantly related from humans as you can get on Earth. Now consider that reproducing with a life form from another planet is far less likely to work out than trying to reproduce with a bacterium from Earth.

For two different species to be reproductively compatible, they cannot have very much evolutionary distance between them. Zebras (Equus zebra), horses (Equus ferus) and donkeys (Equus africanus) can all reproduce with one another. Lions (Panthera leo) and tigers (Panthera tigris) can sometimes produce offspring. The common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) can produce offspring. (As is typical of cross-species hybrids, the offspring of these pairs are not fertile themselves.) I hope you noticed a commonality among all of these matches: they are all in the same genus. Lions and tigers are both in the same genus (Panthera) and they can sometimes reproduce together, but neither the lion nor the tiger can reproduce with a chimpanzee or a zebra because they are not closely related. This is not a hard and fast rule, though. Many species that are in the same genus cannot reproduce together, and occasionally species that are not in the same genus can reproduce together.

The liger is a hybrid between a tiger (Panthera tigris) and a lion (Panthera leo). Image from wikipedia.org

Life on Earth began somewhere between 3 and 4 billion years ago. Since then, life has evolved and diversified to produce the wide range of species that we see today. Another planet with life would have had its own origin and its own evolution to produce whatever diversity that planet has in whatever amount of time it took. An intelligent species on another planet would be unable to reproduce with most of the other species on its planet, just as most species on Earth cannot reproduce with one another. Humans are not able to reproduce with any other species on our planet. Even chimpanzees and bonobos, which are our closest evolutionary relatives, cannot produce offspring with humans (though it is not for a lack of trying). We share almost 99% of our DNA with chimpanzees, and are physically capable of mating, and yet we cannot reproduce with them. So what are the chances of being able to reproduce with a species that evolved from a completely separate origin of life? In this particular case, the likelihood is roughly equal to the percent of our DNA that we share: 0%.


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A Journey Through Plant Evolution at the Lincoln Park Conservatory

I recently spent some time at the beautiful Lincoln Park Conservatory. It is very hot and humid in the greenhouse, which may not have been the most obvious choice on a day that was already 88º outside. Nevertheless, I thought it would be a good way to talk about the evolution of plants. My specialty in biology is animals, but I have always loved the story of plant evolution. Like all other major groups of life on Earth, plant life began in the water. Early plants were very reliant on water, but plants became less and less dependent on water as they evolved. In the water, life is easy. Dehydration is not a problem. Nutrients can be absorbed directly out of the water into the cells. The water will carry sperm for reproduction and disperse the offspring. Life in the water is good, but there was a lot of space to grow on land.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.


One of the first types of plants to live on land were the liverworts. They are very small, with leaves that lie almost flat on the ground. Liverworts have no ability to draw water up out of the ground, as later plants are able to do. As a result, they cannot grow very tall and must be damp all of the time. They cannot survive or reproduce without being wet.

Liverworts are one of the first plants to live on land.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.


Like liverworts, moss also cannot draw water up into their bodies. Are able to grow a little bit taller than the liverworts because they grow in dense mats that can trap water between individual plants. This allows them to grow up to about four inches tall.

Mosses do not dry out as easily as liverworts, but they do rely on water for reproduction. The male sperm must swim through the water to find a female plant.

Like an idiot, I forgot to take a picture of moss. This one form wikipedia.org will have to do.

Club Moss

Club moss are sometimes called “ground pines” because they can resemble pine trees, but they are neither pines nor moss. Modern club moss usually only grow to be a few inches tall, but during the Carboniferous period, when they were the dominant land plant, they grew as tall as modern trees.

Club moss are one of the first type of plants to have vascular tissue, which lets them draw water from the ground up into their bodies. This adaptation is of unparalleled importance for plants on land. For this reason, club moss were one of the first types of plants to be able to grow more than a couple of inches tall. Without vascular tissue, a plant more than an inch or two has no way of getting water to the upper part of the plant. Plants don’t need very much to live, but access to sufficient sunlight is one of their main requirements. When all of the plants around you are only two inches tall, a plant that can grow to be several feet tall or taller has an enormous advantage when it comes to getting sunlight. You can grow taller than your neighbors and spread out to get all the sun you want.

Club moss (not moss). Image from bio.sunyorange.edu

Coal is made of fossilized plants from the carboniferous period. The majority of coal is made up of ferns and club moss. Sometimes coal preserves the structures of the plants it was made from and we can use the coal fossils to learn about ancient plants.

A thin section of coal clearly shows the features of the stem of an ancient plant (in cross-section). Image from http://www.ucmp.berkeley.edu


Evolutionarily speaking, ferns are slightly newer than the club moss. Like the club moss, ferns are have vascular tissue (so does everything else from here on).

Compared to most other plants, ferns grow sideways. The stem lies horizontally underground, and the fronds grow up out of the ground from it. When you see a cluster of fronds sticking up together, they are usually from the same plant.

Fern frond

Fern frond

Structures on the underside of the fronds, called “sori,” contain spores. These capsules break open, releasing the spores, and new ferns grow where the spores land.

Sori are clearly visible on the underside of fern fronds. These contain spores.

Sori are clearly visible on the underside of fern fronds. These contain spores.


Cycads look superficially like palms, pineapples or yuccas, but is not closely related to any of them. They were one of the dominant types of plants during the mesozoic era — the age of the dinosaurs.

Cycads were among the first plants to use pollen in reproduction. Pollen is produced by the male structures on plants, and is responsible for carrying sperm to the ovule in the female structures on other plants. Unlike the earlier plants, which require water for the sperm to swim through, pollen is carried by the wind. This is great for plants that live away from water and want to be able to reproduce with individuals that are far away. The problem is that it is fairly inefficient. Plants whose pollen is carried by the wind need to produce vast quantities of the stuff in order for some of it to get to other plants. I grew up in New Hampshire, where there a lot of white pine trees (which are not cycads, but also reproduce with wind-borne pollen). I got up many a morning to find my car completely covered in yellow pollen. All of that pollen that didn’t end up on the right part of the female plants is wasted energy.

Cycads were also among the first plants to have seeds, instead of spores like the older plants. Spores are fine, but they cannot travel over long distances or lie dormant for a more opportune time to sprout. A seed contains the plant embryo as well as nutrients to keep it alive for months or years. If a spore happens to land on the back of a bird on its way to the other side of the country, the embryo inside may not survive the trip because its mother didn’t pack it lunch. An embryo inside a seed will survive the same journey because it is surrounded in an oil-rich substance called “endosperm.” When we eat nuts, it is the endosperm that we are after.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Flowering Plants

Flowering plants became the dominant plants of the world during the late mesozoic, and today account for the majority of plant species.There are over a quarter million living species of flowering plants, compared to only about 12,000 species of fern, and fewer than 10,000 species of liverwort.

The flowering plants have two evolutionary advancements that allowed them to be so successful: flowers and fruit. These allow plants to solve two big problems in the area of reproduction.

Flowers represent an exchange of goods and services between plants and animals. Big, colorful, aromatic flowers are nature’s equivalent of an “eat here” sign. Flowers produce sugar-rich nectar that animals like ants, butterflies, birds, and bats like to eat. While these “pollinators” are eating the nectar, they get covered in pollen. When they go to the next flower, they drop some pollen off and pick up some more. This results in animals carrying pollen directly from one plant to the next, with very little waste. Remember all that energy that earlier plants wasted trying to pollinate my car? Flowers allow plants to use their energy more effectively. The energy this saves over relying on wind pollination is part of why flowering plants are so successful evolutionarily.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Spreading seeds is another problem for plants. If seeds just fall off the parent plant and onto the ground, some will roll away or get kicked away. The others will sprout right next to their parent and compete for the same nutrients and light. This will reduce the success of both the parent and the offspring. Some (but not all) flowering plants produce fruit to solve this problem.

Fruit is a sugar-rich substance that is easy to get eat, which surrounds the seed, which contains an oil-rich substance that is protected by a hard shell. In human terms, the plant “wants” you to eat the fruit, but does not “want” you to eat the seed. If you eat the seed, you are eating the tree’s offspring. If you (or another animal) eat the fruit, there is a good chance that you will swallow the seeds by accident. The hard shell protects it from being broken in your mouth or digested in your stomach. After eating the fruit, you (or whatever animal) will walk or fly away and eventually deposit the seeds far away in a nutrient-rich pile of fertilizer.

Flowering plants are better at life on land than any other plants. They can draw water and nutrients out of the soil through their roots and vascular tissue, and they are very good at reproducing and spreading their offspring without the aid of water. This is why they are beating all of the other plants.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

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Are Humans Still Evolving?

Image from telegraph.co.uk

I hear this question come up a lot. The subtext is that, because humans have mastered our environment, we are no longer subject to the same pressures of natural selection that we once were.

First, let’s review what evolution is. Evolution is a change in the frequency of alleles or traits in a population over time. Natural selection is the mode of evolution where the change is based on environmental pressures that cause individuals with certain traits to reproduce more than individuals without those traits. Natural selection does not have a goal, or a more or less advanced state. Natural selection improves a population in that it increases the frequency of traits that cause the members of the population to leave more surviving offspring. It does not improve a population by necessarily making the individuals smarter, faster, stronger, or more complex. A cheetah is not more evolved than a sloth because it is faster. Both were designed by natural selection to do what they do.

Infectious disease was a huge problem for our ancestors. Good thing we cured them all. (In case there was any doubt, that last sentence was full of sarcasm.) While it is true that medical advances have greatly reduced the burden of infectious disease in the west, they are still a big problem in most of the world. Malaria, tuberculosis, HIV/AIDS, respiratory infections, and diarrheal disease each kill over a million people a year worldwide, mostly in developing nations.

Even in the medically-advanced United States, tens of thousands of people die every year from influenza alone. We may not have schistosomiasis or malaria, but we still have people dying of infectious disease in large numbers. If there exist any alleles in our population that confer resistance to diseases that we face, however slight, then we are evolving. If for example, people with a particular allele have even a 0.1% increased chance of surviving influenza, then that trait will increase in frequency in subsequent generations.

If diseases are not killing us randomly, then those diseases are causing us to evolve. If we cure those diseases, then the cure will also cause us to evolve. If a disease is killing people who do not have a particular trait, then the frequency of that particular trait will increase. If we cure the disease, then the frequency of that particular trait will begin to decrease again. Both the increase and decrease of the trait frequency are evolution. I think this is where a lot of people get hung up — because we are not “improving” we must not be evolving.

Our evolution is not limited to the effects of infectious disease. If you look at population growth in the world, it is not the same everywhere. Traits that occur at higher frequencies in the parts of the world with higher fertility rates will get passed on at a higher frequency. This is also evolution.

Average fertility rate by country. Most of the world’s population growth is happening in central Africa. Image from wikipedia.org

Evolution takes place over the course of many generations. Individual human observers will therefore have a difficult time observing human evolution. It may not look like we are evolving, but we certainly are.


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The Tragedy of the Commons

In my recent post on greed, I explained how the underlying psychology that leads some people to have an intense and selfish desire to acquire wealth evolved. In doing so, I focused on how these traits favored individuals that had them in the past. One reader pointed out that this leads to resource depletion in humans (which it does), and wanted to know whether this or any other “bad outcomes” occur in other species.

The drive to leave more surviving offspring than other individuals is integral to the evolution of all traits. Reproduction is the only currency of natural selection, and all traits are tied back to this in one way or another. Survival is an important trait, but it is only important so far as living longer allows one to leave more offspring. Social behavior is important for some species, but only if it improves survival and reproduction. The same goes for intelligence, opposable thumbs, wings, or any other trait.

It’s obvious to see how the drive for reproduction can be bad for other species. For example, if an animal enhances its survival and reproduction by being a predator, the survival and reproduction of its prey will suffer — being eaten for lunch certainly qualifies as a “bad outcome.” For this post, though, I’ll focus only on how the evolutionary ambitions of a species can hurt itself.

Parasites have the same drive to reproduce that other organisms have, although the drive is based entirely on physiology instead of having a psychological component. Their lifestyle requires them to be careful (in physiological and evolutionary terms) about how fast they consume their resources (their hosts). If they reproduce too fast inside their hosts, the host will become so sick that it cannot transmit the infection effectively anymore. The virulence (pathogenicity) of a parasite is carefully tuned to suit the host. If a parasite jumps to a new host species, the virulence may be too high to be good for the parasite. This is exactly what happens with ebola.

The ebola virus sometimes infects humans, but we are not its primary host. In humans, ebola is very contagious but it kills the host very quickly — too quickly to infect enough other hosts. For this reason, ebola outbreaks in humans tend to “burn out” fairly quickly and do not infect large portions of our population. Humans are the virus’s resources, and by reproducing so quickly, the resources are depleted and the virus population dies out.

Electron micrograph of an ebola virion. Image from wikipedia.org

In the boreal forests of North America, the Canadian lynx (Lynx canadensis) has a 9- to 11-year cycle of population increase and decrease. The Canadian lynx specializes in hunting the snowshoe hare, and they are very good at it. Lynxes that are better at catching snowshoe hare will get more resources, and are able to have more offspring. These offspring will inherit a superior ability at catching snowshoe hares, allowing them to have more offspring of their own. Hares are renowned for their rapid reproduction, but the lynx is renowned for its ability to kill hares. When the lynx population gets too big, they will kill so many of the hares that there is not enough food left to support the lynx population. The lynxes starve to death and the population crashes.


Canadian Lynx (Lynx canadensis). Image from wikipedia.org

This is obviously not a complete list of how resource consumption can result in problems for non-human species, but it shows that it can be a real problem. Most resources are limited, and the tragedy of the commons applies to everyone, human or not.



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What Causes Greed?

This week’s topic was requested by a reader. If you would like to request a topic, let me know in the comments or on twitter.

Before we begin, be sure to read my post on the naturalistic fallacy. Science cannot condemn or justify any behavior — it can only identify the behavior and explain why it exists. In this post I will attempt to explain greed as I understand it, without mixing in any of my own ideology or the ideology of anyone else.

My dictionary defines greed thusly: “Intense and selfish desire for something, especially wealth, power, or food.” It is the wealth part that I will focus on.

To understand greed, or many other human behaviors, we first have to understand evolution — the human mind, of course, is a product of evolution. Every behavior and thought that we have is not necessarily the direct result of natural selection, but natural selection lays the foundation for our behavior. Natural selection works on a relative level. Traits are successful if they are passed on more frequently than other traits, which means that more individuals with that trait must be born and survive than individuals with other traits. Traits will evolve faster if there is a greater “selective advantage” — that is, the difference in survival and reproduction between individuals with and without the trait is very large.

If a squirrel, for example, has three offspring that survive, it and its traits are doing better than a squirrel that only has one offspring that survives. But it is not doing as well as a squirrel that has 10 surviving offspring. For this reason, natural selection does not give animals a target number of offspring that they want to have over the course of their lives but not exceed. If a squirrel has 10 surviving offspring but could produce more, it will lose the evolutionary race to an individual that could produce more than 10 surviving offspring and does. Said differently, natural selection does not produce squirrels that are satisfied with a particular number of offspring and will not have more. Rather, natural selection produces squirrels that will produce as many surviving offspring as they can. This is true for all organisms, not just squirrels.

For all organisms, resources are the most important thing for reproduction. It takes a lot of calories to produce an offspring, and raising it (in species that provide parental care) takes even more. Animals will produce as many offspring as they can, and it is resources that determine how many this really is. There is a large selective advantage for individuals who can acquire the most resources because they can produce more offspring that are healthy and survive long enough to reproduce themselves.

Modern humans aren’t so different. Estimates vary, but it costs somewhere in the area of $100,000 – $200,000 to raise a child to age 18 these days. This means that you need to have at least this amount of money coming in for every child that you want to have.

But let’s go back in time for a minute. Back in the day, let’s say 50,000 years ago, there was no money. If someone got a windfall of resources by killing a mammoth, they couldn’t put it in the bank, or even horde it underneath their mattress. If they did not use this material wealth, it would rot.

Modern wealth is easily storable, so it can be accumulated in ways that was not possible in the past. Prehistoric people could not stuff an uneaten mammoth under their mattress for later, but modern people can easily stuff the equivalent of a thousand or a million mammoths into a bank account or a mutual fund and keep it for as long as they want.

As animals, we have a strong drive to do better than everyone by as much as possible. Humans obviously have complexities to our behavior that make us more nuanced, but this drive is still rattling around in our brains and affecting our behavior. Just like the squirrel that tries to have more surviving offspring than the other squirrels, people like to have more wealth than others. Natural selection has made us very interested in acquiring more resources than other people.

There is an old joke about a farmer who is granted a wish, but whatever he wishes for will be doubled for his neighbor. He can’t wish for a wealth of gold because his neighbor will be given double the gold. He can’t wish for five strong sons because his neighbor will get ten strong sons. He eventually decides to wish for half of his crops to be destroyed. This solution, or one like it, is the only way he can come out ahead of his neighbor. Coming out ahead, as I have already discussed, is the only way to win at natural selection.

Nobody particularly needs a billion dollars, but lots of people want it. When natural selection built us with an interest in accumulating more wealth than other people (what you might call an intense, selfish desire for it), it did not build an off switch.

And that, in a nutshell, is why we have what we call greed.

See the followup to this post: The Tragedy of the Commons

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