Hypotheses of Asexual Reproduction

It seems that a large portion of the animal kingdom engage in some type of sexual reproduction, and this percentage increases if you look at the more complex species other than those of the amoeba and others.  Sexual reproduction has certain advantages such as recombination of genes, increasing genetic diversity, and allowing natural selection to act on individual genetic traits instead of the entire organism.  Seems like good stuff.  Two individuals with good survival traits can pass them on to their offspring together, thereby giving that offspring both sets of traits and increasing their likelihood of survival.  That’s the most common hypothesis for sexual reproduction, at any rate, and seems to make sense.  So, why is asexual reproduction still around?

Well, one of the benefits of asexual reproduction is the speed at which that reproduction could occur.  Instead of two individuals being required to produce one offspring, each individual can produce their own.  With sexual reproduction, assuming everyone can and does reproduce, with 100 individuals you may get 50 offspring, bringing the population to 150.  If everyone reproduces again, that’s 225 individuals in three generations.  However, with asexual reproduction, again assuming everyone can and does reproduce, if you have 100 individuals you may get 100 offspring, bringing the population to 200.  If everyone reproduces again, that’s 400 individuals in three generations.   This is oversimplified, of course, but you get the idea.

Asexual reproduction is also considered to be less costly than sexual reproduction.  What does that mean?  Well, for sexual organisms they have to produce an egg or a sperm of some sort.  And then they have to seek out another individual they can mate with.  Each
individual must meet whatever criteria the other has for a suitable partner before they mate.  Organisms also compete against others for mates.  Sometimes this comes in the form of physically fighting, which often requires bigger “tools” such as horns to win these matches.  Others compete by being the best singer, or dancer, or having the brightest colors.  Each of these traits are “expensive” in terms of the energy that must be used.  Asexually reproducing animals need to expend the energy to reproduce on their own, but they do not need to use extra energy to seek and win mates.

Since many of the more complex animals that are capable of asexual reproduction are also capable of sexual production, it could be seen as a benefit to have the best of both worlds.  A hypothesis that surrounds the Komodo dragons’ asexually reproductive ability is that an adult female would be able to start a new colony in a place where she may not have any potential mates.  In the dragon’s case all of the offspring would be male, and the mother could then mate with one of the son’s in order to produce more offspring.  Not the best way to get a diverse population, but a quick way to start one.  Other dragons then may come in from the outside and add new genetic material to the already established colony.

For other species, like certain types of sharks, their ranges can be so large that it may not be very easy for them to find mates.  When they do, great, they can reproduce sexually.  However, when they aren’t able to do so, instead of just waiting to reproduce, some species may be able to reproduce on their own.  Many sharks are slow growers.  It takes them several years to reach their reproductive potential and when they do reproduce, their offspring are few.  Being capable of asexual reproduction may allow certain shark species the ability to keep their numbers up in hard times.

Species that are capable of both types of reproduction do seem to have a bit of an advantage, or at the very least we may be able to see why they are capable of both.  But
what about those rare complex species like the New Mexico Whiptails (Cnemidophorus neomexicanus) that only reproduce asexually?  That’s trickier.

It would be my personal guess that their environment has been fairly stable, first and foremost.  It is believed by most scientists that sexual reproduction has a distinct advantage over asexual reproduction in terms of a population’s ability to evolve and adapt more quickly to change.  So, in situations like that of the little Whiptails, it is believed that their isolated communities didn’t give them much choice in terms of exchanging genes.
Perhaps, over time, their males became less suitable, less able to survive, due to this genetic bottlenecking.  Perhaps, over time, males just became fewer and fewer.  The end result would have been to go extinct, at least locally, or find an alternative.

The New Mexico Whiptails have a lot more chromosomes than their sexually reproducing neighbors do.  And their offspring are not genetic clones; the mother’s genes are recombined to produce each new Whiptail.  How they ended up with these extra chromosomes is still a guess at this point, but it’s interesting, yes?  What I also find interesting in all of this is that no matter the advantages, no mammals that we are aware of have the asexual reproduction ability, even in tough times.  Life is a constant mystery, and with science and curiosity, it’s sure fun closing doors and opening new ones.

Into the African Bush, Part 4

Our camp in Swadini was similar to that in Kruger in that we had an area to set up our tents and we had plumbing, but it was quieter there and we had more guests.  The furry kind of guests.  When I made a few phone calls to the States I saw several kudu walk right by me.  They’d stroll through camp as we sat around the camp fire in the evenings.  There were several vervet monkeys (Chlorocebus pygerythrus pygerythrus) trying to steal our food.  And at night you could hear the hyenas just beyond the fence.

Our first stop was to Moholoholo Wildlife Rehabilitation Centre (http://www.conservationafrica.net/projects/moholoholo-wildlife-rehabilitation-centre) where we were given a behind-the-scenes tour of what exactly they do there.  Most of their animals were injured or poisoned.  A few were orphans.  Any animals that could be released would be and if their injuries were too grave, they would be kept at the Centre.  Many of the bird species had fallen victim to power lines while others were poisoned by farmers or herders.  As I’ve mentioned before, poisoning is a big problem that is a threat all over Africa. 

While at the Centre we were able to meet a Bateleur eagle that loved a good scratch, right behind the head please, and feed several White-backed vultures (I got wacked in the head by a wing).  While touring the grounds we were fortunate enough to have their two 5-month old rhinos run up for a visit.  And while visiting the lions we were able to get up close and personal with a few lion cubs, one of which thought my leg looked pretty tasty.  It was great to meet the workers and volunteers too.  The Centre not only tries to rehabilitate the animals and release them but also tries to educate the public about the need to protect them.  It’s an important cause, one that cannot be understated.

Next we visited a reptile centre that had a few other animals too.  We were shown several South African snakes, including a cobra, as well as a baboon spider, scorpion, and a cockroach.  I learned that most snake bites are actually from people trying to handle the snakes.  One demonstration showed that the particular snake used didn’t want to bit the object “stepping” on it, it would rather try to get away.  And many snakes will try to provide ample warning before a strike; bites and the use of venom are costly to the snake and they would rather use it for their prey.

The next step in our wildlife management education was a week of capture-and-release training of large game.  We were also given a tour of a buffalo breeding center and aided in the cleaning of wounds of two of their injured males after they had been tranquilized.  Many buffalo have diseases that easily spread to other animals such as domesticated cattle.  These diseases are given to the calf from the mothers after birth, and the purpose of the breeding center was to produce disease-free buffalo.   

I’m going to go into more detail in a future post about the training we received on capture-and-release, but we participated in the capture and transport of two individual zebra as well as a few larger round-ups of herding species such as impala and waterbuck.  At the end of our week-long experience we were each put into a helicopter with the tranq gun and allowed to shoot at a moving truck with a target in the back, just so we knew what it would be like to try and hit an animal that was on the run.  I did pretty well and it was fun hanging out of a moving helicopter.

Finally we moved on to one more camp, which was tucked away and pretty private.  We were off to another breeding/rehabilitation center where we were able to feed some of the babies wandering around.  One of these “little” guys was a giraffe that was just a few weeks old but already towered over all of us.  There were two young zebra there too, which sadly had been orphaned.  And one of their lizards fell asleep on my arm.  All in all it was a good day.

This was basically the end of our journey.  We had some time to ourselves and I squeezed in a little horseback riding before we had our final test, which consisted of a short essay.  We were given information on a fictional ranch/national park which consisted of size of the area and number and kind of species.  It was our jobs to come up with a plan for making the ranch/national park financially self-sufficient as well as producing a plan for a stable population.  This really made us think about the things we had learned in South Africa and how to implement those lessons.

I very much enjoyed my time in South Africa.  I made friends and felt at peace out there in the bush.  It saddened me, however, to see fences around all of the animals and how many fell victim to things such as poisons.  There are so many aspects of this field that focus on a need to educate the public.  This is one of the things I hope to accomplish with this blog.  There once was a time when animals roamed free and humans lived with them.  Sometimes I think we need to be reminded of that.