Tuesday, February 17, 2009

Evolution can explain sexual reproduction

Can evolution explain Sexual Reproduction?

Probably the best way to understand this complaint is to look at a claim that a creationist made on this subject.

> Darwin talks about natural selection and the survival
> of the fittest, meaning that evolution takes the
> easiest route to continue the species. But how is
> it, if you're a single celled organism that's asexual....
> why would you want to limit your chances of progressing
> your species by relying on another organism for
> reproduction when you can reproduce on your own?
> Seems to me that would lessen your chances than
> increasing them and would be less productive.
> And when did this single celled organism decide it
> was going to go from asexual reproduction to sexual
> reproduction and what kind of chance was involved
> that it actually found a mate??? These are all
> questions evolutionists have no answers for...it's
> quite ridiculous actually.

This creationist raises some very good questions. What evolutionary benefit does sexual reproduction have? How did organisms first evolve the ability to reproduce sexually?

One of the disadvantages of sexual reproduction isn’t even mentioned. That is the advantage of doubling the rate of producing descendents. With sexual reproduction only the females can bear young. But with asexual reproduction, both of the partners can reproduce.

Another disadvantage of sexual reproduction is that each individual doesn’t have to search for and find a mate. To make a somewhat silly analogy, if you can reproduce asexually and you are deserted on a desert island, you can reproduce helpers as well as company.

These questions perplexed evolutionary scientists for a very long time. Here’s a quote from as recently as the year 2000[1]:

“Evolutionary biologists have long puzzled over why sex evolved at all. Asexual organisms simply split themselves to produce offspring and can reproduce twice as fast as sexual organisms. So why go to the trouble of having sex?”

Why indeed?

Sex has been around for a very long time and most organisms do it. So, if you believe in evolution, there must be some good reason for it to take place.

Actually, it turns out that there are at least two – and possibly more - good reasons for sexual reproduction.

First, sex helps to maintain the health of the DNA that you give your offspring[2]. This is the sort of a more technical subject than I promised to avoid, but in a footnote I have provided a reference for anyone interested in learning more.

But there is a significant second advantage to reproducing sexually. That advantage comes from an increased amount of variation allowing faster evolution.

Yeast – a single-celled organism – can reproduce both sexually and asexually. Therefore it is reasonable to look at them in order to see what advantages there might be when they reproduce one way as opposed to the other way.

A scientific study[3] performed in November of 2004 took eight populations of yeast from the same source population. The scientists did some “genetic engineering” to make four of those populations only able to reproduce sexually and the other four populations able to only reproduce asexually. They put sets of each of those populations in an environment to which they were very well adapted (one with a normal temperature). They put the other populations into an environment to which they were not so well adapted (they raised the temperature by 7 degrees centigrade and made a few other changes as well).

As a control they also put samples of the base population into each of the two environments.

The experiment was designed to test the “Weisman effect” which is described in the abstract for that experiment like this:

“…sex allows natural selection to proceed more effectively because it increases genetic variation.”

In other words, in a population under environmental stress, sexual reproduction provides a benefit because it increases the amount of variation available in the population.

With asexual reproduction, every descendent is identical to their parent with the exception of the occasional random mutation.

However, with sexual reproduction, the descendents are a random combination of the genes of the two parents. So there is more variety and it is more likely that the optimal combination of genetic characteristics will result.

As an admittedly simplistic example, consider two parent organisms: one is short with little fur and the other is tall with a lot of fur.

Add to that hypothetical example an environmental change: an ice age is starting and the environment is turning colder as a result.

The ideal body type under those circumstances would be a short body with fur. (The short body minimizes the surface area exposed to the colder air. The fur is self-explanatory.)

If these individuals reproduced asexually, all that you would get is short descendents with no fur and tall descendents with hair (in other words, the descendents would be identical to their parents with only possible minor changes due to random mutations).

On the other hand, if the two parents reproduced sexually, you would see combinations of all of those characteristics. Most specifically, some of the children would have the “short” characteristic from one parent along with the “furry” characteristic of the other parent resulting in the ideal body type for the changing environment.

The scientists testing the yeast found that there was no difference in growth rates in the “benign” environment to which the various populations were well adapted.

But in the “harsh” environment, the yeast that could reproduce sexually was able to grow at a faster rate than those in the same environment that could only reproduce asexually. This confirmed what the Weisman effect. Reproducing sexually does have adaptive advantages when compared to asexual reproduction.

Anyone who understands evolution would expect this result.

Of course, the other question about sexual reproduction is how the ability to do so evolved in the first place.

There are explanations for that as well. Here’s a readable and easy to understand explanation from the Internet[4]:

“Although bacteria simply multiply by dividing into two, they also often exchange genetic material, usually by releasing small fragments called plasmids which can be absorbed by nearby bacteria. In this way, beneficial mutations are shared. This might arguably be considered to be the earliest form of sex.

“After the appearance of plants and animals in the oceans around 600 million years ago, the only practical way for these new multi-celled sea-creatures to reproduce was by releasing seeds or eggs, whose cells would then begin dividing, sticking together, and changing each time they divided, until finally forming a new adult.

“The only practical time to accept potentially beneficial foreign genetic material was before the seed or egg began growing. Many sea plants and animals today release pollen or sperm into the water to be absorbed by nearby seeds or eggs, which will not begin growing until they have been fertilized.

“As plants moved onto the land, their pollen was either carried by the wind or delivered directly to seeds by insects. For almost all land animals and many sea animals, sperm was more effectively delivered by injecting it into an enclosed body cavity containing eggs.

“Evolution generally favors any mutation that increases the drive to find sex partners. While simple creatures like insects follow programmed patterns of behavior, more complex.”

Clearly evolution does have explanations for how and why sexual reproduction evolved in living organisms.

[1] http://news.bbc.co.uk/2/hi/science/nature/969111.stm, referenced on August 6, 2008
[2] http://www.sciencedaily.com/releases/2004/06/040611080718.htm, referenced on August 6, 2008
[3] http://www.nature.com/nature/journal/v434/n7033/full/nature03405.html, referenced on August 13, 2008
[4] http://www.evolutionary-philosophy.net/sex.html, referenced on August 14, 2008

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