Wednesday, April 7, 2010
Gametes and Evolution in Humans
As we all know, males have sperm and females have eggs. Males have lots of sperm and females have only a couple eggs (156). Why is it that males create so much sperm, while women only have a couple eggs? What is the evolutionary significance? Could it be possible that males are suppose to have multiple partners if they produce so much sperm, and could this be a break for evolution stating that we are suppose to produce as many children as possible? If not, why is it that males create so much because isn't it a waste of energy?
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It is true that males create thousands, even millions more sperm cells in a lifetime compared to the females limited amount of eggs. A female is born with the eggs she will develop, where as a male continues to produce sperm his whole life. A high quantity of sperm is an evolutionary advantage for reproduction. Females only need a limited amount of eggs as it takes more time and energy to develop an egg compared to a sperm. The female will stop producing eggs when she is at an age where she can no longer support a fetus as successfully. Sperm, on the other hand, take less energy to create due to their much smaller size and basic structure. The head, which is one tenth of its body, contains the vital information (the 23 chromosomes) that will interact with the egg. Since these sperm cells are so small, chances of reaching the egg may be slim. So rather than having one sperm attempt to reach the egg, chances for successful reproduction are increased by ejaculations with many, many sperm. This does not necessarily mean that males are supposed to have multiple partners if they produce so much sperm because even though the quantity is great, the quality of every sperm is not perfect. Many sperm are weak or are just not fit for reproduction. The male would be able to impregnate many women because of his ability to create new sperm. However, we have learned that humans are nurturing creatures who reproduce fewer offspring in order to devote care and proper upbringing. This ensures the survival of the offspring, and would be much more difficult if the male could not be attentive to every offspring.
ReplyDeleteWhy Evolution is True
Campbell
http://www2.hu-berlin.de/sexology/ECE2/html/sperm.html
Well, Andrew, it obviously isn't too much a waste of energy to become an evolutionary liability, for as Jerry Coyne once said, "The evolutionary difference between males and females is a matter of differential investment--investment in expensive eggs versus cheap sperm..." (157). The reason why males create so much sperm while women create much fewer eggs is simply due to "God, the devil, or evolution," and I think you know which one to attribute it to (Erdmann 4). Evolution selected males that produced higher amounts of sperm, and faster, because these males had much greater chances of successfully reproducing, whether it be with a female of choice, or a "substandard female," with which "males have little to lose by... because they can easily mate again, and repeatedly" (157). The females who produced the strongest (and coincidentally these required the most energy) eggs had a greater chance of successfully passing on their genes because their child would have higher fitness. Whether or not males are to have multiple partners follows the tradition of the species, for there are some species that are monogamous such as some penguins and swans, and others that are promiscuous, such as bonobos. Humans have defined themselves as culturally monogamous; however, human promiscuity occurs in great abundance. It is worth noting that during most of human history, humans lived nowhere near the ages that they live now, which means that sexual behavior that had an advantage in pre-historic human society may not still be the norm in modern day society.
ReplyDeleteSources:
Mr. Erdmann
Why Evolution is True
by: Jerry Coyne
The difference in the number of eggs and sperm produced creates a basis for sexual selection. Since eggs are more limited and more expensive due to the nutrients and such needed to upkeep the egg, they may be deemed as more "expensive." (Coyne) Since they are expensive and limited, the females must be careful with who they use it with, and thus the females are selective while the males have plenty to go around. This allows the male to be more promiscuous and try to spread his genetic line. Males are also able to produce more sperm if needed while females only have a set number of eggs waiting to be fertilized. As far as the energy used to create sperm, it is quite lower than the energy needed to create eggs, and is not a waste of energy. However, the energy needed to maintain the eggs is much more substantial.
ReplyDeleteMales can surely have multiple partners since they produce so much sperm, but then on another hand, so can females. Socially polygamous animals may just find partners to mate and never see each other again. The male, of course, may mate indefinitely while the female must wait for the next mating season if she is impregnated.
Sources:
Campbell Bio book
Why Evolution is True
First off, it is rightly noted above that males do produce significantly more sperm than females do eggs. And it is true that males produce sperm throughout their lives (after puberty of course) and females are born with their eggs, but they mature at different times (about one each month naturally). You're first question is about why this is. Well, as we all know, the male fertilizes the female. This is how sex works. If the male only produced one sperm, the efficiency of sex would be quite low, and more energy would be needed in the actual act. The actual act of sex is much more costly in terms of energy than making more sperm. Therefore, it can be assumed that the amount of sperm produced could have evolved to a point in which sex is efficient enough so that the energy spent didnt become a liability. Then the question goes to the female side. Why one egg? Well, as we already learned, eggs are much bigger than sperm and need to be fertilized to finish meiosis. This means that the sperm need to find the egg. If there were multiple eggs, then there could be multiple zygotes and embryos, resulting in multiple offspring per birth. This can be severly detrimental to an organism if it cannot support that many offspring. The female would most likely die during the birthing process and be unable to go "another round" of reproduction. That is why there is only one egg for human reproduction unless something goes wrong (like if two eggs were released). However, this provides for an evolutionary set back. What if two sperm fertilize the one egg? Eggs have developed defense mechanisms that keep multiple sperm from fertilizing the same egg (which would result in a triploid). The creation of a triploid is called polyspermy, and we have adapted to avoid this with the Fast Block and Slow Block of polyspermy. This is one adaptation caused by the high sperm to egg ratio, but doesn't use enough energy to make it detrimental.
ReplyDeleteYou mentioned in your question about whether or not this suggests that males are supposed to have multiple partners. I don't believe the amount of sperm produced signifies that males are supposed to have more sex because females have more than enough eggs to have the same amount of sex. I believe that the GOAL of life (to pass along one's genes (survive and reproduce)) suggests that males that dont carry their young should have multiple partners. The most efficient way to make sure your genes are passed along is to attempt to reproduce with as many other viable organisms of the same species, but opposite sex. The more eggs you fertilize, the more genes you pass along to the more offspring. This means success.
Why evolution is true, Jerry Coyne
http://en.wikipedia.org/wiki/Evolution_of_sex#Origin_of_sexual_reproduction
http://en.wikipedia.org/wiki/Polyspermy