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70

This is a interesting question and for a long time it was thought that they do not age. In the meantime there are some new papers which say that bacteria do indeed age. Aging can be defined as the accumulation of non-genetic damages (for example oxidative damage to proteins) over time. If too much of these damages are accumulated, the cell will eventually ...


27

You have clearly given this a lot of thought. Unfortunately, as @adam.r said, you are laboring under certain misapprehensions. The quick answer is that each generation does not "improve" on the last. That is a common misconception. In a bit more detail: First of all, your copying metaphor is a bad one. There was no "perfect original", I expand on this ...


21

Sexes (male and female) are generally defined in terms of Anisogamy, which means that there are size differences between the gametes (i.e. the reproductive cells that fuse at fertilization). The sex with smaller gametes is defined as male and the sex with larger gametes is defined as female (individuals that can produce both types of gametes are called ...


19

When dealing with humans, there are only two Biological genders as defined by the presence or absence of the Y-Chromosome. If the Y-Chromosome is not present, or through some process gets totally deactivated, the human will appear and function as a Female. XX = Female XY = Male XXY = Male (Klinefelter's Syndrome) XYY = Male (Aneuploidy - Normal ...


19

Firstly I'll clarify that you are talking about simultaneous hermaphrodites rather than sequential hermaphrodites (1st one sex, then the other e.g. the limpet Patella vulgata). It is perhaps easiest to address the question by countering it and asking why dioecy (2 sex systems/2 gonochoric types e.g. male and female) is better? As you have pointed out there ...


17

I think you might be confusing sex and gender. The terms are often used interchangeably, but strictly speaking, they have different biological meanings. Sex refers to the biological categorization based on genetics, reproductive organs, or similar things, whereas gender is based on social identity. For humans, there are only two sex chromosomes, X and Y, ...


15

The Fisher's principle is not applicable to the fetuses because it has been formulated for parental expenditure and basically states that the ratio of male to female parents (implying that both parties have reached the age of fertility) will tend to 1:1. There are several mechanisms that we can use and that are mentioned in the canonical paper by James ...


14

Having descended testes is a derived characteristic within mammals; monotremes and the Afrotheria (including elephants) all retain the ancestral character state (Kleisner, et al., 2010)2. Among those mammals with descended testes, these can be ascrotal or scrotal. Testicular descent is hypothesized to have only occurred once within Mammalia, with the ...


14

Rice and Salt$^1$ bred fruit flies for 35 generations and from one line of flies created two groups that were isolated from each other reproductively. They could not interbreed because they no longer bred in the same environment. Depending on one's definition of 'species' this could be a case of artificial speciation. $^1$Rice WR, Salt GW (1988), Speciation ...


14

Change in genetic variance From what I have been taught, Natural Selection (or even Artificial Selection) is great for panning favorable genes from a species and bringing them to the fore, however, it does not introduce new genetic changes. Yes, you are pretty much right. In a given population, directional selection will ultimately reduce genetic ...


13

Interfamilial hybrids have never, to my knowledge, been recorded occurring naturally (without human intervention). In plants, somatic inter-familial hybrids have been produced for a wide variety of species pairs in the lab (e.g. between carrot and barley; Kisaka et al. 1997). In animals, there are some historical reports of hybrids between chickens ...


12

The need for birth control has been a concept for thousands of years and remains an important issue for men and women today, the need for contraception helped humans to achieve control over our population and healthy sexual relationships without having the fear of pregnancy. The medieval contraception methods mostly relies on plant based oral ...


12

Short answer People with androgen insensitivity syndrome do not have a functional uterus and cannot bear a child. Background Androgen-insensitive genetic males may develop female genitalia and internal female reproductive organs. However, in both partial androgen insensitivity syndrome (PAIS) and complete androgen insensitivity syndrome (CAIS) the uterus is ...


11

I thought that the only difference between the male and female reproductive cell's DNA code is the X-Y chromosome that determines gender, not differences that can define the species of the offspring. Why does this happen? Well that is not the only difference, there are certain characters that are only transmitted by one of the parents. First of all you ...


11

Sperm are cheaper than eggs According to Bateman's Principle the males of a species are (typically) more able to produce a greater variance in the number of offspring. This is because generally the males can produce many gametes and achieve many matings whereas females are limited by the number of eggs, resources, and time lost to pregnancy. Therefore, ...


11

So most cells require growth factors and survival factors to stay alive in an organism. Cells which stop receiving survival factors, such as sperm, undergo a process of programmed cell death (PCD) called apoptosis. Some cells will also undergo necrosis, or autophagy, among other types of PCD. Lachaud et al. performed a study in 2004 on the contribution of ...


10

The blood of the fetus does not mix with the blood of the mother. Instead, the placenta provides a system where the two separate blood streams flow past each other with thin separation allowing nutrients to flow between the two streams but the not the blood cells and other large components.


9

A common problem for any woman who has experienced pregnancy and childbirth is damage to the levator ani muscle, which comprises a portion of the "pelvic floor." from here This can cause a number of symptoms, including urinary incontinence (due to an increase in pressure on the bladder from less effective muscular support), and possible prolapse of the ...


9

The biological environment in every cell and tissue of your body is an extremely complex, tightly controlled system. There are tens of thousands of genes in a typical eukaryotic genome, and depending on tissue types and environmental conditions, these genes are turned on and off in a very controlled manner. There is regulation at every point in the flow of ...


9

The concept of a human/chimp hybrid is called a humanzee. No humanzees have ever been recorded despite the fact that sexual intercourse between humans and chimps has been recorded on several occasions. This suggests that male-human/female-chimpanzee is not capable of producing viable offspring, at the very least. As to why, several reasons will contribute: ...


9

I’ll add a slightly different perspective, although terdon’s answer already contains the relevant facts. The thing that makes DNA endure in the face of imperfect copying is that, like computer storage, it’s digital. The relevant property of digital data here is that individual pieces of information aren’t given on a scale, they’re drawn from a strongly ...


9

The frequency rises with maternal age due to a peculiarity of meoisis in female mammals. Meiosis is originated in the fetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested ...


9

The maternal and fetal blood circulation systems are completely separate. The embryo's blood cells start developing at around week 5 gestational age (3 weeks after conception), the same way any other tissue is developed by the fetus itself. By around week 7 gestational age, a circulatory system has developed and the heart has started beating. All nutrition ...


8

Daniel's answer excellently explains the issue in an understandable fashion, though you may have been looking for a more cell biological approach. According to Immunological aspects of sperm receptors on the zona pellucida of mammalian eggs, 1976, Dunbar & Shivers, the egg cell has a receptor construction which is necessary for the sperm to attach. If ...


8

The following paper(1) studies the trends in male to female ratios among newborns over the period 1950-1994 in 29 countries (20 major European countries, USA, Canada, Japan, and others). In all countries at any time point, the male to female ratio was always higher than one. In some countries (Mexico and a few northern eastern European countries) the male ...


8

There is a good open access review about the effects of different hormones on sexual arousal. In most instances, the hormone is acting on the brain, which signals the information to gonads and genitals via nerves (i.e., the hypothalamic-pituitary-gonadal axis). However, in a study in which I have participated, we found a more direct and probably ancient ...


8

I want to know can +ve and -ve blood group of a couple could be a cause of miscarriage in pregnancy? Yes. In extreme cases, it can. You are talking about Rh Incompatibility, and can become a big issue when the mother has Rh(-) blood and the father has Rh(+) blood (such as the case with your cousin and his wife). What could have happened (but definitely ...


8

When you say, why don't we see more sexually reproducing species than asexual, I presume you are referring to bacteria, protists, archaea, some fungi, etc. Most multicellular organisims do reproduce sexually, which makes sense since an organism would need to be multicellular to have cells specialized for gametes. As you stated, the advantage is that sexual ...


8

It is called Semelparity. From wikipedia: Semelparity and iteroparity refer to the reproductive strategy of an organism. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. You see semelparity as ...



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