42
Short answer
Shedding or reabsorbing the endometrial lining is energetically advantageous to the female.The advantage of shedding over re-absorption may be that sperm-born pathogens are removed from the uterus. A more parsimonious explanation, however, is that the endometrium in primates has developed into too large of a structure to be completely reabsorbed ...
42
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, and individuals that can produce both types of gametes are called ...
23
One must always be careful not to stretch an analogy further than it can withstand, but since you started these analogies, I will follow up on them and explain the small mistake you've done in their representations.
The two books are not as different as you have in mind. You are not pasting the beginning of Uncle Tom's Cabin with the ending of Harry Potter ...
18
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 ...
17
Yes the males of a group of bony fish Syngnathidae can bear offsprings or as you say produce offsprings. This group includes Seahorses and Pipefishes. In some species the male have either a brood pouch attached to the trunk or its tail while in others the egg is attached to the trunk or tail laid by the female.The fertilisation takes place in the pouch or in ...
16
Homologous DNA recombination does not swap parts randomly in the sense that any two bits of DNA can swap places. DNA recombination is random in the sense that it may or may not occur. In fact DNA homologous recombination is highly specific and its specificity is what it makes a great tool to study genetics in organisms like yeast by swapping a gene of ...
12
You're right that the mutation must be in a germ cell in order to be passed on. Most errors are introduced during DNA replication (at a rate of around 10-10), which occurs a number of times between the zygote stage and mature gametes. This book estimates that there are 24 divisions between zygote and egg and 23n+34 divisions between zygote and sperm, where n ...
10
First Antonio Pardo is not a biologist, he is a professor of bioethics and his degrees are in medicine and philosophy.
Second, Clearly he does not know anything about primates, since there is one famous great ape that has sex even more often than humans do, the Bonobo. Bonobo have sex for everything, hello, goodby, conflict resolution, social bonding, ect. ...
8
Almost all mammals (including mice and humans) have two sexes where the males have a Y chromosome and an X chromosome (whereas females have two X and no Y chromosomes). This is not the only way organisms can determine sexes, but it's the way mammals evolved to do it.
So how does having a Y cause an individual to be male? The Y has only a few genes (less ...
8
As you indicate in your question, the average age of sexual maturity is probably the best way to approach this, since immaturity is usually how juveniles are defined. Age of puberty is also different in boys and girls (the same goes for many animals), and has also decreased in the 21 century. However, as an historical average for humans 15 years is probably ...
7
Assuming that you have studied megasporogenesis and microsporogenesis. To produce a seed, you require the production of pollen(n) and egg(n) and their fusion.
Let's start with pollen grain(n):
4 pollen grains are produced after 1 meiotic division in the pollen sacs.
$$4~\text{pollen grains} = 1~\text{meiotic division}$$
To produce 1 pollen grain.
$$ ...
7
I can only offer a partial answer on the theoretical aspects. I don't know if you are familiar with the mid-90s papers by Otto et al. (Otto & Goldstein,1992, Otto & Marks, 1996), but these are definately relevant to your question. They deal with the "masking hypothesis" of diploidy, i.e. that deleterious mutations can be masked by "healthy" alleles, ...
7
The arithmetic of human sperm
A young, healthy man produces about 1000 sperms every second, which comes to about 90 million per day [1]. These sperms are stored in the epididymis and ductus deferens until ejaculation. With several days of storage, the number can easily become big enough to allow for multiple ejaculations on the same day.
The maturation ...
6
The short answer is that "different species" are defined as "not being able to produce viable offspring". So the answer to your question is that it's a tautology; it's that way because that's how it's defined. You're asking a question about words, not about biology. If you were able to persuade people that a "species" was "anything living in the same area"...
6
So, i want to know if the sex of the resulting clone is determined by
the sex of the donor animal.
Generally yes for mammals and birds. In animals where the sex is determined by sex chromosomes, the sex of the clone will be dependent by the sex of the donor.
In animals (ie turtles and crocodiles) where the sex is determined by external factors (ie ...
6
So I've dug a bit into how a 'normal morphological' form is defined and characterised, since assessing morphology into 'normal' and 'not normal' is always a very tricky thing.
Looking into the method section of the source you cited, we get:
All laboratories generating the data analysed here used standardized
methods for semen analysis [...] The ...
6
The third photo gets to the heart of the matter. Here's an enlargement:
The gold-colored areas are the anthers of some of the stamens, the filaments of which are fused with the tissue of the petal. (This is typical of the Apocynaceae, the family to which A. blanchetti belongs; they have epipetalous stamens, that is the stamens are "adnate" i.e. ...
5
Gerardo is correct that female designates the bigger gamete, so there's nothing important about the terms "male" and "female" here. But I'd like to also respond to your initial question of how the difference in size would've evolved because I don't think it's trivial.
I can provide a rough summary of one explanation that was summarized in Dawkin's The ...
5
Many disease/abnormality causing alleles of a genes are maintained in populations through their recessive nature. That is to say, in order for the disease to manifest itself in offspring they must in have TWO copies of the disease causing allele - they must be homozygous for the disease causing allele. Inbreeding leads to a far greater chance of being ...
5
Apparently they are
Technically speaking, the ovum is not shed because it does not exist unless and until fertilization occurs. Development leading up to the ovum otherwise stops before the final division of meiosis.
Now, you can ask why the body doesn't reabsorb oocytes, but then the answer is that it does, in a process called follicular atresia, with one ...
4
Only a matter of definition
It is only a matter of definition. When it comes to sex, there is such a diversity of mating system that it is very hard to make a good definition that won't be challenged by limit cases.
Sexual reproduction can typically be defined based on the presence of meiosis (but again this will lead to limit-cases), on the need for two ...
4
Meiosis does not determine sexual form. Eukaryotes use meiosis and fertilization to recombine genes to form new combinations. Meiosis does produce haploid cells from diploid cells, but that has nothing much to do with the sexual forms involved.
In the case of the algal genus Chara, the organism's life cycle is entirely haploid except for the single-...
4
For producing $x$ number of seeds (or say zygote) $x$ number of egg cells must fuse with $x$ number of male gametes.
In angiosperms, 1 meiotic division of Megaspore mother cell leads to formation of one egg cell.
So $x$ egg cells are formed from $x$ number of meiotic division.
But 1 meiotic division of Sperm mother cell (2n) leads to formation off 4 ...
4
Let's break it down: The chance that two individuals of distinct gender meet at one location in space time is tremendously higher than for three. We could do some math here (assuming biased random walks of points in three classes on a 2d surface etc.) but that is not necessary I guess. As far as we know sexual reproduction evolved long before complex sensory ...
4
Probably not! There are certainly some physical reasons to doubt it. Looks like the mass difference between X and Y sperm is on the order of 0.1 picograms: X chromosome "weight"?
First, mass differences are not likely to affect swimming and motility of sperm, because sperm are at low Reynolds number -- the forces of fluid drag vastly overwhelm ...
4
Well... since I can't delete this accepted answer... it's going to be a reverse ferret, to some extent. One 2014 review by Ram and Hadany lists a fair number of SIM occurrences outside of bacteria:
Stress-induced mutagenesis (SIM)—the increase of mutation rates in stressed or maladapted individuals—has been demonstrated in several species, including both ...
4
Menstruation is not the shedding of the ovum, it's the shedding of the uterine lining, which prepares for fertilization even if fertilization does not occur. If there is no implantation of a fertilized ovum, the prepared lining is shed. That an ovum might be included in this shedding event is completely inconsequential (a human ovum is about 0.1 mm in ...
3
Quite simply two mates is simpler both functionally and logistically. It is basic statistics you are far more likely to find 1 individual from 50% of the population once, than 2 individuals each representing 33% of the population at the same time. That's three factors making two sexes statistically easier. Also most sexual organisms do not have chromosome ...
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