The fertilization it is the mechanism through which two gametes fuse to give rise to a zygote or egg cell. At the end of this process, said cell will become a new organism or individual..
In general, it can be said that this process is similar in all living beings. However, there may be some differences depending on the organisms that perform it..
For this reason, several types of fertilization can be distinguished: cross-fertilization and self-fertilization. However, there are also two types of fertilization depending on the environment in which it takes place, which is also explained below:
Cross fertilization is the most common modality among living beings. It consists of the fusion of two gametes that come from genetically different individuals.
The goal is to create a zygote, which will have genetic information from both parents. This is the type of fertilization typical of animals, but it is not exclusive to them. Cross-fertilization is also usually common in both vegetables and plants.
In this sense, the gametes that participate in the fusion can be the same or different. And depending on this factor, two types of cross-fertilization are established: isogamic and anisogamic..
This kind of fertilization is produced when the gametes involved, both male and female, are morphologically equal. This means that they have the same size as well as an external shape and an identical physiology..
In this case it is impossible to differentiate the gametes as female or male. Isogamic fertilization occurs in algae, some fungi and protozoa.
On the other hand, anisogamy fertilization occurs when both gametes are different, either in size and / or behavior. In general, the male gamete is the smallest and is the one that moves to meet the female.
This process is the one that usually occurs in multicellular organisms such as higher plants and animals. This type of fertilization together with the previous one are part of the five types of sexual fertilization.
Self-fertilization is characterized by the fact that the two gametes that merge come from the same individual. This is possible because there are hermaphroditic organisms, that is, they have both male and female organs..
In this case, the male gametes are formed in the male organs and later found in the female organ. This is how what is known as self-fertilization occurs.
From the point of view of the evolution of the species, this mechanism presents a problem. And it is that since there are no exchanges of genes with another organism, the process does not produce genetic variability in the descendants, which means that there will not be a significant change between the following generations and therefore the species will not be able to evolve.
In the animal kingdom two types of fertilization are distinguished depending on the place where the fusion of the gametes takes place. It is about internal fertilization and external fertilization.
This fertilization carried out by all land animals occurs within the organs of one of the parents, which is anatomically prepared for this process..
In most species the process is the same, that is, the male introduces his sperm into the female's reproductive system through copulation. During this process the sperm will try to find the ovum, and for this they will have to move towards the interior of the oviducts.
It is worth noting that the movement of the sperm must be carried out quickly since the survival of the ovum is limited..
In the case of mammals, for example, the ovum is capable of surviving up to one day after ovulation. And when it comes to sperm, they have a survival time of no more than a few hours. In the specific case of humans, it does not exceed three days.
Internal fertilization indicates that the fusion of the gametes occurs inside the body of one of the parents, usually the female. This fertilization is usually the one used by animals that have adapted to the terrestrial environment.
In his particular case, the lack of water would affect the gametes and, furthermore, they would be dangerously exposed to environmental conditions..
A remarkable point is that this type of fertilization has a greater probability of reproductive success. During this process there is considerable energy saving, since fewer gametes are produced.
Normally, when it comes to internal fertilization, the survival of the gametes is greater. This is so because the zygote is protected within the body of the parent..
External fertilization is that which occurs in the aquatic environment. Its process is different from the previous type because in this case there is no direct contact between the parents.
In this mechanism, couples, that is, the male and the female, release their eggs and sperm into the water simultaneously. In this way, it is there that the fusion between the gametes occurs..
In general, the liquid medium turns out to be quite favorable for the subsistence of the gametes and especially for the movement of the spermatozoa. In this case the release times of the gametes must coincide.
This is because their life span is very short. However, this is a problem that organisms solve by putting into practice certain patterns of behavior known as sexual courtship..
On the other hand, it is important to bear in mind that gametes are usually susceptible to changes in temperature, pH2 and predators.
This means that their survival rate is not very high. Therefore, it is for this reason that animals that use external fertilization release a large number of gametes at the time of reproduction. This behavior increases their chances of subsistence.
Broadly speaking, this is the type of fertilization typical of aquatic invertebrate animals, as well as fish.
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