Because there has been so much discussion lately about species and hybrids, I thought I would start a thread in which we can these topics the attention they deserve. So now, I am going to start by defining each of these terms, and explaining their significance.
1) Species
As the lowest standard unit in taxonomy, species represents a fundamental unit in biology. This term may be defined in various ways, but two of the most common are the biological/reproductive species concept, and the phylogenetic/cladistic species concept.
A. Biological/reproductive species concept - This is an old but very useful definition of species. Simply put, animals in the same species are those that can successfully reproduce with each other, in other words, those that can produce offspring that are both viable (health) and fertile (capable of reproducing themselves). One qualifier given may be that only animals that would reproduce under their own initiative (without human intervention) may be considered members of the same species.
B. Phylogenetic/cladistic species concept - In this somewhat newer approach, animals may be considered as members of the same species if their genetic makeup is more similar than a given threshold (often originally based on the biological species concept). One standard often used is 97% similarity in RNA genes. Other genes may call for different levels of similarity.
2) Hybridization
The word "hybrid" may come from the Old English "hy" (half) and "brid" (breed), or literally "half-breed," meaning an individual whose lineage is half one type, half another. The world "hybrid" can technically refer to any taxonomic level, whether within a species (intraspecific) or between species (interspecific). In fact, at the molecular level, we can even speak of DNA hybridization, which is simply allowing two different strands of DNA to bond together. We could also say that two wild animals from different subspecies can hybridize, or that two people from different races can hybridize, or that two domestic animals from different breeds can hybridize. However, we usually refer to such crosses as mixes or intergrades, because the offspring are essentially at no risk of defects. Usually when we use the term "hybrid," we are referring to interspecific hybridization, or the crossing of animals from different species. This is controversial because such animals usually cross under human influence, and the offspring often have reduced viability or fertility.
3) Taxonomy
The next taxonomic level above species is "genus." Congeneric animals have many similarities and share a common ancestor, but have diverged to a significant degree, and no longer usually mate. If they do, the offspring may have defects. The level above genus is "family." Confamilial animals are broadly similar, but usually don't mate, and if they do, the offspring are unlikely to survive. The next level up is "order." Animals in the same order are only distantly related, and completely incapable of hybridizing.
4) Genetics
Genes are DNA sequences that code for proteins, which build and operate our bodies. The location of a gene on DNA is known as a locus. DNA is usually strung together in long molecules, which are tightly coiled into structures called chromosomes during replication. The number of chromosomes is a defining characteristic of a species. When two animals from the same species mate (whether or not they are from the same subspecies, race, or breed), basically all the genes line up, resulting in healthy, normal offspring. If two animals from different species but the same genus mate, not all of the genes line up. Therefore, the resulting offspring might not survive, or if they do, their gonads (testes or ovaries) may be unable to produce sperm or eggs. If two animal from different genera but the same family mate, the offspring may die in the egg or uterus, or if it hatches or is born, it may die as an infant.
In birds and mammals, interspecific hybrids of both sexes may be sterile. However, the heterogametic sex is sterile more often than the homogametic sex. This means that, the sex with two different types of sex chromosome is more likely to be sterile than the one with two of the same sex chromosome. In mammals, males are heterogametic (XY) while females are homogametic (XX). In birds, females are heterogametic (ZW) and males are homogametic (ZZ). This phenomenon is known as Haldane's Rule. It probably exists because the homogametic sex has a "backup" copy of every gene, while the heterogametic sex does not.
Most reptiles have temperature-dependent sex determination (TSD), with no heterogametic sex. In that case, interspecific hybrids could be more likely to be viable and fertile than in birds or mammals.
5) Ethics
So knowing all this, the question remains, "Should we or should we not hybridize animals in captivity?" I would argue that no, we should not hybridze animals, and here's why.
A. Conservation - If you think of the pet hobby as a kind of collective "Noah's Ark" project, then it is our responsibility to keep our animal stocks pure, so that if reintroduction is necessary, captive animals could be used.
B. Behavior - Hybrid animals often have conflicting impulses, because each of their parent species (or even subspecies) exhibits different behaviors. For the sake of the animal, it is better to prevent such inner conflicts from arising.
C. Health - As mentioned above, interspecific hybrids may have compromised health or fertility. For the sake of medical and reproductive health, it is best to allow animals to breed with their own species (and even subspecies).
1) Species
As the lowest standard unit in taxonomy, species represents a fundamental unit in biology. This term may be defined in various ways, but two of the most common are the biological/reproductive species concept, and the phylogenetic/cladistic species concept.
A. Biological/reproductive species concept - This is an old but very useful definition of species. Simply put, animals in the same species are those that can successfully reproduce with each other, in other words, those that can produce offspring that are both viable (health) and fertile (capable of reproducing themselves). One qualifier given may be that only animals that would reproduce under their own initiative (without human intervention) may be considered members of the same species.
B. Phylogenetic/cladistic species concept - In this somewhat newer approach, animals may be considered as members of the same species if their genetic makeup is more similar than a given threshold (often originally based on the biological species concept). One standard often used is 97% similarity in RNA genes. Other genes may call for different levels of similarity.
2) Hybridization
The word "hybrid" may come from the Old English "hy" (half) and "brid" (breed), or literally "half-breed," meaning an individual whose lineage is half one type, half another. The world "hybrid" can technically refer to any taxonomic level, whether within a species (intraspecific) or between species (interspecific). In fact, at the molecular level, we can even speak of DNA hybridization, which is simply allowing two different strands of DNA to bond together. We could also say that two wild animals from different subspecies can hybridize, or that two people from different races can hybridize, or that two domestic animals from different breeds can hybridize. However, we usually refer to such crosses as mixes or intergrades, because the offspring are essentially at no risk of defects. Usually when we use the term "hybrid," we are referring to interspecific hybridization, or the crossing of animals from different species. This is controversial because such animals usually cross under human influence, and the offspring often have reduced viability or fertility.
3) Taxonomy
The next taxonomic level above species is "genus." Congeneric animals have many similarities and share a common ancestor, but have diverged to a significant degree, and no longer usually mate. If they do, the offspring may have defects. The level above genus is "family." Confamilial animals are broadly similar, but usually don't mate, and if they do, the offspring are unlikely to survive. The next level up is "order." Animals in the same order are only distantly related, and completely incapable of hybridizing.
4) Genetics
Genes are DNA sequences that code for proteins, which build and operate our bodies. The location of a gene on DNA is known as a locus. DNA is usually strung together in long molecules, which are tightly coiled into structures called chromosomes during replication. The number of chromosomes is a defining characteristic of a species. When two animals from the same species mate (whether or not they are from the same subspecies, race, or breed), basically all the genes line up, resulting in healthy, normal offspring. If two animals from different species but the same genus mate, not all of the genes line up. Therefore, the resulting offspring might not survive, or if they do, their gonads (testes or ovaries) may be unable to produce sperm or eggs. If two animal from different genera but the same family mate, the offspring may die in the egg or uterus, or if it hatches or is born, it may die as an infant.
In birds and mammals, interspecific hybrids of both sexes may be sterile. However, the heterogametic sex is sterile more often than the homogametic sex. This means that, the sex with two different types of sex chromosome is more likely to be sterile than the one with two of the same sex chromosome. In mammals, males are heterogametic (XY) while females are homogametic (XX). In birds, females are heterogametic (ZW) and males are homogametic (ZZ). This phenomenon is known as Haldane's Rule. It probably exists because the homogametic sex has a "backup" copy of every gene, while the heterogametic sex does not.
Most reptiles have temperature-dependent sex determination (TSD), with no heterogametic sex. In that case, interspecific hybrids could be more likely to be viable and fertile than in birds or mammals.
5) Ethics
So knowing all this, the question remains, "Should we or should we not hybridize animals in captivity?" I would argue that no, we should not hybridze animals, and here's why.
A. Conservation - If you think of the pet hobby as a kind of collective "Noah's Ark" project, then it is our responsibility to keep our animal stocks pure, so that if reintroduction is necessary, captive animals could be used.
B. Behavior - Hybrid animals often have conflicting impulses, because each of their parent species (or even subspecies) exhibits different behaviors. For the sake of the animal, it is better to prevent such inner conflicts from arising.
C. Health - As mentioned above, interspecific hybrids may have compromised health or fertility. For the sake of medical and reproductive health, it is best to allow animals to breed with their own species (and even subspecies).