A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.In nature the same thing applies. Once you get a new variety, a subpopulation that is reproductively isolated from its parent population or other populations of the same species, even the result of a "speciation" event, and especially after it has inbred over some generations, its traits are going to be or become fixed. For a new trait to stick, or continue to characterize the new population, competing alleles for that trait must have been eliminated from that gene pool. THIS IS THE NECESSARY REDUCTION IN GENETIC DIVERSITY THAT ALLOWS A NEW TRAIT TO COME TO CHARACTERIZE A NEW BREED OR POPULATION IN THE WILD, THIS MUST OCCUR FOR A NEW TRAIT TO DEVELOP AND STICK IN A NEW POPULATION.
If they are "true breeding", they will be homozygous for their characteristic traits. This HAS to be true whether the population is the result of natural processes such as natural selection or genetic drift, migration and so on, or domestic breeding decisions.
HOMOZYGOSITY MEANS ONLY ONE ALLELE FOR THE GENE, ALL THE OTHER ALLELES HAVING BEEN ELIMINATED FROM THAT PARTICULAR GENE POOL. THAT'S THE SEVEREST CASE OF DECREASED GENETIC DIVERSITY (except for hemizygosity and nullizygosity, mentioned in the article below) AND IT'S NECESSARY TO GETTING A "TRUE BREED." AND WHAT IS A TRUE BREED BUT A NEW PHENOTYPE OR "SPECIES," A SPECIATION EVENT IN ITSELF, THE SUPPOSED STEP ON THE WAY TO OPEN-ENDED EVOLUTION FROM ONE SPECIES TO ANOTHER ACCORDING TO EVOLUTIONISTS.
Take dogs. If you want a Dachshund you have to eliminate all the alleles that specifically produce Great Danes or Golden Retrievers or Chihuahuas etc. If any of those alleles show up in the Dachshund breeding program you'll get a less perfect Dachshund. They make the breed less than what it is supposed to be.
It works the same way in nature, maybe through Natural Selection but probably more often through random events that simply happen to separate a population into two or more subpopulations. A particularly marked salamander emerges because the other markings are genetically decreased by comparison to those for the new marking. The markings of the last species to develop in a ring species of such salamanders should be genetically homozygous. Same with the genetics underlying the last species in the ring of green warblers and so on. You should find decreased genetic variability and probably a lot of homozygosity, just because this is what evolution DOES.
It's NECESSARY to evolution, and if evolutionists weren't always imagining nonexistent mutations into the mix it ought to be obvious even to THEM. The only mutations that are involved are those that contribute diseases to the mix and interfere with the health of the most genetically reduced populations, even to extinction in some cases.
In nature the introduction of disease elements may simply eliminate a new variety, Natural Selection in operation at its most severe, but if the new variety finds a niche it can adapt to it will survive just as a good domestic breed will.
The new variety necessarily comes through a reduction in genetic diversity. That's how evolution WORKS, really, though such an obvious necessity, that must lead to LESS ability to evolve, is simply ignored by believers in evolution who go on spinning evolution out of imaginary mutations.
To repeat the point: If circumstances are such that the populations remain reproductively separate, meaning without gene flow or the sharing of alleles between them, each will develop its own particular characteristics, and as long as there remains no gene flow or reproductive contact between the populations those characteristics will remain. For them to remain means that the alleles for different characteristics have been eliminated. That's what decreased genetic diversity MEANS. This may amount to actual speciation, but at least certainly at the extremes you do get speciation, where the new characteristics are preserved because there is a complete lack of interbreeding with former populations.
Evolutionists regard bottlenecks as events that interfere with the processes of evolution, but they shouldn't. The elephant seal and the cheetah which were produced by severe bottlenecks -- reduction of their former populations to just a few individuals -- that severely reduced their genetic diversity -- really ought to be considered to be examples of speciation, nature doing what domestic breeders do. Bottlenecks are really just one way new varieties or breeds are brought about in nature or in domestic breeding. ALL the processes of evolution tend in the same direction, genetic drift, migration, natural selection, just not as rapidly. Domestic breeding in the past could be described as the artificial creation of genetic bottlenecks for the purpose of developing desired traits for new breeds. You select the desired character and take pains to breed only with others that possess that character. Since a rigid adherence to this formula also usually brings disease problems into the breed, breeders today take care to avoid the most severe bottleneck methods with the most severely decreased genetic variability by mixing with more vigorous but less desirable animals as far as the target trait is concerned, but if it weren't for the threat of disease, these severe methods would be considered the most reliable way of producing the best breeds. SPECIATION.
Yet here we have RAZD at EvC carrying on as if the evolution processes just go on and on producing new phenotypes or varieties or breeds, even past speciation which he treats as the end point of microevolution and beginning of macroevolution, but afterward the same changes continue without a hitch in his scenario.
What separates (micro) evolution from the macro view of evolution (macroevolution) is the process of speciation, as evolution occurs within the breeding population, and nested hierarchies are formed by speciation events, and macroevolution is just a macro view of what occurs over several generations via evolution and speciation.See, he's simply ASSUMING the open-endedness of evolution, the phenotypic changes just go on and on, a neverending ACCUMULATION of changes. He has no evidence for this, though he has charts that give it an aura of authority that are simply meaningless reflections of his false belief. Actually, they are ILLUSTRATIONS of what he believes, they provide nothing in the way of evidence for any of it. And everything he says is also all assumption without evidence. Dawkins does the same thing with his ridiculous computer models of how evolution works, simply programming in his own bias, his assumption of open-ended changes. Sometimes you'll see an evolutionist acknowledging that reduced genetic diversity can sometimes be a problem but they keep that information off in a separate mental compartment, it's something that occurs only with bottlenecks, in extreme scenarios that interfere with evolution, not with evolutionary processes themselves.
If we look at the continued effects of evolution over many generations, the accumulation of changes from generation to generation may become sufficient for individuals to develop traits that are observably different from the ancestral parent population. This lineal change within species is sometimes called phyletic change in species. This is also sometimes called arbitrary speciation in that the place to draw the line between linearly evolved geneological populations is subjective and because the definition of species in general is tentative and sometimes arbitrary.
Just for the record, here is the most pertinent part of the Wikipedia article on Zygosity:
The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an otherwise diploid organism, and nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing.
A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.
An individual that is homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait. This allele, often called the "dominant allele", is normally represented by a capital letter (such as "P" for the dominant allele producing purple flowers in pea plants). When an organism is homozygous-dominant for a particular trait, the genotype is represented by a doubling of the symbol for that trait, such as "PP".
An individual that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".