Classification
The animal kingdom is divided first into Phyla (singular Phylum).
Then phyla are subdivided into Classes, classes into Orders,
orders into Families, families into Genera (singular Genus),
and genera into species. Other subdivisions are also sometimes used which
are self-explanatory, for example, sub-classes, and sub-species (sometimes called races).
Let us see how the human species fits into this classificatory scheme (other subdivisions used given in brackets)
Phylum: Chordata
(Sub-phylum: Vertebrata)
(Super-class: Gnathostomata)
Class: Mammalia
(Sub-class: Theria)
(Infra-class: Eutheria)
(Cohort: Unguiculata)
Order Primates
(Sub-order: Anthropoidea)
(Superfamily: Hominoidea)
Family: Hominidae
Genus: Homo
Species: sapiens.
So the human species is Homo sapiens.
All species have a name in two parts like man.
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Evolution
Evolution operates on the genetic make up of the organism, that is on the inherited factors called genes.
Each organism has thousands of genes. Together they make up the genotype of the organism. In each individual, each gene is present twice in its body cells. Now a gene exists in alternate forms, called
alleles, a1, a2, etc. So for example, for one gene X, one individual of a species may have a1,a1, another individual a1,a2, a third individual a2,a3, etc. Of course, in a whole population, there may be many individuals with say a1,a2 or any other combination. But I think you can see that since there are thousands of genes in any species, every individual is likely to have a unique genotype.
Now with any organism in any particular environment, some genotypes are likely to better adapt individuals to that environment than some other genotypes. Individuals with the more advantageous genotypes are more likely to leave descendants than other individuals. So gradually, over a long period of time, the genotypic compostion of the species changes. This process is called natural selection. These genotypic changes may affect internal physiological processes, behaviour, or the appearance of the organism. This is evolution.
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