What contribute to the variation of protein sequence across different species (e.g. phenotype, environment, evolutionary pressure) and how do those factors contribute to the variation?

I have tried to search using google + google scholar but they generally show papers discussing why and how the protein conserved across species rather than why/how the protein diverged across species. Please help me on this and references/sources are very important and highly appreciated. Thank you!

An example to clarify my question. The gene CD81 is not identical among human, chimp, rat, chicken based on this identity matrix from ClustalW2. Why is CD81 not identical in all species? What factors cause the different in CD81s? How do those factors cause that (e.g. evolutionary pressure)?

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#  Percent Identity  Matrix - created by Clustal2.1
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     1: sp|P60033|CD81_HUMAN    100.00  100.00   93.22   82.55
     2: sp|P60034|CD81_PANTR    100.00  100.00   93.22   82.55
     3: sp|Q62745|CD81_RAT       93.22   93.22  100.00   82.55
     4: tr|F1NW06|F1NW06_CHICK   82.55   82.55   82.55  100.00