1. Similarities in overall sequence:
* High percentage of shared nucleotides: Organisms that share a recent common ancestor will have more similar DNA sequences than those that diverged further in the past. This means a higher percentage of their nucleotide bases will be identical.
* Conserved genes: Certain genes are essential for basic cellular functions and are found in almost all living organisms. These genes are likely to be very similar in organisms sharing a common ancestor.
2. Specific patterns of similarity:
* Synonymous vs. nonsynonymous mutations: Mutations can be synonymous (changing a codon without changing the amino acid it codes for) or nonsynonymous (changing the amino acid). Organisms with a more recent common ancestor will tend to have more synonymous mutations, indicating that changes in the DNA sequence are not being actively selected against.
* Insertions and deletions: The insertion or deletion of nucleotides in DNA sequences can provide evidence of shared ancestry. If two organisms have similar insertions and deletions in the same regions of their genome, it suggests that these changes occurred before their lineages diverged.
* Introns: Introns are non-coding regions of DNA that are removed during gene expression. They are often more variable than coding regions, making them useful for studying evolutionary relationships.
3. Evidence of evolutionary processes:
* Phylogenetic trees: By comparing the DNA sequences of many organisms, we can construct phylogenetic trees that depict the evolutionary relationships between them. The closer two organisms are on the tree, the more closely related they are.
* Molecular clocks: Some DNA sequences mutate at a relatively constant rate. By comparing the number of mutations between two organisms, we can estimate how long ago they diverged from a common ancestor.
4. Specific examples:
* Humans and chimpanzees: These two species share over 98% of their DNA sequences. This high level of similarity is evidence of a very recent common ancestor.
* Humans and bacteria: While humans and bacteria share some essential genes, their overall DNA sequences are quite different. This indicates a very distant common ancestor.
Important Note: It's crucial to remember that these are just some of the pieces of evidence that scientists use to determine evolutionary relationships. It is usually a combination of different lines of evidence that provides the strongest support for a hypothesis.
