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(b) Similarity to human DNA sequences

(b) Similarity to human DNA sequences

African monkey 96.66%

African monkey 96.66%

Human

100%

Gorilla 98.90%

Chimpanzee 99.01%

Human

100%

Gorilla 98.90%

Chimpanzee 99.01%

Figure 8.10 Similar organisms have similar DNA sequences.

(a) The order of DNA bases contains information about the traits of an organism. We can compare the DNA of different organisms by looking at similarities and differences in the DNA sequences for various genes. (b) Species that appear to be more similar to humans have more similar DNA sequences for the same genes compared to species that are less similar.

to a few thousand bases—this sequence contains information about the structure of a protein, the physical product of this stored information.

Many genes are found in nearly all living organisms. For instance, genes that code for the proteins that help neatly store DNA inside cells are found in algae, fungi, fruit flies, humans, and all other organisms that contain linear chromosomes. Among organisms that share many aspects of structure and function, such as mammals, many genes are shared. However, the sequences of these genes are not identical. If we compare the sequence of DNA bases in the same gene found in two different mammals, we find that the more similar their classification, the more similar their genes are (Figure 8.10a). In other words, if classification indicates that two mammals share a recent common ancestor, their DNA sequences are more similar than two mammals that share a more ancient common ancestor.

A comparison of the sequences of dozens of genes that are found in humans and other primates demonstrates this pattern (Figure 8.10b). The DNA sequences of these genes in humans and chimpanzees are 99.01% similar, while humans and gorillas are identical over 98.9% of their length. More distantly related primates are less similar to humans in DNA sequence. This pattern of similarity in DNA sequence exactly matches the biological relationships implied by physical similarity.

At first, this result may not seem especially surprising. If genes are like instructions, you would expect the instructions for building a human and a chimpanzee to be more similar than the instructions to build a human and a monkey. After all, humans and chimpanzees have many more similarities than humans and monkeys. However, remember that the genes being compared perform the same function in all of these species. For example, one of genes in this DNA analysis is BRCA1, a gene associated in humans with an increased risk of breast

Figure 8.11 DNA sequences reflect evolutionary relationships. DNA sequences evolve over time. Species that share more recent common ancestors have undergone less evolution separately than species that share more distant common ancestors, and thus have DNA that is more similar.

Media Activity 8.3 Using DNA to Determine Evolutionary Relationships

3 differences from human

3 differences from human

Figure 8.11 DNA sequences reflect evolutionary relationships. DNA sequences evolve over time. Species that share more recent common ancestors have undergone less evolution separately than species that share more distant common ancestors, and thus have DNA that is more similar.

Ancestral sequence

and ovarian cancer, but which has the general function of helping repair damage to DNA in all organisms. Given this very basic function, there is no reason to expect that differences in the gene sequences should conform to any particular pattern—if organisms show no biological relationship. But the BRCA1 gene of humans is more like the BRCA1 gene of chimpanzees than the BRCA1 gene of monkeys. The best explanation for this observation is that humans and chimpanzees share a more recent common ancestor with each other than either species does with monkeys (Figure 8.11).

The differences in DNA sequence between humans and chimpanzees also allow us to estimate when these two species diverged from their common ancestor. This estimate is based on a molecular clock that has been derived from observations of DNA sequence differences in a number of species groups. The principle behind a molecular clock is that the rate of change in DNA sequences, due to the accumulation of mutations within a species, seems to be relatively constant. According to the molecular clock hypothesis, the amount of time required to generate a 0.99% difference in overall DNA sequence (the difference between modern humans and modern chimpanzees) is approximately five to six million years.

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