Biology Basics: Relationships of Organisms

Biologists draw phylogenetic trees to show the relationships of organisms within a group, much like you’d draw a family tree to show the relationships among your parents, grandparents, and other family members.

The more characteristics two organisms have in common with each other, the more closely related they are. Scientists call the characteristics that organisms have in common shared characteristics.

Although you probably know how your family members are related to one another, biologists have to use clues to figure out the relationships among living things. The types of clues they use to figure out relationships include

  • Physical structures: The structures that biologists use for comparison may be large, like feathers, or very small, like a cell wall. Biologists can examine physical structures on living organisms and in the fossil record. Structures may be similar for two reasons:

    • Homologous structures are similar because they evolved from structures in a common ancestor. A human hand and a bat wing are homologous structures because they have all the same bones and clearly evolved from the same ancestral structure.

    • Analogous structures are similar because they evolved to serve a similar function, but they don’t reflect an evolutionary relationship. A fish fin and a dolphin fin are analogous structures. Both evolved for swimming through water, but they didn’t evolve from a common ancestral structure.

      Analogous structures aren’t useful for constructing phylogenetic trees because they don’t reflect evolutionary relationships.

  • Chemical components: Some organisms produce unique chemicals. The ability to make certain chemicals requires specialized enzymes. When certain enzymes and chemicals exist in some organisms but not in others, this suggests that the organisms are evolutionarily related.

  • Genetic information: An organism’s genetic code determines its traits, so by reading the genetic code in DNA, biologists can go right to the source of differences among species. Biologists often compare sequences of the genes for ribosomal RNA because these genes haven’t changed much over evolutionary time (see the following figure) and because all cells have ribosomal RNA.

    Besides exact sequences, biologists also compare the G-C ratio, which is the percentage of DNA that’s made of G-C base pairs (as opposed to A-T base pairs).

    A phylogenetic tree of life on Earth based on rRNA genes.
    A phylogenetic tree of life on Earth based on rRNA genes.

Test yourself with this practice question: Examine the following chart of characteristics for three different organisms. Which two organisms are most closely related to each other based on the number of shared characteristics?

Organism A Organism B Organism C
Cells have a cell wall? Yes Yes No
Cells have a nucleus? No Yes Yes
G-C ratio in DNA 72% 36% 25%

The following is the answer to the practice question: The answer is that organisms B and C are most closely related.

Although organism C doesn’t have a cell wall, it matches organism B for two out of three characteristics. Organisms A and B only match for one characteristic, and organisms A and C don’t match at all.

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