What makes a good character when determining evolutionary relationships between organisms?
The evolutionary history of a group of organisms is called phylogeny (Campbell, Reece et al, 2008). Phylogenetics is important as they interpret how life has progressed and changed throughout history, helping us understand relationships between both living and extinct species. There are several ways to distinguish between evolutionary characteristics; two of these methods are the biological and morphological species concepts. The biological species concept divides a population up in accordance with a species being a group of individuals who can reproduce together to give fertile offspring, yet are unable to do the same outside of that group (Campbell, Reece et al, 2008). The morphological species concept divides a population up into species by visible characteristics, for example, the number of legs an organism has (Campbell, Reece et al, 2008) A character is an observable feature, which can be passed on to ancestors. In terms of morphology, there are two different groups of characters by which you can classify organisms; quantitative and qualitative. This essay approaches the subject by using the morphological species concept.
Quantitative characteristics are those which can be measured (Campbell, Reece etc al, 2008), for example, the number of legs an organism possesses, also including characteristics like fur, wings and scales. These types of characteristics are good to distinguish evolutionary relationships because there is no discrepancy whether an organism has 2 legs or not. An example of this is brain size/volume in human evolution, below is a graph showing this.
Figure 1. The age of different human ancestors and their expected brain volume
This gives measurable characteristics to show evolutionary relationships. In general, the younger the species, the larger the brain, showing that brain size increases the more related to Homo sapiens the species is. Despite this there are anomalies, for example, the Paranthropus boisei, which has the largest brain volume of around 1600 cm3, yet existed about 3.5 MYA. Also the Pan troglodyte is 0 MYA yet has a very small brain volume of about 500 cm3, which suggests that brain volume may not be a good characteristic to define evolutionary links between species. An equation can be used which would prevent anomalies; the encephalisation quotient. An example can be used where a sample of male and female brain and body mass are recorded.
Body Mass (kg) Brain Mass (g) Brain (% body) Female 57 1475 2.6 Male 73 1639 2.3
Table 1. Showing the differences in brain and body mass between females and males
The problem with this data is that the values vary considerably between the genders, causing sexual dimorphism; the species is split by gender. This shows brain size alone is not a good character to differentiate between species; hence we can use the encephalisation quotient.
EQ= Actual brain massExpected brain mass
Equation 1. The Encephalisation quotient
The expected brain mass can be calculated using:
Expected brain mass =11.22 body mass0.76
Equation 2. Expected brain mass
Using the above equations and data, we can calculate the encephalisation quotients for males and females.
Female 6.1 Male 5.7
Table 2 showing the encephalisation quotient for a sample of females and males
The Enchephalisation quotient is a better characteristic for determining evolutionary relationships compared to brain mass because the females and males are not split into two species as their values are close enough together.
Evaluation of Quantitative Characteristics
There are several advantages for using quantitative characteristics, one being that distinguishing between species is fairly simple because the characteristics are unambiguous. This gives rise to a problem; some species may be indistinguishable from others so it becomes difficult to differentiate between them. An example is the Texas coral snake (Micrurus tener) and the scarlet king snake (Lampropeltis triangulum elapsoides )(Campbell, Reece et al, 2008).
This situation can be reversed as members of the same species can, morphologically, appear different. Another problem is that when approaching evolutionary relationships from a morphological aspect, some characteristics cannot be seen from fossil material, such as eye colour or hair. For example, when using the encephalisation quotient we cannot calculate it for fossil material as we do not know their actual brain mass, thus we cannot create Phylogenetic trees mapping the evolutionary history by using this method.
Qualitative characteristics are those such as longer than... or more than... .This category of characteristics can be quite inaccurate; hence we can create ratios of different measurements to make the characteristic more specific. An example is the humero-femoral index (HFI).This is a measure which shows how a certain species moves; bipedally or quadropedally.
humerus length100femur length
Equation 3. The Humero-Femoral index
This equation can be used to differentiate between species within human evolution, below is a graph showing their different HFIs.
Figure 4. Humero-femoral index values for different species
The table shows that the species we know or predict to walk bipedally have a HFI lower than 80. From human evolution Phylogenetic trees we know that the Homo neanderthalensis is more closely related to the Homo sapien than the Australopithecus afarensis is (Campbell, Reece et al, 2008). This relationship can be seen in fig.4; the HFI of the H.neanderthalensis is closer to the H.sapien than A. afarensis is. This shows that the HFI'S is a good indicator of evolutionary relationships and supports how closely related different species are. The orang-utan has the highest HFI's, which supports its quadropedal locomotion. The values of the gorilla and chimpanzee are similar to the species which walk bipedally, showing that they are more closely related to humans than the orang-utan.
Evaluation of Qualitative characteristics
Qualitative characteristics are good for defining evolutionary relationships as they allow us to utilise non-specific characteristics. Yet this lack of specificity is a problem; it is difficult to distinguish between different species. This problem can be solved by creating ratios of these qualitative characteristics, but this causes another problem; by creating a ratio and definite values the characteristic becomes quantitative.
Conclusion; Quantitative or Qualitative?
Both forms of characteristics have their pros and cons yet, it is apparent from the evidence that qualitative characteristics are harder to use when distinguishing between species as they are not definite characteristics. When defining evolutionary relationships, qualitative characteristic can be converted into a value which is easier to use, making it a quantitative characteristic. Quantitative characteristics appear to be better at differentiating between species and comparing the relationships because the values are fixed and easily comparable with each other. They allow us to develop a clear understanding of how extinct species are related to living species, in a non-complex way. However, qualitative characteristics are useful when there are few quantitative characteristics that are shared by organisms.
Campbell, N. Reece, J. Urry, L. Cain, M. Wasserman, S. Minorsky, P. Jackson, R. (2008) Biology eighth addition, San Francisco: Pearson Education.