Looking back on why we look forward: A special series by Adam Glover, Part 1

Every living thing springs from some parental genetic information. The genetic information encoded in every individual living organism is a modified version of its “parents’” genetic information, which was a modified version of their “parents’” genetic information, and so on. Natural selection dictates that favorable variations in this genetic information will survive within a population; any trait that increases these organisms’ fitness- how the organisms fit into their environment- and their probability of reproductive success will survive to be passed on in the gene pool (De Queiroz, 2011). Extension of this theory allows us to trace family lineages all the way back to the common ancestor of life on Earth and identify where certain characteristic genetic mutations and the body changes that they encoded first appeared, as well as the plausible forces likely to have contributed to their maintenance. These forces- called selection pressures- can be thought of as forces that direct natural selection; they’re the environmental conditions which affect a population’s fit in their environment, and they can affect specific traits either positively or negatively.  Thus, by studying the selection pressures which a lineage has been exposed to, we begin to better understand the origins of that lineage’s unique and characteristic features.

Primary investigations into the selection pressures which have shaped any lineage rely on three major lines of evidence- fossils, genes, and embryos (Shubin, 2008). After all, neither eyewitnesses nor any written record remain for the events and environments which shaped species long, long ago. The study of fossils helps reveal the morphological similarities that genetic information encodes; the study of embryos can reveal stages of development where organisms exhibit ancestral features; and molecular studies reveal that different species share genetic homologies as well as anatomical ones.  It is by studying when and where these homologies occur that we are provided hints as to what selective pressures may have caused certain genetic variations to have an advantage over others.

While the hypothetical environments conceived by such studies may be novel and provide important new insight about the selection pressures which may have operated on a lineage, the experimental methodology underlying them can lend itself to confirmation biases. When a hypothesis is put forth as to why a certain physical structure would be selected for under certain environmental conditions, and then fossil/DNA/embryological evidence is found to support the hypothesis that such a physical structure did exist in what could have been a similar environment, this could be wrongly interpreted as conclusive evidence that the said structure existed under the said conditions for reasons that confirm their pre-conceived hypotheses; however, the physical evidence of the feature in question, when taken in the context of some separate hypothetical environmental evidence that may eventually be found, may not support the pre-conceived hypotheses at all.

Efforts to improve the evaluation of such adaptive hypotheses have been made, however; fossil, genetic, and developmental evidence is now being combined with evidence from “comparative” studies which not only investigate features of the species in question, but also  similar features in similar organisms. For example, when it was first noted that chimpanzees (Pan troglodytes) had relatively large testicles compared to gorillas (Gorilla gorilla), it was hypothesized that the social organization of chimpanzees (multi-male polygyny) was what led to the selection of increased testicle size, because increased testicle size would provide an adaptive advantage to an individual male competing with other males to reproduce.

To test this, the sexual habits of chimpanzees were compared with those of gorillas (one-male polygyny) and orangutans (Pongo pygmaeus) (solitary, females locate single male during breeding season). If testicle size was not correlated with a female’s average number of male sexual partners in the right way, then this hypothesis would be disproved; testicle size was found to becorrelated with a female’s average number of male sexual partners, however, and such studies exemplify the unique and useful nature of the comparative method (Harcourt et al, 1981).

Much work has been done tracing back the origins of characteristic human features. For example, when looking at the fossil record, one can trace the evolutionary rise of our limbs all the way back to a transitional form of organism that shows intermediate states between an ancestral form of fish and that of its amphibious descendants (Shubin, 2008); this matches perfectly a key prediction of evolutionary theory- that evolution from one trait to another will result in intermediates between the two kinds- and the same follows for studies done on the evolution of the human eye, which this paper will focus on reviewing.