Bitter taste evolution
The evolution of bitter taste receptors has been one of the most dynamic evolutionary adaptations to arise in multiple
TAS2R
The bitter taste receptor family, T2R (TAS2R), is encoded on
Members of the T2R family encode alpha subunits of
Toxins as the primary selective force
The primary selective adaptation that arises from bitter taste is to detect poisonous compounds, as most poisonous compounds in nature are bitter. However, this trait is not exclusively positive, as bitter compounds exist in nature that are not poisonous. Exclusive rejection of these compounds would in fact be a negative trait, as it would make it more difficult to find food. Toxic and bitter compounds do, however, exist in different diets at different frequencies.[5] Sensitivities to bitter compounds should follow the requirements of different diets logically, as species that can afford to reject plants due to their low plant diet (carnivores) have a higher sensitivity to bitter compounds than those that exclusively ingest plants. Exposure to the bitter marker quinine hydrochloride supported this fact, as the sensitivities to bitter compounds were highest in carnivores, followed by omnivores, then grazers and browsers.[6] This identifies toxic plants as the primary selective force for bitter taste.
This phenomenon is confirmed with genetic analysis. One measure of positive selection is
TAS2R development in human history
The pseudogenes mentioned earlier are produced by a number of gene silencing events, the rate of which is constant throughout primate species. Several of these pseudogenes maintain a role in modulating taste response, however. By studying the silencing events in humans, it is possible to theorize the selective pressures on humans throughout their evolutionary history. As is the case with the usual distribution of human genetic variation, the highest rate of diversity in TAS2R pseudogenes was often found in African populations. This was not the case with two pseudogene loci: TAS2R6P and TAS2R18P, where the highest diversity was found in non-African populations. This suggests that the functional versions of these genes arose before the human migration out of Africa into an area where selective constraint did not remove non-functional versions of these gene loci. This allowed the pseudogene frequency to increase, creating genetic variance at those loci.[2] This is an example of relaxed environmental constraint allowing silencing mutations to lead to pseudogenization of once important loci.
The gene locus, TAS2R16, also tells a story about bitter taste evolution. Varying rates of positive selection in different areas of the world give an indication of the selective pressures and events in those areas. At this locus, the 172Asn allele is the most common, especially in areas of Eurasia and in pygmy tribes in Africa, where it is nearly fixed. This suggests that the gene has had a relaxed selective constraint in most areas of Africa in comparison to Eurasia. This has been attributed to the increased knowledge of toxic plants in the area that arose around 10,000 years ago. The increased frequency of 172Asn in Eurasia suggests that the migration out of Africa into areas with different climates and foliage rendered the knowledge of toxic plants in Africa useless, forcing the populations to rely once again on the 172Asn allele, causing higher rates of positive selection. The high rate of 172Asn in Pygmy populations is more difficult to explain. The effective population size of these isolated populations is quite small, indicating that genetic drift explained by the founder effect is the cause of these atypically high rates.[8] The different environments that have contained humans have placed different levels of selection on the population, forcing a wide variety in at the TAS2R loci across humanity.
Relaxed constraint
The cause of this relaxed constraint was primarily in lifestyle changes in hominids. Roughly two million years ago, the hominid diet shifted from a primarily vegetarian diet to an increasingly meat-based diet. This led to a reduction in the amount of toxic foods regularly encountered by humanity's early ancestors. Additionally, the use of fire began around 800,000 years ago, which further detoxified food and led to a decreased dependence on TAS2R to detect poisonous food. Evolutionary biologists have theorized how, with fire being an exclusively human tool, relaxed selective constraint has been found in chimpanzees as well. Meat does account for about 15% of the chimpanzee diet, with much of the other 85% being made up of ripe fruits, which very rarely contains toxins. This comes in contrast to other primates whose diets are entirely composed of leaves, unripe fruits, and bark, which have comparatively high levels of toxins.[9] The differences in diets between chimpanzees and other primates accounts for the different levels of selective constraint.