On Tuesday, February 12, students and faculty of the biology department coalesced for a seminar celebrating Darwin Day (which is vehemently regarded as a real holiday among evolutionary biologists at Holy Cross ) and the work of evolutionary biologist Dr. Luis De León. De León, an assistant professor of biology at UMass Boston, is interested in eco-evolutionary dynamics and the anthropogenic impacts that humans wield on the evolution of other species. An alum of both the University of Panama (2002) and McGill University, Canada (2011), where he completed his PhD, De León cited that his curiosity about the mechanisms for promoting biodiversity has remained with him since his early years as a young boy living in a small, Panamanian village.
In his lab at UMass Boston, De León pursues a number of scientific inquiries. Some of these include understanding the means by which speciation events occur, how species adapt to new environments, and how species are maintained in those environments. Beyond this, he is also interested in discerning how humans drive forward the diversification of species in nature. To formulate answers to these scientific queries, De León has done extensive research working with finches in the Galapagos (yes, just like Darwin himself).
To begin his presentation, De León shared a visual displaying the tenets of natural selection to attending students and faculty. He vocalized that, in order for natural selection to take place, some existing environmental variation must be present – one that exerts a selective pressure on a heritable phenotypic trait and alters the extent to which that trait is suitable for the particular environment we are discussing. Following this, there must be some degree of adaptive divergence within the surrounding population. For example, in response to the imposed environmental pressure, selection may favor the survival of individuals displaying one variation of a particular phenotype, and not the other. Those individuals modeling the favored phenotypic characteristic will live to survive and propagate offspring, while those who don’t will not. As a result, the frequency of the expression of this certain phenotype will vary within the population over time, prompting the species to diversify from one another. This continues until the species have become so diverse that they no longer are considered to be members of the same species; hence, a speciation event has occurred.
Following this brief explication, De León transitioned to a discussion about his scientific research. Regarding the Galapagos finches, De León said that variations in food distribution across the islands of the Galapagos have largely contributed to the diversification of the finches over time. He noted that disparities in seed type on each of the islands caused finches to develop an adaptive preference for the food distribution niche particular to their environment. For example, on islands where most of the available seed is small, selection favors the propagation of individuals displaying smaller-sized beaks (which confers a greater advantage to the finches than having a larger beak would, with respect to food collection). On a second island where there are only large seeds, the opposite is true and selection would promote the proliferation of the allele for larger-sized beaks. Over time, as a result of the adaptive divergence of the ancestral finch population, multiple speciation events occurred as members of the population became distinguishable from one another. In his talk, De León referred to this as the process of adaptive radiation, wherein multiple diversification and speciation events precipitate from an ancestral population that has become geographically isolated.
De León next delved into some of the details of his experimental work on El Garrapatero in the Galapagos. At this site, De León and his team are concerned with measuring the variation in beak morphology across the island. To do this, they use a fishing-style net (one that is markedly soft and safe for use) to catch finches for data collection. Once retrieved from the net, they are tagged using a color-coded metal band and measured for beak size, length, and width. Tabulating the data in the form of a graph, De León and his team assess the extent to which selection accounts for the observed variation in beak morphology. According to the data (which De León has repeatedly tried and collected over the course of ten years), small ground and large ground finches (those with small and large beaks) are prolific (have a high probability of living to survive and reproduce) on El Garrapatero. This is an example of disruptive selection: those individuals displaying either extreme form of a heritable phenotypic characteristic are more likely to live to maturity than those who display the intermediate characteristic for beak size.
Going a step further, De León delineated a second, more recent experiment that he and his research team have performed. According to De León, human population growth in the Galapagos has been almost exponential in magnitude over the past seventy years. As a result of this population influx, a host of new non-native plant species have become readily available to the finch populations residing in urban centers across the islands. As more finches continue to rely on human and introduced plant foods, De León and his team postulated that increased preference for human foods among the finch population would weaken the correlation between beak morphology and resource (seed) distribution. The team devised a ‘Cafeteria experiment’ to test their scientific conjecture, in which they prepared plates containing both human (chips, cookies, rice) and natural food types to leave sitting out for nearby finches. They controlled for all factors, excluding the site of the experiment itself – they tested in both urban and rural (unsettled) areas. In urban populations, the finches fed exclusively on the human food, evidencing that the finches have developed an adaptive preference for human food over time. On the other hand, in the natural setting, finches either fed solely on native plant matter or fed on nothing at all. Following the experimental trial, the team conducted a number of statistical tests to make sense of the data. While finches in rural areas overlapped 25% of the time with regards to their diet, finches in the urban center overlapped 96% of the time with regards to theirs. Because the finches here are all essentially feeding off of the same food/resource type, we can deduce that yes, the correlation between beak size and food distribution is much weaker.
In the future, De León hopes to replicate similar experiments on other islands in the Galapagos and further discern how human activities affect the evolutionary process and induce speciation events in nature.