LAB 1: EXPLORING EVIDENCE FOR EVOLUTION BY NATURAL SELECTION IN THE BIOLOGY GREENHOUSES
"There is grandeur in this view of life, … from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
-from Charles Darwin’s closing sentence from On the Origin of Species
With the publication in 1849 of On the Origin of Species, Darwin laid out his case for evolution by natural selection as a way of explaining the amazing diversity of life, of “forms most beautiful and most wonderful.” In the first part of the Biology 102L course you will be studying the evidence for evolution, both as presented by Darwin and as further amplified and expanded during the past 150 years, especially with new fossil discoveries and the recent tool of molecular biology for more clearly recognizing relationships among species. In so doing, our goal is to help you better understand the robust explanatory and predictive power of Darwin’s theory and appreciate why it serves as the underpinning for all of modern biology.
For this week’s lab you will use the wonderfully diverse teaching plant collection of the Biology Department greenhouses to explore three distinctive consequences of Darwinian evolution: adaptation, convergence, and patterns of biogeographical distribution. Darwin’s observations of these phenomena were instrumental in the development of his theory because they could all be accounted for by the same mechanism of evolution by natural selection: differential reproductive success among the individuals of a species living in a given environment according to chance variation in their heritable traits.
In an evolutionary context, adaptations refer to anatomical, physiological, or behavioral characteristics of an organism that enhance its ability to survive and reproduce by making it better suited, a better “fit,” to a particular environment or way of making a living. Adaptations are the most direct and visible consequences of natural selection, and although they can sometimes be so exquisite as to seem intentionally designed, Darwin correctly realized that they instead arise by selection acting on chance variation, gradually and over long periods of time. Evidence of such a process is preserved in the fossil record for a number of adaptive traits, as you can read in Coyne’s Why Evolution is True. As you will see in the greenhouses, it is also apparent in existing species that still retain intermediate, less well-adapted, characteristics and in comparisons of species that are closely related but because they live in different environments, exhibit correspondingly different adaptations and may look very dissimilar.
Conversely, through convergent evolution species that are not at all closely related may come to share many of the same kinds of adaptations and thus look remarkably similar. As Darwin recognized and you will examine in the greenhouses, it is a testimony to the power of natural selection that it can produce similar adaptive "solutions" to the challenges of a particular environment even when acting on species with different genetic backgrounds. Part of the challenge you will face in the greenhouse, and later in the course when you learn about phylogenetic systematics (the use of evolutionary relationships to classify organisms), will be to distinguish between convergence and shared ancestry.
Another consequence of natural selection resulting in adaptations suited for a particular environment and producing similar outcomes in similar habitats is distinctive distribution of species, their biogeography. Some environments are limited to particular geographical regions, and species that have adapted to those conditions are also correspondingly geographically limited. On the other hand, some habitats occur on multiple continents, and although the same kinds of adaptive responses can be seen in different locales, the species expressing them may vary greatly geographically, depending on the ancestral stock from which they were derived.
GREENHOUSE OBSERVATONS AND ACTIVITIES
With the above background you are now ready to apply evolutionary concepts to specimens in the Biology Department greenhouses. The teaching collection there contains an extensive assortment of plants from many families from all over the world showing a broad range of different characteristics. For today’s exercise you will examine how both closely and quite distantly related species of plants from different parts of the world have responded to (i) desert conditions, (ii) low nutrient environments through carnivory (turning the table on animals!), and (iii) specialized pollination by hummingbirds and moths.