NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin.  Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.

Goals: increase understanding of natural selection by considering how it works in various situations, including examples of adaptation to both non-living and living aspects of the environment.  Learn some of the general kinds of adaptation.  Learn about coevolution; evolution of different species in response to one another.

Related Textbook Material: Freeman and Herron (2001) Chapter 3

Lab Manual Questions over this material are in Lab Manual Chapter II

The Lecture:

To increase your understanding of natural selection you need to consider some examples of adaptations -- the results of natural selection.  If you look at any organism, and consider its traits (the characteristics of that organism -- it's color, shape, size, structure, biochemistry, behavior), and how those traits function, you will see that many of them appear to make the organism particularly effective in its specific environment.  Such traits are most likely to be adaptations -- traits that have evolved through natural selection.

To understand these traits as adaptations, we first need to consider what is meant by the environment of an organism.  The enviroment includes all that surrounds and organism -- the non-living conditions it experiences such as the climate, and the living organisms with which it interacts, including other members of its own species, and other species  The environment includes other species such as parasites and pathogens that live within the body of the organism as well as those that are outside of it.  Basically, everything living and non-living with which the organism interacts makes up that organisms environment.  We will see that there are adaptations to all these various aspects of the environment.  The rest of this lecture will present a few examples to illustrate adaptations to these aspects of the environment.the environment -- living and non-living.  As we consider these different examples you should note the different aspects of organisms that can be adaptations, and note that adaptations are specific to particular environments -- a trait that is an adaptation to one environment often would not be an adaption to a different environment.

Adaptations to non-living aspects of the environment:

Plant leaf size and structure shows adaptation to the climate.  Consider plants in the desert, where the climate is hot and dry -- leaves on some plants are small and hard, coated against water loss, leaves on other plants are succulent -- thick with tissue that stores water.  Still other plants, like cacti, have reduced leaves so there is no water loss from leaves and have thick succulent stems instead.  All these are adaptations to a hot dry environment.  In a tropical rainforest, in contrast, leaves are very large and may have pointed tips that slope down; this so-called "drip tips" promote water runoff from leaves so that leaves do not stay wet and potentially mold.

We can see another example of adaptation to climate in mammals.  If we compare similar mammals, such as rabbits of different species or even populations of rabbits of the same species, in warm, southern climates to those in northern climates we find that those in the south have relatively smaller body size, but relatively larger ears compared to their body size, than do those in cold, northern climates.  Mammals put a lot of energy into maintaining a constant body temperature.  A larger body stores heat better, and so is adaptive in a cold climate.  Large ears lose heat -- this is adaptive in a hot climate but smaller ears would save heat and be better adapted to a cold climate.  In both the plant leaf example and the mammal example we see how aspects of shape and size are particularly functional in the climates in which the organisms occur -- these organisms are adapted to their physical environments.

When we find that within one species, populations show different traits that are adapted to their specific environments, we call this local adaptation.  So if within one species of rabbit, northern populations have relatively small ears and southern populations have relatively large ears, we would say that these populations are locally adapted -- they show local adaptation to their environments.  Local adaptation also illustrates how a trait that is adaptive in one environment will not be adaptive in a different environment; we always need to remember that we can only understand adaptation in the context of the specific environment within which a population has evolved.

Adaptations to living aspects of the environment:

All species interact with other species; the other species make up an important aspect of the environment.  Species show adaptations because of their interactions with other species -- traits that help them avoid being eaten by other species, that help them catch and eat other species, that allow them to better compete for resources with other species, that help them avoid paraistes, that help them parasitize hosts if they are parasites, that improve interactions between species that are mutualists (mutually beneficial to one another.)  Often we see that one species has evolved a trait because of an interaction with a second species, and that then the second species has evolved in response to the evolution of that trait in the first.  When species evolve traits in response to each other in this way, it is called coevolution.  We will consider several of the ways in which species interact, note traits that they have evolved in response to one another, and note examples of coevolution.

One way in which species interact is that some species eat, or are eaten by, other species.  We see many adaptations in plants that deter herbivores from eating them; these include structural features such as thorns and biochemical features such as toxins.  The evolution of these features has created an environment in which herbivores have then been selected to be able to eat plants despite these deterents.  Many  herbivorous (plant-eating) insects, for example, have evolved biochemical mechanisms of detoxifying specific plant toxins; these are adaptations in the insects that allow them to eat.  Note that in this example first a trait evolved in plants because of the insects, then a trait evolved in the insects because of the new trait in the plants; since this involves species evolving traits in response to one another it is an example of coevolution.  This coevolution may continue -- once the insects have evolved such detoxifying mechanisms, plants may evolve different toxins, and so on.

Animals that can be prey to predators also have evolved many adaptations that help prevent them from being eating, including cryptic coloratiion (being camouflaged), good sensory organs to detect potential predators, and good escape abilities.  Think, for example, of a rabbit -- its coloration makes it hard to see, its large ears allow it to hear predators as they approach, and its large hind legs give it the running and jumping ability to escape from predators.

Some prey species have evolved toxins that deter predators. An interesting adaptation that has occurred in association with the evolution of toxins is aposematic coloration -- bright coloration that warns potential predators of the presence of toxins.  Even a toxic animal could be harmed if a predator attacked it, but if a predator can be warned away so it will not attack at all, the animal will survive better.  That is the function of aposematic coloration.  We see bright coloration in many poisonous and venomous species including many butterflies, such as monarchs and swallowtails, frogs, such as the poison-dart frogs of Central America and snakes, such as coral snakes.

Another interesting way in which some species are adapted to avoid being eaten is through mimicry -- the presence of traits that resemble something else.  There are non-poisonous butterflies, for example, that strongly resemble poisonous species, and non-venomous snakes, such as king snakes, that resemble coral snakes.  Predators presumably avoid such species because they look like the poisonous ones.  Some moths have eyespots on their wings that, when flashed at a predator, make them resemble the face of an owl -- a potential predator on their predators.  Mimicry can be behavioral, too; for example, there are non-venomous snakes that shake their tails rapidly in leaf litter when approached and make a sound like the rattle of a rattlesnake.

The examples of moths flashing their wings to expose eyespots and of snakes shaking their tails are examples in which the adaptation is behavioral.  These behavioral characteristics have presumably evolved through natural selection, just as biochemical and structural characteristics have.

Species interact with each other in ways other than feeding upon one another.  Flowering plants and pollinating insects, for example, are mutualists; they benefit from one another (the pollinating insects receive food in the form of pollen and nectar; the plants receive sex -- that is, the insects transfer pollen from flower to flower, allowing sexual reproduction of the flowers to occur.)  Mutualists such as these may also evolve traits in response to one another.  Many flowers, for example, have evolved combinations of color, shape, size, and odor that attract specific kinds of pollinator -- they are adapted to attract specific pollinators.  Species also interact by competing with each other for resources.  Species may evolve traits that improve their ability to obtain specific resources because of competition; competing species may also evolve to specialize on somewhat different aspects of their habitats because doing so decreases the negative impact from competition with other species.

In this lecture we have considered just a few of the ways in which organisms show adaptations to their environments.  Now you should consider any organism, look at its characteristics, and see if you can figure out how they have adapted to their environment. In doing so, you will practice what your text book calls "thinking like the organism."  You should also make sure you understand how the process of natural  selection works by applying Darwin's four postulates to the examples from this lecture to explain how natural selection in the past would have led to the characteristics that are present in these species today.