SEE-U 2000: Biosphere 2 - Module 3

## Module 3: Natural Selection and Evolution

### Levels and Types of Ecological Explanation

Why are species located where they are and interact as they do? According to a standard definition, ecology is the attempt to explain the distribution of organisms across time and space. If we accept this definition of ecology, we can cite four general mechanisms that help determine the distributions of organisms.

We will study all four of these mechanisms in the SEE-U class. The first and most longest operating group of mechanisms that could have produced the currently observed ecological distribution is evolutionary ones. Two, more immediate influences are biotic and abiotic factors, or those that are produced by other species and those that are produced as a consequence of the non-living environment, respectively. Lastly, the current ecological setting could be due to human influences. This last factor is increasingly the most important explanation, as the dangerously rapid human population growth continues. While we are treating them separately, it is important to note that human impacts are a biotic factor. These impacts are the result of one species excluding other species from a habitat.

— evolution, abiotic factors, biotic factors, and human influences. The last factor is becoming increasingly dominant as the most important explanation, as the dangerously rapid human population growth continues. We will study all four of these mechanisms in the SEE-U class

These four factors are often grouped into two levels of explanation—the proximate and the ultimate. Proximate explanations or mechanisms are those that can be measured or observed in the lifetime of an individual. These therefore include the biotic, abiotic, and human produced forces. Ultimate explanations are those that occur over much longer time periods, such as across generations or millennia. Evolutionary mechanisms are ultimate explanations for observed phenomena.

Because they explain the original production of species and have influenced the species for all of its existence, evolutionary explanations are tremendously important. Many scientists have persuasively stated that before we can pursue any of the proximate causes to explain the existence of a trait or distribution, we must first explore the possibility of evolutionary explanations. Once the relevant evolutionary explanations are tested and unsupported, then we can proceed to testing proximate explanations. In keeping with this order, then, we will first study evolutionary mechanisms, exploring

Therefore, today we will explore the field of Evolutionary Ecology.

### Evolutionary Ecology

Questions that are answered within this branch of ecology usually address the interspecific (between related species) or intraspecific (between individuals within a single species) origin of a trait or distribution. Interspecific questions about an ecological trait are addressed within the context of either a tree of relationships (called a phylogeny) or a comparison between closely related species. Intraspecific questions about ecological traits are addressed using comparisons among individuals from the same or different populations.

Most non-scientists incorrectly use natural selection and evolution as synonyms. Though distinct, these two important ideas within evolutionary ecology are closely related to each other in that natural selection is a mechanism for evolution. Evolution is often defined as change through time, irrespective of how that change is produced. The idea of evolution preceded Darwin by generations, but was lacking a solid mechanism to explain it until Darwin.

### Natural Selection

Natural selection is the active process whereby some types of individuals survive and reproduce more successfully than others in a given environment. Charles Darwin and Alfred Russell Wallace simultaneously posited the idea of natural selection as a mechanism of evolution. Darwin is usually given credit for the idea because he started formulating his ideas decades earlier than did Wallace, mostly based on observations he made while travelling on the Beagle and from his readings of then-current literature.

While traveling on the ship the HMS Beagle, Darwin made four observations that led him to posit natural selection as the most important mechanism by which evolution can occur. First, he noted that resources are limited—the world is not a vast smorgasbord filled with all of the food, shelter, and mates that an individual needs to survive and successfully reproduce. Second, individuals must compete for these limited resources. Third, a species will tend to expand its population as much as is possible, given the available resources. Last, he concluded that as a consequence of these first three observations there is differential survival and, much more importantly, differential reproduction within a species. Ultimately, the success of an individual under natural selection, also called its fitness, is measured in reproductive contributions to the next generation.

Several of Darwin’s books, including On the Origin of Species and Voyage of the Beagle are online at: http://www.secular.org/library/historical/charles_darwin/

The primary positive outcomes of natural selection are adaptations, or an increased fit between a lineage and its environment. Natural selection operates most strongly and commonly at the level of the individual. Therefore, the differential survival in Darwin’s formulation is the differential reproduction between individuals. The selective value of a trait is always judged relative to the form of that trait in others in the same population.

Obviously when the overlap in resource requirements is greater, competition will increase and the relative values of key adaptive traits will be all-important. As a consequence, we tend to find that intraspecific competition is usually greater than interspecific competition. The resource requirements are always greater within a species than between species. Multiple species with great overlap can coexist using the same resources in the same way if at least one species has great behavioral plasticity and/or ecological flexibility. When one species changes its ecological requirements in the presence of another species, it has been competitively displaced from its natural niche.

### Other Evolutionary Mechanisms

Natural selection, although one of the most powerful evolutionary mechanisms is not the only one. There are others that could also produce evolution—genetic drift (random changes through time) and environmental mediation (the occurrence of changes in a lineage that are environmentally produced and not genetically fixed) are two other important evolutionary mechanisms.

But most of evolutionary ecology focuses on natural selection to explain the current distribution and ecology of existing species Our activity today explores the influence of distance from the most preferred habitat of a species and the amount of within-population morphological variation. There is the possibility that the results that we find are in fact not genetically based and may instead be due to environmental mediation. Keep both of these explanations in mind as we do today’s activity.