Lecture 13
Topics
- Paradigm of the Population - the Darwinian Connection
- Population Attributes
Study Sources
The Main Points
At the beginning of the semester, we asked two kinds of questions about "communities": What are they like? and What do they do? We can ask the same kinds of questions about populations. These are more or less categories of structure and function
The kinds of measurements of "structure" we will be considering are:
- Size, Density and Dispersion are attributes which deal with the spatial arrangement of individuals. We have to draw arbitrary boundaries, we have to sample, in short, we have confront the population as a statistical entity.
- A population has an Age Structure and a Genetic Structure. These result from sampling and placing individuals into categories, e.g. one might group individuals by age, Phenotype or Genotype.
The kinds of measurements of "function" we will be considering are:
- Birth Rate and Death Rate. Together we use these rates to calculate the Intrinsic Rate of Increase or Biotic Potential of the population.
- Survivorship and Fecundity give us insights into the historical dynamic tendancies of the population. From these we can caluculate Net Reproductive Rate (replacement rate).
- Ultimately we will be concerned with the effects of Natural Selection on the phenotypic and genotypic frequencies. When we find a change in the gene frequency we confront directly the process we call Evolution. Can we explain Speciation simply by appealing to the effects of Natural Selection?
In this lecture we need to talk about what a population is. Like a "community", a "population" is an abstract concept.
We can define a population as a localized group of interbreeding organisms, that at least taxonomically are of the same kind. In this sense, these organisms make up a dynamic system that is located, in nature, in some rather arbitrary way. That is, the boundaries around this population are established by the observer.
Except for laboratory cultures or small populations that are isolated in nature, we seldom know the characteristics of the population except in a statistical sense. This means that we know a population from samples. From these we can calulate means and variance of desired variables and from these estimate trends and make comparisons. Our own population of the United States is sampled every 10 years and the measurements of "structure" and "function" (that I mentioned in the first section here) are made from the samples.
One of the fundamental properties of a population is its density. If you make a series of samples and compute the mean number of individuals per sample you have a measure called crude density or simply mean number per unit area. If you divide the number of people in California by the number of square miles in the state, you would have a measure of crude density, or the number of people per square mile.
The problem with crude density is that is doesn't tell us about the dispersion or real arrangement of the individuals in space. The problem becomes even more acute when you work with a population that is dispersed in three dimensions. Consider the problem of understanding the dispersion pattern of a planktonic species arranged over hundreds of square miles of ocean and hundreds of meters of depth.