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How Populations Grow Section One A population consists of all the members of the same species that live in the same place at the same time. A species is a group of populations that are capable of interbreeding and are reproductively isolated from other species. The three key features of a population are: • Population size – Total number of individuals • Population density – The number of individuals in a given area • Population dispersion – How the individuals are grouped: even, clumped, random, etc. The study of populations is called demography. Populations tend to grow as long as their environment can sustain the population size. As populations grow, the number of reproducing individuals and the number offspring tend to lead to exponential growth model. Eventually, predation, disease, habitat, and availability of resources cause the environment to reach a certain limit that the habitat can support. This size limit is called carrying capacity. As carrying capacity is reached, the death rate begins to equal the birth rate and the population is described with a logistic model. Some populations may grow according to density independent factors such as weather and climate. Changes in populations may be caused by the population size. We call the cause of these changes density dependent factors. r – strategists are populations that are usually small and are able to reproduce rapidly when the environment is favorable. Large die offs occur when conditions change. K-strategists are tend to be large animals with slower reproduction, parental care of young, and have a need for stable environments. These populations tend to live near the limits of their carrying capacity. How Populations Change Section Two Populations change thru time. In following classical genetics, biologists often assumed that dominant genes would slowly replace recessive genes in a population. After much mathematic calculation and data collection from populations, the HardyWeinberg Principle was established. The Hardy-Weinberg Principle states that frequencies of alleles in populations does not change unless evolutionary forces act on the population. The five major forces that change populations are: • • • • • Mutation Nonrandom Mating Gene flow Genetic drift Natural Selection Mutation rarely occurs in nature and then, even more rarely, has a noticeable effect on gene pools. Gene flow occurs mainly when individuals migrate. Immigrants bring genes into a population and emigrates take genes away. Genetic drift occurs when a small population is affected by a disaster such as a flood, disease, or volcano and then recovers. Nonrandom mating and inbreeding as in zoos can change the gene frequencies of a population. Natural selection enables individuals who exhibit favorable traits to survive and reproduce. Thus, natural selection does not change genotypes but selects for phenotypes. A favorable trait may be controlled by a large number of genes. This is a polygenic trait. Selection for a polygenic trait leads to a normal distribution. When selection moves toward an extreme such as a larger grizzly bear, it is called directional selection. When selection moves away from the extremes and toward the normal, it is called stabilizing selection.