The organization of genetic variation defines population structure, which is driven by the combined impacts of evolutionary processes like as recombination, mutation, genetic drift, demographic history, and natural selection. The three main components of population structure are: (1) spatial structure - the arrangement of individuals or genotypes within a given region; (2) temporal structure - the pattern of differences over time in the composition or phenotype of a population; and (3) reproductive isolation - any traits that prevent two populations from merging due to genetic incompatibility.
Spatial structure can be defined as "the degree to which the locations of individuals or genes within a population or species are non-random" (Balkenhol's Genetics and Population Structure). Spatial structure results from several factors including physical barriers like oceans, mountains, and rivers, as well as historical events like glaciations and floods. Reproductive isolation leads to the formation of new subpopulations that may become genetically distinct populations depending on how long they remain isolated from each other. For example, humans exhibit remarkable levels of genetic differentiation between geographic regions. This is primarily due to human-mediated migration patterns that occur when people move from one location to another. In addition, some geographical features such as large bodies of water and deserts tend to isolate different groups of humans resulting in different genetic profiles for disease resistance and other traits.
Local populations, or demes, as well as subpopulations. Such populations are organized, but not indefinitely. When populations deviate from Hardy-Weinberg proportions or from panmixia, this is a fair description of population structure. However they often remain divided by geographical features or political boundaries. In these cases, it is useful to think of them as separate populations or subpopulations.
In addition to local populations, many species include within their range more than one geographically isolated population. For example, there may be a northern and southern population of a species with limited interbreeding occurring between them. Or, there may be two nearby forest fragments containing all of the genetic diversity of a larger forest community. These are examples of subpopulations. Subpopulations can also be defined based on differences in habitat quality or other factors that might influence which individuals survive and reproduce.
Finally, some scientists divide populations based on how long they have been separated from each other. If a population has been completely isolated from other populations for many generations, then it is called a "subspecies". If only some members of a species have been isolated from others, then those people are called "breeds" or "strains". For example, there are several dog breeds that have evolved over time without any human intervention. These include Beagles, Biewels, and Dachshunds.
The "make up" or makeup of a population is referred to as population structure. Looking at a place's population structure reveals how the population is divided into males and females of various ages. Also included in this analysis are data on people whose gender is unknown or not specified.
Population structure is one of many factors that influence the size of a population. For example, if more men than women are born, then the male population will increase while the female population will remain the same or even decrease. Likewise, if more men die than women, then the female population will increase while the male population will begin with zero and gradually decline.
People also leave homes for various reasons. Some move to find better jobs or opportunities while others move away from negative influences such as violence or poverty. The type of migration a person makes can be observed by looking at where they want to go and why they're leaving their current home. In general, migrants tend to be younger and more often men than older and female populations.
Finally, some countries lose citizens every year because they move to another country who will take them back when their visa expires. This is called emigration and results in a loss of population numbers over time.
The study of how and why populations vary in size and structure through time is known as population dynamics. Rates of reproduction, mortality, and migration are all important elements in population dynamics. In general, larger populations have more variation in sizes than smaller populations because random genetic changes occur more often in large numbers. Also, larger populations tend to be more stable because they can absorb new individuals without changing their overall composition.
Population density has a big impact on rates of reproduction and death. More space means less competition for resources, which allows individuals to reproduce at a higher rate. Less space means greater competition for resources, which leads to lower rates of reproduction. Denser populations will also have reduced rates of migration because there aren't enough resources available for all individuals to find suitable mates. Long-distance migrants will be particularly affected by limited resources because it would not be efficient for them to travel unless doing so provided some benefit.
Large populations tend to be more stable because they can absorb new individuals without changing their overall composition. Smaller populations are more likely to experience drastic changes when new individuals are added or removed from the group. This is one reason why scientists think that humans were probably not as successful early in our evolution because we had a much smaller first population that later expanded.