The correct answer is: the number of kinds of organisms in an environment.
Species diversity is a measure of the biodiversity of an ecosystem, which is the variety of life forms in a given area. Species diversity can be calculated by counting the number of different species (species richness) and the relative abundance of each species (species evenness) in an environment. A high species diversity means that there are many different kinds of organisms and that they are evenly distributed in the environment. A low species diversity means that there are few different kinds of organisms and that some are more dominant than others in the environment. Species diversity can be affected by factors such as climate, habitat, human activities, and evolutionary history.
The other options are incorrect because:
- The complexity of an ecosystem is a vague term that does not have a clear definition or measurement. It could refer to the structure, function, or interactions of an ecosystem, but it is not synonymous with species diversity.
- The different physical characteristics in a species are related to phenotypic variation, which is the range of observable traits that a species can have. Phenotypic variation can be influenced by genetic variation, which is the diversity of genes in a species' gene pool, or by environmental factors, such as temperature, nutrition, or stress. Phenotypic variation does not reflect species diversity, which is about the differences between species, not within a species.
- The variety of genes in a species' gene pool is related to genetic diversity, which is the amount of genetic variation within a population or a species. Genetic diversity can be measured by looking at the frequency of alleles (different versions of a gene) or genotypes (combinations of alleles) in a population or a species. Genetic diversity can affect the evolutionary potential and adaptability of a population or a species, but it is not the same as species diversity, which is about the differences between populations or species.