Final answer:
Phenotypic variation in species with small genotypic differences, like humans and dogs, is measured by physical and biochemical markers. It can be expressed due to differences in gene expression, with natural selection acting on genetically-based differences. Geographic variations known as clines also contribute to phenotypic diversity.
Step-by-step explanation:
Phenotypic variation can be quantified and measured in species like humans and dogs by assessing both physical characteristics and biochemical markers. Variation within a population, such as the different fur colors and patterns found in domestic dogs, is termed population variation. While genotypic variation in these species may appear small, phenotypic differences can be pronounced due to differences in gene expression.
For example, although every cell in humans contains the same genes, differences in cell types and functions arise from which genes are expressed. Researchers study human biological variation by looking at morphology and biochemical differences, such as variations in skin pigmentation in response to UV ray exposure. These variations, often referred to as polymorphisms, are crucial for a population's adaptation to environmental pressures and evolutionary changes. Genetic variation within a population is essential for natural selection to act upon.Methods of measuring phenotypic variation include morphological measurement (like height and build), genomic sequencing to detect variation at the DNA level, and the study of gene expression profiles. Geographic separation can result in a cline, where species show gradual phenotypic changes across an ecological gradient. Importantly, only genetically-based differences can be selected for across generations, highlighting the importance of genetic underpinnings for phenotypic variation.