Any population has built in variations in characteristics and these variations are raw material for evolution.

Statement-A: Any population has built-in variation in characteristics.

The statement-A is correct.

Explanation:
- Variation refers to the differences that exist among individuals within a population in terms of their characteristics or traits. These traits can be physical, behavioral, or genetic.
- Variation is a fundamental characteristic of any population. No two individuals within a population are exactly alike, and this variation can arise due to genetic, environmental, or a combination of both factors.
- Genetic variation is the result of differences in the genes or genetic material of individuals. It can arise through processes such as mutation, recombination, or gene flow.
- Environmental variation, on the other hand, is the result of differences in the environment experienced by individuals. Factors such as diet, climate, and habitat can influence an individual's traits and characteristics.
- Variation is important because it provides the raw material for evolution. It allows populations to adapt to changing environments and can lead to the development of new traits or the elimination of existing ones.
- In a population, individuals with traits that are advantageous in a particular environment are more likely to survive and reproduce, passing on their traits to future generations. This process, known as natural selection, can lead to changes in the frequency of traits within a population over time.
- Therefore, any population, whether it be a population of organisms or a population of genes, will exhibit variation in characteristics. This variation is essential for the process of evolution to occur.

Statement-B: According to Darwin, the fitness refers ultimately and only to reproductive fitness.

The statement-B is correct.

Explanation:
- Fitness, in the context of evolution, refers to an individual's ability to survive and reproduce in a given environment. It is a measure of how well an individual is adapted to its environment.
- According to Charles Darwin's theory of natural selection, the ultimate goal of fitness is reproductive success. Individuals with traits that increase their chances of surviving and reproducing are more likely to pass on their genes to future generations.
- Darwin argued that natural selection acts on individuals, favoring those with traits that enhance their reproductive success. These traits can include physical characteristics, behaviors, or physiological adaptations that increase an individual's ability to find mates, compete for resources, or successfully raise offspring.
- Darwin's concept of fitness is often referred to as "reproductive fitness" or "Darwinian fitness" to distinguish it from other forms of fitness, such as physical fitness or health.
- It is important to note that reproductive fitness does not necessarily mean having the most offspring. It refers to the ability to pass on one's genes to future generations, and this can be achieved through various strategies, such as producing a few high-quality offspring or many low-quality offspring.
- Overall, according to Darwin's theory of natural selection, fitness ultimately and primarily refers to reproductive fitness, as it is the key determinant of an individual's evolutionary success.