Heredity and Evolution

DNA stores hereditary information, so comparing DNA sequences helps reveal evolutionary relationships. When two organisms have greater similarity in their DNA, they are generally likely to have shared a more recent common ancestor than organisms with less similarity. However, similarity alone does not prove that they are the same species, and it does not mean that they lack variation. Therefore, option B is correct.

An evolutionary tree represents lines of descent and relationships among organisms. If two branches split close to one another, the organisms on those branches are interpreted as having a relatively recent common ancestor. This does not mean they are identical organisms or that they lack genes; it only indicates closer evolutionary relatedness. Thus, option B gives the correct inference.

Evolution by natural selection does not require a complex organ to appear suddenly in its final form. Heritable variations that provide even small advantages can be preserved because their bearers are more likely to survive and reproduce. Across many generations, successive useful changes may accumulate and produce a complex structure such as an eye. Therefore, option B is correct.

Convergent evolution occurs when organisms that do not share the same basic structural origin develop similar functions because they face similar environmental demands. The resulting structures are called analogous organs; for example, wings of birds and insects perform flight but have different origins. Mendel’s law and sex determination address inheritance, not this pattern. Therefore, option A is correct.

Phenotype means the observable expression of a trait, whereas genotype means the allele combination carried by an organism. If a dominant allele is present, a homozygous dominant individual and a heterozygous individual can show the same phenotype even though their genotypes differ. Thus, visible similarity does not prove identical genetic combinations. Option A states this distinction correctly.

Let the normal dominant allele be A and the recessive disease allele be a. Two unaffected carriers have the genotype Aa, so the cross is Aa × Aa. The possible offspring genotypes are AA, Aa, Aa, and aa. Only aa expresses a recessive disorder, giving 1 affected outcome out of 4, or 25%. Therefore, option A is correct.

The adaptive value of a trait depends on the environmental conditions in which an organism lives. A thick coat may help an animal survive in a cold region but become disadvantageous in a hot region. Since natural selection favors traits that improve survival and reproduction under local conditions, the same trait can be useful in one environment and neutral or harmful in another. Option A is correct.

Variation means differences in traits among individuals of the same population. Some variations may help an organism survive or reproduce more successfully in a particular environment. Natural selection preserves and increases such advantageous heritable variations over generations, while less useful forms may decline. Thus, variation supplies the different forms on which evolution can act; heredity then helps transmit selected traits to descendants.