Heredity and Evolution

Let T represent the dominant tall allele and t the recessive dwarf allele. A hybrid tall plant has genotype Tt. In the cross Tt × Tt, the possible combinations are TT, Tt, Tt, and tt. The tt offspring receive the recessive allele from both parents and therefore show the dwarf phenotype. Thus option A is correct; tallness is not destroyed and the environment does not create this inherited combination.

The 3:1 phenotypic ratio in the F2 generation is characteristic of a monohybrid cross between two heterozygotes, such as Tt × Tt. The alleles separate when gametes form, and fertilisation recombines them. The genotypic ratio is 1 TT:2 Tt:1 tt, which produces a 3 tall:1 dwarf phenotype ratio when T is dominant. Hence option A is correct; recessive alleles are hidden, not erased.

In the Mendelian pea example, tallness is dominant and dwarfness is recessive. A dominant allele would mask a recessive dwarf allele in a heterozygous plant, so a plant showing the dwarf phenotype cannot contain a dominant tall allele for that trait. Its genotype must therefore be tt, with two recessive alleles. Option A is correct; genes are present, and the phenotype is not explained as merely an acquired change.

The governing distinction is between phenotype and genotype. If T is the dominant tall allele and t is the recessive dwarf allele, both TT and Tt plants appear tall. Only TT is pure or homozygous, whereas Tt is hybrid or heterozygous and can pass the recessive t allele to offspring. Since appearance alone cannot distinguish TT from Tt, a tall plant is not necessarily pure. Therefore option A is correct.

A dihybrid cross follows two pairs of contrasting traits at the same time. In the classic cross RrYy × RrYy, independent assortment produces four equally likely gamete types, and their combinations give the F2 phenotypic ratio 9:3:3:1. This pattern indicates that the allele pair for one trait can separate independently of the allele pair for the other, under the conditions of Mendel’s model. Thus option A is correct.

The governing concept is natural selection acting on heritable variation. If a variation improves survival or reproductive success in a particular environment, individuals carrying it may leave more offspring. Because the variation can be inherited, its frequency may increase over several generations. Option A is correct, but this is a tendency rather than a guarantee. B is unjustified, C denies heredity, and D confuses an inherited trait with an acquired skill.

Genetic drift is a random change in the frequency of alleles in a population. In a small group, each individual represents a larger fraction of the gene pool, so the accidental loss or reproduction of only a few individuals can change allele frequencies substantially in one or a few generations. Isolation prevents much gene flow from restoring the earlier frequencies. Thus A is correct; the other statements are biologically false.

Natural selection is non-random with respect to fitness: individuals with inherited traits that improve survival or reproduction tend to contribute more offspring. Genetic drift changes allele frequencies through random sampling, especially in small populations, and the change need not improve fitness. Both can alter populations over generations, but their causes differ. Therefore A states the essential distinction; C reverses it, while B and D are incorrect.

Long-term reproductive isolation stops or greatly reduces gene flow between populations. Once separated, mutation, recombination, genetic drift and natural selection can act differently in the two environments. Differences may accumulate until the populations can no longer produce fertile offspring with one another, a condition associated with separate species. Thus A is a possible result. The other options incorrectly claim instant identity, chromosome loss or a complete absence of change.

Geographical isolation separates populations and reduces interbreeding and gene flow. Evolutionary differences usually arise through many small changes, including mutation, recombination, drift and selection, rather than appearing in one generation. Over many generations these differences can accumulate and produce reproductive isolation, making speciation more likely. Therefore A is correct. B and C describe impossible instant events, while D contradicts the role of geographical isolation.

A biological species is commonly identified by the ability of its members to interbreed and produce viable, fertile offspring. External appearance alone may be misleading because unrelated groups can look alike, while closely related populations may differ internally. If two groups cannot breed successfully, reproductive isolation may indicate that they are different species. Hence A is the best answer; B and C ignore genetic or reproductive evidence, and D does not explain the failure to interbreed.

Homologous organs share a basic structural plan because they were inherited from a common ancestor, but they may perform different functions in different organisms. For example, the forelimbs of humans, whales and bats have corresponding bones adapted for distinct uses. This pattern is called divergent evolution: one ancestral structure becomes modified in different directions. Therefore A is correct; B describes analogous organs more closely, and C and D do not explain the structural relationship.

Analogous organs perform similar functions but differ in their basic structure and evolutionary origin. Wings of birds and insects, for instance, both support flight but are built differently and did not arise from the same ancestral wing structure. Their similarity results from convergent evolution, in which unrelated or distantly related organisms face similar functional demands. Thus A is correct. B describes the pattern of homologous organs, while C and D are unrelated or scientifically incorrect.

A sudden disaster can sharply reduce the size of a population, leaving only a small, possibly random sample of the original individuals. Because the survivors may carry certain alleles by chance, the frequencies of those alleles can change in the remaining population. This random change is genetic drift, and the disaster scenario is specifically called a bottleneck effect. Option A is correct; B, C and D are physiological processes, not population-level changes in allele frequency.

The founder effect is a form of genetic drift that occurs when a new population begins with only a few individuals. The alleles carried by these founders may not represent the frequencies of the original large population, so their descendants can show noticeably different trait frequencies. Thus option A is correct. Analogous organs concern functional similarity, an acquired trait concerns an individual’s lifetime change, and respiration is energy release, so none explains the island population pattern.

The governing concept is natural selection acting on heritable variation. A population already contains differences produced by genetic variation. If an existing difference helps survival or reproduction, individuals carrying it may leave more offspring, so that trait can become more common over generations. Selection changes the frequency of available traits; it does not consciously design or instantly manufacture a trait because an organism needs it. Therefore option A is correct, while B and C are meaningless and D expresses the rejected idea of instant change by need.

This example illustrates natural selection only when colour differences already occur and are heritable. In a polluted habitat, dark insects may be less visible to predators, so they survive and reproduce more successfully. Their offspring can inherit the colour tendency, causing the trait to become more common over generations. Therefore option A combines variation, differential survival, reproduction, and inheritance. B confuses acquired features with inheritance, while C and D are biologically unrelated.

The governing distinction is between homologous and analogous organs. Homologous organs have a similar basic structural plan because of common ancestry, even if their functions differ. Analogous organs may perform similar functions but have different structural origins, as in the wings of birds and insects; their similarity reflects adaptation to a similar function, not necessarily close ancestry. Thus A is correct. B, C, and D wrongly reject the value of comparative anatomy or evolutionary history.

Descendants of a common ancestor may begin with similar inherited features, but separate environments expose them to different conditions and selection pressures. Heritable variations that are useful in one environment may be less useful in another. Repeated selection over many generations can therefore produce divergent adaptations, especially when populations are isolated and do not exchange genes freely. Option A is correct. Exact sameness, absence of variation, or absence of reproduction would prevent such evolutionary divergence rather than cause it.

The chapter connects heredity, variation, natural selection and evolution. Genes carried on chromosomes transmit information that helps produce traits. Mutations and recombination can create variation, and heritable differences may affect survival and reproduction. Over many generations, the accumulation of such population-level changes can produce evolution. Therefore A gives the complete conclusion; B confuses acquired traits with inherited traits, while C denies essential requirements and D is biologically incorrect.

Genetic drift is a change in allele or gene frequencies caused by chance rather than by a consistent selective advantage. Its effect is especially strong in small populations because random sampling can greatly alter which alleles are passed to the next generation. A sudden reduction may produce a bottleneck effect, while a small founding group may produce a founder effect. Therefore A is correct; natural selection is non-random differential success, and the other choices do not describe this population process.

When the underlying forelimb bones are compared, bird and bat wings show the same basic skeletal arrangement, even though the wings have adapted for flight in different evolutionary lineages. A shared structural plan is evidence of common ancestry, so they are homologous in this comparison. Option A is correct. Analogous organs would have a similar function but different basic structural origin; vestigial and recessive are not suitable terms here.

The governing concept is speciation through reproductive isolation. If separated groups no longer interbreed successfully, gene flow between them is reduced or stopped. Mutations, natural selection, and genetic drift can then cause the groups to accumulate different inherited characteristics. Over many generations, the divergence may become large enough for them to function as separate species. Thus, option A is correct; isolation does not instantly remove variation or end sex determination.

An acquired trait develops during an organism’s lifetime because of use, exercise, nutrition, injury, or environmental conditions. Evolution depends mainly on heritable variations, which arise from changes or recombination in genetic material passed through reproductive cells. Since an acquired change usually does not alter those genes, it is not normally transmitted to offspring. Therefore, option A is correct; the other options make absolute or factually incorrect claims.

Let round seed be represented by R, the dominant allele, and wrinkled seed by r, the recessive allele. Two hybrid round-seeded plants have genotype Rr. In the cross Rr × Rr, the possible genotypes are RR, Rr, Rr, and rr. Only rr expresses the recessive wrinkled phenotype, so the seeds appear wrinkled when both parents contribute r; this is about one-fourth of the offspring.