Unit 24: Evolution — Short Questions

Evolution is the gradual change in the genetic makeup (allele frequencies) of a population over successive generations, which may lead to the formation of new species. In its earliest form, the concept held that the universe and organisms did not always exist in their present form, that today's organisms were not created all at once, and that they are the result of innumerable changes from lower to higher forms, occurring due to variations in organisms and natural selection.

Homologous structures are body parts that are similar in structure but different in function, e.g., vertebrate forelimbs (bird wing for flight, whale flipper for swimming, horse leg for running, human arm for grasping) share a common bone arrangement. This structural similarity indicates that these organisms arose from a common ancestor and then diverged, modifying the same ancestral structure for different functions as they adapted to different environments — this is divergent evolution.

Analogous organs are similar in function but differ in structure, e.g., the wing of a bird and the wing of an insect are both adaptations for flight but are not homologous. Since the organisms bearing them are not closely related, the similarity arises because independent lineages evolved comparable structures to meet the same functional need (e.g., a broad, flat gliding surface for flight) — this is convergent evolution.

The Hardy-Weinberg theorem, presented in 1908 by Godfrey Hardy and Wilhelm Weinberg, states that the frequencies of alleles and genotypes in a large, randomly mating population remain constant from generation to generation, provided no evolutionary forces are acting — i.e., the population size is large, mating is random, there is no migration, no mutation, and all genotypes have equal chance of survival and reproduction. If any of these conditions is violated, evolution occurs.

Allele frequencies change when Hardy-Weinberg assumptions are not met, mainly due to: (1) Genetic drift (neutral selection) — random chance events altering gene frequencies, most evident in small populations; (2) Gene flow — migration of individuals (immigration/emigration) introducing or removing alleles from a population; and (3) Mutation — the origin of new alleles/genetic variation, which disturbs mutational equilibrium.

Jean-Baptiste Lamarck (1744-1829) was among the earliest scientists to formally propose that life evolves. In his 1809 book 'Philosophie Zoologique,' he argued that organisms change from simple to complex forms as they adapt to their environment, through use and disuse of organs and inheritance of acquired characters. This was presented 50 years before Darwin's 'On the Origin of Species' (1859), making Lamarck an early proponent of the idea of evolutionary change, even though his proposed mechanism was later disproved.

Lamarckism proposes that organisms acquire characters during their lifetime through use and disuse of organs, and that these acquired characters are inherited by offspring, directly driven by the organism's needs. Darwinism proposes that heritable variations arise randomly through mutation (not driven by need), and that natural selection then favors individuals with advantageous variations, which are passed on, while disadvantageous variations are eliminated over generations.

Homologous organs are similar in structure but different in function, arise in organisms sharing a common ancestor, and indicate divergent evolution (e.g., vertebrate forelimbs). Analogous organs are similar in function but different in structure, arise in organisms with different ancestors, and indicate convergent evolution (e.g., wings of birds and insects).

Natural selection is a non-random process in which individuals with advantageous (fitness-enhancing) traits survive and reproduce more successfully, changing allele frequencies in a directional way. Neutral selection (genetic drift), by contrast, is a random, chance-driven change in gene frequencies unrelated to fitness, and its effects are most pronounced in small populations.