Unit 24: Evolution — Long Questions

Individuals within a population show heritable genetic variations, arising mainly through random mutations; no two individuals are exactly alike. Because organisms have a far greater reproductive potential than the environment can support, they must struggle for survival against limited resources. In this struggle, individuals whose variations are advantageous (adaptive traits) are more likely to survive and reproduce, while those with disadvantageous variations are less likely to do so. Since adaptive traits are inherited by offspring, their frequency increases in the population over successive generations, while disadvantageous traits become rarer and may eventually be eliminated. This gradual, cumulative change in the genetic makeup (allele frequencies) of the population, driven by variation and natural selection, is evolution, and over long periods it can lead to the formation of new species.

Charles Darwin published 'On the Origin of Species' in 1859, proposing that evolution occurs through natural selection, with four key elements: (1) Overproduction — organisms have a far greater reproductive potential than can ever be realized (e.g., a female sea star releases about 1 million eggs each season); if all survived, oceans would soon overflow with sea stars. (2) Variation — genetic variations arise through random mutations, so no two individuals are exactly alike; variations may be advantageous, harmful, or neutral, and inherited variations can pass to offspring. (3) Struggle for survival — since more offspring are produced than resources can support, individuals compete for survival; those with disadvantageous variations die or fail to reproduce, while those with advantageous variations (adaptive traits) succeed in reproduction. (4) Inheritance of adaptive traits — adaptive traits are passed on to the next generation, while disadvantageous traits become less frequent and eventually eliminated, resulting in gradual evolutionary change.

Biogeography, the study of the geographic distribution of plants and animals, shows that life forms in different parts of the world have distinctive evolutionary histories, supporting evolution over special creation. First, similar groups of organisms separated by vast geographic barriers show obvious similarities suggesting common ancestry, while obvious differences reflect millions of years of independent evolution — e.g., the African lion closely resembles the American mountain lion, their similarities indicating shared ancestry and their differences indicating independent evolution after separation. Second, organisms separated by geographical barriers can be very different despite living in similar environments, because they evolved independently — e.g., Australia and Tasmania have many unique marsupials such as kangaroos as major herbivores, unlike animals anywhere else in the world. Third, islands often have relatively few but unique species, because colonization by a few ancestral species followed by isolated evolutionary change (as Darwin observed with Galápagos finches and tortoises having island-specific beak shapes and shell shapes) produces species found nowhere else. Such patterns are explained by evolution and isolation acting on common ancestors, not by independent creation at each location.

Comparative anatomy, the study of structures of fossils and living organisms for evolutionary similarities, provides three lines of evidence. (1) Homology (divergent evolution): homologous structures are similar in basic structure but different in function, e.g., vertebrate forelimbs share a common bone arrangement though used for flight (birds, bats), swimming (whales, dolphins), running (horses), or climbing (arboreal lizards) — indicating these organisms share a common ancestor and diverged, adapting the same structure for different functions. (2) Analogy (convergent evolution): analogous organs are similar in function but different in structure, e.g., the wings of birds and insects are both adaptations for flight but are not homologous, arising independently in unrelated lineages facing similar functional demands. (3) Vestigial organs: these are poorly developed, functionless remnants of once-useful ancestral structures, e.g., minute pelvic bone remnants in boas and pythons (from limbed reptilian ancestors), and in humans the vermiform appendix (vestige of the cecum), coccyx (vestige of the tail), wisdom teeth, and non-functional ear muscles — such remnants are clear indications of evolutionary change.

Divergent evolution is revealed by homologous structures — organs similar in structure but different in function. Organisms possessing them share a common ancestor, and the underlying structure (and the genes controlling it) was inherited from that shared ancestor, subsequently being modified and used for different functions as lineages diverged into different environments, e.g., vertebrate forelimbs. Convergent evolution is revealed by analogous structures — organs similar in function but different in structure. Organisms possessing them do not share an immediate common ancestor for that trait; the similar structures were NOT inherited from a shared ancestral structure but arose independently in separate lineages because both faced similar environmental/functional demands, e.g., wings of birds and insects. Thus, homology reflects shared inheritance with later modification, while analogy reflects independent origin without shared inheritance of that particular structure.

Molecular biology provides strong evidence for evolution because, just as organisms can have homologous anatomical structures, they can also have homologous biochemicals. Ultimately, the structure and function of every organism is based on the genetic blueprint carried in its DNA. Related organisms inherit similar DNA sequences from a common ancestor, and because DNA codes for proteins, related organisms also have similar proteins. By comparing the DNA and protein structures of different organisms and looking for dissimilarities, and by assuming relatively constant mutation rates, scientists can estimate how much time has elapsed since two organisms diverged from a common ancestral molecule — the greater the molecular difference, the longer the time since divergence. An applied example is antibiotic resistance in bacteria: a mutation in a bacterial gene that confers resistance evolves via natural selection and is passed on to successive generations, directly demonstrating evolution occurring at the molecular/genetic level.

According to Lamarckism, the evolution of the giraffe's long neck proceeded as follows: (1) The ancestors of giraffes had short necks and fed on grass and leaves of small plants. (2) Environmental change made these food sources scarce. (3) Some individuals began stretching their necks to reach leaves on tall trees as an alternative food source. (4) Constant stretching during an individual's lifetime caused its neck to become stronger and longer — an 'acquired character' due to increased use of the neck. (5) This acquired character was inherited by the offspring, which were born with slightly longer necks. (6) These offspring, on reaching maturity, continued the same neck-stretching behavior, making their necks even longer and stronger during their own lifetimes — a further acquired character. (7) Through repeated inheritance of acquired characters across many generations, the long-necked giraffe as seen today gradually evolved.

The Endosymbiotic Theory, proposed by Lynn Margulis, is the most widely accepted explanation for the evolution of eukaryotes (~2 billion years ago) from prokaryotes (which originated ~3.5 billion years ago). According to this theory, large prokaryotic cells engulfed smaller prokaryotic cells, which were not digested but instead lived within the larger cells in a mutually beneficial (symbiotic) relationship. Small prokaryotes capable of aerobic respiration supplied energy to themselves and to the host cell, and became the mitochondria of eukaryotic cells. Other small prokaryotes capable of photosynthesis made food and shared it with the host cell, becoming the chloroplasts of eukaryotic cells. Flagella are thought to have arisen similarly, from ingested spirochete-like prokaryotes. The resulting eukaryotic cells were more powerful and efficient, enabling the evolution of multicellularity, cell specialization, and larger cell size, giving rise to the great diversity of animals, plants, and fungi today. Evidence supporting this theory includes: mitochondria and chloroplasts are the same size as prokaryotic cells; they contain their own circular DNA organized like a prokaryotic chromosome; they reproduce by binary fission (unlike eukaryotic cells, which reproduce by mitosis); and their membrane composition more closely resembles that of prokaryotes than of eukaryotes.

Jean-Baptiste Lamarck (1744-1829), in his 1809 book 'Philosophie Zoologique,' proposed that organisms progress from simple to complex forms as they adapt to their environment, through two linked mechanisms. (1) Use and disuse of organs: when an organism's environment changes, it begins using some body parts/organs more and others less; organs used more frequently become developed and strong, while organs not properly used deteriorate or become weak. Such changes, arising from long-term use or disuse, were termed 'acquired characters.' (2) Inheritance of acquired characters: Lamarck believed that characters acquired during an organism's lifetime were passed on to its offspring. When such acquired characters are inherited across successive generations, they ultimately bring about modifications that help the organism better cope with changes in its environment — this is how Lamarck explained evolutionary change (illustrated by the evolution of the giraffe's neck).

According to the chapter, at the time Lamarck proposed his theory, no evidence was yet available to explain the actual mechanism of evolution. Because of this gap in available evidence, Lamarck was criticized more for advocating the very idea of evolutionary change (that species are not fixed and transform over time) than for the specific mechanism (use and disuse, inheritance of acquired characters) he proposed to explain it. It was only later, after scientists discovered the true mechanisms of inheritance (genetics), that Lamarckism's proposed mechanism was specifically disproved, on the grounds that acquired characters have no genetic basis and organs are not modified simply by an organism's wish or need.

Although both Wallace's and Darwin's papers on natural selection were jointly presented to the Linnaean Society in 1858, the theory came to be attributed to Darwin mainly because Wallace himself agreed to credit Darwin, recognizing that Darwin had already been developing the idea in much greater depth for about twenty years, supported by extensive evidence gathered during his five-year voyage on HMS Beagle (fossils, biogeographic patterns, the Galápagos finches and tortoises, etc.). Wallace, whose own work was a shorter, more recent paper/letter, convinced Darwin to publish his far more comprehensive book, 'On the Origin of Species' (1859), which presented the detailed evidence and reasoning behind natural selection. Because Darwin's book was the thorough, well-substantiated published account that the scientific community engaged with, the theory became permanently associated with his name, even though Wallace had arrived at an essentially identical mechanism independently and at the same time.