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A rich inheritance. Heredity and Homologies. All cells from cells ( Omnis cellula e cellula). Rudolf Virchow argued for this principle based on microscopic observations of cell division. (1858) Nuclei were seen to be duplicated as well, and nuclei of germ cells unite at fertilization.
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A rich inheritance Heredity and Homologies
All cells from cells (Omnis cellula e cellula) • Rudolf Virchow argued for this principle based on microscopic observations of cell division. (1858) • Nuclei were seen to be duplicated as well, and nuclei of germ cells unite at fertilization.
August Weisman • Argued that, since the hereditary material is contained in the nucleus, and the nucleus of a cell comes from the nuclei of ancestral cells, Darwin’s pangenesis is impossible: No combination of elements from all the body can be combined and placed in the nuclei of the germ cells. (ca. 1887)
Chromatin • A material in cell nuclei that stained strongly with new (synthetic) dyes. • As a cell begins to divide, the chromatin is gathered in rod-shaped ‘chromosomes’, which then duplicate (splitting along their length). • One set of chromosomes goes to each daughter cell. • The process is called mitosis; this material, Weisman concluded, must be the ‘hereditary substance’.
Meiosis • Germ cells combine to make a fertilized egg– each supplies chromosomes, so they must each carry ½ the usual complement. • Weisman predicted a division would occur without duplication, to create germ cells. • In fact, meiosis duplicates the chromosomes once, but then divides twice to produce four germ cells. • Weisman worked to connect these facts to natural selection and evolution.
Still more trouble for inheriting acquired traits • The cell lineage that leads to germ cells in the hydra (Hydromedusae) is distinct from somatic cell lines from very early in development. • So there is no way for somatic changes to alter the nuclei of germ cells. • Weisman tried tail amputations in mice to test this notion; no inheritance of the mutilations was found (such inheritance was widely believed in up to this point).
Translation • More broadly, Weisman saw no way that information from the rest of the body could be received and ‘translated’ by the germ cells to produce similar effects on progeny. • This reinforces the notion that variation is undirected (we often say ‘random’ here, but it’s a sloppy way to describe it). • Given undirected variation, natural selection seems to be the only way to produce adaptive change over time– Weisman concluded that NS is the key mechanism in evolution.
Lamarckians • Lamarckians resisted this conclusion, insisting that acquired traits could be passed on, somehow. • Related to this was Copes notion of orthogenesis, the idea that there were ‘trends’ in evolution (towards larger size or altered shapes of some parts for instance) that were not driven by natural selection. They could even drive a species to extinction. • Cope thought that inheritance of acquired characteristics could explain these trends.
The Irish Elk • Largest known member of the deer family. • An ice-age animal. • Cope and others believed it had been driven to extinction by an evolutionary ‘trend’ that led to ever-bigger antlers, which eventually made it unable to avoid obstacles or escape predators
Patterns of inheritance • If inheritance was so clear and ‘precise’ a process, surely there should be patterns of inheritance that we could detect. • Bateson and de Vries studied variation and its patterns of inheritance. • For Bateson, variation (not selection) was the key to evolution. • Bateson thought that sudden, dramatic, discontinuous variation could explain the discontinuities distinguishing even closely related species. • De Vries favoured a ‘particular’ account of inheritance.
De Vries • Inheritance of one such particle (pangene) would be independent of inheritance of others. • So we should be able to follow different ones through the population, generation by generation. • Observations of plants showed a definite pattern of inheritance; de Vries observed it in 30 or so species. • As he prepared to publish, he looked back into the literature and found that he’d been scooped.
Mendel • Gregor Mendel was an Augustinian monk who worked as a teacher in Brno, Austria. • He experimented with peas, breeding them in controlled ways by hand-pollination. • The pea plants showed 7 discrete characteristics: tall or short, with round or wrinkled peas, etc. • Selection could make them breed true for these characteristics. • Cross-breeding the results showed a simple pattern.
Particles • If separate particles, independently inherited, cause various traits, and some particles ‘dominate’ others in their effects, then the pattern Mendel observed is easily explained. • The result is that when we cross the pure forms, the offspring will have one of each particle type (for a given trait) and will ‘show’ the trait that is dominant. • But breeding this second generation will produce a mix– ¼ will inherit two dominant particles, ¼ will inherit two ‘recessive’ particles, and ½ will inherit one of each. • So ¼ of the second generation will display the recessive trait while ¾ display the dominant trait. More complex patterns appear, but they all fit this theory.
De Vries again • De Vries wanted to make his theory of heredity into a theory of evolution. • His idea was that sudden, dramatic shifts in the ‘genome’ could lead to a new species forming in a single step (a ‘mutation’). • One example seemed to fit: the evening primrose. The result was the mutation theory of evolution (like Bateson, the main force in evolution for de Vries was the force of variation, not selection).
Further developments • Thomas Morgan began his career as a disciple of Bateson and de Vries, and focused on studying variation and how it arises over time. • Defenders of natural selection, including Wallace, were unimpressed: the phenomenon of continuous, slowly changing variation in wild populations seemed undeniable to them. • But in the end it turned out that discontinuous genetic variation (genotypes), combined with ‘random’ effects from the environment, could easily produce a continuous range of phenotypic traits; Johannsen showed this in 1909; more details were revealed by Nilsson-Ehle, in his multiple-gene analysis of grain colours in wheat.
Smaller discontinuities • The distance between natural selection advocates and the geneticists was diminished by the gradual recognition that genes could produce small differences, not just major/sharp ones. • Morgan’s work reinforced this lesson. Small mutations in his fruit flies appeared regularly. This steady supply of small changes allowed Morgan to study inheritance in great detail, recognizing many more patterns: links between genes that tend to be inherited together (but not always) and other complexities were identified.
Lamarckism’s demise • The success of this particulate view of inheritance put the final nail in the coffin of ‘inheritance of acquired traits’. • Further, it became apparent that there was no hard and fast distinction between continuous and discontinuous variation. Mutations could produce fine steps and small changes as easily as sharp, large ones. • And study of selection effects in the wild showed that small differences could have a significant effect on survival.
Anatomy and descent • Meanwhile, detailed studies of anatomy and development in the vertebrates showed that the similarities already known were just the tip of the iceberg. • Evolution was reinforced by this evidence, which consistently extended and unified the tree structure found in anatomy, development and the fossil record. • The segmental theory of the head was a major accomplishment along these lines; observations of development showed that all vertebrates develop on the same segmental plan.
Evolution of the head • Here we see consilience– not just development, but the evolution of the vertebrate head showed shared patterns throughout the group, and a gradual transition from one pattern to another in transitional fossils. • Transitional forms linking fish to amphibians, amphibians to reptiles, and certain reptiles (the ‘mammal-like’ reptiles) to mammals were found. • Jaw morphology is a key line distinguishing mammals from reptiles; here and in other traits, though, the transition can be seen in the fossils. • Development later secured the identification of the two extra mammalian middle ear bones with the extra two reptilian jaw bones.
The Branching Process • The diversity of evolution, the fact that it has no set goal, no ‘destination’ (such as ourselves) also became apparent. • Divergence or radiation, not a linear ladder of progress, is what we find. • Groups start out small, with few forms (or one) that are not very specialized. • Later (when successful) we find much wider variation, more striking specializations and many more forms.
