Revisions of darwin's theory

REVISIONS OF DARWIN’S THEORY

Neo-Darwinism

The most serious weakness in Darwin’s theory was his failure to identify correctly the mechanism of inheritance. Darwin saw heredity as a blending phenomenon in which the hereditary factors of parents melded together in their offspring. Darwin also invoked the Lamarckian hypothesis that an organism could alter its heredity through use and disuse of body parts and through the direct influence of the environment. August Weismann rejected Lamarckian inheritance by showing experimentally that modifications of an organism during its lifetime do not change its heredity, and he revised Darwin’s theory accordingly.

We now use the term neo-Darwinism to denote Darwin’s theory as revised by Weismann. Mendelian genetics eventually clarified the particulate inheritance that Darwin’s theory of natural selection required. Ironically, when Mendel’s work was rediscovered in 1900, it was considered antagonistic to Darwin’s theory of natural selection. When mutations were discovered in the early 1900s, most geneticists thought that they produced new species in single large steps. These geneticists relegated natural selection to the role of executioner, a negative force that merely eliminated the obviously unfit.

Emergence of Modern Darwinism:

The Synthetic Theory

In the 1930s a new generation of geneticists began to reevaluate Darwin’s theory from a mathematical perspective. These were population geneticists, scientists who studied variation in natural populations using statistics and mathematical models. Gradually, a new comprehensive theory emerged that brought together population genetics, paleontology, biogeography, embryology, systematics, and animal behavior in a Darwinian framework. Population geneticists study evolution as a change in the genetic composition of populations. With the establishment of population genetics, evolutionary biology became divided into two different subfields. Microevolution pertains to evolutionary changes in frequencies of different allelic forms of genes within populations. Macroevolution refers to evolution on a grand scale, encompassing the origins of new organismal structures and designs, evolutionary trends, adaptive radiation, phylogenetic relationships of species, and mass extinction. Macroevolutionary research is based in systematics and the comparative method. Following the evolutionary synthesis, both macroevolution and microevolution have operated firmly within the tradition of neo-Darwinism, and both have expanded Darwinian theory in important ways.

MICROEVOLUTION:

GENETIC VARIATION AND

CHANGE WITHIN SPECIES

Microevolution is the study of genetic change occurring within natural populations. Occurrence of different allelic forms of a gene in a population is called polymorphism. All alleles of all genes possessed by members of a population collectively form the gene pool of that population. The amount of polymorphism present in large populations is potentially enormous, because at observed mutation rates, many different alleles are expected for all genes. Population geneticists study polymorphism by identifying the different allelic forms of a gene present in a population and then measuring the relative frequencies of the different alleles in the population. The relative frequency of a particular allelic form of a gene in a population is called its allelic frequency. For example, in the human population, there are three different allelic forms of the gene encoding the ABO blood types). Using the symbol I to denote the gene encoding the ABO blood types, I A and I B denote genetically codominant alleles encoding blood types A and B, respectively. Allele i is a recessive allele encoding blood group O. Therefore genotypes I A I A and I Ai produce type A blood, genotypes I B I B and I B i produce type B blood, genotype I AI B produces type AB blood, and genotype i i produces type O blood. Because each individual contains two copies of this gene, the total number of copies present in the population is twice the number of individuals. What fraction of this total is represented by each of the three different allelic forms? In France, we find the following allelic frequencies: I A ! .46, I B ! .14, and i ! .40. In Russia, the corresponding allelic frequencies differ ( I A ! .38, I B ! .28, and i ! .34), demonstrating microevolutionary divergence between these populations. Although alleles I A and I B are dominant to i, i is nearly as frequent as I A and exceeds the frequency of I B in both populations. Dominance describes the phenotypic effect of an allele in heterozygous individuals, not its relative abundance in a population of individuals. We will demonstrate that Mendelian inheritance and dominance do not alter allelic frequencies directly or produce evolutionary change in a population.

MACROEVOLUTION: MAJOR

EVOLUTIONARY EVENTS

Macroevolution describes large-scale events in organic evolution. Speciation links macroevolution and microevolution. Major trends in the fossil record are clearly within the realm of macroevolution. Patterns and processes of macroevolutionary change emerge from those of microevolution, but they acquire some degree of autonomy in doing so. The emergence of new adaptations and species, and the varying rates of speciation and extinction observed in the fossil record go beyond the fluctuations of allelic frequencies within populations. Stephen Jay Gould recognized three different “tiers” of time at which we observe distinct evolutionary processes. The first tier constitutes the timescale of population genetic processes, from tens to thousands of years. The second tier covers millions of years, the scale on which rates of speciation and extinction are measured and compared among different groups of organisms. Punctuated equilibrium is a theory of the second tier, explaining the occurrence of speciation and morphological change and their association over millions of years. The third tier covers tens to hundreds of millions of years, and is marked by occurrence of episodic mass extinctions. In the fossil record of marine organisms, mass extinctions recur at intervals of approximately 26 million years. Five of these mass extinctions have been particularly disastrous. The study of long-term changes in animal diversity focuses on the third-tier timescale.

Speciation and Extinction

Through Geological Time

Evolutionary change at the second tier provides a new perspective on Darwin’s theory of natural selection. Although a species may persist for many millions of years, it ultimately has two possible evolutionary fates: it may give rise to new species or become extinct without leaving descendants. Rates of speciation and extinction vary among lineages, and lineages that have the highest speciation rates and lowest extinction rates produce the greatest number of living species. The characteristics of a species may make it more or less likely than others to undergo speciation or extinction events. Because many characteristics are passed from ancestral to descendant species (analogous to heredity at the organismal level), lineages whose characteristics increase the probability of speciation and confer resistance to extinction should dominate the living world. This species-level process that produces differential rates of speciation and extinction among lineages is analogous in many ways to natural selection. It represents an expansion of Darwin’s theory of natural selection. This expansion is particularly important for macroevolution if one accepts the theory of punctuated equilibrium, which states that the evolutionarily important variation occurs primarily among rather than within species.

References : Hickman.2008.INTEGRATED PRINCIPLES OF ZOOLOGY, FOURTEENTH EDITION. New york. McGraw-Hill Companies