The Modern evolutionary synthesis is combination of Darwinian evolutionary theory and Mendelian genetics. It is impossible to understand the theory and it's importance to the scientific community unless one understands the history behind the theory.
From 1902 to 1953 major publications in the areas of systematics, developmental biology, botany, population genetics, and paleontology sucessfully integrated Darwin's four postulates and Mendelian genetics into a reformation of evolutionary theory. The new theory is referred to as the Modern Synthesis, Evolutionary Synthesis, or the Modern Evolutionary Synthesis. These terms can be used inter-changeably.
Before one can understand the Modern Synthesis and this analysis there a few defintions that must be explained in order to grasp the concept, espically if a person is not familiar with biological terminology. This list of definitions does not have to be read fully but is provided to refer to when a biological term is not understood... of course maybe not every term that isn't understood, but the majority.
-Allele-one of a pair, or series, of alternative forms of a gene that occur at a given locus (location) in a chromosome.
-Fitness-the number of offspring left by an individual, often compared with the average of the population or with some other standard, such as the number left by a particular genotype.
-Gene-a hereditary determinant of a specific biological function; a unit of inheritance (DNA) located in a fixed position on a chromosome.
-Genotype-the genetic constitution (gene makeup) of a an organism.
-Phenotype-The observable characteristics of an organism.
-Chromosomes-darkly staining nucleotide bodies that are observed in cells during division. Each chromosome carries a linear array of genes.
-Mutation-a change in DNA at a particular locations in an organism. The term is used loosely to include point mutations involving a single gene change as well as a chromosomal changes.
-Variation-in biology, the occurrence of differences among individuals.
-Taxon (plural: Taxa)-any named group of organisms.
-Macroevolution-large evolutionary change ,usually in morphology; typically refers to the evolution of differences among populations that would warrant their placement in different genera or higher-level taxa.
-Microevolution-changes in a gene frequencies and trait distributions that occur within populations and species.
-Inheritance-the hypothesis that phenotypic changes in the parental generations can be passed on intact, to the next generation.
-Population-for sexual species, a group of interbreeding individuals and their offspring; for asexual species, a group of individuals living in the same area.
-Natural Selection-a difference, on average, between the survival or fecundity of individuals with certain phenotypes compared to individuals with other phenotypes.
-Definitions from the glossaries of Campbell;Freeman & Herron; and Jenkins, Simmons, & Snustad.
The Modern Synthesis in its most basic from is actually a progression of ideas that have been passed on from ancient civilizations and ancient minds by the hands of time to modern man. The idea of evolution dates back far before Charles Darwin, but if one was to try and trace the total all inclusive history of how the Modern synthesis came about from the very beginning then a person would have two options, first being to devote their whole life to the project and when they reached the age of 105 they might have the complete history totaling thousands and thousands of pages, and second to just loose your mind and save yourself some trouble. Fortunately for us, in order to understand the Modern Evolutionary Synthesis, a full history of evolution is not necessary.
The initial change that started people thinking about evolution was the old world view versus the new world views. There were five basic issues that challenged the traditional thinking in the world. The first was the expansion of the time-scale. The traditional view was that the earth was only a few thousand years old, however the new views said that the earth was at the very least a few million years old. The current age of the earth is estimated at between four and five billion years old. The second issue challenging the past thinking was the idea of a changing universe. The old view was that the geological formations had been there since Noah's flood. The idea was that nature only maintains the original forms created by God. The new view is that there is and always has been constant change taking place on and to the earth. The third idea challenged the existing notion that each individual life form had been designed by the Creator. The new idea was that the ever changing forces of nature forces life forms to change and adapt to it's changing environment, that natural processes account for the origin of life. The fourth idea was that divine spontaneous appearance of species does not occur. The fifth idea was that man was a part of nature. Earlier ideas were that man was separate from the rest of the animal world (Bowler, 4-8). These were the first ideas that got the ball rolling on the modern evolutionary theory. Next was Charles Darwin's announcement of his theory of evolution.
Charles Darwin made the announcement of his theory of evolution by natural selection public at the Linnaean Society in 1858 (Meadows, 151). Then in 1859 he published his book On the Origin of Species by Means of Natural Selection, in which he stated his four postulates.
Darwin's original four postulates on the theory of evolution through natural selection are:
Individuals within species are variable.
Some of these variations are passed on to offspring.
In every generation, more offspring are produced than can survive.
Survival and reproduction are not random: The individuals that survive and go on to reproduce, or who reproduce the most, are those with the most favorable variations. They are naturally selected.
-(Freeman & Herron, 35).
The main factors that influenced Darwin's evolutionary theory were the use of judging the earth's age by the use of new methods of geology. It was also his understanding of geological barriers such as oceans, mountain ranges, and deserts that helped him understand the change of species in order to adapt to their environment. He began to initiate his thoughts on evolutionary theory during his voyage on the H.M.S. Beagle and his study of the Galapagos Islands, especially with his study of the various finch populations (Meadows, 151). The four postulates were the core of Darwin's theory of evolution, they were what modern synthesis is partially based upon, the other part being genetics.
The start of modern genetics was initiated by Gregor Mendel, who published his theory of genetics in 1865 but was not really popularized until the rediscoverory of his work in the 1900's by biologists. It took another 30 years until a unification of Mendelian genetics and Darwinian evolution.
The core of Mendel's theory lies within population genetics. Population genetics' main concern is with whether a particular allele or genotype will become more or less common in a population over time and why. From a population genetic point of view, evolution is change that has occurred over the course of generations and time in the frequency of alleles (Jenkins, Simmons, & Snustad, 722). The law that Mendel came up with to explain his findings is called the law of segregation, which states that each individual carries two unblending copies of each gene, and that each gamete receives only one copy of each gene chosen at random from the individual's two. This allowed biologist to predict the fate of alleles with in families (Freeman & Herron, 122). Mendel did work for more than thirty years on pea plants to establish his principles.
Mendel's law of segregation does allow the following of alleles if a person assumes a population in which:
there is no selection (that is, all individuals have equal rates of survival and equal reproductive success);
there are no mutations converting one allele to another or creating new alleles;
there is no migration of individuals into or out of a population;
there are no random events that cause come individuals to pass on more of their genes than do other individuals;
individuals choose their mates at random.
If all of these hold true then the relative abundance of different alleles will not change across generations. Evolution will not take place in this population (Freeman & Herron, 122). If any one of the five assumptions above is violated then the alleles in a group may change and evolution might take place.
The Hardy-Weinburg Principle
The initial application of Mendelian genetics to interbreeding populations began almost immediately after Mendel's work was rediscovered. The first person to pick up Mendel's work and put it into application was Udny Yule in 1902. His main point was as follows: " ...when members of an F2 population, segregating for a single pair of alleles (A and a), interbreed at random, the three possible genotypes (AA, Aa, and aa) are represented in the same proportions in the F3 and all succeeding generations" (Jenkins, Simmions, & Snustad, 722). The F2 is the second filial generation (grandchildren) produced by crossing the F1. Thus F3 could be considered the great grand children. In 1903, another man by the name of William Castle, added to Yule's work. He found that "genotypic ratios would change in each generation if the aa class were selected out at each generation", i.e. they were physically removed or prevented from mating. In 1908 G.H. Hardy, a mathematician, found that in order for "a 3:1 phenotypic ratio to occur, the frequency of the dominant and recessive alleles each had to be 0.5, as they are a monohybrid cross between AA and aa parents", thus both alleles equal 1. The reason that he noticed this is due to Yule not observing that phenotypic frequencies are a direct function of the allele frequency. Also in 1908 Wilhelm Weinberg, a German physician, did ground breaking work using Mendel's principles. Shockingly enough, he was the first person to take Mendel's genetic principles and apply them to the human population. His work in addition to Hardy's basically related the frequency of genotypes to the frequency of alleles in populations. These two insights are known the Hardy-Weinberg Principle.
Two conclusions come from this:
The allele frequencies in a population will not change, generation after generation.
If allele frequencies in a population are given by p and q the genetic frequencies will be given by p², 2pq, and q².
-Basically this means p=A1(allele 1) and q=A2 (allele 2): they are both part of a larger statistical equation. Hardy and Weinberg used the p and q equations to find the genetic frequencies and the allelic frequency which is steady at all values between 0 and 1, as long as p+q=1.
-(Jenkins, Simmions, & Snustad, 722) and (Freeman & Herron, 126-127).
The Hardy-Weinberg principles will only hold true if all the assumptions of Mendel's law of segregation are not violated.
In general they proved that, all things being equal (ceteris paribus), in the limits of those assumptions evolution does not happen. Yes, they proved that evolution does not happen. The fact is, however, that things do not remain equal. In other words, it has been consistently shown with vast amounts of evidence and scientific data that those assumptions almost never hold. They may only in a rare circumstance and then it is only for a very short period. Variations occur.
The Modern Evolutionary Synthesis
As mentioned earlier the idea of the Modern Evolutionary theory was formed from the publication of articles in vast biological disciplines. There were a great number to people who contributed to it's formation. However there are a few main players that certainly made a huge contribution to the theory, obviously a few have already been mentioned.
Thomas Hunt Morgan was a geneticist in the early 1900's who began his work with fruit flies and genetic mutation. He showed that mutation occurs in every generation and in every trait. This dispelled the earlier notion that variability inside a population was strictly limited. Morgan's experiments were key because until he arrived on the genetic scene mutation was a little known and little understood phenomena.
August Weismann was a German developmental biologist who did work on germ lines and somatic cells. Weismann observed that germ line and somatic cells are separated early in development and do not interact. The basic idea before was that particles migrated from different body tissues to the germ cells and then would influence inheritance. Weismann proved this idea to be wrong (Freeman & Herron, 52).
Theodosius Dobzhansky was an evolutionary biologist who worked at Morgan's Lab in California in the 1930s. Dobzhansky's first work was on translocation but then in the mid-1930s, collaborating with mathematical theorists, he formulated his synthesis between genetics, adaptation, and biological diversity (Smocovitis, 1632).
Advances in botany lead to some remarkable new insights into evolution as well. G. Ledyard Stebbins, Jr. did work on genetic and selective theory in plants between 1920 and 1950. His research ended in 1950 with the publication of his book Variation and Evolution in Plants (Smocovitis, 1625).
Ernst Mayr was an evolutionary biologist that effectively brought systematics into the progressing evolutionary theory. This was brought about by the publication of his book in 1942 titled, Systematics and the origin of Species. His contributions to the modern evolutionary theory were vast due to the various roles he played such as historian, philosopher, organizer, and general promoter of evolutionary biology. Mayr sponsored and was editor of the scientific journal Evolution (1939-1950) and he had a large part in the formation and organization of the Society for the Study of Evolution (Smocovitis, Evolution, 1).
George Gaylord Simpson was a paleontologist who also made new scientific connections between paleontology, genetics, and evolutionary biology (Campbell, 417).
G. Ledyard Stebbins, Jr., Ernst Mayr, Theodosius Dobzhansky, and George Gaylord Simpson are the four people that are directly credited with actual formation of the Modern Synthesis itself (Campbell, 417).
The theory (Modern Synthesis) is based on two central ideas:
Gradual evolution results from small genetic changes that are acted upon by natural selection.
The origin of species and higher taxa, or macroevolution, can be explained in terms of natural selection acting on individuals, or microevolution.
-(Freeman & Herron, 54).
According to the new theory, Darwin's four original postulates could be restated in the following ways:
As a result of the mutation creating new alleles, segregation, and independent assortment shuffling into new combinations, individuals within populations are variable for nearly all traits.
Individuals pass their genes on to their offspring intact and independently of other genes.
In most generations, more offspring are produced than can survive.
The individuals that can survive and go on to reproduce, or who reproduce the most, are those with the alleles and allelic combinations that best adapt them to their environment.
The major out come of these ideas are that the alleles associated with the greater fitness increase in frequency form each generation to the next.
-(Freeman & Herron, 54).
The Modern Evolutionary Synthesis has been a culmination of ideas that have taken a great amount of time. The effect that it has had on the scientific community has been far reaching and dramatic. This however, is nothing compared to the controversy it has raised in society. Within the scientific community no one doubts that evolution takes place.
The importance of evolution is summarized in this quote:
"Nothing in biology makes sense except in the light of evolution"
-Theodosius Dobzhansky (evolutionary geneticist)
These are a few good books about evolution and the controversy around it:
Evolution: The History of an Idea by Peter J. Bowler
Science on Trial: The Case for Evolution by Douglas J. Futuyma
The Blind Watchmaker: Why the Evidence of Evolution reveals a Universe without Design by Richard Dawkins
A few Good evolution/genetic web links to look at: