An Easy To Understand Explanation for the Mechanisms of Human Evolution.
One thing we know for certain is that everything always changes. Ever since the big bang the universe has been changing however long the changes take. It can take billions of years for stars, planets and galaxies to form. On earth it can take hundreds of millions of years for continental drift to move the earth's land masses around the globe and it can take million years for a species to evolve from one to another. But mostly not. Mostly not a million years, in fact it can take a relatively short period of time for even mammals to evolve from one species to another. A period of tens of thousands of years while quite long to us is not all that long in the big scheme of things and it can be plenty of time for a species to change. After all some small organisms will evolve in matter of years and observing them has given us the research to theorize on evolution in general.
We can define evolution as change and we know all things change around us, even mountain ranges grow and decline. When it comes to living organisms we can define evolution as the process of DNA changing through the generations.
Before we talk about the processes of evolution let’s take a look at DNA itself. Humans have 46 chromosomes, 23 pairs. Chromosomes are made up of DNA and Genes are a certain stretch along the DNA with the blueprint to make proteins. Humans have approximately 22,000 genes which comprises only 3% of our DNA. Genes are the basic unit for genetics and all evolutionary change is genetic change.
DNA makes the organism what it is and the characteristics of a life form are coded in the genes and given enough time and change in the DNA a life form will evolve from one species to another.
The function of the 97% of DNA that is not genes is a mystery and used to be known as ‘junk’ DNA. However, this ‘junk’ DNA might not be so junky after all. It is now theorized that long stretches of non-coding DNA can provide a rich source of material for quickly adapting and evolving.
To summarize this one point, DNA with its genetic code defines the nature of a species and there are known mechanisms which can modify the genetic code giving us the basis of evolution.
In biological terms evolution is the change in DNA through the generations of an organism.
Mutation and Natural Selection
Since Charles Darwin science has embraced the concept of evolution. The accepted vehicle for evolution has long been considered to be natural selection as outlined by Darwin. In recent years however there has been a new understanding about the initial impetus for the evolution of a species.
It stands to reason that if the DNA never changes the species would never change. Small changes to the DNA that are passed on to offspring over time can lead to big changes to a species. The mechanism for significant or even drastic change in the DNA is mutation. Mutations are random and can be detrimental or beneficial or have no effect at all, in fact the vast majority of mutations are neutral and do not evolve the species in one direction or another.
Mutations in the genetic code which give the species a survival advantage can take hold and spread in a given gene pool relatively rapidly.
The following is reproduced from the website The Genetic Science Learning Center.“The whole human family is one species with the same genes. Mutation creates slightly different versions of the same genes, called alleles. These small differences in DNA sequence make every individual unique. They account for the variation we see in human hair color, skin color, height, shape, behavior, and susceptibility to disease. Individuals in other species vary too, in both physical appearance and behavior.
Genetic variation is useful because it helps populations change over time. Variations that help an organism survive and reproduce are passed on to the next generation. Variations that hinder survival and reproduction are eliminated from the population. This process of natural selection can lead to significant changes in the appearance, behavior, or physiology of individuals in a population, in just a few generations.
Once new alleles arise, meiosis and sexual reproduction combine different alleles in new ways to increase genetic variation.
Without mutation there would be no variation, and without variation there would be no evolution.”
Gene Flow and Genetic Drift
Gene Flow and Genetic Drift are both mechanisms for distributing changes across a species. So in effect they are not agents of change in the DNA like a mutation would be but rather promoters of that change in a given gene pool.
It is pretty clear to me that the initial trigger for change would be a mutation. Then once that takes place if the mutation is advantageous natural selection goes to work and gives that change a chance to spread throughout the species.
The process by which organisms that are better suited to their environment than others produce more offspring. As a result of natural selection, the proportion of organisms in a species with characteristics that are adaptive to a given environment increases with each generation. Therefore, natural selection modifies the originally random variation of genetic traits in a species so that alleles that are beneficial for survival predominate, while alleles that are not beneficial decrease. Originally proposed by CharlesDarwin, natural selection forms the basis of the process of evolution. (from The American Heritage® Science Dictionary)
Masatoshi Nei, world renowned molecular biologist. (From an interview in Discover Magazine)
“First you have to have change, and then natural selection may operate or may not operate. I say mutation is the most important, driving force of evolution. Natural selection occurs sometimes, of course, because some types of variations are better than others, but mutation created the different types. Natural selection is secondary. My position is mutation creates variation, then natural selection may or may not operate, it may or may not choose the good variation and eliminate the bad one, but natural selection is not the driving force. For example, if blue eyes are better for some reason in Scandinavia, that mutation has a selected advantage, and then of course that advantage will occur more in that population. But first you have to have the mutation.”
Until the study of DNA in evolution natural selection was thought to be the main driving force. As it turns out natural selection is just one of the forces driving evolution.
Now that we understand the actual mechanism that precipitates change there is still the question of how one species changes into another. How do we go from Australopithecus africanus to Homo erectus? To get from one early man species to another speciation had to occur.
A Dictionary definition. Speciation – The formation of new species as a result of geographic, physiological, anatomical or behavioral factors that prevent previously interbreeding populations from breeding with each other.
Scientists consider four main types of speciation, allopatric, peripatric, parapatric, and sympatric. Let’s look at each one.
1. Allopatric Speciation
This occurs when a species gets to be separated and isolated into two groups by some environmental or physical divide like a river or mountain range or large desert, canyon or riff develops, even a long distance between the two groups, etc. These barriers prevent interaction and gene flow between the two groups and they can no longer interbreed. Over time the genetic mutations from each group will move them in separate ways and eventually when and if they come back into contact they can no longer interbreed successfully.
2. Peripatric Speciation
This type is very similar to allopatric in that a given population will be separated by a geographical barrier. The difference between the two is with peripatric speciation a small group splits off or becomes separated from the larger group. The term ‘founder effect’ can apply to this small population which has been reproductively isolated from the larger parent population group and is subject to ‘genetic bottlenecking’. The term genetic bottlenecking refers to the potential for change in a reduced population size.
It seems to me that when talking about early man, peripatric speciation is the most likely form of speciation that occurred. Small mobile groups become isolated by some geographical barrier or a large distance and because the group is small beneficial genetic mutations that occurred would, through natural selection, be able to change the smaller population more rapidly than a larger one. Over the course of thousands of years a new species would have evolved.
3. Parapatric Speciation
In this type of speciation there are no geographical isolating barriers to inhibit mating between groups. It appears that individuals are much more likely to mate with those in close proximity than in a different part of the given population’s range. This type of speciation is not likely for the human linage.
4. Sympatric Speciation
Sympatric speciation is when a given breeding population separates from each other due to some sort of breeding preference and not because of any geographical barrier. This is considered very rare and until recently only observed in plants. I say until recently because scientists have discovered sympatric speciation occurring in the hominin population of the United States currently. Due to the polarizing divisiveness of the political climate it has been noticed that Democrats and Republicans have stopped mating with each other. Most, if not all, mixed couplings when they happen are from an alcohol induced stupor. It is expected that in another 4 to 5 presidential cycles any such sexual encounters will not be able to produce offspring. In the meantime if any offspring are produced they will become a hybrid species called an ‘Independent’.
In part one of this essay on human evolution we talked about time and how it relates. In this second part we’ve looked at the actual mechanisms involved in the process of evolution and how it is possible for one species to become two. In part three we are going to look at the species of man and see what evidence paleoanthropologists have unearthed so far.