Origin of Life and Evolution Theories
Life’s Origin
Miller’s Experiment (The Primordial Soup Theory).
In 1953, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed on early Earth from inorganic precursors. The now-famous “Miller-Urey experiment” used a highly reduced mixture of gases – methane, ammonia and hydrogen – to form basic organic monomers, such as amino acids.
Joan Oró
However, the current scientific consensus is that such was not the case. The next most important step in research on prebiotic organic synthesis was the demonstration by Joan Oró that the nucleic acid purine base, adenine, was formed by the simple heating of solutions of ammonium cyanide.
Haldane and Oparin
In 1924 when Aleksandr Ivanovich Oparin reasoned that atmospheric oxygen prevented the synthesis of the organic molecules that are the necessary building blocks for the evolution of life. In his The Origin of Life, Oparin argued that a “primeval soup” of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight
EVOLUTION
Biological evolution is descent with modification. This definition could be understood as a small-scale evolution (changes in gene frequency in a population from one generation to the next) or large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren’t examples of biological evolution because they don’t involve descent through genetic inheritance.
THE THEORIES OF EVOLUTION
Despite the great diversity of theories, they can be grouped into two basic ideas:
Those that consider that species are created exactly as we know them today and that they are fixed forms of life. They are all creationist and fixist explanation of the origin of species. They do not consider the possibility of being proved or modified. For this reason they are not used in modern scientific activities.
Those that consider that species have been transformed and continue to be transformed throughout the history of life. The origin of the different species is caused by the progressive accumulation of these transformations. These are evolutionist theories.
ANATOMY AND RELATIONSHIPS
Homologies and Analogies
We use homologous characters as characters in different organisms that are similar as indicators of common ancestry because they were inherited from a common ancestor that also had that character. These characters have a certain similarity in their internal anatomy although they are different in their morphology and their functions.
An example of homologous characters is the four limbs of tetrapods. Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature—so the presence of four limbs is a homology.
Not all characters are homologies. For example, birds and butterflies both have wings. Does that mean that birds and butterflies are more closely related to one another? No.
Bird and butterflies wings are analogous—that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function. Analogies are the result of convergent evolution.
Descent with Modification
We’ve defined evolution as descent with modification from a common ancestor, but exactly what has been modified? Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are heritable and can be passed on to the next generation—which is what really matters in evolution: long term change.
Compare these two examples of change in beetle populations. Which one is an example of evolution?
Beetles on a diet
Imagine a year or two of drought in which there are few plants that these beetles can eat. All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in the population are a little smaller than the preceding generation of beetles.
Beetles of a different color
Most of the beetles in the population (90%)have the genes for bright green coloration and a few of them (10%) have a gene that makes them browner. Some number of generations later, things has changed: brown beetles are more common than they used to be and make up 70% of the population
Which example illustrates descent with modification—a change in gene frequency over time?
The difference in weight in example 1 came about because of environmental influences—the low food supply—not because of a change in the frequency of genes. Therefore, example 1 is not evolution. Because the small body size in this population was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply.
The changing color in example 2 is definitely evolution: these two generations of the same population are genetically different.
MECHANISM OF EVOLUTIONARY CHANGE
Each of these four processes is a basic mechanism of evolutionary change.
Mutation
A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make the genes for brown beetles more frequent in the population.
Migration
Some individuals from a population of brown beetles might have joined a population of green beetles. That would make the genes for brown beetles more frequent in the green beetle population.
Genetic Drift
Imagine that several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation—but just by chance. These chance changes from generation to generation are known as genetic drift.
Natural Selection
Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration on to their offspring. So in the next generation, brown beetles are more common than in the previous generation.