Towards establishing evolution


History of Evolution:

The idea of evolution goes back to Jean Lamarck in the early 1800's. Lamarck published a theory of evolution in 1809. Lamarck thought that species arose continually from nonliving sources. These species were initially very primitive, but increased in complexity over time due to some inherent tendency. This type of evolution is called orthogenesis.

Lamarck proposed that an organism's acclimation to the environment could be passed on to its offspring. For example, he thought proto-giraffes stretched their necks to reach higher twigs. This caused their offspring to be born with longer necks. This proposed mechanism of evolution is called the inheritance of acquired characteristics. Lamarck also believed species never went extinct, although they may change into newer forms. All three of these ideas are now known to be wrong.

Even when resources are plentiful, the size of a population tends to increase geometrically until the population outstrips its food supply. This led Malthus to believe that poverty, disease, and famine was a natural and inevitable phenomenon, leading to a "struggle for existence".

Evolution came of age as a science when Charles Darwin published "On the Origin of Species." Darwin's contributions include hypothesizing the pattern of common descent and proposing a mechanism for evolution -- natural selection.

In Darwin's theory of natural selection, new variants arise continually within populations. A small percentage of these variants cause their bearers to produce more offspring than others. These variants thrive and supplant their less productive competitors. The effect of numerous instances of selection would lead to a species being modified over time.

Darwin also recognized several critical facts:

It then follows logically that certain variants will be preserved over time over other variants and that populations will change over time in their composition. This is evolution by natural selection.

The greatest weakness in the theory of evolution by natural selection was the fact that Darwin knew neither how variation among individuals was generated nor how it was inherited.

Biology as a science made its move from an Arisotitlean stage to a Newtonian one with the development of the theory of evolution. Evolution is a change in the gene pool of a population over time. A gene is a hereditary unit (the microscopic `atom') that can be passed on unaltered for many generations. The gene pool is the set of all genes in a species or population (the macroscopic `object').

Populations evolve, not individuals. In order to understand evolution, it is necessary to view populations as a collection of individuals, each harboring a different set of traits. A single organism is never typical of an entire population unless there is no variation within that population. Individual organisms do not evolve, they retain the same genes throughout their life. When a population is evolving, the ratio of different genetic types is changing -- each individual organism within a population does not change. For example, in the previous example, the frequency of black moths increased; the moths did not turn from light to gray to dark in concert.

The process of evolution can be summarized in three sentences: Genes mutate. Individuals are selected. Populations evolve.

Evolution requires genetic variation. In order for continuing evolution there must be mechanisms to increase or create genetic variation and mechanisms to decrease it. Mutation is a change in a gene. These changes are the source of new genetic variation. Natural selection operates on this variation.

Considerable variation is present in natural populations. Levels of genetic variation in animals range from roughly from 15% in birds, to over 50% in insects. In most populations, there are enough genes so that every individual, identical twins excepted, has a unique combination of gene traits.


Selection:

Some types of organisms within a population leave more offspring than others. Over time, the frequency of the more prolific type will increase. The difference in reproductive capability is called natural selection. Natural selection is the only mechanism of adaptive evolution; it is defined as reproductive success of classes of genetic variants in the gene pool.

The most common action of natural selection is to remove unfit variants as they arise via mutation. In other words, natural selection usually prevents new genes from increasing in frequency.

Natural selection can be broken down into many components, of which survival is only one. Sexual attractiveness is a very important component of selection, so much so that biologists use the term sexual selection when they talk about this subset of natural selection. Sexual selection is natural selection operating on factors that contribute to an organism's mating success.

Three examples of selection are shown before stabilizing, disruptive and directional. The black dots are individuals that die out before passing on their genes.

Stabilization: Removes the extreme ends of the distribution. Moves everything closer to the middle.

Directional: Depopulates preferentially one extreme side of the distribution (usually the disfavorable side) has disastrous social consequences if believed (e.g. Nazi Germany).

Disruptive: Occurs when individuals at both ends of the curve have a higher survival probablity than those in the middle. Ultimately drives mutation and new, isolated species. Argueably this is what happens on Earth. Stimuli to Evolove:

Dispersal of Parental Units by a variety of mechanisms:

Passive: (slowly alters gene pool over time)

Active: (catastrophe "overnight" change in ecosystem)

Survival of the fittest works in well-defined, slowly evolving ecosystems (like the ocean); On long time scales, the role of random catastrophe is more important

Mass Extinctions

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Geological History:

Mass extinctions mean:


What Can we Conclude from the Above Graph:

It seems likely than when terrestrial ecosystems developed, the flow of nutrients to the oceans became impaired which served to suppress that ecosystems growth rate over the rest of geologic time.