Introduction to evolution

Overview

The goal of life forms is to reproduce.

Reproduction with variation creates variety.

Some varieties of life survive better.

Some of these life forms produce more offspring.

Some differences are passed on to offspring.

Over time new species result.

This story, forming the history of life on the planet, is recorded in fossils.

Evolution is the term used to describe the change of organisms over generations.

In a general way, evolution is described as the changes that have transformed life from its earliest origins into the diverse forms of life represented today. More specifically, the Scientific Theory of Evolution states that all living things share, at some point in their evolutionary history, a common ancestor. Evolution depicts life as a tree, with many branches arising from a single trunk. The tips of the branches represent present-day life forms. Each fork in the branch represents ancestors common to all lines arising after the split.

The idea of common ancestry has its roots in the Darwinian revolution. Charles Darwin (1809-1882)saw unity among all life, and he proposed that all living things are related and have descended from a common ancestor. Darwin describes these events as descent with modification. The Darwinian explanation for the mechanisms of evolution is based on his theory of natural selection, which in turn is based on five basic ideas:

- Organisms will produce more offspring than their habitat can sustain. There will be a 'struggle to survive'.
- Not all the offspring will be identical.
- Some of the differences between the offspring will be due to variations in their genetic makeup.
- Genetic variations that allow an organism to survive and reproduce will be passed on to the next generation. Genetic variations that do not help an organism to survive and reproduce will be lost.
- Over time, these genetic variations will accumulate until a new species results.

Darwin’s Theory on Natural Selection laid the ground work for the evolutionary theory. However, it was the emergence of the field of Genetics, pioneered by Gregor Mendel(1822-1884, that provided the missing information on hereditary processes which now solidifies the Theory of Evolution. The mergence of Darwin’s ideas with our current understandings of heredity lead to the birth of Population Genetics.

Population Genetics

In terms of Population Genetics, the definition of Evolution may be further refined to mean: A generation to generation change in the frequencies of alleles within a population that share a common gene pool. A population is defined as: A localized group of individuals belonging to the same species. An example would be all the bass sharing a single pond. A gene pool is described as: The sum of all the alleles (variations in genes) shared by members of a single population. Thus, a population is united by its gene pool. Small scale changes in the frequencies of genes in a gene pool is referred to as microevolution.

To understand the mechanisms that allow a population to evolve, it may be beneficial to first describe the circumstances that would prevent microevolution. Such a situation is described by the Hardy-Weinberg theorem.

The Hardy-Weinberg Theorem states that: The frequencies of alleles in a population’s gene pool will not change over time unless acted upon by forces other than both random reshuffling of alleles during sex cell formation and random combination of sex cells during fertilization.

For example: In a population of mice that inhabit a barn, there are only two variations in the gene that controls fur color. One allele codes for black and accounts for 75% of the gene pool, the other for white, which accounts for the remaining 25%. According to the Hardy-Weinberg Theorem, if the only factors determining an allele’s potential to be passed on to the next generation are random formation of sex cells and fertilization; the frequencies will remain 75% black and 25% white. In this case, no changes in the frequencies of alleles in the mouse population, thus no microevolution, at least in regards to fur color. Such a population is said to be in Hardy-Weinberg Equilibrium

For a population to exist in Hardy-Weinberg Equilibrium the following conditions must be met.

The population must be large. Small populations are subjected to chance fluctuations in the gene pool, a condition known as genetic drift.
There can be no exchange of members between populations. Such migration between populations is called gene flow.
There can be no mutations. Mutations introduce new variations in a trait will result in changes in frequencies; although the initial effect would be minimal.
Mate selection must be random. There can be no preference for any particular allele during mate selection.
Natural selection can not be a factor. The probability of the survival and reproduction of all the alleles must be equal.

Such criterion rarely if ever, exists in a natural population. Therefore, frequencies of alleles in a gene pool are always changing, resulting in the microevolution of populations over successive generations.

Evidence For Evolution

Fossil Record

Paleontology (the study of fossils) supports Darwin's original idea that all living creatures are related and that accumulated changes over long periods has lead to the diverse forms of life we see today.

The field of Paleontology was developed by Georges Cuvier (1769-1832). Cuvier noted that each layer of sediment was defined by a specific group of fossils. The deeper (older) the layer, the more simplistic the life forms. He also noted that many forms of life that existed in the past are no longer represented in the present. Cuvier proposed the idea of catastrophism, which he used to explain the fossil record in the light of the theologian views of his time. He proposed that catastrophes occurred in localized areas throughout the earth’s history. Such areas were then repopulated by species that migrated in from nearby locations. Such explanations were supported by the biblical text, such as the case of Noah’s flood. This view is no longer held in favor.

Today, the fossil record is more complete. The fossil record now serves as a chronological record documenting the emergence of new, more complex species from simpler ancestral forms. The fossil record also provides some examples of transitional species that provide evidence of ancestral links between species that exist today.

Perhaps the best know of these transitional fossils is ''Archaeopteryx'', an ancient fossil that has the distinct characteristics of a reptile, yet clearly possess the feathers of a bird. The implication from such a find is that modern reptiles and birds arose from a common ancestor.

Comparative Anatomy

Artificial Selection

Molecular Biology

Evolution in action

Ways we can see evolution in action include convergent evolution and adaptive radiation.

Convergent evolution results in unrelated organisms looking similar because they have evolved to survive in similar habitats. Dolphins, sharks, and ichthyosaurs all look very similar, but are not at all closely related. One is a mammal, one a fish, and one a reptile, but evolution has shaped their bodies to look similar because they all lived in the ocean and swam quickly after fish.

Adaptive radiation means that a single part of the body can be re-shaped to be used in different ways. A good example of this is the hand of a mammal. Bats, humans, whales, and cats all have very different looking hands, but they are all made up of similar patterns of bones. Another example is the wing of a bird. Over time some birds' wings evolved into more of a flipper, so that they were better suited for swimming (penguins). Some birds' wings evolved for soaring, so that they would not use as much energy when flying for long periods of time (vultures), and some birds' wings have been adapted simply for showing off and making the bird appear larger (ostriches).

Images

The diversity of life.......(expand here)