Darwin and Mendel
Darwin’s theory of evolution via the process of natural selection handily explained how species change over time, a process which had mystified scientists who examined specimens from the fossil record, and noted the extreme diversity of life on earth.
It wasn’t until the 1930s that Mendel’s theories of inheritance, combined with Darwin’s evolution work, provided the connection between the mechanism of evolution and the units of evolution. Mendel’s work showed that natural selection was a process which involved changes in genes: units of information passed from parents to offspring over the generations.
Evolution is based on Variations in Heritable Genetic Information
Inherited traits are those which are passed down from generation to generation via the passage of genetic material, in the form of chromosomes and genes. Examples of these are eye color, hair color, and skin color.
These traits are passed to successive generations via DNA, a linear macromolecule which is made up of a phosphate backbone attached to molecules called bases. There are four bases, and the sequence of these along the DNA is ordered to create units called genes. A single gene codes for one protein.
Over time, DNA can change via a number of mechanisms, such as the reshuffling which occurs during sexual reproduction and genetic mutations (and in some species such as plants and bacteria, through processes such as hybridization and horizontal gene transfer).
In a given population, these genetic changes, coupled with evolutionary forces (such as habitat, climate, or the existence of predators), are what serve to drive evolution.
Variations in genetic material come from random mutations—changes in the DNA sequence of the genome. However, the end result of a mutation is not always beneficial to the organism. In fact, the majority of mutations are either harmful or have no effect. It’s only rarely that a mutation will have even a weakly beneficial effect on an organism/
Mutations can involve changes in a single gene, or much larger sections of DNA which get duplicated or rearranged to a new location on the same chromosome, or even a different chromosome.
The human genome is full of locations where such mutations have occurred. One such example is where, long ago, two chromosomes in the genome of an ancestral species of Homo fused, to become what we now call human chromosome 2. This didn’t occur in any other species, and our closest relatives, the great apes, still retain the two separate chromosomes. (In evolutionary terms, this type of mutation is beneficial because it means species which undergo these large-scale mutations are less likely to be able to interbreed with species they once bore a closer resemblance to.)
Reproduction and Recombination
The development of two separate sexes is an evolutionary change which is thought to have accelerated the pace of evolution itself. This occurred because the coming together of DNA from two parents (in the form of egg and sperm) allows for a process called recombination, in which the two sets of chromosomes pair off and swap sections of DNA.
Apart from speeding up the pace of evolution, the actual function of sexual recombination is unclear. The process does, however, help to remove harmful mutations from a population, while retaining beneficial mutations.
Mechanisms and Results of Evolution
Evolution is a process of genetic change over time, causing a change in the frequency of certain genes from generation to generation. There are three mechanisms by which this genetic change occurs: natural selection, genetic drift, and gene flow.
Natural Selection is a process which sorts the beneficial from the harmful mutations, in the context of the environment and habitat of the species in question. In a given population, more offspring are produced than can survive on the available resources, and due to genetic variation they vary in their ability to thrive and reproduce. Offspring with beneficial mutations are more likely to survive and reproduce than are offspring without those mutations. (Similarly, offspring with harmful mutations are less likely to survive than non-mutated offspring.)
Genetic Drift is a change in the frequency of a particular gene over the generations. It occurs because the genetic material passed along to offspring is a more or less random sampling of those which came from the parents. In the absence of selective forces, gene frequencies ‘drift’ fairly randomly, halting only when the gene becomes fixed in the population or disappears entirely.
Gene Flow is an exchange of genetic material between two different populations of the same species. This occurs when, for example, a male lion leaves its own pride to mate with lionesses of a different pride.
The results of all this genetic mixing and shuffling is that organisms are able to adapt to their environments—a process which generally occurs very slowly, over long periods of time. In fact, approximately 4,000 million years has passed since organic molecules first appeared on the earth.
Evolution is by no means a fast-paced or even a progressive process (contrary to the idea that evolution has ‘concentrated’ on producing progressively more complex organisms, complex species are still far outnumbered by infinitely more simple species of microorganisms). There is no “end-goal” for evolution; it is not something that will ever stop.