Asked in ScienceBiologyEvolution
How do you know evolution is true?
September 28, 2012 12:21PM
First of all, the explanation is fairly simple and obvious. Living things reproduce with variation. Reproduction is part of the definition of life, and variation is evident in a number of ways. You are not exactly the same as either of your parents. You probably have some genetic information that they don't have. There is actually too much genetic information in the human gene pool to fit into one or even several individual humans. Most changes don't have any real effect, because most genetic "information" has no function. This is called junk DNA. Many changes are malignant and even fatal, and on rare occasions they are positively useful. However, simply because the harmless changes are not lethal, they can be passed on. Since they are mostly unique, forensic scientists use these changes to identify people from DNA samples, and even determine paternity, etc.
This is where natural selection steps in, and random chance ends.
Natural selection is tendency for living things with genes beneficial for surviving and (more importantly) reproducing in its environment to occur more frequently. Members of a species that are better able to find food, mate, evade predators, and ensure the survival of other members of their species (because they have mostly the same genes) will generally do so. By this means, only genes which make their hosts more suitable to their environment are passed on. Therefore, nature imposes "selective pressure" on living things and, in a way, forces them to change on average.
Since the explanation works so well, what evidence do we have that this has occurred?
The fossil record is full of beautiful examples of extinct species, some of which gave rise to species that still exist today. There are examples of early reptiles, early mammals, early jawless fish, etc. all the way up the tree of life. Just recently, for example, Tiktaalik was discovered as a link between fish and amphibians. Sometimes the fossils are so ambiguous that taxonomists argue fervently over classifying them as, say, mammallian reptiles or reptillian mammals. Speciation is slow and smooth, and it does not occur in abrupt steps, as naming conventions might lead one to think. Often, naming a specimen as a particular species is somewhat arbitrary. Lions and tigers are separate species, but they can mate and produce ligers (yes they are real) which live but are unable to reproduce with tigers, lions, or even other ligers. Speciation simply occurs when two portions of a population remain separate long enough that they can no longer produce fertile offspring with each other. It is gradual, and often species can form a chain of genetically compatible individuals with incompatable members far enough along the chain from one another. Imagine a slow gradient from red to blue. Red can mate with reddish purple, which can mate with purple, which can mate with indigo, which can mate with blue, but blue and red are unable to reproduce. There is a species (or two) of newt in California that display this well, for instance.