Basics of Evolution

  1. Evolution is a unifying theory in the biological sciences.  It attempts to provide an explanation for the both the diversity of organisms and their similar genetic and chemical properties. It also attempts to explain the observations made in the fossil record.
  2. Biological evolution in its simplest form means change over the course of time. This change occurs over the course of generations in the properties of populations NOT individuals.  Commonly it is referred to as decent with modification as no generation will be the same as the prior generation, and thus will be “modified” to some extent.
  3. At its root evolution is caused by effects of heritable variation (being able to be passed down through generations) of mutations and recombination of genes.  These heritable changes are then sorted out by random changes and by non-random differences in the survival and reproduction of individuals in a population.  This latter process is called natural selection and is the cause of adaptations which are simply changes in response to the surrounding environment.  For example in a population of lions, when new cubs are born if one has a new version of gene for better running it may catch more prey and have a better chance to survive.  Thus we can refer to “nature” selecting that cub to survive and pass on its genes. If it does so, the next generation will be genetically different than the previous generation.
  4. Decent with modification from common ancestors is certainly a proven fact from observations over the past several thousand years as we know that all organisms are born of prior parents and that descendants will change genetically.  The fossil record also suggests that decent with modification has occurred into the distant past with changes over time resulting in many new species.  The theory of evolution is not the same as the observation of decent with modification but rather involves a large number of hypotheses regarding the causes of how this descent with modification occurs.

Evidence that suggests that organisms have charged over time

Charles Darwin and others during his time made a number of observations about the world that suggested to them that organisms had evolved over time.  The fact that organisms had changed or that the earth had changed over time was of little or no dispute; the question was what was the cause of this change that was observed?  While few scientists debate that organisms do change over time, even today there is much discussion about the mechanisms of that change. 

Some of the observations that people made that convinced them that organisms have been different over time are:

A. Fossils:

Fossils are thought to be the remnants of post living things. 

  1. It has been shown that many fossils most closely resemble the living organisms that are in the same areathat the fossils are found.  Almost always the fossils do not appear to be identical to the living organisms, suggesting that they represent former populations of those living and so gave rise to the modern taxa bydescent through modification.

    For example, consider the lemurs of Madagascar.  Fossils of organisms that are definitely lemurs but are not the same species as currently alive have been found in rocks in Madagascar but lemur fossils (other than some bones found in India of a possibly very primitive lemur) are not found elsewhere in the world.  No lemur fossils have been found in Africa but thousands of other monkey fossils have been found there.  These fossils suggest that at some point in the past, lemurs where on Madagascar that were different than today’s and now the ancestors of those lemurs are present there today.  This is an observation that the general theory of evolution attempts to answer.  
  2. In rock strata (layers), changes in organisms can be seen in fossils from earlier and earlier layers.  These are often referred to as the “facts” of evolution, or change over time, as the progression of fossils in the fossil record (the fossils at the bottom of the layers of rocks are always different from those at the top) is recognized by everyone, though they may disagree about the meaning of that progression.

B. Observations from Geographical distributions:

  1. Different continents may have habits that are very similar (South Africa and California are very similar) and yet they have completely different plants and animals.  Why would each continent have different plants and animals when the habitats could support the exact same organisms?
  2. Similar to the above point, plant and animals of each continent are often more similar to one another than they are to others on other continents. For example, there are rodents on every continent but all of the rodents in South America are much more similar to one another than they are to ones from Africa.
  3. Oceanic Islands often have very different animals and plants compared to continental land masses.  In particular it is observed that the farther an island is from a continent the more different the animals and plants on that island will be.  For example about 90% of all the species of plants and animals on Hawaii are unique to Hawaii. On the Galapagos islands, where Darwin worked for some time, he noticed that each island had its own species of some animals and that those animals were different from those found in central America. 

More evolutionary examples.

Evolutionary theory as described by Darwin provides a fairly simple explanation for these differences.   If an organism is isolated from its parents, it cannot exchange genes with those parents.  Mutations and recombination in their genomes will create new combinations which cannot be shared with the original population on the continent (or another island), and so over time they will inevitably become increasingly genetically divergent from one another. 

The Mechanisms of Evolution

If evolution may be defined as “change over time,” then it is apparent that at least some evolution has occurred and is always occurring in organism. However we may distinguish between degrees of evolution:

  1. Microevolution – changes in gene frequencies in populations over geologically short time spans
  2. Macroevolution – large scale change over long periods of time

Some other terms that are good to know:

Adaptation:  Changes that increase the ability of an organism to survive in a particular environment.  

In 1859 both Darwin and Wallace published an explanation for how evolution might take place based on a theory called “Natural Selection.”  This theory is based on principles derived from basic observations about the natural world:

Variation: variation exist in all populations of individuals (i.e., all individuals of a population are similar but not identical)

Variation is heritable:  Many aspects of variation are due to genetic differences and thus are inherited

Unlimited biotic potential:  All species have the ability to produce excess progeny – far more than can actually survive. 

Limited resources:  In any environment, resources are limited.  Therefore, no organism can reproduce to its biotic potential

Competition:  Organisms with unlimited biotic potential within a common environment must compete for limited resources

If you put all of these observations together you can see where the theory of natural selection comes from.

Natural selection:  Those organisms that are most successful at securing resources and nutrients are most likely to survive and to reproduce. 

As a result of this natural selection, which is the interaction between individuals and the environment, favorable characteristics will be represented at a higher and higher frequency in successive generations; this is what we have called microevolution. Microevolution has been tested and verified in countless biological experiments, and thus has been proven true.  It has been tested both in the biological realm, but also in computer environments where the "organism" is computer code that replicates within computer memory.  This proves that the process of natural selection will hold regardless of the medium provided, as long as the above conditions are met.

One misconception of evolutionary theory is that individuals are evolving.  That is not what evolutionary theory (both micro and macro evolution) is saying.  Rather POPULATIONS change over time - not individuals.  Natural selection simply describes the process by which organisms are selected by natural forces, thus causing the next generation to be slightly changed.

The theory of natural selection can be tested (and has been tested MANY times) and has been shown to be very important in determining how organisms change over time.  In addition, evolutionary theory makes many predictions about the distribution of organisms, their genetic makeup, and the fossil record that are directly testable.   

So if it is genetic variation between organisms that is being selected by the environment, what is the source of that genetic variation?   This is one of the questions that scientist do debate.  If there were no genetic variation (all organisms were clones) then obviously natural selection would not work. Where does the genetic variation originally come from?

Ultimately it is mutations in genes that is the source of genetic variation.  Selection determines if mutations are harmful, helpful, or "neutral" (neither helpful or harmful) in their effect.

As microevolution selects upon genetic variation (originally created by mutations), the accumulation of those changes is predicted to result in the evolution of new species.  Such long-term changes are term macroevolution. The difference between micro- and macroevolution is difficult to discern.  Even those that don’t accept macroevolution find it difficult to interpret where microevolution ends and macroevolution starts, or how far microevolution may proceed before it should be called macroevolution.  For example, did all dogs come from a single dog ancestor or only domestic dogs?  Some will say that all the animals and plants on Hawaii evolved from the same kinds of animals and plants on the mainland (i.e., that they all started out as birds, plants, fish, insects etc.. and that they have not changed what “kind” of organism they are in Hawaii).  Thus, almost everyone agrees that some evolution has taken place, even if "only" for a few thousands years.