What Evidence Do We Have That All Species Continue to Evolve

Evidence of Evolution

Biological evolution is genetic change in a population from one generation to another.  The speed and direction of change is variable with different species lines and at different times.  However, continuous evolution over many generations can result in the development of new varieties and species.   Likewise, failure to evolve in response to environmental changes can result in extinction.

S ufficient research data has been accumulated over the last two centuries to safely conclude that evolution is a fact, not just a theory, and that all life forms, including people, evolved from earlier species.  Furthermore, all still living populations of organisms continue to evolve today.  There are a number of different natural processes than can cause evolution to occur.  These are presented in later tutorials of this series.

How Do We Know
That Evolution Has Occurred ?

T he evidence has primarily come from four sources:

1.	fossil record of change in earlier species
2.	chemical and structural similarities of related life forms
3.	recorded genetic changes in living organisms over many generations
4.	geographic distribution of related species

Fossil Record

geological strata containing an evolutionary sequence of fossils

R emains of animals and plants found in sedimentary rock  deposits give us an indisputable record of past changes through time.  This evidence attests to the fact that there has been a tremendous variety of living things.  Some now extinct species had traits that were transitional between major groups of organisms.  Their existence confirms that species are not fixed but can evolve into other species over time.

T he evidence also shows that what have appeared to be gaps in the fossil record are due to incomplete data collection.  The more that we learn about the evolution of specific species lines, the more that these so-called gaps or "missing links in the chain of evolution" are filled with transitional fossil specimens.

Chemical and Structural Similarities

L iving things on Earth are fundamentally similar in the way that their basic structures develop and in their chemical compositions.  No matter whether they are simple single celled protozoa or highly complex organisms with billions of cells, they all begin as single cells that reproduce themselves by similar division processes.  After a limited life span, they also all grow old and die.

A ll living things share the ability to create complex molecules out of carbon and a few other elements.  In fact, 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements.  This is not a mere coincidence.

A ll plants and animals receive their specific characteristics from their parents by inheriting particular combinations of genes.  Molecular biologists have discovered that genes are, in fact, segments of molecules in our cells.

section of a DNA molecule

T hese segments of DNA are chemically coded recipes for creating proteins by linking together particular amino acids in specific sequences.

protein molecule

A ll of the tens of thousands of types of proteins in living things are made of only 20 kinds of amino acids.  Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things.  This is evidence of the fundamental molecular unity of life.

I n addition to molecular similarities, most living things are alike in that they either get the energy needed for growth, repair, and reproduction directly from sunlight, by photosynthesis, or they get it indirectly by consuming green plants and other organisms that eat plants.

A ll of these major similarities between living things can be most logically accounted for by assuming that they either share a common ancestry or that they came into existence as a result of similar natural processes.  These facts make it difficult to accept a theory of special and independent creation of different species.

Genetic Changes Over Generations

T he Earth's environments are constantly changing, usually in subtle and complex ways.  When the changes are so great as to go beyond what most members of a population of organisms can tolerate, death occurs.  As Charles Darwin observed, however, not all individuals always perish.  Fortunately, natural populations have genetic diversity.  Those individuals whose characteristics allow them to survive an environmental crisis likely will be the only ones able to reproduce.   Subsequently, their traits will be more common in the next generation--evolution of the population will have occurred.

T his process of natural selection resulting in evolution can be easily demonstrated over a 24 hour period in a petri dish of bacteria living in a nutrient medium.  When a lethal dose of antibiotic is added, there will be a mass die-off.  However, usually a few of the bacteria are immune and survive.  The next generation is mostly immune because they have inherited immunity from the survivors (i.e., the purple bacteria in the petri dishes shown below).  In other words, this bacteria population has evolved.

evolution of antibiotic resistant bacteria

dog variety resulting from selective breeding over many generations

P eople have developed many new varieties of plants and animals by selective breeding.  This process is similar to the bacteria experiment.  Selection of specimens to breed based on particular traits is, in effect, changing the environment for the population.  Those individuals lacking the desirable characteristics are not allowed to breed.  Therefore, the following generations more commonly have the desired traits.

insect with a high reproductive potential

S pecies that mature and reproduce large numbers in a short amount of time have a potential for very fast evolutionary changes.   Insects, bacteria, and other microorganisms often evolve at such rapid rates that our actions to combat them quickly lose their effectiveness.  We must constantly develop new pesticides and antibiotics in an ever escalating biological arms race with these creatures.

I f evolution has occurred, there should be many anatomical similarities among varieties and species that have diverged from a common ancestor.  Those species with the most recent common ancestor should share the most traits.  The fact that wolfs, dogs, and other members of the genus Canis are anatomically very similar, for instance, is due to the fact that they are descended from the same ancient canine species.  Wolves and dogs also share similarities with foxes, indicating a slightly more distant ancestor with them.

Geographic Distribution
of Related Species

A nother clue to patterns of past evolution is found in the natural geographic distribution of related species.  It is clear that major isolated land areas and island groups often evolved their own distinct plant and animal communities.  For instance, before bats and humans arrived, Australia had more than 100 species of kangaroos, koalas, and other marsupials but none of the more advanced placental mammals such as dogs, cats, bears, and horses.

L and mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand.  Each of these places had a great number of plant, insect, and bird species that were found nowhere else in the world.  The most likely explanation for the existence of New Zealand's and Hawaii's unique biotic environments is that the life forms in these areas have been evolving in isolation from the rest of the world for millions of years.

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W hile it is clear that evolution has occurred in the past and still continues in all living populations of organisms, the mechanisms that cause evolution to occur were not easy to discover.  In fact, they were not all known until the last half of the 20th century.

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Source: https://www.massey.ac.nz/~wwpapajl/evolution/assign2/LB/Evidence.htm