Earlier I made the statement that while evolution is accepted as a law by virtually all biologists, a sizeable proportion of the general public still does not believe in evolution. I want to conclude our discussion of evolution looking at some of the evidence supporting the theory of evolution.
So, what is the evidence that evolution has produced the various forms of life on earth?
A few points before we look at the evidence -- remember the following:
1. no one has witnessed the entire history of life of earth.
2. no one has observed the process of speciation from start to finish.
Does this mean we can't study evolution or draw conclusions about the evolutionary process?
Evolutionary biology is a historical science that often involves looking at the end products events that occurred long ago or that occurred over long periods of time.
Other historical sciences include geology and anthropology/archaeology -- they are still sciences.
Q: If no one has observed the entire process, how can we determine if evolution has/is occurring?
A: Deduction.
If evolution has been going on for 3.5 billion years on earth, then we can make predictions about the patterns that we should observe in the natural world.
Stated another way: "If process X has been going on, then pattern Y should be observed." It also follows that "If pattern Z is observed instead of Y, then process X hasn't be been operating" -- the hypothesis is falsified."
Remember that evolution is a hypothesis (theory or law) and if the predicted patterns are not found, then we reject evolution as a hypothesis and come up with something else as a hypothesis.
If evolution is responsible for the diversity of life, a number of predictions about patterns can be made. Here are two broad predictions can be made:
1. species should be capable of change over time (descent with modification).
2. all species have evolved from a common ancestor and are related in a genealogical sense.
So, let's look at the evidence.
Can species change over time? Several lines of evidence suggest they can.
1. artificial selection -- man has used artificial selection to drastically alter the appearance of many plant and animal species.
a. archaeological evidence shows that corn has changed dramatically over the last 10,000 years of agriculture in the Americas.
b. vegetables broccoli, cauliflower, cabbage and brussel sprouts have all been derived through artificial selection from a wild species of mustard plant.
c. selective breeding of dogs over the past 200 years has produced the various breeds now in existence -- breeds of domestic dogs belong to the same species, but breeds are more different from one another (e.g. Great Dane vs. Dachshund) than are wild canids of different species (e.g. wolf vs. coyote).
2. evolution in nature -- there are many examples of descent with modification that have occurred without deliberate breeding by people -- countless examples of gene pools being modified by mutation, gene flow, genetic drift and, especially natural selection.
a. Many pest species ranging from bacteria to insects to rats have evolved resistance to the various compounds used to control them -- have adapted through natural selection to changes in their environments -- e.g. antibiotics such as penicillin, pesticides such as DDT and warfarin.
b. Industrial melanism -- genetic changes in populations of animals (primarily moths) where dark phenotypes increase in frequency in response to air pollution -- classic example is the pepper moth (Biston betularia) in England -- melanistic carbonaria form replaces betularia form in industrial regions of England over a 40 year period -- natural selection (bird predation) changing the gene pool -- has been observed in other species in Europe and in the U.S.
Figure 15.12:
What is the evidence that life forms have all evolved from a common ancestor and are therefore related?
1. fossils.
a. simplest fossils found in oldest strata (3.5 b.y.a), while more complex forms are found in progressively younger strata.
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b. fossils in younger strata are more similar to living organisms than are fossils in older strata.
c. gradual changes are seen in many fossil lineages (e.g. horses) and intermediate forms have often been found.
2. morphological similarities.
a. embryonic development -- different species go through similar stages of embryonic development -- for example, all vertebrate embryos have gill slits on either side of the throat -- develop into gills in fish, but develop into other structures in terrestrial vertebrates -- suggests that all vertebrates are related and descended from a common ancestor that had gills.
Figure 18.7.
b. transitional forms -- species possessing primitive traits indicating ancestry -- e.g. Archaeoptyerx -- one of the first birds -- had feathers and was able to fly -- also had teeth and a long jointed tail.
e.g. platypus -- primitive mammal that lays eggs and has pelvic and pectoral girdle like a reptile -- these examples suggest that birds and mammals both evolved from reptilian ancestors.
c. vestigial structures -- structures with no apparent function but are similar to useful structures in other species -- "evolutionary baggage" reflecting evolutionary history -- e.g. pelvic girdles in snakes, whales; appendix in humans.
d. homologous structures -- similar structures modified for different functions in different species -- features that are similar due to common ancestry -- e.g. forelimb bones in reptiles, birds and mammals -- Figure 18.4 -- all have same bones, but they have been modified for different purposes -- same bones suggest common ancestor, in this case, a reptile.
Figure 18.4.
Evolutionary biologists need to be wary of similarities due to convergent evolution and not common ancestry -- e.g. analogous structures.
Similarities may be the result of natural selection "solving" a particular ecological "problem" in the same way -- e.g. wing structure of insect, bird and bat is an analogous structure not a homologous structure -- similarity not due to common ancestry, but rather to convergent evolution.
3. genetic similarities.
a. universal genetic code -- all species utilize DNA-RNA system and the same genetic code -- suggests all life evolved from an ancestor that used this system.
b. specific genes are shared by different species -- e.g. gene for ABO blood groups found in humans and various other higher primates -- genes for hemoglobin found in all mammals -- genes coding for metabolic enzymes (e.g. Krebs cycle) are found in many species.
c. differences in DNA/amino acid sequences reflect phylogeny -- if evolution has occurred, closely related species (recent common ancestor) should have fewer genetic differences than species that are more distantly related -- genetic similarity can be measured by determining the DNA base sequence for the same gene in different species or the amino acid sequence of the same protein in different species -- species sequences compared in a pairwise fashion and numbers of differences recorded -- phylogenetic trees based on these differences are similar to those based on morphology.
4. biogeography -- study of the geographical distributions of organisms -- species thought to be closely related are found in the same parts of the world e.g. marsupials are primarily confined to Australia -- if species have evolved from one another, they should be expected to be located near one another -- e.g. South American tropical animals are more closely related to South American desert animals than they are to African tropical animals -- e.g. islands have endemic species (species found nowhere else) that are similar to species on the nearest mainland (e.g. Darwin’s finches).
Much of evidence supporting evolution is circumstantial -- this is by necessity because evolution is largely a historical science -- in studying evolution we are often limited to studying the end products of events that occurred along time ago.
Evidence supporting evolution comes from many different areas of science: paleontology, embryology, comparative anatomy, molecular biology and biochemistry, biogeography -- evidence from each of these areas corroborates the evidence in the others.
Next Time: EXAM II !!