Evidence for Evolution, part 1: Claims and Historical Observations

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1. “Nothing in Biology Makes Sense Except in the Light of Evolution”

Evolution is the idea that life changes over time. You can think of this in terms of looking back to the past, and into the future.

• Looking backward: the animals, plants, and other organisms around you are the modified descendants of their ancestors that lived in the past.
• Looking forward: these same organisms are members of populations that will either continue to evolve into the future or, like the vast majority of the species that have ever lived, become extinct.

Evolution’s importance to biology was memorably stated by biologist Theodosius Dobzhansky who wrote that “Nothing in biology makes sense except in the light of evolution.”

What’s the evidence that evolution is true? 

In what follows below and in the next three tutorials, we’ll look at the evidence supporting evolution. Because evolution is a scientific theory, let’s start by clarifying what a theory is, and how the word theory is used in a scientific context.

2. A theory is much more than a hypothesis

In common speech, the word theory is often used in a way that’s synonymous with the word “opinion” or “belief.” For example, you’ve probably heard someone say something like: “That’s your theory. Let me tell you mine.”

That use of the word theory is closer to the scientific idea of a hypothesis. A hypothesis is an idea that requires testing. In science, a theory is an explanation that’s already been tested many times. Unlike a hypothesis, a theory is supported by a significant amount of evidence.

To sharpen your ability to distinguish between a scientific theory and a hypothesis, read the following passages.

A theory explains a phenomenon, accounts for all available data, is supported by a huge body of evidence. Hypotheses are just guesses that need testing.

Adapted from: Ruth Levy Guyer, Professor of Immunology and Bioethics, Johns Hopkins University and Haverford College.

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A theory is a well-established principle that has been developed to explain some aspect of the natural world…while a hypothesis makes a specific prediction about a specified set of circumstances. A theory has been extensively tested and is generally accepted…A hypothesis is a speculative guess that has yet to be tested.

Adapted from Kendra Cherry, About.com, Introduction to Research Methods

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So, before we look at the evidence for the theory of evolution, let’s make sure that the distinction between theory and hypothesis is clear.

3. To prove evolution true, we need evidence of descent with modification

When Darwin talked about the process of evolution, he used the term descent with modification. Descent with modification means that as a population produces descendants, these descendants might evolve in ways that make them different from their current form. Or, if you run the clock backwards, it means that the current form of a population (or an entire species) might be different from its ancestral form.

Let’s think about this in terms of a familiar animal: the dog. Based on archeological and genetic evidence, we know that dogs are domesticated wolves. If you could look back 10,000 years, the ancestors of dogs would be different from the dogs you see today. That’s because 10,000 years ago, the process of domestication of dogs was just beginning. People had dogs, but many of the breeds we know today (like poodles or dachshunds) didn’t exist. If you went back 100,000 years there wouldn’t be any dogs at all: just wolves. If you went back 100,000,000 years, there wouldn’t be wolves, but rather an ancient, earlier mammalian species that was in the line that, millions of years later, led to wolves.

So if we want to prove that evolution has happened, we need to find evidence of descent with modification. We can find this in two ways.

• We can look for changes in a population’s phenotype over time. Through studying fossils and comparing the anatomy of existing organisms, scientists have been gathering this type of evidence since the start of the 1800s (even before Darwin and Wallace formulated their idea of natural selection).
• We can look for changes in a population’s genotype. As we saw in the last module, evolution is about changes in the allele frequencies in a population’s gene pool. So, by comparing the genes of existing organisms, we should be able to see evidence of mutations emerging, and alleles rising and falling in frequency.

As we consider this evidence, we’re going to look for two patterns. We can track these in the image to the left, a sketch by Darwin in one of his notebooks. Darwin made this sketch in 1837  (twenty two years before his publication of The Origin of Species), and it’s thought to represent his “Eureka” moment (when he first figured out the pattern of evolution).

The first pattern is change in a single lineage, over time. A lineage is a line of descent that links ancestors and descendants. If “1” at left represents an ancestral species, then A, B, C, and D are all descendant species. If you follow the line from 1 to any of its descendants, you should be able to observe descent with modification along the way.

The second way that evolution unfolds is shown by the diagram’s branching pattern. Since species 1 is the ancestral species that gave rise to A, B, C, and D, then all of these descendant species should have shared features that result from their common ancestry. If this diagram were of a family, then A, B, C and D would be cousins.

Flipping that logic gives us a way to find evidence of evolution. Pretend this is a family tree. If you didn’t know that B and C were related, but you did a DNA test and and found that they shared a certain percentage of their genes, you would have to conclude that they were related: siblings, cousins, etc. And while it’s less direct, if you found certain shared inherited phenotypes, that would lead you to the same conclusion.

With species, you can do the same thing. Shared DNA sequences can serve as evidence of common descent. So can shared inherited phenotypes. After all, if these species weren’t the descendants of a common ancestor, why would they share DNA sequences, or other inherited traits?

So, to prove evolution true, we’re looking for evidence of descent with modification. We can look for that in

1. Changes in phenotypes within a lineage of organisms
2. Genetic change in a population’s gene pool.
3. Shared inherited structures, molecules, or gene sequences that confirm life’s branching pattern.

4. Observations of Evolution in Recent Times

When Darwin first formulated his theory of evolution by natural selection, he saw change in the natural world as something that would occur very slowly. Hence, his view of evolution was one in which changes would be imperceptibly slow, over hundreds of thousands or millions of years, and only discernible in hindsight.

However, a variety of studies during the late 19th, 20th, and 21st century have shown that evolution can proceed rapidly. In other words, scientists have observed both descent with modification, and changes in a population’s genetic structure. Here are a few examples.

4a. Evolution of Antibiotic resistant bacteria and the rise of MRSA.

Staphylococcus aureus is a bacterium. It’s one of many bacterial species that usually live harmlessly on our skin and other parts of our bodies. But there are also pathogenic forms that can cause skin and sinus infections, along with food poisoning, pneumonia, and wound infections. Up to 50,000 deaths/year are associated with S. aureus infections, and about 500,000 hospital patients become infected with S. aureus each year.

Bacterial infections by S. aureus can be treated with antibiotics: substances that kill bacteria or inhibit bacterial growth. However, widespread use of antibiotics has created selective pressure for S. aureus (and other infectious bacteria) to evolve antibiotic resistance, making antibiotics increasingly ineffective. The first widely used antibiotic, penicillin, was introduced in 1943. By the 1950s, penicillin-resistant strains of S. aureus were common.

Natural selection was at work. S. aureus populations have genetic variation. Most bacterial cells can’t reproduce when exposed to penicillin, a molecule that inhibits bacterial cell wall formation. But some bacterial cells produce an enzyme called penicillinase, which breaks apart penicillin, making it ineffective. These individual bacterial cells were able to survive the doses of penicillin that killed off the rest of the population. When the survivors reproduced, they passed along their genes for resistance. By 1950, only seven years after penicillin was introduced, 40% of S. aureus studied were penicillin-resistant. By 1960, this had risen to 80%. Now it’s about 98%.

Some other features of bacterial reproduction have enhanced the spread of resistance. Bacteria don’t only pass their genes on to their descendants. Through tiny, transferrable circles of DNA called plasmids, bacteria can also pass genes to other members of their population. This is called horizontal gene transfer. Transfer of antibiotic resistance genes through plasmids has further increased antibiotic resistance.

Here’s an infographic that illustrates the process.

1. Image “A shows the initial population. Most bacterial cells (the gray ones) are killed by the antibiotic. A small percentage are antibiotic-resistant. Note that the resistance is a pre-adaptation. It didn’t arise in response to the selective pressure, but was part of the population’s pre-existing genetic variation.
2. Application of the antibiotic kills all the bacteria that are susceptible, leaving only those that are resistant.
3. As the bacteria reproduce, the new population will be mostly composed of antibiotic-resistant cells.
4. Horizontal gene transfer of genes for antibiotic resistance increases the proportion of antibiotic resistant individuals in the population.

Widespread use of antibiotics in agriculture has made the problem worse.

In agriculture, antibiotics are used to prevent outbreaks of infection in animals that are kept in close confinement. In 2013, the US Department of Agriculture estimated that 32 million pounds of antibiotics were used in food animal production. Another way to think of that amount is that 80% of the antibiotics used in the United States are used to raise animals (not to treat people with infections).

As with humans, the antibiotics fed to animals (A) kill only the susceptible bacteria. That selects antibiotic resistant strains (B and C). These can be transferred to people through animal products (contaminated meat), through produce (often lettuce) that’s been in contact with animal feces-contaminated soil or water, or during food preparation (which is why hand-washing is so important).

Methicillin is an antibiotic that was introduced in 1959. Like penicillin, methicillin inhibits bacterial cell wall formation, but through a different mechanism than penicillin. However, within two years of its introduction, methicillin-resistant bacteria had been observed. In this case, the resistant bacteria were able to use an enzyme to synthesize their cell walls through a mechanism that wasn’t inhibited by methicillin.

These methicillin-resistant bacteria have come to be known by the acronym MRSA (methicillin resistant S.aureus), and they’re much more dangerous than non-MRSA S. aureus. Often, MRSA infections can be controlled by other antibiotics, such as vancomycin. But the concern is that our arsenal of effective antibiotics is growing increasingly thin, as the bacteria mutate around whatever selective pressure our use of antibiotics creates.

Is this evidence for evolution? Absolutely. In the rise of antibiotic resistant bacteria, we have a clear example of descent with modification.

4b. Evolution of Pesticide Resistance in Mosquitoes

Mosquitoes are the primary way that the Plasmodium parasite that causes malaria spreads. Malaria is the number 1 killer of children worldwide, and killed 445,000 people in 2016, over half of whom were under the age of 5. The disease is particularly prevalent in sub-Saharan Africa (source: Forgotten Children Worldwide)

One approach to controlling malaria has been to suppress mosquito populations with insecticides (insect-killing chemicals). One such pesticide is DDT (dichlorophenyl trichloroethane). DDT was developed in the 1940s, and works by opening sodium channels in the membranes of insect nerve cells, causing these cells to misfire, leading to spasms and death.

Based on the story of antibiotic resistance in bacteria, you should be able to predict what happened.

Here’s a graph showing what happened when scientists measured the resistance of mosquitoes to exposure of 4% DDT solution for one hour. Measurements were done under controlled conditions, over a period of 18 months. Study the graph and answer the questions that follow. The term “mortality” on the Y axis means “certainty of death.”

4c. Evolution of new phenotypes in response to host changes in the Soapberry bug

The soapberry bug (Jadera haematoloma) lives in the southeastern United States, with its range stretching from Florida to Oklahoma. It uses a needle-like beak to pierce the outside skin of fruit, and then pierce the seed coat of seeds within the fruit. Enzymes liquify the seeds, and the bugs suck up the liquified seed contents.

Here’s a drawing by UC Davis Professor Scott P. Carroll of a soapberry bug feeding on the fruit that’s native to Florida, Cardiospermum corindum. Note the distance between the fruit’s skin and its seed, and the corresponding length of the soapberry bug’s beak.

In historical times (within the last 150 years), several plant species have been introduced from Europe and Asia into the range of the soapberry bug. These introduced species have fruit in which the distance between the outside skin and the seeds inside is different from that of the native species. In Florida, the introduced species (Koelreuteria elegans) had a fruit structure in which the seeds were much closer to the skin of the fruit than in the Florida native, C. corindum.

In the interactive reading below, you can interpret what happened.

5. Cumulative Quiz: Historical Examples of Evolution

There are numerous examples of rapid evolutionary change that’s been observed by biologists. To learn more, use any of the following links

1. This article in Discover Magazine has several examples.
2. The evolution of the Peppered Moth. This is an example of industrial melanism first identified in the 1800s, confirmed by B.D. Kettlewell in the 1950s, subsequently attacked by creationists, then validated by experiments at the start of the 21st century by Michael Majerus). You can read about the scientific history and biology on Wikipedia. I have also produced a music video about the peppered moth. 
3. Species formation (also based on host shifts and introduced species) in the apple maggot fly.

 

Links

1. Homologous and Vestigial Structures (the next tutorial in this module)
2. Evidence for Evolution Menu