PART FOUR Evolution
The Theory of Natural Selection
One of the most important topics in biology is the concept of evolution. This elegant process explains how we ended up with such a staggering variety of life-forms on Earth. It’s important to first understand the difference between evolution and natural selection. The term evolution means “change over time”; natural selection is how this slow change occurs.
Organisms that are born with a trait that allows them to survive and thrive have a greater chance of surviving, reproducing, and passing on their traits.
Natural selection is based on traits organisms are born with (genetic); traits acquired during an organism’s lifetime are not genetic and cannot be passed on to the offspring.
Natural selection is the mechanism for evolution.
The Development of the Theory of Natural Selection
We give Charles Darwin credit for being the “father of evolution,” for explaining how the process of evolution occurs in nature. Any good idea, however, grows from a foundation of previously learned knowledge; Darwin had the help of many other excellent minds who also studied evolution. In 1809 (the year Darwin was born!), a French biologist named Jean-Baptiste Lamarck realized that evolution was a slow process that occurred because of environmental changes. He explained evolution based on two principles: (1) use and disuse, the idea that body parts used frequently are made stronger and more robust, and parts that aren’t used deteriorate; and (2) inheritance of acquired characteristics, the idea that the most-used and strengthened body parts would then be passed on to the offspring.
Giraffes provide an excellent example for Lamarck’s theory. The giraffes’ ancestors looked like antelope or deer. Lamarck explained that modern giraffes came about based on inheritance of acquired characteristics. There was competition for food, and soon the only food available was the leaves high up in the trees. The giraffes stretched their necks in order to reach those tasty leaves, and would then pass on their newly acquired long necks to their offspring. Lamarck was totally right about the role of competition and the environment in the evolutionary process, but incorrect about the inheritance of acquired characteristics; traits obtained during an organism’s lifetime would not be genetic, and would not be inherited by the offspring. For example, if Lamarck was correct, a person who lifted a lot of weights and developed really strong muscles would then have a really strong, muscle-bound baby. Nope. That’s not to say there isn’t a component of truth in that scenario. An adult may indeed have children who are also strong, but it’s because they are genetically predisposed for increased muscle mass. Even with the missteps, Lamarck did an amazing job proposing a logical explanation for evolution that included the importance of the environment and competition.
Two geologists also played important roles in Darwin’s formation of a theory of evolution. Understand that the prevailing belief during Darwin’s day was that Earth was very young (only a few thousand years old) and populated by unchanging species. In the 1700s, a geologist named James Hutton suggested that Earth was, in fact, very old. Furthermore, geologic features were formed by very gradual mechanisms that could occur because the planet had been around a long, long time. During Darwin’s time, another geologist—Charles Lyell—suggested that those slow geological processes described by Hutton were still happening today, as they had in the past. Darwin had the realization that Earth must be significantly older than a few thousand years—otherwise, how could valleys be carved out by rivers, or mountains stretch up into the sky? The timescale of Earth was long enough (billions of years!) to allow gradual changes to happen in geology … why not biology?
Another significant insight came from an unlikely source: an economist named Thomas Malthus. Darwin read a 1798 essay by Malthus describing how an increase in the human population would lead to a strain on resources such as food and space. This would lead to disease, famine, and death. Darwin realized that this applies to all life, not just humans. All species have the capacity to overpopulate and run out of resources, leading to a struggle to survive.
Artificial selection is the process of changing a species by selectively breeding individuals that have certain traits a person wants to foster. Humans have been benefitting from artificial selection for thousands of years. For example, the different breeds of dogs we have today arose from artificial selection, starting with a gray wolf. Plants such as broccoli, kale, and cauliflower were all created by artificial selection, starting with a wild mustard plant. The big difference between artificial selection and natural selection? In natural selection, the environment applies the selective pressure; in artificial selection, we determine what is “most desirable” and select who gets to survive and reproduce.
In 1831, Darwin was invited to sail on the ship HMS Beagle as it took a 5-year journey to map the coast of South America and the Pacific Islands. This presented an amazing opportunity for Darwin to collect all sorts of samples of plants, animals, and rocks. He kept an extensive diary of all of his observations, such as insightful comparisons between fossils found in different locations. The best stuff came at the end, when he arrived at the Galapagos Islands. Here, Darwin was blown away by the similarities (and differences!) between the species found on one island compared to another (or even the mainland). The life-forms were so perfectly suited to their environments, yet maintained similarities to comparable species on a neighboring landmass. For example, the finches on each island had beaks perfectly shaped for the primary source of food in their environment: narrow, sharp beaks for catching insects; thick, strong beaks that can crush hard seeds; or beaks able to tear at vegetation. Although all of these finches had remarkable similarities to one another, they each also had adaptations that allowed them to survive and reproduce in their particular environment.
During his voyage, Darwin also found fossils of sea life buried high up in the mountains. He recalled what he learned from Hutton and Lyell, that Earth was very old and underwent slow geological processes that could result in huge changes (such as an ocean disappearing and being replaced by a mountain range!).
One of the coolest things happened earlier, when they visited Argentina. Some of his crewmates brought back an armadillo, which Darwin had never seen before. He had, however, found a fossil of a giant armadillo (now named Glyptodon) while on a fossil-hunting trip in the area. So, here’s this mind-blowingly huge version of an extinct critter that looks eerily similar to a version walking around today. The similarity just screamed “evolutionary ancestor.”
Fossil remains of Glyptodon
Author: Dellex. https://commons.wikimedia.org/wiki/File:Glyptodon_Skelett.JPG
It’s a common misconception to think that by using the phrase theory of evolution, it suggests that it’s nothing better than a guess. Please understand that in science, the term theory carries a lot of weight. A theory is supported by a large body of evidence and has been repeatedly confirmed through experimentation. I mean, gravity is just a theory … and we feel pretty confident about that one.
Once Darwin returned home, it took him more than 20 years to study the significance of his notes and comb through the specimens gathered on his voyage. He came up with a theory explaining how evolution could give rise to such a rich and varied selection of life, and why a species ended up being so perfectly fit for its environment. Darwin published The Origin of Species in 1859, explaining how evolution occurs through a process called natural selection. This is our must know concept. Even though he saw the results of evolution (different—but related—species), he didn’t know how it had happened. The how of evolution is the process of natural selection.
The Mechanism of Natural Selection
Although natural selection is responsible for some pretty remarkable things, the process isn’t overly complex. Our must know concepts can be simplified to these four steps:
1. Individuals in a population are born with genetic differences (variation).
Alleles refer to different variations of the same gene (as we learned back in Chapter 14). These variations are the basis for natural selection, because some alleles provide an advantage over other forms. Please understand that the different forms of a gene are not made through natural selection, they are chosen by natural selection. New alleles are made by random mutations! Another thing to keep in mind is that natural selection acts on phenotypes, not genotypes. The genes are only specific sequences of nucleotides kept safely in the cell’s nucleus. The environment doesn’t interact directly with the genes (the genotype); the environment instead interacts with the physical trait the DNA codes for (the phenotype).
Genetic variation comes about by many different means (which is great, because variation in a population means a healthy population). When gametes are formed during meiosis, an organism’s genes are lined up and shuffled in different ways. This—along with crossing over—yields a huge number of different eggs or sperm with different combinations of alleles. To mix it up even further, the process of sexual reproduction takes one of those genetically unique eggs and fertilizes it with a genetically unique sperm. The offspring receives two copies of each gene, one from each parent. So many combinations. Finally, what if there happens to be a mutation in one of the sex cells? That would create a brand-new genotype! And if that heritable mutation happened to have a beneficial phenotype, it could go on to create a new adaptation.
Genetic variation is naturally found in populations, unless the population is undergoing asexual reproduction. This type of reproduction relies on the “parent” producing genetically identical copies of themselves, without the use of gametes (egg and sperm). Bacteria undergo asexual reproduction by first copying their chromosome and then splitting themselves in half (binary fission), and many plants can clone themselves. Humans have, in fact, cloned plants for thousands of years by cutting off a part of a desirable plant and coaxing it to grow as a new plant. A cloned crop plant can increase food yield and save millions of lives, but a cloned crop can also be more susceptible to disease (the lack of genetic variation limits the population’s ability to adapt). Cloning is much more difficult in vertebrate animals, but it is becoming more successful as advances in genetic engineering continue.
2. There are usually too many offspring for the environment to support, because resources are limited.
The idea of overproduction rolled around in Darwin’s mind after he read Malthus’ essay on human population growth. This is a really important thing to consider for the process of natural selection, because if the environment would support unlimited numbers of offspring, natural selection wouldn’t occur. As it is, however, exponential growth is only possible with unlimited resources, no predators, and no disease … and that’s not likely.
3. There’s a lot of competition, and any individual who happened to have been born with a helpful trait is able to outcompete others in the population.
Environments place limits on population growth because, as we mentioned above, an environment just can’t provide unlimited space and resources. So, this leads to an epic struggle for food, water, and shelter … and some organisms are more successful than others.
4. This most-fit individual gets to survive and reproduce. By doing so, the survivor’s “most-fit” genes are now passed onto the next generation, resulting in an adaptation.
Certain variations allow an individual to outcompete others and win that struggle for resources. An adaptation is a helpful trait found in a population that is the product of natural selection. Natural selection accumulates these adaptations because they help an individual survive and reproduce.
As so often happens in science, this simple concept has hidden pitfalls in the form of misconceptions. I will cover three important points that are easy to misunderstand.
■ You gotta be born with it.
It’s super, super important to understand that any trait that helps an individual survive must be genetic and heritable, otherwise it doesn’t count (sorry, Lamarck). Only those qualities that can be passed onto the next generation (in their genes) have an impact on the next generation’s genetic makeup (gene pool). If, for example, an octopus developed super-stretchy legs during its lifetime and allowed it to better reach a crab hiding in the rocks, that doesn’t result in evolution. Sure, that one octopus will be better able to find food and survive, but it cannot pass on its improved stretchiness to its offspring unless it was a genetic trait it was born with.
The reason I keep stressing that “you gotta be born with it” is because even though we accumulate genetic mutations during our lifetime, that doesn’t mean your kids will inherit any of those mutations. What if, for example, you got radioactive waste on your hands and it caused genetic mutations in your fingertips so they grew suckers that allowed you to climb walls. Super cool, right? But would this mutation be passed on to the next generation? To answer that question, think about this, where are the genes to make the next generation coming from? Yup, the egg and sperm.
In order for you to pass on some helpful mutation, you need to be born with it because it needs to be in your gametes (egg or sperm). And in order for a certain gene to be found in the cells that create egg and sperm, it needs to have been there when you were nothing but a single-celled zygote. Not some random mutation you got because you handled glowing, bubbling radioactive waste (please don’t ever handle glowing, bubbling radioactive waste).
■ Populations evolve, not individuals.
Another important thing to keep in mind is the fact that populations evolve, not individuals. Adaptations occur in populations as most-fit genes (as selected for by natural selection) start to accumulate. An individual can’t evolve because you are born with the genes you have and they don’t change, beyond a random mutation. The best an organism can do is survive, have lots of offspring, and contribute its excellent genes to the next generation.
■ The story isn’t over once the population adapts.
A population’s adaptations that are considered “most fit” can change as the environment changes. The selective pressures of evolution are the conditions in which an organism lives, and if those conditions change, so do the qualities that best help a critter survive. For example, European tawny owls come in two colors: brown and gray. The gray color helps them blend in better when winter brings the snow, providing an advantage when hiding from predators. Because of climate change, however, the winters are becoming milder and there is less snow. Natural selection has shifted to favor the brown color, and there has been an increase in brown owls.
1. _____________ is change over time. ______________________ is the mechanism that explains how it happens.
2. Why were the geologists’ (Hutton and Lyell) insights into geological processes so helpful to Darwin when forming his theory of evolution?
3. Lamarck had his own theory of evolution, but it was flawed due to the principle of inheritance of acquired characteristics. Why was it wrong?
4. A(n) _________________ is a helpful trait found in a population that is the product of natural selection.
5. A species of woodpecker that evolved to possess brightly colored head feathers is an example of [choose one: macroevolution/microevolution]. The ancestral woodpecker that gave rise to many different species better adapted for their particular environment is an example of [choose one: macroevolution/microevolution].
6. Match the person with the correct influential insight:
7. How does artificial selection differ from natural selection?
8. Choose True or False for the following statements:
a. Any beneficial genetic mutation will be passed on to the next generations.
b. Populations, but not individuals, are able to evolve.
c. Once a most-fit phenotype is selected through evolution, the population’s adaptations are “fixed” and will no longer change.
9. Briefly describe the four steps of natural selection.