Study Guide

Plant Evolution and Diversity In the Real World

  • Biotechnology

    Up until now, we have been talking about evolution by natural selection. But humans are taking evolution into our own hands through selective breeding and genetically modified organisms (GMOs).

    Selective breeding is the way people develop new or improved varieties of crops. This works by exploiting natural variation that already exists. Just like tall parents often have tall children, large pumpkins often have large seeds. And large seeds grow into large pumpkins. Those 1700-pound pumpkins that take home blue ribbons at pumpkin weigh-offs didn't just spontaneously come into being. They were bred to be big.

    A really good example of artificial selection is the various dog breeds humans have come up with. All dogs are the same species, Canus lupus. But through years and years of breeding little fluffy dogs with other little fluffy dogs, and big hunting dogs with other big hunting dogs, we ended up with the whole spectrum of dog shapes and sizes:


    In the same way that humans have been breeding dogs, and cows, and prize-winning pumpkins, we have also bred many different plants so that we have the fruits, vegetables and grains that so many of us eat on a daily basis. Humans have been doing this for thousands of years—This is also called artificial selection, since humans are choosing the traits instead of natural selection.

    Genetically modified organisms build on the same principles of artificial selection but take it a step farther. Instead of picking a prize-winning plant and harvesting its offspring, scientists modify the specific DNA sequence that will produce a desired trait.

    Instead of artificial selection happening on the farm, it can now happen in a lab. What once took generations of plant growth can now happen very quickly. Some of the most exciting prospects for GMOs is using the technology to improve crops to be more nutritious, particularly for areas of the world that don't have access to a wide variety of foods.

    One example of a promising, yet controversial GMO is golden rice. Golden rice is a variety of rice that contains beta-carotene, the same pigment that makes carrots orange. Beta-carotene is the precursor to vitamin A in the human body. In areas where rice is a main food source, people are often deficient in vitamin A because it does not occur naturally in rice. Proponents of golden rice hope that their product can reduce malnutrition in developing areas where vitamin A deficiencies cause blindness and mortality.

    Many people don't like the idea of GMOs, because the genes that are imported into plant genomes would not get there naturally. Who knows what might happen to these unnaturally mutant plants in the wrong circumstances; the DNA that was transferred to the plant might be transferred to other organisms. Certain sources of DNA might give people allergic reactions, too. If you are allergic to strawberries, then you might also be allergic to a fruit that had strawberry DNA added. On the other side is the argument that if it doesn't hurt us and has the potential to solve problems, why not? Of course the real issue is that we don't know what the impact of eating food with modified genomes is, and not too many people are willing to volunteer to find out.

    One could argue that modifying DNA in a lab is only an extension of the artificial selection we have been doing since the agricultural revolution 10,000 years ago, but sped up in laboratories and using more technology. Usually though, people do not like to think about eating something that has been developed in a lab instead of a farm. Another argument against GMOs is the fear that biotechnology companies are taking over the food industry. We'll leave it up to you to decide how you feel about GMOs, but keep in mind that there are many players involved with differing goals, opinions and backgrounds. Understanding the science behind genetic modifications is an important first step to being an informed decision-maker.

  • Research

    As biotechnology improves, we know more and more about plant genetics and evolutionary relationships. We can use the tools available to resolve evolutionary relationships between plants, at least theoretically. In practice, plant evolutionary research can be really challenging. Plants are notoriously promiscuous—many plants can hybridize, and others will spontaneously double all their chromosomes and become a new species. This makes it tough to know who is most closely related to whom.



    When scientists want to know how two plants are related, they have a few choices. They can look at the morphology of the plant, compare it to other species, and conclude that species that look the same must be closely related. This is how plant classification used to be done. But if you know a little about evolution, you know that species living in the same habitat tend to share traits and look alike because of convergent evolution. Luckily, now we have technology on our side.

    Evolutionary biologists use phylogenetics to study how species are related to each other. Phylogenetics is the study of the evolutionary history of a group of organisms using genetic information. Scientists look at the DNA sequences of organisms and compare them to see how different they are. The more similar a DNA sequence, the more closely related two organisms are.

    Phylogenetics uses phylogenetic trees, also called phylogenies to show how different species or genera or families are related. These are similar in concept and look to a family tree you might use to describe the relationships between your relatives.

    Every so often, a group of scientists called the Angiosperm Phylogeny Group revises the phylogenetic tree for the flowering plants based on new studies that have been done. For more information on current plant classifications, see the Angiosperm Phylogeny website or the wikipedia page about the APG.

  • Environment

    A major way humans are changing the environment is by moving plants around to new habitats. We are taking plants out of the habitats in which they evolved and are putting them in new ones, which has a huge effect on plant diversity—some plants become invasive, and invasive plants threaten native plant diversity by outcompeting the plants native to an area. Invasive plants reproduce very quickly and spread to new areas. These plants don't only decrease native diversity, but can cost a lot of money in damage control. Invasive species are thought to be the second greatest threat to biodiversity, after habitat loss (Wilcove et al. 1998).

    When plants are moved into an environment in which they did not evolve, sometimes they just don't survive or reproduce there, and they die. However, some plants are good at dealing with a wide range of conditions. These plants are the ones that might be problematic.

    Some famous invasive plants are kudzu in the southeastern United States, purple loosestrife in most of the United States, and prickly pear cactus in Australia. In the United States, over $100 million a year is spent controlling invasive plants in waterways. That doesn't include control efforts in forests, farms, or national parks. So how do these plants get so out of control?
    Kudzu is native to Japan and China but was introduced to the United States in the late 1800s. It has spread across much of the eastern US, and forms dense thickets, covering other plants in its path.


    There are lots of different theories about how invasive plants succeed in new habitats, but two in particular have to do with evolution:

    • Enemy Release Hypothesis (ERH)
    • Evolution of Increased Competitive Ability Hypothesis (EICA)

    The idea behind Enemy Release is that in their native habitat, plants have animals that like to eat them or pathogens (fungi or bacteria) that infect them. These "natural enemies" probably co-evolved with the plant. This means some plants die from the enemy attacks, but a big battle of natural selection is going on. The plants that survive longer and produce more offspring than other plants (that get eaten or infected sooner) may have some sort of resistance to the enemy. Maybe they are less tasty, or hairier, or more resistant to disease. They will pass these traits on to their offspring, and the next generation will be slightly more resistant to enemy attack. However, natural selection is happening on the enemy's side too. Let's say the plants have chemicals in their leaves that are toxic to bugs. Most bugs die when they eat the toxic leaves. Yet some bugs, for some reason, are resistant to the toxin and can eat the leaves and survive to reproduce. They might pass this trait onto their offspring, and all of a sudden there are lots of bugs that can eat the leaves without dying.

    The way natural selection works, plants that produce more potent toxins will survive in greater numbers than plants producing less toxic leaves. However, the bugs' natural resistance can keep growing from generation to generation. This whole scenario is an example of what's called an evolutionary arms race—each side keeps building up their defense strategy but each side catches up with the other.

    Now let's say we take the plant out of its natural environment (e.g. Florida) and take it to a new one (e.g. Hawaii). The plant still has its toxic leaves, but the little bug that was developing resistance to it is not in the new environment. In the new environment, there is nothing that can eat the leaves of the plant because they are so toxic. The plant has escaped its natural enemies and can survive and reproduce quite well because nothing has evolved the ability to eat it.

    The Evolution of Increased Competitive Ability (EICA) Hypothesis builds on the co-evolution idea proposed in the Enemy Release hypothesis, but involves the plant evolving in its new habitat. A fundamental difference is that EICA assumes that the plants are not as successful when they first arrive in the new habitat as they are in later generations.

    The creators of EICA, Bernd Blossey and Rolf Notzold, realized that there is often a lag between the time a plant is introduced somewhere and the time it starts spreading rapidly around the land. They proposed that because the plant was taken out of its natural habitat and no longer has to produce energetically costly defenses against its natural enemies, it could put those resources into growing bigger and stronger (or bigger and more prolific in its seed production). In other words, the plant evolves to be a better competitor in its new environment.

    Blossey and Notzold tested the EICA Hypothesis by comparing plant growth of purple loosestrife from European seeds (native range) and American seeds (introduced range) in the same conditions. They found that the American seeds grew into bigger plants even though they were growing in the same soil and under the same conditions as the European plants. But hey, you could just say the plants were following the American Dream. The American plants were also less resistant to a beetle that attacks purple loosestrife in its native range.

    Not all invasive species become bigger in their introduced ranges, so EICA does not hold for all invasives. Either the ERH or EICA could apply to different invasive plants, and some invasive species are successful for other reasons. This is an active area of research, so scientists don't have all the answers yet.