Evolution 101: Natural Selection

This week’s Evolution 101 post is by an interdisciplinary group of BEACONites, all of whom rely on the principles of natural selection in their research: MSU graduate student Nikki Cavalieri (Zoology), MSU postdoc Prakarn Unachak (Evolutionary Computation), and NC A&T graduate student Patrick Wanko (Industrial & Systems Engineering).

If animals are closely adapted to their habitats, why do we see overlap?

Photo credits: Gray Tree Frog by Heidi Bakk-Hansen; Green frog by Trish Coxe; Background by Kahunapule Michael Johnson; Illustration by Prakarn Unachak

For instance, Gray Treefrogs (Hyla versicolor) and Green Treefrogs (Hyla cinerea) in the Southern United States seem to  be ecologically equivalent. Both species eat insects, live off the ground on vegetation and lay their eggs in small pools. So why is there not just one species?

Adapted Roger Conant and Joseph T Collins. 1998. A Field Guide to Reptiles & Amphibians of Eastern & Central North America (Peterson Field Guide Series).

While Gray Treefrogs and Green Treefrogs seem to occupy the same habitat, when we look closer we can see that though their ranges overlap, Gray Treefrogs live farther north than Green Treefrogs. We can also see that Gray Treefrogs prefer wooded areas with temporary pools, while Green Treefrogs prefer more open wetland areas with cattails and other aquatic vegetation.

CC google Hyla versicolor (LeConte, 1825). Cryptic colored adults clinging to a tree trunk. Photo © Painet, Inc.

 On a tree in a wooded area, the Gray Treefrog is much harder to detect.

Photo by Richard Crook

In a wetland, the situation is reversed.

What is Natural Selection?

Natural selection is the process in nature by which organisms better adapted to their environment tend to survive and reproduce more than those less adapted to their environment.

For example, treefrogs are sometimes eaten by snakes and birds. Gray Treefrogs blend well in dark wooded areas on tree bark and Green Treefrogs blend in well with green vegetation found in marshes and swamps. A Green Treefrog on the bark of a tree is easier for a predator to find, compared to a Green Treefrog on a green leaf. So, Green Treefrogs that go into habitats where they are not camouflaged are more likely to be eaten by predators. Since Treefrogs that have been eaten do not live to have any more baby Treefrogs, natural selection has favored Treefrogs that live in habitats in which they are more camouflaged.

This explains the distribution of Gray and Green Treefrogs. The wooded habitat of the Grey Treefrog is larger and extends farther North, while the Green Treefrog’s swamp and marsh habitat is concentrated in the South.  In the area in which Gray and Green Treefrogs overlap, both habitats occur but in different places.

However, natural selection does not always go to the optimum. It only goes to what works. For instance, rabbits are herbivores, which have hind gut fermentation (fermentation of food after passing through the stomach). They have a special organ called a caecum which helps them digest their food. Unlike other animals, the caecum of the rabbit is located too far down the intestines of the rabbit for the rabbit to get all the nutrients out of its food. So when digested food is expelled from the body, it still contains a high quantity of nutrients. To compensate for losing these nutrients, rabbits are coprophagous (they eat their own fecal pellets).  They have two types of fecal pellets: 1) pellets which have been digested only once, which they place in a special latrine to consume later, and 2) those than have been digested twice and are not stored. Rabbits have evolved to get maximum nutrients from their food despite having a non-optimum arrangement of digestive organs.

To be more general, natural selection is a process that results in some animals and plants with certain characteristics being better adjusted than others to their natural environment. Those plants and animals then have a higher chance to survive, reproduce, and increase their population more than the ones that are less adapted to their environment. The better adapted plants and animals are therefore able to pass on their advantageous characteristics (coded for by genes) to their offspring through inheritance.

However, genes are not always passed on to the offspring in the exact same form as the parents’ genes. Change in a gene sequence can occur through two mechanisms known as crossover and mutation.

Crossover? Mutation? What are those?

We cannot see genes with our naked eyes, but we can observe the products of them through physical traits, known as phenotype (type of hair, color of eye/skin, sex…).  Gregor Mendel, the “father of modern genetics,” experimented with pea plants between 1856 and 1863. Mendel showed that by fertilizing a given shape of green pea plant with the pollen of a different shape of yellow pea plant, one would get a variety of green and yellow peas of many shapes. The resulting peas will share their color or their shape with the original peas. What Mendel did is today called cross-pollination, and the fact that the resulting peas will share some common traits is due to inheritance.

Genes are grouped together on chromosomes. For crossover to occur, we need two chromosomes that exchange material.  A mutation, on the other hand, is a change or error within a gene or chromosome that can result in a change of genetic functions and expressions. When that error occurs, it modifies a gene which can change the phenotype of the plant or animal, which may be more than just a change in appearance. Mutations can involve deletions, duplications, insertions, inversions or translocations of sections of genetic sequence. Mutations and crossovers supply the raw material for natural selection to work with by creating variation among organisms.



How Do We Get So Many Different Organisms by Natural Selection?

Natural selection results in organisms with different characteristics (caused by mutations and crossovers) thriving in different environments. Beside the Green and Gray Treefrog (our example above, showing adaptation through camouflage), there are many ways natural selection shapes organisms:

  • Some bacteria can live at temperatures of 60°C (140°F) and higher. One species, Methanopyrus kandleri, can even prosper under extreme heat as high as 120°C (248°F)! Other bacteria also adapt to seemingly inhospitable environments –  acidic, radioactive, or under the deepest part of the sea, where there are no conventional source of food. No matter how hostile an environment is, it is very likely that you will find some kind of microorganisms evolved to live there.
  • Penguins, at first glance, are birds that cannot fly, which does not seem to make them good candidates for survival. However, in place of flying, penguins have adapted to be master swimmers, which benefits them greatly in finding food and escaping predators.  Furthermore, in Antarctica, and other places penguins live, there are no natural predators on land, thus losing the ability to fly is not a disadvantage. There are other flightless birds, and all have adapted to compensate their lack of flight in other ways. Either by being a fast runner (Ostrich), hiding well (Invisible Rail), or able to defend themselves effectively (Cassowary).

  • Some plants, such as the Venus Fly Trap, are carnivorous. Usually plants obtain nitrogen, a chemical element vital to a plant’s survival, from the soil through their roots. These plants, however, usually grow in areas where soil is lacking in nitrogen. They cannot get enough nitrogen just by taking it out of the ground. In order to thrive in such an environment, these carnivorous plants capture insects in trap-like leaves. These insects become an alternative source of nitrogen for the plant allowing it to survive in a nitrogen-poor habitat.

Environments change over time, and natural selection acts on the genetic diversity in species. Individuals with better traits for the new environment have more offspring. After many generations in this new environment, the current population may not look like their ancestors because natural selection has changed them – they have evolved – to survive in the new environment.

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