Natural SelectionTechnical Definition (Actual One)

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Understanding Natural Selection

By: Mark Goldy-Brown

Date: 10/17/2013

Audience: Freshmen students in a general biology class at a larger

university such as Penn State. The students are assumed to have taken

a basic biology course in high school and are therefore familiar with

many of the fundamental principles of biology such as evolution and

genetics.

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Understanding Natural Selection

What is Natural Selection?

Natural Selection is one of two mechanisms of

evolutionary change. Specifically, natural selection defines

a process by which organisms with certain, inheritable

characteristics survive and reproduce more frequently than

organisms without them. This process results in adaptive

evolution, by which from generation to generation, a

population of organisms becomes better adapted to its

environment. Essentially, you can think of life as one big

game of chutes and ladders, with only those organisms bestadapted to their environments getting to keep on climbing

up.

Charles Darwin (Image 1) first proposed the theory

of natural selection, which he termed survival of the

fittest, all the way back in the 1860s after his famous trip

to the Galapagos Islands, and to this day, the theory still

holds. How strange to think that a theory, created by a man

without any of the tools of modern science, still holds up

over 150 years later. According to Darwin, and furtherembellished by modern science, natural selection requires

three basic conditions in order to occur in a species:

1) Variation (genetic) must exist in a specific trait among individuals of a given species2) Variation in this trait must lead to non-random differences in reproductive success3) Variation in this trait must be heritableOver the next few paragraphs, I will break down each component of natural selection in a nice

and simple manner. While natural selection may seem daunting right now, by the end of this article,

you will be an expert, able to call home and show off your new knowledge to your parents!

But before we jump in to a discussion of natural selection, lets first talk about a system in

nature that will help you to understand natural selection and the importance of each of its three

components. While most scientists and textbooks explore biological theories using generic organisms

such as mice, fruit flies or bacteria, we will explore natural selection using something far more

interesting; fence lizards and fire ants.


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A Model System for Natural Selection: Lizards, and Fleeing, and Ants, oh my!

Fence lizards (Sceloporus undulatus)are a species of lizard native to the southern and eastern

United States. They range from Alabama and Florida, all the way up to Pennsylvania and New Jersey.

The lizards all along this range constitute the same species, but each population of lizards along this

great range shows differences in various traits such as size, color and behavior.

Down in Alabama, fence lizards live in areas also inhabited by an invasive ant, the Red Imported

Fire Ant (Solenopsis invicta) which was accidentally introduced into the South over 80 years ago. Im

sure youve heard of fire ants before, whether by new or by a personal encounter! Just 12 attacking fire

ants can paralyze and kill an adult fence lizard. Keep in mind, the average fence lizards is about 72 times

larger than the average fire ant (Image 2).

So, among populations of fence lizards in Alabama, intense pressure exists for lizards to

respond to attack by fire ants; if they dontrespond, they may die. A lizards survival and reproduction

directly depends on how it responds to a fire ant attack. When attacked by fire ants, fence lizards

exhibit one of two responses: fleeing or sitting still. Those that flee have an improved chance of survival,

while those that sit still will likely die. Figure 1 summarizes the lizard-fire ant system for reference.


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Now that you understand the model system, lets explore the three components of natural

selection and how they function in our lizard-fire ant system.

The Three Components of Natural Selection

1) Variation must exist in a specific traitNatural selection requires variation on which to act. Variation comes in many forms (behavior,

appearance, fecundity, etc) and for our purposes we will assume all variation has a genetic basis.

Apart from clones, every organism differs slightly from each other, even within the same species. Just

take a look at eye color in humans; some people have blue eyes, some have brown and others, green!

So returning to our lizard-fire ant system, which specific trait shows variation? Well if you

remember Figure 1, then you know not every lizard responds the same when attacked by fire ants; some

lizards flee while others just sit still and endure the pain. So in this case, natural selection acts upon the

behavioral response to fire ant attack; bet you didnt know behavior could evolve. In other words, the

response to fire ant attack is the specific trait that varies.

Lizards that flee have a better chance of survival, while those that just sit still will either die or

become severely damaged. If all of the lizards fled, or all of the lizards just sat still, then natural

selection would not affect this trait.


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*Remember: Natural selection is not a conscious process; it happens naturally as an organism interacts

with its environment.

While natural selection might still be a bit confusing now, it will start to get clearer as we

progress so lets move on to the second requirement of natural selection.

2) Variation must lead to differences in reproductive successFor natural selection to act, variation in a trait must lead to differences in the reproductive success

of those individuals possessing the trait compared to those that lacking it. Assuming natural selection

occurs in our lizard-fire ant system, how does the previous sentence relate to it?

If you said the lizards which flee from fire ant attacks will have more offspring than those which sit

still when attacked, you were correct! For those of you whocouldnt connect the dots, lets explore this

a little further.

We are still talking about variation in the behavioral response to fire ant attack, but now we are

examining how that variation relates directly to reproductive success. Think of it this way; if you die,

you cant have babies. The lizards which flee from fire ants survive to reproduce more frequently than

those which sit still when attacked. Since more fleeing lizards survive to reproduce than lizards that sit

still when attacked, fleeing lizards will produce more offspring than non-fleers. Thus, in the next

generation of lizards, the offspring of fleeing lizards will make up a greater percentage of the population

than the offspring of lizards which sit still (see figure 2 below).

Now can you begin to see the importance of the link between variation and reproduction?

3) Variation must be heritable


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The last component of natural selection states that the variation in a specific trait must be

heritable, or able to be passed on from parent to offspring. If we look at our lizard-fire ant system, the

lizards that flee must produce babies that flee and the lizards that sit still must produce babies that sit

still. Without this vital link, natural selection would not lead to adaptive evolution.

If response to fire ant attack were not a heritable trait in fence lizards, then we would expect infigure 2 to find the ratio of fleeing lizards to sitting still lizards remaining fairly constant from generation

to generation. However, this is not the case! Fleeing lizards produce offspring that flee, and so over

multiple generations of lizards we see more and more lizards responding appropriately to fire ant attack.

Consequently, the trait for fleeing becomes more common among the lizard populations. Natural

selection leads to organisms well-adapted to their environments, but if these beneficial adaptations

(such as fleeing when attacked by fire ants) cannot pass on from generation to generation, then they

cannot lead to change within a species.

Now do you see, with this last piece of the puzzle, how natural selection works?

Final Conclusions

As stated initially, natural selection is the mechanism of evolutionary change. As we discussed,

natural selection has three requirements:

1. Variation must be present in a specific trait2. Variation in this trait must lead to non-random differences in reproductive success3. Variation in the trait must be heritableIf a trait meets all three of these components, then natural selection will act upon it, thus

resulting in organisms better adapted to their environments. Taking the fence lizard as an example, the

specific trait we examined was whether the lizards fled from fire ant attack or merely sat still. Since

natural selection acts in this system, we expect that the frequency of fleeing lizards should increase from

generation to generation as long as the fire ants remain a significant threat. Figure 3 summarizes the

direct relationship between natural selection and the fence lizard-fire ant system.


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As we have explored in this brief article, natural selection is a means of bringing about change, a

mechanism by which life continues to move on, despite all of the challenges our world presents. When

fire ants were first introduced to the US, fence lizards were not able to respond appropriately. Most of

the lizards just sat still and died, but there were some, just a small few, that fled when attacked. Over

many generations, the lizards that fled had more offspring than those that didnt, and so, in general, the

species adapted. Isnt that amazing? I hope by reading this article, you haveboth gained an

understanding of natural selection, but also an appreciation for it as well. The most beautiful and

intricate relationships in nature, such as that of the fragrant flower, tantalizingly attracting the

persistent pollinator, the charming crocus creeping out from below the shimmering snow, or even the

little lizard fleeing from the fierce fire ant, arose by the guiding force of natural selection.

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Natural SelectionTechnical Definition (Actual One)