Introduction to Natural Selection

Natural selection posits that only a certain percentage of offspring born will survive to reproduce another generation. Individuals with certain genotypes will be more likely to survive, mate, and reproduce their genotypes in subsequent generations. Thus, natural selection acts upon individuals, whereas evolution occurs at the level of a population. There are four aspects to natural selection we need to consider.

1. Random mutations cause variations in alleles, or different amino acid sequences, which in turn cause variations in DNA sequences, producing new gene products (proteins) that serve different functions.
2. The mutated alleles may code for proteins that are either beneficial or detrimental to the individual’s survival.
3. Beneficial mutated alleles are more likely to be passed on to subsequent generations because the survival of the parent organism has been enhanced beyond other organisms without the beneficial allele.
4. Over many generations, the frequency of the mutated beneficial allele may increase because of the increased survival and reproduction of organisms carrying the allele over organisms who have other variations of the gene.

Darwinian Fitness

Darwinian Fitness is a quantitative measurement of the reproductive fitness of individuals with certain genotypes. This should not be confused with physical fitness in organisms. Physical fitness may give organisms an advantage in their environment, but if their genes are not passed to offspring, their Darwinian Fitness is lower. See the pictures below for an example of the difference between physical fitness and Darwinian Fitness (reproductive fitness).

Arnold Schwarzenegger is known for his physically fit image. He has produced four offspring.

Octomom may be less physically fit than Arnold Schwarzenegger in his bodybuilding days, but she has produced 14 offspring.

Who has more copies of their genes present in the population, Arnold Schwarzenegger or Octomom? Octomom certainly has passed on more genes and thus has a higher Darwinian Fitness.

Let’s calculate Darwinian Fitness numerically. Let’s assume three genotypes in our population. The first genotype we’ll call AA, a dominant homozygote. We’ll say Schwarzenegger, with four offspring produced, has the genotype AA. The second genotype is Aa, a heterozygote. We’ll say Octomom, with fourteen offspring produced, has the genotype Aa. Let’s throw in a third genotype aa, a homozygous recessive genotype. We’ll say that I have the genotype aa, and I have produced no children.

To calculate the Darwinian fitness of each genotype, we will assign the genotype with the highest reproductive success the value 1.00. The other genotypes will be compared relative to the genotype with the highest rate of successful reproduction

Octomom has the highest reproductive success with the genotype Aa. So we have:

Fitness of Aa = 1.00

The next step is to divide the number of offspring in our other genotypes by the number of offspring produced by our genotype with the fitness value 1.0. This will tell us what the Darwinian Fitness of each genotype is relative to the other genotypes. Octomom produced 14 children, so we will divide each value by 14.

Fitness of Aa = 1.0

Fitness of AA = 4 / 14 = 0.29

Fitness of aa = 0 / 14 = 0.o

So we can see that the highest Darwinian Fitness is assigned to Octomom with fourteen children and a Darwinian Fitness value of 1.0. The next closest in Darwinian Fitness is Arnold Schwarzenegger with four children and a Darwinian Fitness value of approximately 0.29. The least reproductively fit is myself, with no children and no Darwinian Fitness at all, or 0.0.

Hopefully from this example you can see that physical fitness does not equal Darwinian Fitness. Also, it merits mentioning that the values of Darwinian Fitness do not necessarily need to add up to 1. The Darwinian Fitness represents ratios of the genotypes relative to each other, and Darwinian Fitness based on genotypes does not necessarily come out to nice round numbers every time.

What Natural Selection Does Not Do

Perfect organisms do not result from natural selection. There are many reasons why there will never be a “perfect” organism resulting from the mechanisms of natural selection. For example, random mutations cause new alleles to be acted upon by natural selection; natural selection does not cause new, beneficial alleles to be formed. Natural selection is not purposeful in its events; rather it is a process that flows with the events that occur in any given environment.

Patterns of Natural Selection

There are four major types of natural ecological selection.

1. Directional Selection- one extreme of a phenotype has an advantage over the other extreme of a phenotype. For example, there may be phenotypes that express a very tall organism or a very short organism. If a certain environment favors only the very short organism, the tall organism will have a disadvantage and the number of tall organisms will be small compared to the large organisms.
2. Stabilizing Selection- the “middle of the road” phenotype is favored over either extreme. If an environment favors organisms with a medium height, rather than very tall or very short, then most organisms will be of medium height and only a few will be very tall or very short.
3. Disruptive Selection- two or more phenotypes are favored over any other phenotypes. If the environment favors either very tall organisms or very short organisms but not those of medium height, there will be more very tall or very short organisms than there are of medium height.
4. Balancing Selection- heterozygote phenotypes are favored over homozygote phenotypes. Homozygotes are present in few numbers in the population than heterzygotes. Environments with balancing selection is often said to present a heterozygote advantage because the environment favors genetic diversity and keeping as many alleles as possible present in the gene pool.

We can also consider one additional type of natural selection: sexual selection. There are two types of sexual selection.

1. Intrasexual selection- occurs between members of the same sex. Usually, this manifests as direct competition between males.
2. Intersexual selection- occurs between members of the opposite sex. Usually, this manifests as males vying for the attention of a female.

Intrasexual selection often results in direct competition between males for females or for territory.

Intersexual selection often results in showy traits in males to attract the attention of females.

Related posts:

1. Population Genetics
2. Important Biology Terms
3. Mendelian Genetics Basic Definitions