Natural Selection

The EoE article on Natural Selection (that I wrote) is still under review, so I will give you a copy of the info in that article here. The Evolution article in the EoE has some useful information about natural selection as well.

Natural Selection

Biologists are interested in understanding the amazing diversity of life that we observe around us. Fortunately, we have the process of natural selection to aid us with this task.

It is my experience that most people have a poor understanding of how natural selection works, so it might be useful to briefly discuss how natural selection can cause organisms to become adapted to their environment. First, natural selection is not best defined as “survival of the fittest” (it is a shame that the one thing that apparently every student remembers from school is wrong). Instead, natural selection is best defined as a process.

Natural selection is a process where if-

1) there is variation in traits among individuals in a population,

2) this variation in traits is heritable (i.e., there is a resemblance in traits between parents and offspring), and

3) this variation in traits affects survival, fecundity (the number of babies produced), or mating ability,

then the trait frequency varies between the parent and offspring generation.

Variation in traits among individuals in a population can occur because different organisms have different genes or because they are found in different environments. Genes are molecules (deoxyribose nucleic acids, DNA) that are found in chromosomes. Genes play an important role in determining phenotypes because (1) genes influence which proteins are produced inside cells, (2) proteins can act as enzymes or “biological catalysts” (catalysts act by speeding up the rate of chemical reactions), and (3) phenotypes are influenced by which chemical reactions are taking place in the cells. Thus, if two individuals have different genes then they can produce different proteins that act differently as enzymes. Differences in enzymes leads to differences in the types, or rates, of chemical reactions occurring in the cell which can produce distinctly different phenotypes.

Genes also cause traits to be heritable. We tend to resemble our parents because we receive genes from both of our parents. We are not exactly like our parents because we only get half of our genes from our Mom and the other half from our Dad. When organisms reproduce sexually they produce gametes (eggs – female gametes, sperm- male gametes) by the process of meiosis. The male gamete is mobile (usually sperm are able to swim) so they move to the egg where fertilization occurs to produce a zygote. Crossing over of chromosomes during meiosis and the random combination of gametes during fertilization results in the production of offspring that are genetically different from both of their parents and all of their siblings.

Genes get passed on from one generation to the next by reproduction. Obviously, genes that produce traits that allow organisms to be good at surviving and reproducing should get passed on more often than genes that produce traits that make organism bad at surviving or reproducing. Thus, over time we would expect genes that produce traits that make organisms better at surviving and reproducing to become more common in the population (this is what is meant by the change in trait frequency over time in the definition of natural selection). We might expect that these genes would get more and more common until all individuals in the population have these genes (the gene is “fixed” in the population). If this occurred there would be no more heritable phenotypic variation (assumptions 1 and 2 would not be met) so natural selection would cease.

The creation of new genetic variation by mutation will be needed for natural selection to continue. Mutations are changes in the genes (that typically occur as the result of mistakes produced during replication of chromosomes or the production of gametes) that lead to changes in the phenotypes. Mutations are random. If a mutation occurs that causes an individual to have higher survival or reproductive success, then we would expect the frequency of that gene to increase in the population. A sequence of selection followed by the introduction of new mutations repeated over time should produce organisms that are good at surviving and reproducing in their environments. We call traits that make organisms good at surviving and reproducing “adaptations”. We expect that over time, natural selection should cause organisms to be adapted to their environments.

Because conditions vary between different environments, it is not surprising that the traits that maximize survival and reproduction in differ between different environments. Because of the differences in environmental characteristics of aquatic and terrestrial environments, it is not at all surprising that we see very different types of organisms living in the water and on land.

Expected Learning Outcomes

At the end of this course a fully engaged student should be able to

- explain how the process of natural selection has produced a trait that has increased an organism's survival or reproduction in a particular environment (TEKS 112.43. 7B).

- identify and describe behavioral, physiological, and morphological adaptations to a particular environment (TEKS 112.43. 7B).

- develop curricular materials to teach students how and why the traits of similar organisms can be different across different environments (TEKS 112.43. 7B & 12C).