Natural Selection and Evolution

Natural selection is the way that a species evolves through adaptations within individuals. According to Charles Darwin’s foundational development of the natural selection theory, competition for limited resources causes evolution. This is because the individuals with the adaptations most fit for the environment will pass on their adaptations to future generations. Reproductive success (that is, having children) measures success in natural selection.

Natural selection acts on phenotypes in a population since the physical, outward characteristics most directly influence an organism’s interaction with its environment.

Mechanisms of Variation

Mutation

Random mutations are often the cause of adaptations that can lead to natural selection and cause evolution. Mutations occur when there are changes in an organism’s genetic code. Read more about mutations on the DNA Replication and Genes to Proteins page

Genetic Drift

Genetic drift is the process of how random changes in genotypes within a population cause changes in the most common phenotype. As certain genotype frequencies vary by random chance, certain phenotypic traits can become more or less common. The bottleneck and founder effects can accelerate this process. In the bottleneck effect, a natural event like an earthquake or fire wipes out a large portion of a species. Then, as a species reproduces, certain traits may have been eliminated with the loss of a large portion of the species, thus causing evolution. Similarly, the founder effect accelerates genetic drift. If a small subset of a population colonizes a new place, such as a new island, only a small portion of the original genetic diversity is present which changes the overall traits of the population. Genetic drift often results in the creation of a new species.

Gene Flow

Gene flow is the movement of genes between two populations. Two populations may be part of the same species but inhabit slightly different niches and therefore have distinct adaptations. The movement of organisms between the two populations through immigration or emigration changes allele frequencies and causes adaptation.

Extinction

When a large extinction or other clearing event occurs, new niches open. Through adaptive radiation, different species move to inhabit the open area. As the populations move, they undergo natural selection to adapt to their new environment.

Artificial selection

Artificial selection occurs when humans take advantage of evolution. Humans use artificial selection to breed preferred traits in anything from crops to dogs.

Speciation

When studying natural selection and evolution, speciation occurs when two organisms can no longer have fertile offspring. For example, a horse and a donkey can mate and have a mule, but mules are infertile. Therefore, a horse and a donkey are two different species.

Sympatric speciation occurs when a new species arises in the same area as the species it arose from. This most often occurs in plants when mistakes in DNA replication more commonly affect the total number of chromosomes in a plant. If, for example, a species is diploid but a few plants become tetraploid, then only triploid plants can reproduce with one another, causing a new species to arise

Allopatric speciation occurs when there is a physical barrier between two populations. When a species becomes split between the two areas, the new populations will evolve separately which can lead to speciation.

Prezygotic and postzygotic barriers accelerate speciation and ensure that two species cannot interbreed. A zygote is a fertilized egg cell. Prezygotic barriers occur before fertilization, meaning that they prevent fertilization from happening in the first place. The following are examples of prezygotic barriers:

Habitat isolation: two species live in separate habitats and therefore don’t come into contact
Temporal isolation: two species are active at different times of day or during different seasons of the year which prevents them from mating
Behavioral isolation: certain species do not carry out the same mating courtship procedures as other species.
Gametic isolation: sperm and egg cells are not compatible.
Mechanical isolation: bodies and reproductive structures are not compatible

Postzygotic barriers prevent a zygote from being viable. This means that a zygote will either not be born, or like in the case of the mule as described above, the offspring will not be able to have its own offspring.

Modes of Selection

Disruptive

Disruptive selection occurs when the traits at both extremes of a spectrum are selected for. For example, this might occur in fish if small fish are able to hide within rocks that big fish can’t reach, but big fish can eat bigger prey that small fish can’t eat. Medium fish have no advantage and are therefore more likely to die and not pass on their genes, causing selection for both ends of the extremes.

Directional

Directional selection occurs when the selection for a trait shifts towards one extreme. For example, the peppered moths are a classic example of directional selection. The moths originally were white, but as the industrial revolution took over and pollution caused a dark soot to cover their environment, the darker moths were able to reproduce more frequently, causing a shift in the population towards darker coloring.

Stabilizing

Stabilizing selection occurs when the middle or average trait is selected for. For example, childbirth undergoes stabilizing selection. A birth weight that is too low is dangerous for the survival of the baby, while a birth weight that is too high is dangerous for the survival of the mother. Therefore, more babies are born at an average weight.

Convergent and divergent evolution

Divergent evolution occurs when a common ancestor evolves into two or more species. This is an example of cladogenesis where one species splits in the evolutionary tree to multiple species.

Convergent evolution occurs when similar selective pressures from an environment push species to develop similar phenotypic adaptations. For example, both bats and birds have wings, but they developed the ability to fly separately as seen by the distinct structure of their wings.

In between divergent and convergent evolution is anagenesis which occurs when one species evolves without branching off the evolutionary line of descent. This means that either an entire species evolved into a similar but distinct new species, or that some members of a species accumulated an adaptation and the rest of the species went extinct.

Evidence of Evolution and Common Origins

DNA/Proteins

On a microscopic scale, changes in DNA which result in changes in protein structure can be analyzed to show evolution. The total number of changes in DNA or a protein show the length of time that one species split from another and the diversity between the species. Even small changes can be significant, as humans and chimpanzees share about 99% of our DNA.

Geography/fossils

Fossils show evidence of evolution by giving a look into the skeletal structure of organisms from long-past times. Looking at strata (the layers in the rocks) or using carbon-14 dating can show how old an organism is and how much its structures have changed from its time to present species.

Embryos

Similarities in embryonic development give evidence for evolution. In the early stages of an embryo, many distinct and different species look strikingly similar, with specific characteristics only developing later.

Anatomical

Certain anatomical structures give evidence for evolution.

Vestigial structures are parts of the structure of an organism that are no longer used and point back to a time when the structure was used. The appendix in humans is a vestigial structure because it no longer serves a purpose but it is still present from past evolution.

Homologous structures are structural elements in an organism that are made similarly even if they do not serve the same function. For example, the bone structure in human arms, whale flippers, cat paws, and bat wings are all similar, even though they don’t serve the same purpose. This shows evolution from a common ancestor with the trait that all the organisms share.

Analogous structures, as discussed above with convergent evolution, show how selective pressures in an environment cause similar adaptations even if they come from different sources.

Evidence of common origins of life

All eukaryotes contain certain characteristics that support the common ancestry theory. All eukaryotes have membrane bound organelles, linear chromosomes, and genes that contain introns. The presence of introns shows that DNA sequences themselves were adapted to better suit the needs of the organism as it evolved.

Additionally, the three domains (eukaryotes, bacteria, and archaea) all show common ancestry though metabolic/energy pathways (cellular respiration), the presence of cell membranes, and the presence of ribosomes

Hardy Weinberg Equilibrium

Hardy Weinberg Equilibrium is a model used to predict allele frequencies in a population that meets the following conditions:

Large population size
Absence of migration
No mutation
Random mating
No natural selection

These conditions are almost never met, but they allow for valuable hypothetical analysis.

There are two important equations when studying Hardy Weinberg equilibrium, pictured above. p and q represent certain alleles for a trait, while p^2, pq, and q^2 represent certain genotypes. p represents the dominant allele while q represents the recessive allele. The first equation is derived from the fact that the two possible alleles represent all the total alleles possible for a trait within a population. The second equation is derived from crossing two heterozygous organisms (pq) for the trait in question.

When solving a Hardy Weinberg question, first write down what the problem tells you. When the question gives information about the allele frequency, you know either p or q, depending on whether the allele is dominant or recessive. If the question gives information on the percent of a population with a certain trait, you are given p^2, pq, or q^2. Most often you are given q^2 since the population with the dominant phenotype could be homozygous dominant or heterozygous which complicates the problem. Visit the calculations page to practice example Hardy Weinberg questions.

Phylogeny and Cladograms

Cladograms and phylogenetic trees are visual ways of mapping out evolutionary relationships. Cladograms differ from phylogenetic trees in that phylogenetic trees are dated such that the splits between species are dated based on chemical or geological evidence, while cladograms are undated. Additionally, a cladogram shows the development of certain adaptations. When constructing cladograms, it is best to use molecular or genetic data, but physical characteristics can be used as well. The example below describes the steps to draw a cladogram.

Origins of Life Theories

There are many theories about the origins of life. Geological evidence shows that the world formed about 4.6 billion years ago. The first life formed around 3.9 billion years ago, and fossil evidence dates back to 3.5 billion years ago.

One theory for the origins of life is the transport of organic molecules to Earth from space by a meteorite or other space event.

Another theory for the origins of life involves the synthesis of monomers which were eventually built into single celled and more complex organisms. These reactions have been synthesized in lab conditions in the following steps:

Monomers: the Miller-Urey experiment successfully created monomers in an environment like early earth with limited atmospheric oxygen and using electric sparks to simulate lightning.
Polymers: another experiment produced polymers from monomers using catalytic surfaces like hot sand
Protocells: these early cells organized polymers in a concrete area defined by a membrane
Molecule of heredity: RNA was likely the first form of heritable information which allowed single cells to become more complex

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