BIO 112 Notes: Evidence of Evolution

Where are fossils found?

Fossils are found in **sedimentary rocks** formed in the sand and mud that settle to the bottom of seas, lakes, and swamps. New sediment layers cover old ones and compress them into layers of rock called **strata.**

What do fossils give evidence to?

The fossils found in the strata provide a glimpse to the organisms that populated the earth at the time of the layer forming. This contributes to the theory of evolution, because originally it was thought the earth was only a few thousand years old and floods were the cause of valleys and canyons, but with the theory of evolution, it was proposed they were formed by slow, continuous actions, and the earth must be much older than people thought.

* This supports the theory of evolution because it takes organisms millions of years to evolve into what they are today, and because the earth is so old as well, it is probable evolution is likely.

Evolution:

the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.

Why does evolution occur?

It occurs due to a changing environment, forcing organisms to adapt over time and show altered genes, novel traits, and new species.

Artificial selection:

when humans modify species over many generations by selecting and breeding individuals with desired traits. People have been artificially selecting animals and plants for years.

Direct observations:

Biologists have documented evolutionary change in thousands of scientific studies

Homology:

Homology: Similarity resulting from common ancestry

The fossil record:

documents the pattern of evolution, showing that past organisms differed from present-day organisms and that many species have become extinct.

Biogeography:

the scientific study of the geographic distributions of species.

Closely related species share the features used to determine their relationship, but they also share many other features that make little sense except in the context of evolution. What is an example of this evidence?

For example, the forelimbs of all mammals, including humans, cats, whales, and bats, show the same arrangement of bones from the shoulder to the tips of the digits, even though the appendages have very different functions: lifting, walking, swimming, and flying. Such anatomical resemblances would be highly unlikely if these structures had arisen anew in each species. Rather, the underlying skeletons of the arms, forelegs, flippers, and wings of different mammals are homologous structures that represent variations on a structural theme that was present in their common ancestor.

Microevolution:

a change in allele frequencies in a population over generations. The three main mechanisms that cause it are natural selection, genetic drift, and gene flow.

Natural selection:

a differential success in reproduction which results in certain alleles being passed to the next generation in greater proportions.

What does natural selection cause?

Natural selection causes adaptive evolution, a process in which traits that enhance survival or reproduction increase in frequency over time.

Genetic drift:

a process where allele frequencies fluctuate unpredictably from one generation to the next.

bottleneck effect

An example of genetic drift is the bottleneck effect, which can eliminate alleles from the entire gene pool of the species due to a drastic reduction in population size from a sudden environmental change.

founder effect

Another example of genetic drift is the founder effect, which is when a few individuals in a population colonize a new location that's separate from the old population. This also greatly reduces the population size, as well as reduces the genetic variability of the population.

Directional selection

The first main mode of natural selection is **directional selection.**

* Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, causing a population to become more frequent in that phenotype.

Disruptive selection

The second main mode of natural selection is **disruptive selection.**

* Disruptive selection occurs when conditions favor individuals of two different extreme phenotypes over individuals with intermediate phenotypes. Therefore, a population begins to become more frequent in extreme phenotypes.

Stabilizing selection

The third main mode of natural selection is **stabilizing selection,** which favors intermediate phenotypes over extreme phenotypes.

* The result is a population becoming more frequent in intermediate phenotypes rather than extreme.

Gradualism

**Gradualism** states that evolutionary change happens continuously and gradually.

* Many genetic changes that would be insignificant by themselves build up until there is a significant phenotypic difference and an organism distinctly different from the original arises, and it can be considered a new species.

Punctuated equilibrium

**punctuated equilibrium** suggests that the pace of speciation is much more random and dynamic.

* It explains that species maintain a relatively unchanged phenotype and genotype for many generations, followed by an extremely rapid change in traits, resulting in a new species over a short amount of time.

Reproductive isolation

**Reproductive isolation** is the existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring.

Prezygotic barriers:

a reproductive barrier that impedes mating between species or hinders fertilization if interspecific mating is attempted.

Postzygotic barriers

Postzygotic barriers: a reproductive barrier that prevents hybrid zygotes produced by two different species from developing into viable, fertile adults.