Evidence of Evolution

So we know that species with the best traits for survival are able to survive and reproduce to pass on those traits and over time, the population will evolve to have those favorable traits. But how did scientists come up with this theory? What evidence do they have to suggest this is how it works? The following are examples scientists have studied that has led them to the modern theory of evolution.

Fossils

Fossils are the remains of past life that are preserved by natural processes. Often, they are trapped within layers of rock, hidden from the surface for millions of years. These can be entire bones, shells, or even imprints of plants left in rock. Check out this website that does an excellent job showing how fossils are formed and discovered. 

Fossils from related species from different time periods can be compared to evaluate the evolutionary history associated with them. To determine how old a fossil is, scientists do what is called radiometric dating. The rock layer that the fossil is in contains radioactive isotopes with calculable half lives. A half life refers to how many years it takes half the radioactive material to decay. If you calculate how many half lives the rock has undergone, you can determine how old it is. So if you know two species are related and they had fossils that were found in rock 3 million years apart, you can make conclusions as to what evolutionary mechanisms led to that. Fossils are perhaps the best piece of evidence as to what the Earth used to be like, and what modern organisms arose from. 

Homologous Structures

Homologous structures are structures that species have that have similar internal structure based on common ancestry, but different function based on selective evolutionary pressures. Internally, homologous structures are very similar, but externally perform completely different functions.

The structural anatomy of the arm bones of these four species is fascinating. Humans, dogs, birds, and whales all have a similar upper arm bone (humerus in humans), 2 lower arm bones (radius and ulna in humans), wrist bones (carpel bones in humans, and phalanges.  The similarity in these structures suggest these species are related through a common ancestor.

Волков Владислав Петрович [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Vestigial Structures

Vestigial structures are structures an organism has that shows evidence of something that was once used, but is no longer needed. Vestigial structures are evidence of structures that ancestors used to have,but have not totally been selected against. The following are examples of vestigial structures seen in modern organisms:

• Coccyx in humans:
• The coccyx is the anatomical term for the tailbone. The coccyx is located at the bottom of the vertebrae (backbone), and points inward toward the center of the body. It serves no major function to humans and can really be a pain in the butt if damaged. Other species such as apes and monkeys which have a coccyx, have a coccyx that points outward with a tail protruding through their posterior end. Humans having a coccyx shows they share a common ancestor with other primates, but natural selection has to humans no longer needing a tail for function.
• Appendix in humans:
• The appendix is a small hallow opening in the large intestine. Scientists struggle to determine if it has any real function in the modern human body. Appendicitis is a common disorder when the appendix becomes inflamed. Appendicitis often requires the appendix to be removed. Removal of the appendix has no effect on the body. It is theorized that the appendix was once useful in our ancestors immune systems, but now it is no longer necessary.
• Hip bones in snakes:
• Believe it or not, many snakes have hip bones. Some have hip bones that are so defined that they poke through the bottom of their skin. Snakes having hip bones, which serve as the foundation for limbs, show that snakes may have evolved from 4 legged tetrapods. In the past, they may have had legs for walking but over time, they began to slither and no longer needed legs to move around. The hip bones are remnants from past ancestors that walked on legs. 

Mechanisms of Evolution

Evolution refers to the cumulative adaptations of a species as the result of descent with modification of genes through many generations. Evolution has no motive or goal but is the result of different mechanisms and selective pressures based on environmental factors. The following are 5 mechanisms of evolution:

Natural Selection

Natural selection is probably the most prominent mechanism of evolution. Proposed by Charles Darwin, the theory of natural selection gives an answer to how modern organisms evolved from past life. Individuals with the best traits for survival are able to survive and pass on their genes until the majority of the population possesses that advantageous gene, thus evolving the population. Additional information about natural selection can be found here.

Mutations

Mutations are chance alterations in nucleotide sequences (DNA) that can result in new phenotypes in resulting offspring. For example, two green beetles can reproduce and a mutation in nucleotide sequences could lead to the offspring being brown. Mutations lead to variation in the population which is the first step of evolution. There are various type of mutations, some being more drastic than others. It is also possible that a mutation can happen, and it have no real effect on the individual. Mutations tend to occur during DNA replication. The following are five different types of mutations: 

Deletion

A segment of DNA is removed from the original DNA sequence. Can have no effect or drastic effects on an individual, it depends on what segment of DNA is effected.

Duplication

A segment of DNA is copied twice is repeating segments. Duplication may or may not have significant effects on the individual.

Inversion

When copied, the DNA segment is reversed in orientation. It would be like if you went to sleep with your head on one end and then when you wake up, your head is on the other end. 

Insertion

A section of nucleotides is added in a place it should not. This could throw off the reading from and have significant effects on the individual.

Translocation

A portion of a nucleotide sequence is copied onto another nucleotide sequence. 

DNA models depicting different types of mutations

Genetic Drift

Genetic drift refers to chance changes in the allele frequency that leads to the evolution of a species with a resulting allele frequency not necessarily indicative of the original population. There are two main types of genetic drift; founders effect and bottleneck effect. Founders effect when a smaller group of individuals become isolated from the original population and become the founders of a new one. The population then evolves based on selective pressures of their new environment and it could become completely different from the original population. Bottleneck effect refers to a chance event killing a group of individuals from the population. This could be as major as a storm wiping out much a population on an island, or as simple as a human stepping on a group of beetles. The resulting population is different from the original because of a chance encounter. The population will then recover and evolve based on the resulting allele frequencies after the bottleneck event, and the selective pressures in the given environment. The following video simulates genetic drift by depicting both founders and bottleneck effect.

Gene Flow

Gene flow refers to the changes in allele frequency in a population by individuals coming into, and leaving a population. This is similar to founders effect except the only difference is founders effect is a result of chance happenings. Gene flow is simply the influx and outflux of alleles in the population based on habitable area, availability of resources, and mates. Certain advantageous genes may enter a population as a result of gene flow and the population may evolve to acquire that advantageous trait.

Sexual Selection

Lastly, sexual selection refers to the selection of mates based on physical characteristics. It is usually female choice and they select mates based off what they find attractive. It could be anything from feather color, to length of tail, to size of muscles, all of these could be characteristics desirable to the opposite sex and cause them to reproduce. Sexual selection can result in the evolution of a species because if many females are drawn to the same aesthetic, they will continually reproduce with the males that present that trait. Over time, the population will evolve to mostly have that attractive trait.

Understanding Natural Selection and Artificial Selection

The driving force behind the origin of all living species and all species to have ever lived is theorized to be natural selection. Natural selection is simply a mechanism that drives the evolution of organisms in an environment based on their traits. Through humans understanding of natural selection, they have been able to forcefully evolve certain animals to have more desirable traits; this is called artificial selection or domestication. Continue reading for clarity on natural and artificial selection.

Natural Selection

The Theory of Natural Selection was first published by Charles Darwin in 1859 in his book The Origin of Species. After decades of research on the Galapagos Islands in South America, Darwin's observations led him to this theory of natural selection. What Darwin proposed was this:

-There will be variation of traits in a population

- More organisms are born in a population than are able to survive

- Organisms with trait(s) best adapted for the environment will survive

- Survivors will reproduce and pass on those advantageous traits to offspring, thus evolving the population over time

Hopefully that gives you some understanding to the name natural selection. Organisms fit with the best traits for survival are "naturally selected" to survive and pass on those traits to offspring. Over time, the population will evolve to have a majority of individuals that have the advantageous trait.

The Industrial Revolution began in Great Britain in the late 1800s. Trees were once white allowing white colored peppered moths to blend in better. White moths could then survive and reproduce evolving the population to be more white.  Over time, fossil fuel emissions stained the trees black. This black substrate was advantageous for the black colored peppered moths. The black colored moths were then naturally selected to survive and evolve the population. 

By Khaydock - Own work, CC BY-SA 3.0, Link

There are a few misconceptions about natural selection that trip some students up when learning it:

- Individual organisms don't evolve, populations evolve. Over the course of many generations, advantageous traits, as selected by the environment, will be passed down by survivors to their offspring until the majority of the population has evolved to have that trait. Variation in the population arises from chance mutations in DNA that may or may not be advantageous in a given environment. Individual organisms do not spontaneously evolve into something better; it is a long gradual process.

- Evolution takes a long time. The Earth has been around 4.6 billion years and all of Earth's biological processes, including natural selection, have been at work during this time, creating the planet we know today. Natural selection happens over thousands of years gradually modifying species based on traits from past ancestors. The only species we can see evolve over a human life time is bacteria. They reproduce asexually at a rapid rate allowing them to produce millions of generations in a relatively short amount of time. That's why microorganisms such as yeast and e. coli are model organisms in the lab for understanding natural selection.

- Natural selection doesn't fashion perfect organisms. It simply selects for for whatever trait is beneficial in a given environment.

- Certain organisms aren't more "highly evolved" than any other. Organisms will thrive in whatever environment they have evolved to live in. For example, it may seem easy to say that a human is more highly evolved than an arctic copepod (a tiny sea insect). Humans are bigger, stronger, and smarter so of course we are the superiors species! However, imagine LeBron James gets dropped into the middle of the arctic ocean without a life vest on. I don't care how well you can dunk a basketball, an arctic copepod is going to outlive a human in that environment because it is better evolved for it.

Artificial Selection

Artificial selection, whether understood or not, is a practice humans have engaged in since long before Darwin came up with the Theory of Natural Selection.  In artificial selection, humans select which organisms to reproduce based on the desired characteristics they want the offspring to have. This is a practice used often in agriculture to yield the most crops/livestock as possible. This has led to much bigger produce and animals to meet the needs of our growing human population. The mechanism behind the evolution in artificial selection is similar to natural selection, with the only difference being who does the selecting. In natural selection, the environment selects for the best traits, but in artificial selection, humans select for the best traits. The traits may not even be the best, they just might be what humans find to be most desirable. Both natural and artificial selection lead to the evolution of a population over time.

Believe it or not, Great Danes and Chihuahuas are the same species, Canis familaris. Every breed of dog descended from grey wolves and falls under the same umbrella of species. Humans domesticated dogs and artificially selected for different traits when breeding them resulting in the variation in dog breeds we see today.

By Ellen Levy Finch / en:User:Elf (uploaded by TBjornstad 14:51, 17 August 2006 (UTC)) - http://en.wikipedia.org/wiki/Image:IMG013biglittledogFX_wb.jpg, CC BY-SA 3.0, Link

Why Understanding Natural Selection and Artificial Selection Is Important

Understanding natural selection is essential to humans understanding the origin of life on Earth. It is such a perplexing question, how could all things on Earth arise from nothing? The idea that all living organisms arose from a shared common ancestor and that all living organisms are a result of the descent of genes with modifications based on selective pressures of the environment is the best theory we have. Understanding the history of life gives us a better understanding of ourselves and how our bodies function. It can also help us predict the future based on biological processes that have occurred in the past. We can predict the evolution of bacteria to become resistant to drugs, so scientists must continually come up with new, effective, antibiotics. I'd say the theory of natural selection has laid a foundation for how we perceive science and has certainly aided the survival of humans.

Having a general understanding of genetics and natural selection has allowed humans to virtually master artificial selection. Mastering artificial selection has been monumental in providing food needs for an exponentially growing population. The business of agriculture would be entirely different if not for artificial selection.