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.