This topic deals with how all species are interconnected and descended from a common ancestor, and how all species are named and organized. It gives evidence for the theory of evolution and explains its significance for life on earth. It is one of the more challenging chapters as it is lengthy and includes many terms. It builds extensively on previous knowledge, specifically on genetics. I would therefore revise topic 3 if something is unclear. Otherwise, it is a very interesting chapter even though it might be hard at times. My advice would be to take some time and learn the terms. I will include a list to make it easier.
Points for revision:
- Evolution and natural selection
- Evidence for the theory of evolution
List of terms:
Evolution – cumulative changes in the hereditable characteristics of a population
Species – Organisms that can produce outcome capable of sexual reproduction
Population – A group of individuals of a species living and interbreeding within a specific area
Artificial selection – Selective breeding to produce species with more desirable traits
Adaptive radiation – Evolution of a single species into various types to adapt to new environments
Homologous structures – Similar structures evolved from a close common ancestor
Analogous structures – Similar structures not evolved from a close common ancestor
Transient polymorphism – Strong environmental pressure slowly replacing an allele for another allele
Variation – diversity in gene frequencies on a species
Resistance – Population not affected by certain biotic or abiotic factors
Immunity – Individual not affected by certain biotic or abiotic factors
Natural classification – Classification based on similarities, then characteristics
Artificial classification – Classification based on arbitrary characteristics, then similarities
Evolution and natural selection
Evolution is a gradual change in the cumulative hereditable characteristics of a population. If the change in characteristics is significant enough, a new species might evolve. A species is defined as two organisms that can produce outcome capable of sexual reproduction. The driving force of evolution is natural selection. Natural selection means that species with more favorable traits have a higher percentage chance of survival, thus passing the favorable traits on to the next generation. It is necessary for species to have a big variation in the gene pool to increase the chances of favorable traits developing. A varied gene pool is closely related to the success of the species.
Charles Darwin categorized evolution due to natural selection into 5 steps. The first step is overproduction of offspring, which leads to the second step, variation. A big population increases the genetic variation, and thus the chance of favorable traits. The third step is competition, or a struggle for survival. Competition between or within a species, and environmental factors limits the resources needed for survival, leading to the fourth step, differential survival. The individuals with the most favorable traits have the biggest chance of survival. The surviving individuals goes on to the fifth and last step, reproduction. The favorable traits are passed on to the offspring, which in turn passes them on to future generations, starting the process over again. It is this continuous exchange of traits that builds the basis of evolution, because with time an accumulation of traits leads to a speciation split, thus creating a new species.
Evidence for the theory of evolution
The best evidence for the theory of evolution we have comes from fossil evidence and the presence of homologous structures. According to cell theory, all cells are descended from other cells, which means that all organisms are descended from other organisms. However, ancient fossil records show that life at that time was very different from life today. This implies that species must have changed over time, which supports the theory of evolution. The theory is further strengthened by increasing rates of homologous structures the younger the fossils are. Homologous structures means similar structures that has evolved from a common ancestor. Having 5 fingers for example, is a homologous structure between apes and humans. Using carbon dating looking at half-lives, scientists have found that younger fossils have more homologous structures than older ones, which suggests that life gets more and more similar to today, the further it is away from the origins of life. Genetic evidence also supports connections between past species and present species by looking at their genetic codes. More recent species share more DNA with present species than ancient species. Life does not change by itself, which means that there has to be some driving mechanisms that causes species to diverge and evolve. Darwin hypothesized that natural selection was that driving force, which led to the theory of evolution.
Genetic variation
New traits are developed as a result of three primary mechanisms: Mutations, sexual reproduction, and meiosis. Random mutations in the genetic material of an individual might turn out to be favorable and hereditary, thus passing itself on to the next generation. However, the probability of useful random mutations is very small. Sexual reproduction is therefore the main driving force in genetic variation, evident by the fact that almost no humans are identical, and that no pregnancies produce the same offspring (except identical twins). The many factors in sexual reproduction results in an outcome largely influenced by chance. The huge variability of genetic material is made possible by meiosis. Because of recombination during prophase 1 and random orientation during metaphase 1 and 2, the probability of two gametes being identical is extremely small. Meiosis thus ensures that no two individuals are identical.
Hierarcy of taxa
The hierarchy of taxa developed by Carolus Linnaeus is a system of classification that sorts all living organisms into categories. It is based on 7 principal taxa with an overarching category called the domain. A taxon is essentially a level of organization. The structure of the system is reminiscent of Russian nesting dolls, with one doll containing the next doll, getting smaller and smaller until it gets to the smallest unit. In the case of the hierarchy of taxa, the smallest units are the species. In the time of Linnaeus, knowledge about genetics was still very vague, so his classification was largely based on artificial classification, which means classification by arbitrary characteristics. Fortunately, he did a good job at categorizing, since his system is still in use today. However, with increasing knowledge about genetics, natural classification is taking the place of artificial classification, thus reclassifying misplaced organisms. Furthermore, the naming convention used for organisms today, the binomial nomenclature, is based on the hierarchy of taxa. Binomial nomenclature names all organisms with their genus as their first name, and their species as their surname. Homo sapiens for example belongs to the genus homo, and the species sapiens. A good mnemonic to remember the order of taxa, is: “King Philip Came Over for Good Soup”. The first letter represents the first letter of the taxa.
For more information, visit:https://biologydictionary.net/taxonomy/
Cladistics
While the Linnaeus system of classification is a system sorting and organizing organisms into groups, cladistics is a system organizing organisms to trace back their evolutionary past. Cladistics use homologous traits to deduce how closely related two species are. In the process, primitive and derived traits are evaluated closely. A primitive trait is a characteristic that has the same structure and function, that evolved a long time ago. And a derived trait is essentially a primitive trait that has evolved more recently. Organisms with more similar derived traits have a bigger probability of being closely related. Cladistics often use cladograms to represent the relationships between species. A cladogram shows how closely related organisms are, and consists of different organisms on the top line, and various characteristics along the line of the right. As we move further up the line, the traits get more and more exclusive, and the organisms get more and more closely related to each other.
There are some terms related to cladograms, visit this website for more information:
I very much enjoyed this chapter even though I found it to be the most difficult so far. I had to go back and revise terms very frequently, and I got a little confused about the Linnaeus classification system and cladistics. I see this chapter and the previous chapter as a break from biology on the microscopic level and instead focusing on the bigger picture, even though evolution is fundamentally based on genetics.
Helpful videos:
Evolution: https://www.youtube.com/watch?v=dyiZaHIRM6w
Taxonomy: https://www.youtube.com/watch?v=F38BmgPcZ_I
Cladograms: https://www.youtube.com/watch?v=ouZ9zEkxGWg