A collection of notes on evolution and genetics
- Charles Darwin was the first to propose the theory of evolution by natural selection.
- Darwin defined evolution as "descent with modification". The idea is that all living things are related and have descended from a common ancestor.
- The mechanism of evolution is natural selection. This is the process by which organisms with certain inherited traits are more likely to survive and reproduce than others.
- Natural selection causes populations to become adapted to their environment. Adaptation is the process by which a population becomes better suited to its environment.
- Natural selection is a random process. It does not favour any particular individual or trait. It is the result of the interaction of many factors, including mutation, genetic drift, migration and sexual selection.
- Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it saltation.
Darwin also proposed a mechanism for evolution: Natural selection.This mechanism was elegant and logical, and it explained how populations could evolve in such a way that they became better suited to their environments over time.
2.1 Concepts of natural selection
- traits are often heritable. This means that they are passed on from parents to offspring.
- More offspring are produced than can survive. This is because not all offspring survive to adulthood. This is because of predation, disease, competition and environmental factors.
- Offspring vary in their traits. This is because of mutation and recombination. Mutation is the process by which a gene is changed. Recombination is the process by which genes are shuffled during the formation of gametes.
- Traits are not randomly distributed. This is because some traits are more useful than others. For example, a bird with a longer beak is more likely to survive than a bird with a shorter beak (Under certain environmental conditions ofcourse).
As the above indicates, those alleles(one of two or more versions of DNA sequence) of a species that are favored in the environment will be more likely to be passed on to the next generation. This is the process of natural selection.
An adaptation is a characteristic that enhances the survival or reproduction of an organism that bear it, relative to alternative character states. The word "adaptation" also refers to the process whereby the members of a population become better suited to some feature of their environment through change in a characteristic that affects their survival or reproduction.
3.1 The Nature of Adaptations "An adaptation is a phenotypic variant that results in the highest fitness among a specific set of phenotypic variants in a given environment." This definition is a bit complicated, so let's break it down.
The first part of the definition is the phenotypic variant. This is a characteristic of an organism that can be observed. For example, the colour of a bird's feathers is a phenotypic variant.
The second part of the definition is fitness. This is the ability of an organism to survive and reproduce in its environment. For example, a bird with a longer beak is more likely to survive than a bird with a shorter beak.
The third part of the definition is environment. This is the surroundings in which an organism lives. For example, the environment of a bird is the air, the ground and the trees.
"The sutures(surgical stitches) in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition (birth), and no doubt they facilitate, or may be indispensable for this act, but as sutures occur in the skulls of all mammals, and as the young of all mammals are born alive, it is clear that the sutures are not an adaptation for birth, but are an adaptation for the young to be born alive." - Charles Darwin (The origin of species,chapter 6). preadaptation is a feature that fortuitously(by luck) exists in an organism, but is not used for its current function. For example, the human appendix is a preadaptation. It is a vestigial organ that is no longer used for its original function. It is thought that the appendix may have been used for digestion in our ancestors, but is no longer used for this purpose.
3.2 Adaptive Radiation The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas is called adaptive radiation.
The three core principles of evolution - variation,heredity and differential fitness - are the foundation of evolutionary theory.
4.1 Role of Extinction in Evolution In the origins of species, Darwin made his view of exinction clear: "The extinction of a species is the permanent loss of a certain number of varieties, and the extinction of a variety is the permanent loss of a certain number of individuals." He saw three essential features.
i) Extinctions of species have ocurred gradually and continuously. This is because the rate of extinction is greater than the rate of speciation. This is because of the mass extinction that occurred at the end of the Permian period. This mass extinction is thought to have been caused by a volcanic eruption.
ii) Species extinction is usually, though not always, caused by the failure of a species in competition with other species. This is because of the competitive exclusion principle. This principle states that two species cannot occupy the same niche in the same environment for long. This is because one species will outcompete the other.
iii) The extinction of species is closely tied to the process of natural selection and is thus a major component of progressive evolution. Thus as it seems to me, the manner in which single species and whole groups of species become extinct accords well with the theory of natural selection.
In Darwin's finally summary of the Origin, Darwin listed the fundamental components or laws of the evolutionary process: reproduction,inheritance,variation,selection,extinction and adaptation.