Evolution

phenotype-selection

Adaptive radiation is a related concept that describes the rapid rise of a number of different species from a common ancestor. The benefit of adaptive radiation is that it allows the different species present to occupy different niches, thus decreasing competition for limited resources. Group selection refers to natural selection acting on the group, not the individual. Altruism is the performance of an action that benefits another individual at a cost to the actor of the altruic behavior. Altruism sacrifice the fitness of the individual to benefit the group (family), which shares similar genes with the individual. When it is more beneficial than costly, the altruistic behavior is usually selected. If the frequency of an allele increased, then that’s evolutionary success for that allele. If the frequency of alleles of an individual increased in a population, then that’s evolutionary success for that individual.4

species is defined as the largest group of organisms capable of breeding naturally to form fertile, viable offsprings. The formation of a new species through evolution is called speciation. Two populations from the same species that have been separated geographically for a long period of time, would experience different evolutionary pressures that would lead to different adaptive selections. If a long enough time period transpires, the changes would be enough to lead to isolation, in which the offsprings of these populations could no longer freely interbreed. The two groups would now be considered as separate species. Reproductive isolation may occur either prezygotically or postzygotically. Prezygotic mechanisms prevent formation of the zygote completely and postzygotic mechanisms allow for gamete fusion but yield either nonviable or sterile offspring.4 A dog and a cat cannot mate with each other and produce an offspring, so they belong to different species. A horse and a donkey can interbreed, but their offspring, the mule, is sterile. So horses and donkeys aren’t the same species. Some species of flowers can cross pollinate to produce fertile offspring. However, this never occurs in nature because one is bee-pollinated and the other is bird-pollinated. As a result, they are classified as different species despite being able to produce potentially fertile offspring.

The genetic change in a population results in the occurrence of adaptation and it is caused by natural selection. A giraffe’s neck is long because long necks increase the survival rate, so more long-necked giraffes survive to reproduce, and over many generations, the population evolved long necks. Specialization is the process of adapting certain traits to better fill a certain niche. Inbreeding is mating between relatives. Inbreeding increases the frequency of homozygotes but it reduces heterozygotes as well as genetic diversity. Inbreeding depression occurs because of the increase in the frequency of homozygous recessive detrimental alleles. Some species (naked mole rats) naturally inbreed because they stay in one small area and don’t migrate much. Detrimental homozygous recessive alleles are eliminated because of many generations of natural selection. Outbreeding is mating with non-relatives, which is just the opposite of inbreeding. Outbreeding increases heterozygosity. Genetic drift is the random changes in allele frequencies and the effect of genetic drift increases as population size decreases. Bottlenecks increase the effect of genetic drift.

Evolution is a slow process, that incorporates the changes in the environment and subsequent changes in genotypes and phenotypes of a population over a given period of time.5 The rate of evolution is found by the rate of change of a genotype over a period of time and is influenced by the severity of evolutionary pressures that will come against a species. If a species is already adapted to its habitat and there are virtually no significant changes to the conditions in which it lives, the rate of evolution will be very slow, although there will still be some small base rate of genetic mutation. However, if an organism lives in a rapidly changing environment, the rate of evolution will be much greater, since selection for and against certain genetic traits will be actively occurring within that population. Scientists can now quantify the level of similarity between two organisms by comparing DNA sequences between different species. For example, 95% of the chimpanzee’s genome is similar to humans, whereas mice have about 70% similarity to humans. The proportion of shared genome will decrease as species move further taxonomically. Molecular evolutionists have matched that the degree of genomic similarity with the amount of time since two species split off from the same common ancestor: the more resemblance between the genomes, the more recently the two species were separated from each other.5 This is sometimes referred to as the molecular clock model.

 

References

1) Churchill, F. (1974). William Johannsen and the genotype concept. Journal of the History of Biology, 5 -30.

2) Freeman, S. (2011.). Biological Science (6th ed.). Hoboken, NY: Pearson.

3) Hartl DL, C. A. (2007). Principles of population genetics. Sunderland, MA: Sinauer.

4) Baker, J. M. (2005). Adaptive speciation: The role of natural selection in mechanisms of geographic and non-geographic speciation. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 303 – 326.

5) S. Blair Hedges, S. K. (2003, April). Genomic clocks and evolutionary timescales. Retrieved from Hedges Lab: http://www.hedgeslab.org/pubs/146.pdf

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