In Darwin’s theory of evolution, all of life is the descendant of a common ancestor and natural selection is the main mechanism of adaptation for evolving species. That is made possible as Darwin so clearly stated in The Origin of Species that every population has heritable variation, an observation by Darwin which has been confirmed by later biologists and that a variation can either be beneficial or harmful and natural selection selects the individuals with the beneficial variation while removing the ones with the harmful variations. Variations , small and large, easily observable and also subtle are present in every population in every stage of its life cycle.
Darwin made the fact of variation and natural selection clear in his great work, and it is because of the combination of variation and natural selection that makes evolution possible. There was one problem that Darwin himself was made aware and if you have read some of my blogs on evolution, you will probably have guessed it by now but if you have not, then the big problem that Darwin faced was where did the variation come from?
Only natural selection, with its ability to create adaptation in every evolving population, can work only if there are variations for without variations, there can be no natural selection and hence no evolution. Even though, natural selection is powerless without variations, Darwin had difficulties in trying to explain how variations arise.
Despite his admitting his ignorance on the biological source of variations, Darwin was unaware that somewhere in what is now the Czech Republic, there was this monk whose hobby was breeding plants and although Darwin never even heard of him, this monk was not only breeding plants as a hobby but he was establishing a science by doing careful experiments, making observations, and even analyzing the results, and was even published in a journal in biology and although when the paper was published, it attracted little attention but more importantly this paper documenting years of hard work of observing how certain traits are inherited in these kinds of flowering plants are transmitted and it turns out that later, this provided the answer to Darwin’s problem. This problem that Darwin faced would eventually solved by this monk curious about inheritance and his name was Gregor Mendel.
Although a monk by training, Mendel was also a botanist and while as monk at an Augustinian monastery, he also had time to do gardening and he became adept at raising garden peas. He was determined to find patterns of inheritance in the garden peas and how he was able to do this was that he noticed that flower color, which can be purple or white tended to be inherited as either purple or white. What about other traits? One other trait that Mendel look after was plant height and pea plants can either be tall or short and like flower color, there would either be a tall plant or a short plant. In both plant height and flower color there were either one trait or a different trait, there were no continuous traits which made studying inheritance relatively easy.
Suppose you had a pea plant with purple flowers and another with purple flowers. If you were to cross these plants together, what would the offspring be like? Mendel crossed these flowers with the same color and got offspring with purple flowers or that the offspring was the result of the same trait, which is flower color or that the purple flowers “bred true” resulting in offspring with purple flowers. Likewise, if you crossed white flower plants with pea flower plants, the offspring will also have been bred true and pea plants with white flowers will result. The same was true if tall plants was breed with tall plants, and the offspring were all tall plants. Mendel found that when you cross pea plants with the same traits, then you get offspring with the same traits.
What would happen if you crossed a pea plant with a purple flower plant and another pea plant with white flowers? Will the offspring be a blend of these two colors resulting in pink flowers or will the offspring inherit one color over the other? Mendel did the cross and the offspring had purple flowers in the first generation but there was no white flowers.so it seemed that in the first generation, the white flowers disappeared after the cross with the purple flowers so maybe the white flowers did disappear and to check to see if it had happened, Mendel self pollinated the first generation flower and found that about 3/4 of the offspring of the second generation had purple flowers but 1/4 had white flowers. He found a similar pattern when he crossed tall plants with short plants and in the first generation, there were only tall plants while in the second generation, through self pollination, 3/4 were tall while 1/4 was short.
Mendel found that when two varieties of the same species of pea plants were crossed, one trait tends to be dominant while the other trait does not really disappear but stays hidden in one generation only to appear in the next generation and that trait would be called recessive. From these breeding experiments, the pea plants come in two varieties such as purple or white flowers and tall or short and since one of these traits are inherited, it occurred to Mendel that traits, in general, should be inherited separately and because flower color and plant height tend to be inherited, Mendel gave a name to these inheritable traits by hypothesizing something intangible to the organism responsible for these traits, which he would have called “factors” and Mendel observed that there were two varieties of each factor inherited separately through either the pollen or the egg and when pollen and egg combine together to form the offspring, these two versions of the same factor such as plant height will show up in the offspring as either short or tall. In modern language, we know now that Mendel’s heritable factors are the genes and that each gene has two forms called alleles.
Mendel in a way discovered the alleles of genes using pea plants as his model organism and he found the rules of heredity which later proven to be correct. An allele of a gene is either dominant or recessive, which in the first generation will either be dominant if the offspring was the cross of two organisms, one with one different alleles of the same gene and the other with another version of the gene for whichever trait while if you cross two organisms with the same alleles, then you are likely to get offspring with the same inherited alleles which the offspring does resemble both parents in succeeding generations.
Mendel proved that traits are inherited separately or that alleles of a gene encode various traits in a discrete manner. Previously, it was believed that traits were inherited in the manner of mixing two different colored pigments which would result in offspring with blends of different parents or that trait were inherited with mixing features from each parents, a belief that even Darwin supported. This was known as blending inheritance but Mendel’s experiments proved otherwise, in that traits tend to be inherited in a discrete manner. No blending was observed in that one version of allele will appear in another generation which would not be the case if traits blended together.
Recall also that Darwin was right in stating that populations that reproduce inherit traits form each generation, resulting in variations and if blending inheritance was correct, then in each generation, whatever variety was present would end up disappearing which would mean that there would be no variation for natural selection to work on. Mendel disproved the blending inheritance and so whatever variety is present and if it either makes a difference in the survival or death of an organism in each generation, then natural selection would have to favor the trait because traits are inherited separately which ultimately makes evolution possible.
Of course, when Mendel published his results, it did not garner much attention nor did it ever reached Darwin since the particulate inheritance hypothesis or inheritance of discrete traits such as in Mendel’s experiment was confirmed, it nonetheless did not have much of impact in Darwin’s later work nor was he even interested for he did had his own explanation of how traits were inherited which later turn out to be false. It took about 80 years for the deep implications of Darwinian theory to be accepted and that had to wait when around 1900, Mendel’s work was rediscovered by three biologists who did research in botany and once Mendel’s work was finally given serious attention, first it was applied to botany then zoology and after genetics became a mature science in how traits were inherited, eventually Darwinism, with its emphasis on the slow gradual accumulation of heritable changes in populations, was combined with Mendelian genetics, which focused on how changes in populations are inherited as well as how they originate, in what is called the Modern Synthesis and after that, the rest was history.
Darwin, C. (1859) On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London, England: Murray London
Gould, S.J (2002) The Structure of Evolutionary Theory . Harvard, MA : Harvard University Press Belknap
Martinez, A ( January, 27, 2016). The Modern Synthesis and Evolutionary Biology. Retrieved from http://unityoflifeblog.com/the-modern-synthesis-of-evolutionary-biology/
Martinez, A (September,5, 2015). The Ever Changing Concept of the Gene. Retrieved from http://unityoflifeblog.com/the-ever-changing-concept-of-the-gene/
Mayr, E (2001) What Evolution Is. New York, NY: Basic Books
Pierce, B. (2012). Genetics: A Conceptual Approach (4th ed.) New York, NY W.H Freeman and Company
Michael MK Khor https://www.flickr.com/photos/mk-creatures/12853575873/in/photolist-kzPWi8-85VkKC-oaQT8k-9NkeXZ CC BY 2.0