Father Gregor Mendel’s Courageous Scientific Undertaking

At the beginning of June, I attended the Annual Conference of the Society of Catholic Scientists in Mundelein, near Chicago. I was deeply moved by the spiritual and academic depth of the conference—this is why I have now attended for the third time.

Before taking up some of the ideas and insights I received there, I would like to provide my talk in written form. Here is Part I (Timeline and his 1866 paper). Part II (Family, teachers, friends and colleagues) and Part III (Integrating priesthood and research) will follow.

Timeline of Mendel’s Life

Johann Mendel was born in 1822 in a small village in Silesia, then part of the Austrian Empire. In 1843, he entered the Augustinian monastery of St. Thomas in Brno, where he received the religious name Gregor. He was ordained a priest in 1847.

After a short period of parish ministry, Mendel began teaching science at the secondary-school level. New educational regulations required teachers to pass a state examination in Vienna. Mendel took the examination and failed—rather miserably, in fact—most likely because he had prepared using outdated textbooks he had found in the monastery library.

Despite this setback, one of the examiners[i] was impressed by Mendel’s intellectual capacities and recommended that he attend the University of Vienna for further education. Mendel spent two years there, studying mathematics, physics, and the natural sciences. When he later repeated the examination, however, he failed a second time.

As a result, Mendel remained a substitute teacher rather than becoming a fully certified instructor. This setback worked to the benefit of science, because it gave him the time and flexibility to pursue his hybridization experiments. Or can we even see providence here?

He devoted eight years to these experiments before presenting his results in 1865 [ii]. The paper was published the following year under the modest title Versuche über Planzen-Hybriden (Experiments on Plant Hybridization). [iii]At the time, it attracted very little attention.

In 1868, Mendel was elected abbot of his monastery. This brought substantial administrative responsibilities, which increasingly limited his scientific work, although he never abandoned scientific inquiry entirely. Mendel died in 1884 at the age of sixty-two.

Mendel’s 1866 Paper

Mendel used thousands of experimental plants, including thirty-four different lines of peas of the genus Pisum which had been tested for their genetic purity (“true breeding lines”), in research extending over eight years. In the introduction to his paper, Mendel thus wrote:

“Es gehört allerdings einiger Muth dazu, sich einer so weit reichenden Arbeit zu unterziehen” – “Some courage is certainly required to undertake such an extensive work.”

Mendel’s Methodological Innovation

Mendel was the first to apply a rigorously quantitative and statistical framework to experimental botany—or, more broadly, to experimental biology. He also chose his model organism with remarkable care. He worked with Pisum sativum, the garden pea, for several important reasons. One crucial advantage was that the plant is naturally self-fertilizing, which allowed traits to remain stable across successive generations and made observations over two or three generations possible.

In addition, unlike many researchers of his time, Mendel deliberately selected traits that were discrete and clearly contrasting. Mendel did not use the term “gene,” since the concept did not yet exist; the term would only emerge in the early twentieth century. Instead, he spoke of “elements” or “factors,” which he clearly distinguished from the observable “Merkmale”, or “traits” (today we call this the phenotype).

Experimental Design and Procedure

First, Mendel used carefully controlled artificial cross-fertilization in the parental generation. He deliberately selected and crossed true-breeding lines with contrasting traits. Second, he then allowed the subsequent generations to proceed through self-fertilization. This enabled him to follow the inheritance of traits in a stable and reproducible way across multiple generations. A third important point is his quantitative recording of phenotypic distributions, particularly in seeds and flowers. Mendel did not rely on vague impressions or isolated examples. He counted large numbers of plants and carefully documented the numerical ratios in which traits appeared. In an era when botany was largely concerned with the description, classification, and comparison of plants, Mendel introduced a markedly different approach: controlled experiments combined with quantitative analysis. This methodological innovation distinguished his work from much of nineteenth-century natural history.

Finally, Mendel performed what we would today call a longitudinal analysis across multiple generations. Rather than stopping after the first hybrid generation, he followed the patterns of inheritance over time.

This graph illustrates the last point. When hybrids are followed over many successive generations, they split in a certain way: The orange line represents the hybrids—the heterozygous forms, which Mendel would have described as mixed forms. In the first generation, all individuals are hybrids. But with each subsequent generation of self-fertilization, the proportion of hybrids steadily decreases by half. At the same time, the two true-breeding forms—the constant dominant and the constant recessive types—gradually reappear and increase in frequency. Eventually, each approaches fifty percent of the population, while the hybrids become increasingly rare.

Mendel derived a mathematical expression for it. He showed that after n generations of self-fertilization, the population is divided in the ratio (2ⁿ−1) : 2 : (2ⁿ−1), representing the constant dominant forms, the hybrids, and the constant recessive forms, respectively.

The graph also confirms an observation made by previous researchers (as Mendel specifically mentioned): the original true-breeding forms “come back” after hybridization. The parental forms were not lost or permanently blended. Instead, they re-emerged in mathematically describable proportions over successive generations.

Laws of Inheritance

Today, we summarize Mendel’s findings in the form of the three classical laws of inheritance.

• The first is the Law of Dominance. In German it is traditionally called the Uniformitätsregel, the “law of uniformity,” which is perhaps the more precise description. It states that the first filial generation is uniform: when two true-breeding parental lines with contrasting traits are crossed, all offspring in the first generation display the same dominant trait.
• The second is the Law of Segregation. In the following generation, the traits separate again, producing the well-known phenotypic ratio of approximately three dominant to one recessive form. This demonstrated that hereditary factors are not blended (like paints mixed together), but instead remain discrete (like marbles) and can reappear in later generations.
• Finally, there is the Law of Independent Assortment. Mendel did not study only single traits in isolation; he also combined multiple traits and analyzed how they were inherited together. He observed that the inheritance of one trait was independent of another. We now know that Mendel was also somewhat fortunate in his choice of experimental traits. The traits Mendel selected were either located on different chromosomes or sufficiently far apart on the same chromosome to assort independently [iv].

Extension and Refinements

Mendel also carried out experiments with Phaseolus (bean plants), a genus related to Pisum (peas), indicating polygenic inheritance (in today’s nomenclature):

Another aspect of Mendel’s work was his use of reciprocal crosses. In these experiments, he performed a cross in one direction—for example, using a plant with a particular trait as the female parent and another as the male parent—and then repeated the cross with the parental roles reversed. At the time, it was not yet generally accepted that male and female gametes contributed equally to offspring characteristics. Mendel’s reciprocal crosses indeed proved this fact. He thus weighed in on an ongoing scientific debate, particularly among researchers in Vienna[v], concerning the respective roles of male and female parents in heredity.

Mendel’s conceptual breakthroughs can be seen in:
– the demonstration of particulate inheritance following mathematical laws and
– the refutation of blending inheritance theories, thus implicitly challenging Charles Darwin’s pangenesis hypothesis.

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Selected Biographies:

Daniel J. Fairbanks. Gregor Mendel: His Life and Legacy. Simon and Schuster 2022

Jiří Sekerák, Pavlína Pončíková. Iconographia Mendeliana 2022 – Half a Century of International Research Into the Life and Work of Gregor Johann Mendel and the Beginnings of Genetics in Pictures and Documents: in Memory of Gregor Johann Mendel on the 200th Anniversary of His Birth. Moravian Museum, 2022.

Vollmann J. Mendel in Vienna, a source book [Internet]. Zenodo; 2022 [cited 2026 Apr 30]. Available from: https://doi.org/10.5281/zenodo.7973743. doi:10.5281/zenodo.7973743.

Weiling F. Historical study: Johann Gregor Mendel 1822-1884. Am J Med Genet. 1991 May;40(1):1-25. doi:10.1002/ajmg.1320400103

Sladek P. Zur inneren Gestalt Johann Gregor Mendels. Augustiniana. 1984;34(3-4):236-243.

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[i] Andreas von Baumgartner (1793–1865), professor for physics, director of the examination committee

[ii] Timeline of his pea experiments can be found in: van Dijk PJ, Jessop AP, Ellis THN. How did Mendel arrive at his discoveries? Nat Genet. 2022;54(7):926-933. doi:10.1038/s41588-022-01109-9.

[iii] The paper was published in “Verhandlungen des naturforschenden Vereines in Brünn“. The historically important English translations are: Druery and Bateson (1901), Royal Horticultural Society Translation (1913) and Sherwood and Stern (1966). Two translations were published recently: (a) Fairbanks DJ, Abbott S. Darwin’s influence on Mendel: evidence from a new translation of Mendel’s paper. Genetics. 2016 Oct;204(2):401-405. doi:10.1534/genetics.116.194613. (b) Müller-Wille St., Hall K, Dostal O. Experiments on Plant Hybrids: Versuche über Pflanzen-Hybriden. New Translation with Commentary, Masaryk University Press. 2020.

[iv] Auffray C, Noble D. Gregor Mendel at the source of genetics and systems biology: celebrating the relevance of Gregor Mendel’s experiments on the development of hybrid plants on the occasion of his bicentenary. Biol J Linn Soc Lond. 2022;137(4):720-736. doi:10.1093/biolinnean/blac105.

[v] The dispute was between Franz Unger and Eduard Fenzl. Gregor Mendel sided with Franz Unger, and proved him correct in his experiments. see: Fairbanks DJ. Demystifying the mythical Mendel: a biographical review. Heredity (Edinb). 2022 Jul;129(1):1-10. doi:10.1038/s41437-022-00526-0.

Maria Sibylla Merian – Science and Beauty in the Metamorphosis of Butterflies

Maria Sibylla Merian (*2 April 1647 in Frankfurt am Main; † 13 January 1717 in Amsterdam) was a naturalist and artist. She was among the first to systematically observe insects. She demonstrated that caterpillars depend on a small number of specific food plants and lay their eggs only on these plants.

Although some scholars were aware of the transformation of caterpillars into chrysalises and then into adult butterflies, the process was unfamiliar to the wider population—even to many educated people. Merian helped to change this with her book Der Raupen wunderbare Verwandlung und sonderbare Blumennahrung (“The Caterpillars’ Marvelous Transformation and Strange Floral Diet”), published in 1679 and 1683. Written in German, it reached a broad audience. For the same reason, however, many scientists of her time denied her recognition—the scholarly language of the learned world was Latin.

With this little book, she also sought to praise the Creator in and through nature:

„Suche demnach hierinnen nicht meine sondern allein Gottes Ehre Ihn als einen Schöpfer auch dieser Kleinsten und geringsten Würmlein zu preisen.“

“Therefore, seek herein not my honor but only God’s glory, to praise Him as the Creator even of these smallest and humblest little worms.”

The book stands within a tradition of nature piety that was widespread in Nuremberg at the time—the search for God precisely in the most insignificant creatures. This search also led her to make an unusual decision: she left her husband and moved with her widowed mother and her two daughters to Amsterdam to join the Labadist community, a pietist Protestant group.

In 1700, at a time when it was almost unthinkable for a woman to travel without male protection, she set out for Suriname in South America, accompanied only by her daughter, the then 21-year-old Dorothea Maria. After weeks on a merchant ship, she spent two years working in the humid heat of equatorial rainforests, relying on the support of a few Indigenous people. There, she discovered and documented a number of previously unknown animals and plants.

Upon her return to Amsterdam, her book Metamorphosis insectorum Surinamensium was published in 1705, containing 60 colored copper engravings along with her own texts. Her classification of butterflies into day-flying and night-flying species (which she referred to as “Kapellen” and “Eulen”) remains valid to this day.

Her artistic work was already appreciated by her contemporaries. Unfortunately, the few copies of the first edition quickly disappeared into university libraries and the collections of scholars and collectors. It was not until the 20th century that widespread interest in her drawings and colored plates re-emerged. By then, it had become possible to reproduce the first edition faithfully using advanced printing techniques and to distribute it more widely.

Galileo, Castelli, and Stellar Parallax

The story of Galileo Galilei versus the Catholic Church is usually told as a simple drama: enlightened science crushed by religious prejudice. We know this narrative is wrong, as I discussed previously on my blog (Revisiting the Galileo Affair).

But did you know that Galileo tried to measure stellar parallax to prove the Copernican heliocentric system, that his experiment failed, and that he did not tell anyone? – This really struck me.

Here is the story:

In 1617, Benedetto Castelli asked Galileo to observe Mizar, a star that appears single to the naked eye but reveals itself as a double star when seen through a telescope.

Benedetto Castelli (wikicommon)

Galileo was an important figure, and so was Castelli. He was a Benedictine monk, mathematician, and astronomer — first a student and later a close friend of Galileo. He defended Galileo and the Copernican system throughout Galileo’s troubles with Church authorities. In 1613, when Galileo decided to defend himself publicly against some of his accusers, he chose to do so by means of an open letter addressed to Castelli.

Galileo realized that the double star Mizar could be used to observe stellar parallax and thus confirm heliocentrism. “He observed them [both stars] meticulously for a year, but saw no parallax. He recognized this as observational evidence, given the state of astronomical knowledge at the time, that Earth does not go around the sun,” explains Tim Thompson, former physicist at NASA’s Jet Propulsion Laboratory in California [1].

What Galileo did not know was that the real distances of these stars from us are orders of magnitude greater than his calculations suggested. “Nobody understood optics well enough to realize that the apparent stellar disk they saw in telescopes was just an optical artifact (the Airy disk). They thought it was the physical disk of the star, and therefore all of them, Galileo included, seriously underestimated the distances to the stars, assuming they must be nearby and therefore must also show parallax,” says Thompson.

Galileo could not observe stellar parallax, and this was strong counter-evidence against heliocentrism. What did Galileo do? “He hid his observations and never told anyone what he had discovered.”

Hide observations and tell nobody? Scientists perform experiments and make observations to test their hypotheses, and based on these results, they refine their hypotheses in order to arrive at a strong theory. I told this at various occasions to my high-school students during my recent 18-month assignment teaching Biology and Chemistry. I used this graph for illustration of the scientific method:

(modified from the German version)

And if Galileo hid his results, how do we know?

Several historians of astronomy [2] investigated Galieo’s notes and the correspondence between Galilei and Castelli and found reasonable evidence that Galileo tried for several years to find stellar parallax, not only looking at Mizar but also other double stars between 1617 and 1627, on his own and upon Castelli’s request.

But there is even more to say about Galileo’s lack of scientific transparency. In 1632, Galileo wrote:

“I do not believe that the stars are spread over a spherical surface at equal distances from one center; I suppose their distances from us vary so much that some are two or three times as remote as others. Thus if some tiny star were found by the telescope quite close to some of the larger ones, and if that one were therefore very very remote, it might happen that some sensible alterations would take place among them.”

Chris Graney, astronomer and historian of science from the Vatican Observatory, indicates [3] that Galileo proposes — using the word “if” — that parallax might be observable with double stars at different distances from each other. Yet, he does not mention that he had already found such a pair in 1617 and had not observed any differential parallax. He says, “Should Galileo have published his double star observations? Certainly. From a scientific standpoint, if he is going to promote the ideas that Earth circles the sun, and that the stars are sun-sized bodies at varying distances from Earth — and that for these reasons a double star might reveal differential parallax and thus appear in court to give witness that Earth does in fact move — then, yes, he should also mention that he has already observed exactly the sort of double star system that he describes, and that those observations contradict the ideas he is promoting. When Galileo wrote in the Dialogue about how ‘if some tiny star were found by the telescope’ close to a large star, he was sitting on exactly that sort of observation, right in his notebooks. Scientifically speaking, that’s definitely not cool.”

And he concludes: “Galileo was a great scientist. He should be honored as one of the greats. But, in trying to prove that the Earth moved, he did stuff that scientists are not supposed to do. Galileo was not punished for proving that Earth moved. But some of the things he did while trying to prove Earth’s motion would get him in trouble in the scientific world today.”

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Stellar parallax was first measured by Friedrich Bessel in 1838. In 1851, the Earth’s rotation was demonstrated with Foucault pendulum. The decisive evidence Galileo sought did exist — but it required instruments, theoretical insight, and physical understanding that lay beyond his century.

Galileo was right about the Earth’s motion. He was wrong about the distances of the stars. And when his own careful observations failed to support his expectations, he chose silence instead of transparency.

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[1] Tim Thompson on facebook (16 Feb 2026)

[2] Umberto Fedele (1949). Le prime osservazioni di stelle doppie. Leos Ondra (1999), A New View of Mizar.
Overview articles: Harald Siebert (2005). The Early Search for Stellar Parallax: Galileo, Castelli, and Ramponi. Journal fothe History of Astronomy, 36(3), 251–271. doi:10.1177/002182860503600301; Christopher Graney (2017). Strange Tales of Galileo and Proving: Telescopic Evidence for Earth’s Immobility through Double Stars, on the blog of Vatican Observatory. Christopher Graney (2024). The View of the Double Star Mizar, Twenty Years Later, on the blog of Vatican Observatory 

[3] Christopher Graney, (2017), op.cit.

St. John Henry Newman and Science

St. John Henry Newman will soon be declared a Doctor of the Church. Newman’s most influential theological idea is the development of doctrine—the belief that Catholic teachings grow and unfold over time under the guidance of the Holy Spirit. His book An Essay on the Development of Christian Doctrine prepared his own conversion to the Catholic Church and had an impact on Vatican II.

St. John Henry Newman was, by vocation and training, a theologian and pastor. However, given his Oxford education (which included lectures in mineralogy and geology) and his role as Rector of the Catholic University of Ireland, Newman was deeply immersed in the intellectual world of his time, including the rising influence of the natural sciences.

Prior to his canonization in 2019, I prepared an article for the Society of Catholic Scientists, https://www.catholicscientists.org/idea/saint-john-henry-newman-a-co-patron-for-scientists, reflecting on his views on science in general and on the theory of evolution in specific.

And I share here a recent comment by Heinz-Herman Peitz (in a translation from German):

“Thank you for the link to a thoughtful tribute of [St. John Henry] Newman by Berta M. Moritz. She presents Newman as a great interdisciplinary pioneer and as one of the first theologians who did not see Darwin’s theory as a contradiction to the Christian faith. For Newman, apparent conflicts between science and faith do not necessarily arise from scientific error but may just as well spring from a misunderstanding of revelation. In this respect—one might add—he was ahead of the First Vatican Council. That Council, to be sure, also recognized that, since there is only one truth, there can ultimately be no genuine conflicts. But when it came to apparent conflicts, it acknowledged only the opposition between “erring reason” and “infallible revelation.”
Moritz also shows Newman’s continuing relevance today, for example in his understanding of “design.” Here Newman inverts the logic employed by many modern proponents of Intelligent Design: “I believe in design because I believe in God, not in God because I see design.”

Henrietta Swan Leavitt – Measuring Stars

“She had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning.” – Solon I. Bailey in the obituary of Henrietta Leavitt

Henrietta Leavitt (04 July 1868 – 12 December 1921, a dedicated researcher at Harvard College Observatory, made groundbreaking contributions to astronomy. Henrietta Leavitt was tasked with cataloging, observing, and tabulating stars that varied in brightness over time, known as Cepheid variables. Photographic plates taken over an extended period at the Arequipa Observatory in Peru captured night sky images, which were then sent to Boston. Leavitt identified many new Cepheid variables, increasing the number of known Cepheid variables in the Large and Small Magellanic Clouds to nearly 1,800. She discovered that these stars oscillated over periods ranging from days to weeks and that the duration of these oscillations was related to their brightness (or luminosity): the longer the period from minimum to maximum brightness, the brighter the star. She found this pattern, since all stars she observed were at approximately the same distance from Earth. She published her findings in 1908 and in 1912.

The comparison of the apparent brightness of a Cepheid star to its absolute brightness – based on its pulsation period – could now be used to measure the distance of these stars. Up to this time, the only possibility to measure distances was using the star’s parallax, a method that only allowed to measure distances up to a few thousand light-years. With using Leavitt’s relationship between luminosity and pulsation period in Cepheid stars, distances up to 13 million light-years could be measured: she thus revolutionized astronomy.

In 1924, Edwin Hubble discovered a Cepheid variable in the Andromeda galaxy and, with the help of Leavitt’s finding, showed that this object lies far beyond our Milky Way.

Her father was a Congregationalist minister. While we know she was a deeply committed Christian throughout her life, details about her faith are scarce. She was passionately devoted to her work, carrying it out with meticulous precision. Despite significant health challenges, including hearing loss and a prolonged battle with cancer, she persevered. Her legacy is a testament to her kindness, intellectual curiosity, and the power of scientific inquiry.

Had she lived longer, she might have been awarded the Nobel Prize. The mathematician Gösta Mittag-Leffler, a member of the Swedish Academy of Sciences, planned to nominate her for the Nobel Prize in 1924, but her untimely death three years earlier prevented this. The Nobel Prize is not awarded posthumously.

Henreitta Leavitt has a moon crater and an asteroid named after her.

St. Augustine on the Literal Meaning of Genesis

St. Augustine was the first to describe that Revelation is given to us in the Book of Nature and in the Book of Scripture. The metaphor of the two books is routed in God’s Truth as the source of both [1].   

He expresses true concern that a literal misinterpretation of the book of Genesis – like the erroneous claim by Young-Earth-Creationists that the universe was created in six (24 hour) days and that earth is 6.000 years old – may lead unbelievers to ridicule Holy Scripture. This warning should be taken seriously:

Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of the world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds to as being certain from reason and experience.

Now, it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn. The shame is not so much that an ignorant individual is derided, but that people outside the household of faith think our sacred writers held such opinions, and, to the great loss of those for whose salvation we toil, the writers of our Scripture are criticized and rejected as unlearned men. If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe those books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods on facts which they themselves have learned from experience and the light of reason?

Reckless and incompetent expounders of Holy Scripture bring untold trouble and sorrow on their wiser brethren when they are caught in one of their mischievous false opinions and are taken to task by those who are not bound by the authority of our sacred books. For then, to defend their utterly foolish and obviously untrue statements, they will try to call upon Holy Scripture for proof and even recite from memory many passages which they think support their position, although “they understand neither what they say nor the things about which they make assertions” (Tim 1,7).”

St. Augustine, The Literal Meaning of Genesis, Book 1, Chapter 19, Section 3

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[1] For an in-depth scholarly article, see: Oskari Juurikkala, The Two Books of God:  The Metaphor of the Book of Nature in Augustine, Augustinianum 61/2 (2021), p. 479 – 498  

Thomas Aquinas: Fierce animals in Paradise?

Have you heard that all animals were tame in Paradise and only ate plants? I did, but it sounded strange to me. Here is a reply from Thomas Aquinas, 13th century:

“In the opinion of some, those animals which now are fierce and kill others, would, in that state, have been tame, not only in regard to man, but also in regard to other animals. But this is quite unreasonable. For the nature of animals was not changed by man’s sin, as if those whose nature now it is to devour the flesh of others, would then have lived on herbs, as the lion and falcon.”
– Thomas Aquinas, Summa Theologiae, part I q 96, Reply to Objection 2.

Picture: Jan Brueghel the Elder: Earth, or The Earthly Paradise, detail of animals, 1607-08

The 2024 list: Catholic Women Pioneering in Science

I am preparing a talk for the 2024 Conference of the Society of Catholic Scientists, my contribution is titled: “With the Future in Mind: Catholic Women Pioneers in Science”.

The women presented here come from a variety of scientific disciplines and lead lives as diverse as life itself can be, but they all had two characteristics in common: (1) they have had an impact on the science and/or scientific education at their time and (2) their lives were informed, transformed, and inspired by their Catholic faith.

Twenty-four of these women were presented in the 2023 post, and 12 new short biographies have been added.

Lise Meitner, Eva von Bahr, and Elisabeth Schiemann: the power of friendship

Memorial for Lise Meitner, Humboldt Universität Berlin, created by Anna Franziska Schwarzbach

These three female scientists were unique in their personalities and bound together in friendship. Did you know that Lise Meitner – who discovered the principle of nuclear fission – was supported by her previous colleague Eva von Bahr to build up a new life in Sweden when she needed to flee Germany in 1938? And that Elisabeth Schiemann was a leading botanist and an active member of the “Confessing Church”, active in the resistance to the Nazi regime?

I explored this and more in my recent article “Three pioneering women in science: a story of science, faith, and the power of friendship” on the website of the Society of Catholic Scientists.

Darwin and Mendel on Inherited Variability

Darwin’s Origin of Species took many years of compilations and it was even accelerated by Alfred R. Wallace paper on the same subject in 1858. Origin of Species was really groundbreaking, because Darwin not only proposed common ancestry – that was in the air since Humboldt and Lamarck, and “Vestiges of the Natural History of Creation” -, but he proposed also a process for the underlying mechanism of the diversity of species.

His theory of evolution had 2 principles: common ancestry and natural selection. Natural selection can be described with 5 words: Variation, Inheritance, Selection, Time, Adaptation. Inherited variability and a selection pressure to adapt to the environment.

In Darwin’s time, the source of inherited variability was unknown. Darwin considered heredity as a “blending” process and the offspring were seen as essentially a “dilution” of the different parental characteristics.