Learning objectives

• Understanding Mendel's model of inheritance

• Familiarizing with the mechanism of evolution through natural selection

It is fascinating to appreciate the rich diversity of lifeforms we see on our planet. It is even more awe-inspiring to understand the underlying principles that maintain this diversity and lead to the origin of newer forms. The theory of genetics and evolution - two major pillars of modern biology- deal with these areas respectively.

In other words, we all know that the offspring of dogs are always dogs, and the offspring of cats are always cats. But, how does the parent dogs faithfully transfer the set of all the traits (or features or phenotypes) relevant to being a dog (let's call it 'the blueprint') to the next generation? In this context, Gregor Johann Mendel proposed a working model to explain the inheritance patterns of several traits in his experiments with pea plants, and this is the beginning of the era of genetics.

After Mendel, molecular and cell biology has advanced a lot. And, now we know that 'the blueprint' is stored inside the nucleus of a cell in the form of DNA sequences, and often called the genetic information. A gene is essentially a stretch of DNA that serves a specific function and contributes to the phenotype of the organism. Typically, this mapping of genetic information to the phenotypic information occurs by transcribing the specific DNA sequence of the gene to messenger RNA sequence, which is eventually translated into proteins (amino acid sequence) - the workhorses of a cell. These advancements essentially reaffirmed Mendel's model of inheritance, and this exemplifies the power of a model that was constructed based on empirical observations and logical deductions.

In contrast, evolution focuses on details. In our earlier example of dogs and cats, we know that although offspring of a pair of parent dogs will be all dogs, but the offspring are not identical (or clones) of the parents, neither they are identical to themselves. Maybe one has black spots on otherwise white coat colour, while others don't.

Suppose, this trait variant of having black spots arises due to a slightly different version of the 'coat colour gene' (different versions of the same gene are called alleles; alleles differ in their DNA sequence, how does this difference in sequence appear that we will cover later). Now, if, for whatever reason, this black-spotted dog has a greater reproductive success (maybe other dogs of opposite gender like this coat colour and preferentially mate with it, or maybe a dog-breeder takes fancy to this new variety and ensures preferential reproduction of this dog over other varieties) and there is a faithful transfer of the genetic information of being black-spotted to the next generation (significant heritability of the trait), then we will find more number of such black-spotted dogs in the subsequent generations. Therefore, both the phenotypic distribution (phenotypes in X-axis and frequency in Y-axis) of the coat colour in these dogs and underlying genotypic distribution (genotypes in X-axis and frequency in Y-axis) will alter over generations. Reiterating this process across one or multiple different traits for several generations may lead to lifeforms that are completely divergent from the original parental lifeform from which the entire process began in the first place. This mechanism of generating newer lifeforms in nature is conceptualized independently by Charles Robert Darwin and Alfred Russel Wallace, and is popularly known as Darwin's theory of natural selection.

Thus, the existence of phenotypic variation, a genetic basis of the observed phenotypic variation, differential survival and/or reproductive success of the phenotypic variants, and faithful transmission of the genetic basis of the phenotype to the next generation (heritability) are the prerequisites of evolution.

Historically, it is interesting that Mendel and Darwin were working at the contemporary times, in the mid-1800s. Mendel had some idea of Darwin’s work and broadly appreciated it, except those notions that contradicted his observations from experiments with pea plants. He collated his notes and wrote a letter to Darwin, but that letter remained on Darwin’s desk, unopened.

Darwin’s theory of natural selection and Mendel’s laws of inheritance were integrated into a single conceptual framework only in the early 1900s. This framework is called modern synthesis. Several pioneers of the field, R. A. Fisher, Sewall Wright, Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson, and others, contributed to the construction of this framework, which is still considered as the foundations of transmission genetics or Evolutionary Genetics.