Heredity

The process by which traits and characteristics are passed down reliably from parents to offspring is called heredity. Offspring show similarities to their parents (basic human features in human offspring) but also exhibit differences, contributing to the variation within a species.

Inherited Traits

Traits are characteristics that are passed down from parents to offspring through inheritance. Examples of inherited traits in humans include earlobe type (free or attached), hair colour, eye colour, height, etc.

Observing and tracking the inheritance of specific traits within families or populations (like earlobe type in a class) can help identify patterns and rules of heredity.

Rules For The Inheritance Of Traits – Mendel’s Contributions

Gregor Johann Mendel, an Austrian monk, conducted experiments on garden pea plants in the 19th century and formulated the fundamental rules of inheritance. His work was crucial in understanding how traits are passed from one generation to the next.

Mendel studied various contrasting traits in pea plants (e.g., tall/short plants, round/wrinkled seeds, violet/white flowers). He crossed plants with contrasting traits and observed the characteristics of the progeny over generations.

Mendel's Experiments and Findings:

• Monohybrid Cross (studying one trait): When Mendel crossed a tall pea plant with a short pea plant, all the plants in the first generation ($\text{F}_1$ progeny) were tall. There were no plants of intermediate height. This showed that only one of the parental traits was expressed in the $\text{F}_1$ generation.
• Self-pollination of $\text{F}_1$ progeny: When the tall plants of the $\text{F}_1$ generation were self-pollinated, the second generation ($\text{F}_2$ progeny) was not all tall. Instead, about one-quarter of the $\text{F}_2$ plants were short, and three-quarters were tall. This indicated that the shortness trait, which was not expressed in the $\text{F}_1$ generation, had been inherited and reappeared in the $\text{F}_2$ generation.

Mendel's Postulates (based on Monohybrid Cross):

1. Traits are controlled by factors (now called genes).
2. Each sexually reproducing organism has two copies of each gene, one from each parent. These gene copies can be identical or different.
3. When two different gene copies for a trait are present, only one is expressed (the dominant trait), while the other is not expressed but is still inherited (the recessive trait). In the pea plant example, 'tallness' (T) was dominant over 'shortness' (t). A plant with at least one 'T' gene copy (TT or Tt) is tall, while a plant must have two 't' copies (tt) to be short.
4. During gamete formation, the two gene copies for each trait separate, so each gamete (sperm or egg) receives only one gene copy.

Dihybrid Cross (studying two traits simultaneously): Mendel crossed pea plants with two contrasting traits, like tall plants with round seeds (dominant traits) and short plants with wrinkled seeds (recessive traits). The $\text{F}_1$ progeny were all tall with round seeds, confirming tallness and round seeds as dominant traits.

When $\text{F}_1$ progeny were self-pollinated to produce the $\text{F}_2$ generation, Mendel observed plants with new combinations of traits (tall with wrinkled seeds and short with round seeds), in addition to the parental combinations. This showed that the inheritance of one trait (e.g., tallness/shortness) is independent of the inheritance of another trait (e.g., round/wrinkled seeds).

Law of Independent Assortment: Genes for different traits are inherited independently of each other.

How Do These Traits Get Expressed?

Genes, which are segments of DNA, contain the information for making proteins. Proteins perform various functions in the cell and body, and ultimately control the characteristics or traits we observe.

Example: Plant height is influenced by the amount of a specific plant hormone. The production of this hormone is controlled by an enzyme, which is encoded by a gene. If the gene is efficient, more enzyme is produced, leading to more hormone and a taller plant. If the gene has an alteration (variant) that makes the enzyme less efficient, less hormone is produced, resulting in a shorter plant.

In sexual reproduction, offspring receive one copy of each gene (on a chromosome) from each parent. These two copies (alleles) can be the same or different. The interaction and expression of these two gene copies determine the trait seen in the offspring, following Mendel's rules of dominance and recessiveness.

The fact that organisms have two sets of chromosomes (one from each parent) and that these chromosomes are paired explains why offspring receive one copy of each gene from each parent. During gamete formation (meiosis), homologous chromosomes separate, so each gamete (sperm or egg) receives only one chromosome from each pair, thus containing a single set of genes. When two gametes fuse during fertilisation, the normal diploid number of chromosomes (two sets) is restored in the zygote, ensuring the stability of the species' DNA content across generations.

Sex Determination

In many species, the sex of an individual (male or female) is determined at the time of fertilisation, usually based on the inheritance of specific sex chromosomes.

Different species have different mechanisms for sex determination. Some rely on environmental factors (e.g., temperature during egg incubation in certain reptiles), while in others (like humans), it is primarily determined by genes inherited from the parents.

In human beings, there are 23 pairs of chromosomes. 22 pairs are autosomes, and one pair is the sex chromosomes. Women have a pair of XX sex chromosomes, while men have a pair of XY sex chromosomes.

During gamete formation (meiosis):

• Females (XX) produce eggs, each containing one X chromosome.
• Males (XY) produce sperms, half containing an X chromosome and half containing a Y chromosome.

When an egg (always X) is fertilised by a sperm:

• If the sperm carries an X chromosome, the resulting zygote is XX (develops into a female child).
• If the sperm carries a Y chromosome, the resulting zygote is XY (develops into a male child).

Thus, the sex of the child is determined by the type of sperm (X or Y) that fertilises the egg, which is contributed by the father. There is approximately a 50% chance of having a boy (XY) and a 50% chance of having a girl (XX) in each pregnancy.

Evolution

Evolution is the process by which life on Earth has changed over millions of years through the accumulation of variations in populations of organisms, leading to the diversification of species.

Evolution is driven by several factors, primarily natural selection and genetic drift, which act upon the variations generated during reproduction.

An Illustration

Consider a population of red beetles living on green bushes. Crows eat these red beetles. Imagine variations arise in the population:

• Situation 1 (Natural Selection): A green beetle variant appears, and its green colour is heritable. Crows cannot see green beetles on green leaves, so they eat fewer green beetles than red ones. Over generations, more green beetles survive and reproduce, increasing the frequency of the green trait (and the gene for green colour) in the population. The environment (crows) selects for the advantageous trait (green colour). This is natural selection, leading to adaptation to the environment.
• Situation 2 (Genetic Drift): A blue beetle variant appears, and its blue colour is heritable, but it offers no survival advantage against crows (crows eat blue beetles as readily as red ones). If a random event (like an elephant stamping on the bushes) drastically reduces the beetle population, and by chance, most of the surviving beetles are blue, then the new population will be predominantly blue. The frequency of the blue trait (and its gene) changed, not due to survival advantage, but due to a random event affecting a small population. This is genetic drift. Genetic drift can introduce diversity but is not necessarily adaptive.
• Situation 3 (Environmental Effect, Not Evolution): The bushes suffer from a plant disease, reducing the available food. Beetles are poorly nourished and have lower average weight. If the plant disease is eliminated, the next generation of beetles, with sufficient food, will have normal weight. The change in weight due to starvation is an acquired trait during the lifetime of individuals and is not inherited; the genes of the germ cells are not altered. Therefore, this is not an example of evolution.

In both natural selection and genetic drift scenarios, the frequency of an inherited trait (controlled by genes) changes in the population over generations. This is the core idea of evolution – a change in the genetic makeup of populations over time.

Acquired And Inherited Traits

It is crucial to distinguish between traits that are acquired during an individual's lifetime and those that are inherited through genetic material.

Acquired Traits: Characteristics developed by an individual during their lifetime due to environmental factors, experiences, or activities. Examples: learning a skill, building muscles through exercise, a scar from an injury, low weight due to starvation. Acquired traits affect the non-reproductive tissues and are generally not inherited by offspring because they do not cause changes in the DNA of the germ cells.

Inherited Traits: Characteristics passed down from parents to offspring through genes (DNA) in the germ cells. Examples: eye colour, hair colour, height (influenced by genes), genetic predispositions. Inherited traits are encoded in the DNA and can be passed on to the next generation, contributing to variation and evolution.

The experiences of an individual during its lifetime do not directly alter the genetic information in its germ cells and therefore cannot guide evolution.

The distinction between acquired and inherited traits is fundamental to understanding evolution and was clarified through the work of geneticists following Mendel.

Charles Darwin developed the theory of evolution by natural selection, but the mechanism of inheritance was unknown to him. Gregor Mendel's work on inheritance provided this mechanism.

Speciation

Evolution involves not just changes in the common characteristics of a species (micro-evolution) but also the formation of entirely new species. This process is called speciation.

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A new species is considered to have formed when a population splits into two or more groups that can no longer interbreed and produce fertile offspring, even if they are in the same geographic area. This reproductive isolation is the key event in speciation.

Speciation can occur when variation within a population is combined with factors that isolate sub-populations, preventing gene flow between them. Over generations, genetic differences accumulate in these isolated sub-populations due to genetic drift and potentially different selection pressures from the environment.

Factors Contributing to Speciation:

• Genetic Drift: Random changes in gene frequencies that are more pronounced in small populations. If populations are isolated, different random changes can accumulate in each.
• Natural Selection: If isolated populations experience different environmental conditions, natural selection may favour different traits in each, leading to genetic divergence.
• Geographical Isolation: Barriers like rivers, mountains, or vast distances can prevent gene flow between populations. This isolation allows genetic differences to accumulate independently in each separated group.
• Reproductive Isolation: Genetic changes can lead to mechanisms that prevent interbreeding even if populations come into contact. Examples include changes in mating behaviours, reproductive timing, or the inability of gametes to fuse or the zygote to develop successfully.

When the genetic differences between isolated populations become significant enough, they may lose the ability to interbreed, resulting in the formation of distinct species.

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Evolution And Classification

The classification of organisms into hierarchical groups (Kingdom, Phylum, Class, Order, Family, Genus, Species) reflects their evolutionary relationships. Species that share more common characteristics are considered more closely related and likely shared a common ancestor more recently.

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By studying the similarities and differences in the characteristics of organisms, we can trace their evolutionary history, going back in time to identify common ancestors.

Characteristics used for classification are details of form or function. Some characteristics are more fundamental (appeared earlier in evolution) and are used for broader groupings (e.g., presence of a nucleus, unicellular vs. multicellular). Less fundamental characteristics are used for smaller subgroups.

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Tracing Evolutionary Relationships

Evolutionary relationships can be traced by identifying characteristics in different species that are similar because they were inherited from a common ancestor. Such characteristics are called homologous characteristics or homologous organs.

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Example: The basic structure of the limbs (forelegs, wings, arms) in vertebrates like amphibians, reptiles, birds, and mammals is similar, even though these limbs are adapted for different functions (walking, swimming, flying, grasping). This underlying structural similarity suggests that they were inherited from a common ancestor and are therefore homologous organs.

Not all similarities in organ shape or function indicate common ancestry. Analogous characteristics or analogous organs perform similar functions but have different basic structures and evolutionary origins.

Example: The wings of birds and bats both function in flying. However, the structure of a bird's wing (feathers along the arm) is fundamentally different from a bat's wing (skin folds stretched between elongated fingers). These are analogous organs; they evolved separately to serve the same function.

Comparing the structures of living organisms and their fossil records helps distinguish between homologous and analogous traits and trace evolutionary relationships.

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Fossils

Fossils are the preserved traces of living organisms that existed in the past but are now extinct. They provide direct evidence of evolutionary history and help bridge gaps in our understanding of how different groups of organisms are related.

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Fossils form when dead organisms (or parts of them) are preserved in environments that prevent complete decomposition, often by being rapidly buried in sediment that later hardens into rock. The hardened sediment retains the impression or mineralised remains of the organism.

Dating Fossils: The age of fossils can be estimated in two main ways:

• Relative dating: Fossils found in deeper layers of rock are generally older than fossils found in layers closer to the surface.
• Absolute dating (Radioactive dating): Based on detecting the ratios of different isotopes of elements (like Carbon-14) in the fossil material. Radioactive isotopes decay at a known rate, allowing scientists to calculate the approximate age of the fossil.

By studying fossils from different geological layers, scientists can reconstruct the sequence of evolutionary changes over millions of years.

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Evolution By Stages

Complex organs or features, like the eye or feathers, are unlikely to have evolved in a single step. Instead, they are believed to have evolved gradually through intermediate stages, where each stage provided some survival advantage, even if it was different from the final function.

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Example: Evolution of the Eye: Very simple organisms like Planaria have rudimentary 'eyes' (eye-spots) that can only detect light (Fig. 9.11). More complex eyes evolved through stages, where intermediate structures (like a cup-shaped eye) provided increasing visual capability and conferred fitness advantages, even before a fully developed eye with a lens and retina evolved.

Example: Evolution of Feathers: Feathers are thought to have initially evolved for insulation in cold weather. Later, they may have been adapted for flight in birds. Some dinosaurs (reptiles) had feathers but could not fly. This suggests an evolutionary link between birds and reptiles.

The study of living organisms (comparing structures and DNA sequences) and the fossil record allows scientists to trace the evolution of features and understand how seemingly dissimilar structures evolved from common ancestral designs.

Artificial Selection: Human breeding practices also illustrate how diverse traits can evolve from a common ancestor through selection. Wild cabbage, for example, has been selectively bred by humans over centuries to produce various vegetables like cabbage (selection for short leaves), broccoli (arrested flower development), cauliflower (sterile flowers), kohlrabi (swollen parts), and kale (large leaves). All these variations originated from the wild cabbage plant.

Comparing the DNA sequences of different species provides a direct way to estimate how much genetic change has occurred over evolutionary time and is widely used in tracing evolutionary relationships (molecular phylogeny).

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Evolution Should Not Be Equated With ‘progress’

When studying evolution, it's important not to view it as a linear progression from 'lower' or 'primitive' forms to 'higher' or 'advanced' forms, with humans at the pinnacle.

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Evolution is primarily about the generation of diversity and the shaping of this diversity by environmental selection. It is not necessarily a ladder of progress. Multiple branches occur at each stage, and while new species emerge, older ones may also continue to exist if they are well-suited to their environment.

The only apparent trend might be towards increasing complexity in body designs over time, but simpler organisms (like bacteria) are still very successful and inhabit diverse environments, including extreme ones. Humans are just one species among many, resulting from evolutionary processes.

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Human Evolution

The same scientific tools (fossils, DNA studies, excavation, dating) are used to study human evolution. The vast diversity in human appearance (size, colour, features) led to the idea of 'races', but genetic evidence has shown that all humans belong to a single species, Homo sapiens, with no biological basis for distinct races.

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Genetic studies and fossil evidence strongly suggest that modern humans originated in Africa. The earliest fossils of Homo sapiens have been found there. From Africa, humans migrated in stages across the world over hundreds of thousands of years, adapting to different environments. This migration was not a simple, linear journey but involved complex movements, separations, and mixing of groups.

Like all other species, humans evolved through the processes of variation, inheritance, and natural selection, shaped by environmental factors and genetic drift.