Life Sciences Grade 12

Variation in Evolution

Variation refers to the differences that exist among individuals of the same species. These differences can be seen in both genotypes (the genetic makeup) and phenotypes (the observable characteristics such as height, eye color, or blood type). Without variation, natural selection and evolution could not occur, as all individuals would be genetically identical and respond the same way to environmental changes.

There are several key sources of variation in sexually reproducing organisms. These mechanisms ensure that offspring are genetically unique and not mere clones of their parents. This diversity within populations is essential for survival and adaptation.

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Sources of Variation

Variation arises mainly from the processes involved in sexual reproduction and from mutations. The following mechanisms each play a specific role in creating new genetic combinations:

a) Crossing Over During Prophase I of Meiosis

During Prophase I of meiosis, homologous chromosomes pair up and exchange segments of genetic material in a process known as crossing over. This results in chromosomes that contain a mixture of maternal and paternal genes. As a result, the gametes formed (sperm or eggs) carry unique combinations of genes. For example, this explains why siblings may share certain traits but still look quite different—they inherit different recombined segments of DNA.

b) Random Arrangement of Chromosomes During Metaphase I

Another major contributor to variation is the random alignment of homologous chromosome pairs at the equator of the cell during Metaphase I of meiosis. This random positioning means that each gamete receives a different mix of maternal and paternal chromosomes. For instance, one sperm cell may carry chromosome 1 from the father and chromosome 2 from the mother, while another sperm may carry both from the mother. The total number of possible combinations increases exponentially with the number of chromosome pairs.

c) Random Fertilisation

Even after meiosis produces unique sperm and egg cells, fertilisation introduces more randomness. Any sperm can fertilise any egg, and since each gamete is genetically distinct, the fusion of two gametes leads to a completely unique zygote. This is why siblings born to the same parents still have different traits—fertilisation is a random process involving millions of possible sperm and one of many egg cells.

d) Random Mating

When organisms within a species mate without selective pairing, random combinations of genetic material are brought together. This is especially common in large, freely breeding populations. For example, in wild populations of birds or fish, mates are not selected based on genotype, so the variety of pairings introduces more genetic variation into the gene pool.

e) Mutations

Mutations are permanent changes in the DNA sequence of a gene. These can occur spontaneously or be triggered by environmental factors like radiation or chemicals. A mutation can change how a gene functions or is expressed. Some mutations have no visible effect, while others can cause dramatic changes in the phenotype. For instance, a mutation might result in a different eye color or even confer resistance to a disease. Mutations are crucial to long-term evolution as they introduce entirely new genetic material into a population.

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Types of Variation

Variation among individuals can be categorized into continuous or discontinuous variation, depending on how the traits are distributed across the population.

Continuous Variation

Continuous variation refers to traits that show a range of phenotypes, with no clear-cut boundaries between one extreme and another. These traits are usually influenced by multiple genes (polygenic inheritance) and can also be affected by environmental factors. An example of continuous variation is human height, where individuals can be very short, very tall, or anywhere in between. Other examples include skin color, weight, and intelligence. These characteristics form a smooth distribution curve when plotted on a graph.

Discontinuous Variation

Discontinuous variation refers to traits that fall into distinct, separate categories, with no intermediate forms. These traits are usually controlled by a single gene and are not influenced by the environment. A good example is blood group in humans—you are either type A, B, AB, or O; there are no partial or in-between blood types. Other examples include the ability to roll your tongue or having attached versus free earlobes. Discontinuous traits are easily grouped and typically analyzed using bar graphs rather than curves.

Evolution Theories

Theories of evolution are scientific explanations that describe how living organisms have gradually changed over long periods of time, leading to the vast diversity of life seen today. These theories investigate the mechanisms behind the origin of species, how they adapt to changing environments, and the processes that lead to the development of new species or the extinction of others. One of the earliest formal ideas was Lamarck’s theory, which proposed that characteristics acquired during an organism’s lifetime could be passed on to offspring. This idea was later replaced by Charles Darwin’s theory of natural selection, which remains the most widely accepted explanation

Lamarck’s Theory of Evolution

The study of biological evolution aims to understand how species change over long periods of time. Among the earliest scientists to propose a formal evolutionary theory was Jean-Baptiste de Lamarck (1744–1829). At a time when most believed species were fixed and unchanging, Lamarck suggested that life forms adapted to their environments and transmitted these changes to their offspring. Although his ideas were later rejected with the advancement of genetics, his contributions laid the groundwork for future evolutionary theories, particularly Darwin’s.

Lamarck’s theory is now known as Lamarckism, and it attempted to explain the mechanism behind species transformation. It was the first comprehensive attempt to link environmental change to biological adaptation and evolution.

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2. Core Principles of Lamarck’s Theory

Lamarck based his theory of evolution on two fundamental concepts:

a) Use and Disuse of Organs

Lamarck believed that frequent use of a body part would cause it to grow stronger and more developed, while body parts that were rarely or never used would shrink, weaken, or disappear over time. This principle was based on the idea that organisms respond directly to their environment by adjusting the way their bodies function.

Example: According to Lamarck, flightless birds like ostriches lost their ability to fly because their wings were not used over generations. Instead, their leg muscles became stronger from running, and these changes would be passed to their offspring.

b) Inheritance of Acquired Characteristics

The second principle was that characteristics an organism develops during its lifetime—whether due to behavior, use, or environmental pressures—can be inherited by its offspring. This meant that physical changes acquired after birth were encoded into future generations.

Example: A lizard that climbs trees might develop stronger claws. If this trait was acquired during its life, Lamarck believed its offspring would be born with stronger claws.

These ideas formed the backbone of Lamarck’s evolutionary explanation: organisms are not fixed; rather, they respond to their environment, and these responses are passed on to future generations, leading to gradual evolution.

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Why Lamarck’s Theory Was Rejected

a) With the rise of genetics in the 20th century, especially after the discovery of DNA, Lamarck’s theory was discredited for lacking genetic evidence. It was proven that acquired traits do not alter the genetic material (DNA) in reproductive cells and therefore cannot be inherited.

Scientific Evidence: A person who becomes skilled at drawing or who develops strong muscles from gym workouts does not pass these changes to their children. This is because these changes affect the somatic (body) cells, not the germ (reproductive) cells that carry heritable genetic information.

Lamarck also did not explain the existence of variation within populations or the randomness of genetic changes, which are now understood as key components of evolution.

b) Another flaw in Lamarck’s theory was its failure to explain how traits are stored and passed on at the molecular level. At the time, genes, chromosomes, and DNA were not yet discovered, so Lamarck had no way of understanding the mechanisms of inheritance. As a result, he mistakenly assumed that effort or behavior could alter the heritable traits of organisms.

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Darwin’s Theory of E

Introduction to Darwin’s Theory

Charles Darwin developed the theory of evolution by natural selection to explain how species gradually change over time. According to his theory, organisms are not fixed or unchanging; instead, they slowly evolve as beneficial traits are passed from parents to offspring. Over many generations, these small inherited differences can result in the development of entirely new species. This process accounts for the wide variety of life forms seen on Earth today.

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Key Principles of Natural Selection

1. Natural Variation in Populations

In every population, individuals differ from one another. These variations occur naturally due to mutations and the mixing of genes during reproduction. Some individuals may have traits—like better eyesight, faster speed, or stronger immune systems—that give them a survival edge in their environment.

Example: In a herd of deer, those with sharper hearing are more likely to detect predators early and escape.

2. Favourable Traits Offer Survival Advantages

Certain traits make individuals better suited to survive in their surroundings. These beneficial traits increase an organism’s chances of living long enough to reproduce, passing these advantages to the next generation.

Example: Arctic foxes with thick fur coats can withstand freezing temperatures better than those with thinner coats.

3. Overproduction and Competition

Organisms often produce more offspring than the environment can support. As a result, there is competition for limited resources like food, water, and shelter. Only the individuals that are best adapted are likely to survive.

Example: Sea turtles lay hundreds of eggs, but only a few hatchlings survive due to predators and other threats.

4. Survival of the Best-Adapted (Natural Selection)

The individuals whose traits give them an advantage are more likely to survive and reproduce. These traits become more common in the population over time—a process Darwin called “survival of the fittest.”

Example: In dry areas, plants with deeper roots are more likely to survive droughts than those with shallow roots.

5. Elimination of Unfavourable Traits

Traits that reduce an organism’s chances of survival tend to disappear from the population. Individuals with these traits often do not survive long enough to reproduce.

Example: A cheetah with weak legs will have trouble catching prey, making it less likely to survive and reproduce.

6. Inheritance of Beneficial Traits

When advantageous traits are passed to offspring, they become more common in the population. Over time, this leads to a population that is better adapted to its environment.

Example: Giraffes with longer necks are more successful at feeding on high leaves, and their offspring inherit this trait.

7. Gradual Evolution Over Time

As favourable traits accumulate across generations, the species slowly changes. This gradual process may eventually lead to the formation of a new species—a process known as speciation.

Example: Arctic and desert foxes evolved from a common ancestor but adapted differently to survive in extreme cold or heat.

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Important Concepts in Darwin’s Theory

Common Ancestry

Darwin proposed that all living organisms descend from a common ancestor. Over millions of years, species diverged from these early forms due to environmental pressures and adaptations.

Example: Humans and chimpanzees share around 99% of their DNA, pointing to a recent common ancestor.

Adaptation

Adaptations are traits that improve an organism’s ability to survive and reproduce. These can include body structures, behaviours, or internal processes.

Example: Camels have adapted to desert life by storing fat in their humps, allowing them to go without food for long periods.

Speciation

Speciation occurs when populations of the same species become isolated—by geography or other factors—and evolve separately into new species.

Example: Birds separated by a mountain range may adapt differently over time and become distinct species.

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Evidence Supporting Darwin’s Theory

1. Fossil Record

Fossils provide a timeline of how species have changed. Transitional fossils show features shared by both ancient and modern species.

Example: Fossils of early humans show changes from ape-like ancestors to modern human features.

2. Comparative Anatomy

Different species have similar body structures, even if used for different purposes. This suggests they share a common origin.

Example: The forelimbs of humans, whales, and bats have the same basic bone structure, although used for grasping, swimming, or flying.

3. Embryology

The embryos of many animals look alike in early development stages, showing that they may have evolved from a common ancestor.

Example: Human and fish embryos both show gill slits early on, even though humans do not retain them in adulthood.

4. Molecular Biology

Comparing DNA shows how closely related different species are. The more similar the DNA, the more recent the common ancestor.

Example: Humans and gorillas share many identical genes, supporting their close evolutionary relationship.

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Darwin’s theory of evolution by natural selection provides a powerful explanation for how life on Earth has changed and diversified. It shows that individuals with beneficial traits are more likely to survive, reproduce, and pass on those traits. Over many generations, this leads to the gradual evolution of species. The theory is strongly supported by multiple lines of evidence, including the fossil record, anatomy, embryology, and genetics, and remains one of the cornerstones of modern biology.

Punctuated Equilibrium

Punctuated equilibrium is an evolutionary theory proposed by paleontologists Niles Eldredge and Stephen Jay Gould in 1972. It offers an alternative explanation to how species evolve, differing from the traditional idea that evolution occurs at a slow, continuous pace. Instead, it suggests that species remain relatively unchanged for long periods (a state known as stasis) and that most evolutionary change occurs in short, rapid bursts, often linked to speciation events.

Stasis: Long Periods of Stability

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Punctuated equilibrium proposes that once a species appears in the fossil record, it remains relatively unchanged for most of its existence. During these long stable periods, natural selection may still act, but any changes are usually minor and do not result in the formation of new species. This period of little or no evolutionary change is known as stasis or equilibrium.

Example (Animals): Crocodiles are a classic example of evolutionary stasis. Despite existing for millions of years, their physical form has changed very little, as they remain well-adapted to their environments.

These long periods of stasis are occasionally punctuated by short intervals of rapid evolutionary change. These bursts are often triggered by environmental changes, such as climate shifts, migration, or the isolation of small populations. In these short geological timespans, natural selection acts more intensely, leading to the emergence of new traits or even entirely new species.

Example (Animals): After major events like ice ages or volcanic eruptions, some mammals may develop adaptations such as thicker fur or changes in size in a relatively short period to survive the new climate.

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A core idea in punctuated equilibrium is that speciation—the formation of new species—can occur quickly in evolutionary terms, sometimes over thousands of years instead of millions. This rapid change usually occurs in isolated populations where genetic drift and selection pressures differ from those in the main population.

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One of the main processes that drives these rapid changes is allopatric speciation, which happens when a small group of organisms becomes geographically isolated from the larger population. Because these isolated groups experience different environments and limited gene flow, they can evolve rapidly and independently into new species.

Example (Humans & Animals): Isolated groups of early humans, such as Homo erectus or Neanderthals, developed unique features due to geographic separation and different environments, eventually becoming distinct species.

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Phyletic gradualism is the older, traditional theory of evolution that proposes that species evolve slowly and continuously over long periods, with small changes accumulating to produce new species. Punctuated equilibrium challenges this view by suggesting that most significant evolutionary changes happen in short, dramatic episodes, not gradually.

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Punctuated equilibrium was proposed to help explain patterns observed in the fossil record, where many species appear suddenly, remain unchanged for long periods, and then disappear—with few transitional forms. This supports the idea that evolution does not always leave behind a complete sequence of gradual changes, as rapid speciation in small, isolated populations is less likely to be preserved as fossils.

Example (Fossils): Fossils of early whales, like Pakicetus, appear relatively abruptly in the fossil record, followed by long periods of little change—supporting the punctuated model of evolutionary change.

It’s important to understand that punctuated equilibrium does not reject Darwin’s theory of natural selection or common ancestry. Instead, it proposes a different pace or tempo of evolution—where natural selection still operates, but significant changes often occur in brief, intense bursts rather than slowly and steadily.

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