Variation and Evolution

In this guide, we will cover the key concepts of variation and evolution. These are crucial in understanding how species change over time and how life on Earth adapts to its environment. The guide will explore the sources of variation, the theory of evolution, the evidence for evolution, how fossils are formed, and the causes and consequences of extinction.

Variation

Variation refers to the differences between individuals of the same species. These differences can be in terms of physical traits, behaviours, or physiological characteristics. Variation is the foundation for the process of evolution and natural selection.

Causes of Variation

Genetic Variation: This arises due to differences in the genetic material (DNA) between individuals. There are several factors that contribute to genetic variation:

• Mutations: Random changes in DNA sequences that can introduce new alleles (versions of a gene) into a population. Mutations can be beneficial, harmful, or neutral.
• Recombination: During sexual reproduction, the mixing of genetic material from both parents leads to offspring with unique combinations of alleles.
• Gene flow: The movement of genes between populations, typically due to migration, which introduces new genetic material into a population.

Environmental Variation: Differences in traits that are influenced by the environment. For example, plants growing in different environments may develop different sizes or shapes, even though they share the same genetic makeup. Environmental factors such as temperature, nutrition, and exposure to disease can influence traits, but these changes do not alter the genetic code.

Types of Variation

• Continuous Variation: This type of variation shows a range of differences in a characteristic, such as height, skin colour, or weight. Traits like these are controlled by multiple genes and are often influenced by both genetic and environmental factors.
• Discontinuous Variation: Traits that have distinct categories, such as blood type or the ability to roll your tongue. These traits are controlled by a single gene and are not influenced by environmental factors.

Evolution

Evolution is the process by which species change over time through the accumulation of genetic changes in successive generations. It is driven by the mechanism of natural selection and results in the adaptation of species to their environments.

Natural Selection

The theory of natural selection, first proposed by Charles Darwin, explains how evolution occurs. According to natural selection:

• Variation: Within a population, individuals exhibit genetic variation.
• Competition: Organisms compete for limited resources such as food, shelter, and mates.
• Survival of the Fittest: Individuals with characteristics that give them an advantage in their environment are more likely to survive, reproduce, and pass on their advantageous traits to their offspring.
• Adaptation: Over many generations, these advantageous traits become more common in the population, leading to the gradual adaptation of the species.

Example: The peppered moth (Biston betularia) in the UK provides a classic example of natural selection. During the Industrial Revolution, soot from factories darkened the trees, making the darker (melanic) form of the moth better camouflaged from predators. As a result, the frequency of dark moths increased in polluted areas, while light moths remained more common in cleaner, unpolluted areas.

Speciation

Speciation occurs when a population of a species becomes reproductively isolated from the rest of the population, leading to the formation of a new species. This isolation can occur through geographical barriers (e.g., rivers or mountains) or behavioural differences (e.g., different mating rituals). Over time, genetic changes accumulate, and the two populations can no longer interbreed, even if they are brought back together.

Evidence of Evolution

There are several lines of evidence that support the theory of evolution, showing how species have changed over time and providing insight into their common ancestry.

Fossil Evidence

Fossils are the preserved remains or traces of ancient organisms. The study of fossils, known as palaeontology, provides valuable evidence for evolution. Fossils show a sequence of organisms that appear in layers of rock, with older fossils found deeper underground than younger ones. This allows scientists to trace the changes in species over time and see how species have evolved.

• Transitional Fossils: These fossils show characteristics of two different groups of organisms, indicating how one group may have evolved into another. For example, the fossil Archaeopteryx shows features of both reptiles and birds, providing evidence for the evolution of birds from dinosaurs.

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Comparative Anatomy

By comparing the structure of different species, scientists can identify similarities that suggest common ancestry. There are three types of comparative anatomy:

• Homologous Structures: Structures that are similar in different species due to common ancestry. For example, the forelimbs of humans, bats, and whales have a similar bone structure despite their different functions, indicating that these species share a common ancestor.
• Analogous Structures: Structures that have a similar function but do not have a common evolutionary origin. For example, the wings of birds and insects serve the same function but evolved independently.
• Vestigial Structures: Structures that have lost their original function through evolution. For example, the human appendix or the pelvic bones in whales are vestigial structures that provide evidence of evolutionary change.

Comparative Embryology

The study of embryos of different species can reveal similarities in early development, suggesting a common origin. For example, the embryos of vertebrates, including fish, birds, and mammals, all share similar features in their early stages, such as pharyngeal pouches, indicating a common ancestor.

Molecular Evidence

The comparison of DNA and protein sequences across different species has provided strong evidence for evolution. Species that are closely related show more similarities in their genetic material. For example, humans and chimpanzees share about 98% of their DNA, indicating a close evolutionary relationship.

How Fossils are Formed

Fossils are formed when an organism is buried quickly after death and its remains are preserved over long periods of time. The process involves several stages:

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Death: An organism dies and is buried by sediment, such as mud, sand, or volcanic ash.

Decay and Mineralisation: Over time, soft tissues decay, but hard parts like bones, teeth, and shells are more likely to be preserved. Water carrying minerals seeps into the remains, slowly replacing the organic material with minerals, turning the remains into stone.

Exposure: Over millions of years, the fossil may be exposed by erosion, or it may be discovered during excavation.

Extinction

Extinction occurs when a species no longer exists. This can happen for a variety of reasons, including:

• Environmental Changes: Significant changes in climate, food supply, or habitat can lead to the extinction of species that cannot adapt quickly enough.
• Competition: A species may be outcompeted by another species for resources, leading to its decline.
• Human Activity: Overhunting, habitat destruction, and the introduction of invasive species by humans can cause the extinction of native species.
• Catastrophic Events: Events such as asteroid impacts or volcanic eruptions can cause massive environmental changes that lead to mass extinction.

Mass Extinctions

Throughout Earth’s history, there have been several mass extinction events in which large numbers of species disappeared in a short period of time. The most well-known event is the Cretaceous–Paleogene (K–Pg) extinction 66 million years ago, which wiped out the dinosaurs, likely due to an asteroid impact.

Summary

Variation and evolution are key concepts in biology that help explain how life on Earth has changed over time. Variation within populations provides the raw material for natural selection, which leads to the evolution of species. Evidence for evolution comes from fossils, comparative anatomy, embryology, and molecular biology, all of which point to the common ancestry of living organisms. Fossils are formed through the slow process of burial and mineralisation, and their study provides important insights into the history of life. Finally, extinction is a natural part of the evolutionary process, with species disappearing due to environmental changes, competition, or catastrophic events. Understanding these processes helps us appreciate the diversity of life and the mechanisms that shape it.