Evolution — natural selection and evidence (KCSE Biology Form 4)

TL;DR

Evolution is the gradual change in living organisms over time, driven primarily by natural selection. Natural selection favors individuals with advantageous traits, allowing them to survive and reproduce more successfully. We see evidence for evolution in fossils, comparative anatomy, embryology, and genetics.

1. The Mental Model

Imagine a population of animals where some are slightly better at something important, like finding food or hiding from predators. These better-adapted individuals are more likely to live long enough to have babies, passing on their good traits. Over many generations, these advantageous traits become more common in the population, leading to a gradual change in the species.

2. The Core Material

What is Evolution?

Evolution is the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the Earth. It's not about an individual changing during its lifetime, but about changes in the genetic makeup of populations over generations.

Natural Selection: The Driving Force

Natural selection is the main mechanism proposed by Charles Darwin and Alfred Russel Wallace that explains how evolution occurs. It's often summarized as "survival of the fittest," but it's more accurately described as "differential reproductive success." Here's how it works:

1. Variation: Within any population, individuals show variations in their traits (e.g., some giraffes have longer necks, some beetles are darker). These variations are often due to random mutations.
2. Overproduction: Organisms produce more offspring than can possibly survive.
3. Competition (Struggle for Existence): Due to overproduction, resources like food, water, and space are limited, leading to competition among individuals.
4. Differential Survival and Reproduction: Individuals with traits that make them better adapted to their environment are more likely to survive this competition, reproduce, and pass on those advantageous traits to their offspring. Individuals with less favorable traits are less likely to survive and reproduce.
5. Inheritance: The advantageous traits are heritable, meaning they can be passed from parents to offspring.

Over many generations, this process leads to an increase in the frequency of advantageous traits in the population, and the population as a whole becomes better adapted to its environment.

graph TD
    A[Variation within a population] --> B{Overproduction of offspring};
    B --> C[Competition for resources];
    C --> D{Differential Survival & Reproduction};
    D -- Favorable traits passed on --> A;
    D -- Unfavorable traits decrease --> A;
    D --> E[Population becomes better adapted over generations];

Evidence for Evolution

We don't just believe in evolution; there's a lot of evidence supporting it.

1. Fossil Records

Fossils are the preserved remains or traces of organisms from the past.
* Transitional Fossils: These show intermediate forms between different groups of organisms, like Archaeopteryx which has features of both reptiles (tail, teeth) and birds (feathers).
* Dating Fossils: Scientists can determine the age of fossils using methods like radiometric dating. Older rock layers generally contain simpler life forms, while newer layers contain more complex ones, showing a progression of life over time.

2. Comparative Anatomy

This involves comparing the body structures of different species.
* Homologous Structures: These are structures that have a similar underlying anatomy but may have different functions. For example, the forelimbs of humans, bats, whales, and cats all have the same basic bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges), suggesting a common ancestor.
* Analogous Structures: These are structures that have similar functions but different underlying anatomy. For example, the wings of a bird and the wings of an insect both allow flight, but their internal structures are very different. This shows convergent evolution, where different species adapt to similar environments in similar ways.
* Vestigial Structures: These are reduced or non-functional structures that were once important in an organism's ancestors. Examples include the human appendix, wisdom teeth, and the pelvic bones in whales.

3. Comparative Embryology

This compares the embryonic development of different species. Early embryos of many vertebrates (fish, amphibians, reptiles, birds, mammals) look remarkably similar, possessing structures like gill slits and tails that may not be present in the adult form. This suggests a common ancestry.

4. Geographical Distribution (Biogeography)

The distribution of species across the Earth provides evidence.
* Unique Species on Islands: Islands often have unique species that are similar to, but distinct from, species on the nearest mainland. This suggests they evolved from mainland ancestors that colonized the island and then adapted to the new environment.
* Related Species in Different Continents: The presence of similar fossil forms on continents now separated by oceans supports the idea of continental drift and shared ancestry.

5. Molecular Biology and Genetics

This is some of the strongest evidence.
* DNA and Protein Similarities: All living organisms use DNA as their genetic material and share a common genetic code. The more closely related two species are, the more similar their DNA sequences and protein structures will be. For example, human DNA is over 98% similar to chimpanzee DNA.
* Universal Genetic Code: The fact that the same codons specify the same amino acids in almost all organisms points to a single origin of life.

3. Worked Example

Let's consider the classic example of Industrial Melanism in Peppered Moths ( Biston betularia ) in England.

Before the Industrial Revolution, most peppered moths were light-colored with dark speckles, blending in well with the light-colored lichen-covered tree trunks. A small number of dark-colored (melanic) moths existed due to random mutation.

During the Industrial Revolution (18th and 19th centuries), factories released huge amounts of soot and pollution. This pollution killed the lichens and blackened the tree trunks.

1. Variation: The moth population had both light and dark forms.
2. Overproduction: Moths produced many offspring.
3. Competition: Birds were predators, hunting moths on tree trunks.
4. Differential Survival: On the blackened trees, the light-colored moths were now highly visible to predators, making them easy targets. The dark-colored moths, however, were camouflaged against the sooty trunks. They were less likely to be eaten.
5. Inheritance: The surviving dark moths reproduced more successfully, passing on their genes for dark coloration to their offspring.

Over several generations, the frequency of the dark-colored moths in polluted areas increased dramatically, becoming the dominant form. In contrast, in unpolluted rural areas, the light-colored moths remained dominant. This is a direct, observable example of natural selection in action, driven by environmental change. When pollution controls were introduced later, and tree trunks lightened again, the light-colored moths started to increase in frequency once more.

4. Key Takeaways

• Evolution is the change in heritable characteristics of biological populations over successive generations.
• Natural selection is the primary mechanism of evolution, favoring individuals with advantageous traits.
• Variation, overproduction, competition, and differential reproduction are the key steps in natural selection.
• Fossil records provide a historical sequence of life forms and transitional species.
• Homologous structures show common ancestry, while analogous structures show convergent evolution.
• Similarities in embryonic development and DNA sequences strongly support common ancestry.

Common mistakes to avoid:
- Thinking that individuals evolve during their lifetime; evolution happens to populations over generations.
- Confusing natural selection with "survival of the strongest" – it's about being "fit" for the specific environment.
- Believing that evolution has a goal or direction; it's a blind process driven by environmental pressures.
- Mixing up homologous (same structure, different function, common ancestor) and analogous (different structure, same function, no recent common ancestor) structures.

5. Now Try It

Imagine a population of rabbits living in a snowy environment. Due to climate change, the snow cover is rapidly decreasing, and the ground is becoming mostly brown soil for longer periods of the year. Describe, step-by-step, how natural selection would likely affect the fur color of this rabbit population over many generations. What would the "advantageous trait" be, and why? What would success look like in this scenario?