The Theory of Evolution

What was the conventional view of life during Darwin’s time?

That species were fixed and unchanging, created in their present-day form.

Which cultural and philosophical influences supported the idea of fixed species?

Aristotle’s idea of fixed forms and Judeo-Christian literal interpretations of Genesis.

Who proposed the theory of evolution through natural selection?

Charles Darwin.

What phrase did Darwin use to describe evolution?

"Descent with modification."

What is natural selection?

A process where individuals with traits suited to their environment survive and reproduce more successfully than others.

What are adaptations?

Accumulated traits that enhance an organism’s ability to survive and reproduce in its environment.

How did Darwin gather evidence for his theory?

Through observations during a sea voyage, notably on the HMS Beagle.

What is a scientific theory?

A well-supported and widely accepted explanation of a phenomenon that is broader than a hypothesis and supported by a large body of evidence.

Why was Darwin’s idea of evolution considered radical for his time?

It contradicted the prevailing belief that species were unchanging and created exactly as they are.

What distinguishes Darwin’s theory from earlier ideas of evolution?

He provided a mechanism—natural selection—to explain how evolution occurs.

How do adaptations form according to Darwin?

Through the accumulation of favorable traits over generations via natural selection.

Why is natural selection considered a mechanism of evolution?

Because it explains how advantageous traits increase in a population over time, leading to evolutionary change

How does Darwin’s concept of "descent with modification" explain the diversity of life?

It suggests all species share common ancestors but have diverged over time through accumulated adaptation

A population of moths lives in a forest. After pollution darkens tree bark, darker moths are less visible to predators and reproduce more. What process is occurring?

Natural selection.

A student argues that Darwin was the first to propose evolution. How would you correct this misconception?

Darwin wasn’t the first to propose evolution, but he was the first to explain how it happens through natural selection.

A biologist finds that desert animals often have light-colored fur and water-conserving adaptations. How would Darwin explain this?

These traits were naturally selected over generations because they increased survival and reproduction in desert environments.

If scientists observe a gradual change in a bird’s beak size over several generations in response to food availability, what evolutionary mechanism is most likely responsible?

Natural selection acting on beak size.

Why do scientists refer to Darwin’s idea as a "theory" and not just a "hypothesis"?

Because it is a comprehensive explanation supported by extensive evidence and capable of generating new testable hypotheses.

A theory

is a widely accepted explanation of a phenomenon that is broader than a hypothesis,

Hypothesis

is able to generate new hypotheses, and is backed by a large body of evidence.

Evidences of Evolution

fossil evidence, comparative anatomy, comparative embryology, biogeography, and molecular biology. 

geographic distribution

or proximity is a better predictor of relationships among organisms than the similarity of environment. For example, plants and animals in temperate regions of South America resemble tropical species in the continent more than those temperate living species in Europe.

Fossils

are the remains of organisms that lived in the past. They allow us to document changes between past and present organisms and identify many species that have become extinct. Many fossils are found in fine-grained sedimentary rocks in layers atop another called strata.

Paleontologists,

scientists who study fossils, gain access to very old fossils when erosion carves through the upper, younger stratum into the deeper, buried layers.

Another evidence for evolution is the presence of anatomical structures that may have different functions but are structurally similar because they come from a common ancestor.

Similarity coming from this common ancestry is called homology, and these features are called homologous structures. An example is the forelimbs of mammals. Comparing the limb of a human to that of a whale (that has become a flipper) or that of a bat (that has been modified for flight), the functions differ but when observed anatomically will have similarities.

This differs from analogous structures which are similar features that evolved independently for distantly related organisms. For example, the marsupial sugar glider from Australia is able to glide like the flying squirrel, a eutherian, because they modified their flap of skin tissues to allow gliding.

Analogous structures come to be because of convergent evolution, the independent evolution of similar features in different lineages.

Some homologous structures turned into remnant structures that have become marginal or perhaps of no importance to the organism. Called vestigial (or rudimentary) structures, these features served important functions for the ancestors of the lineage. An example of vestigial structure found in the human body is the appendix. In ancestral species, diet is composed primarily of plant material hence the appendix then functioned as site where symbiotic microorganisms would help digest cellulose from plant material.

Having proposed a mechanism for evolution, Darwin conceived that artificial selection – selective breeding of domesticated plants and animals to get desirable traits in their offspring – was key to understanding change brought by evolution. By talking to farmers about livestock breeding, he learned two essential components of artificial selection: variation and heritability.

Variation,

defined as differences among individuals in the same group, allows breeders to select animals or plants with the most desirable combination of characters as breeding stock.

Heritability,

meanwhile, refers to the transmission of a trait from parent to offspring. Despite lacking underlying knowledge of genetics, breeders are already aware of the importance of heritability in artificial selection.

phenotypic variation

the inherited traits expressed in the individual such as appearance

genotype

the genetic makeup, and many influences from the environment.

The mutation

is the change in the genetic information encoded in DNA. Genetic information is stored in genes that can have different versions, structures referred to as alleles, that may affect the phenotype.

population

refers to a group of individuals of the same species that occupy the same area and can reproduce with each other.

gene pool

which contains all copies of every allele at every locus in all members of the population.

Genes that encode specific traits can have alleles that allow them to manifest or not. These alleles vary in frequency in the population and when the frequency shifts over a number of generations, evolution occurs on its smallest scale called

microevolution.

The shuffling of alleles during sexual reproduction does not alter the genetic makeup of the population. In other words, unless an external factor causes the genetic makeup to change, the gene pool will remain constant. This condition is known as the Hardy-Weinberg equilibrium, and for a population to be in this equilibrium, it must satisfy five main conditions:

1. The population must be large. Smaller populations are more prone to have their allele frequencies fluctuate due to chance events or encounters, depending on the magnitude of the event.

2. No gene flow between populations. When individuals move into or out of populations, they add or remove alleles that alter the gene pool. Think of animal hybrids as clear examples.

3. No mutations.

4. Random mating. As in the case of artificial selection, if individuals mate preferentially, specific traits are favored, and this modifies the gene pool.

5. No natural selection.

Genetic drift

refers to chance events that cause allele frequencies to fluctuate unpredictably. Two examples of events that lead to genetic drift are the bottleneck effect and the founder effect.

bottleneck effect

can be visualized when a catastrophe kills large numbers of individuals in a population. The remaining population will not have the same genetic makeup as the original population. Analogous to when shaking marbles through a bottleneck, only a few marbles will be able to escape. These living individuals may have specific alleles, translating to higher allele frequency. Thus, the living population’s genetic pool becomes skewed in favor of the increased allele. 

Genetic drift also likely takes place when a few individuals colonize a new habitat referred to as the

founder effect.

Gene flow  

causes change in a population’s allele frequencies due to the migration of fertile individuals into or out of the population or when gametes, in the case of plant pollen or fungal spores, are transferred between populations. tends to reduce differences between populations, but introduces new alleles into previously isolated populations.

sexual selection

natural selection that favors individuals with certain traits fashioned for obtaining mates.

Secondary sexual characteristics, noticeable differences not directly related to reproduction, manifest as sexual dimorphisms.

These may come in the form of size difference; but can also include forms of adornment like horns, manes, and plumage with males usually being the showier sex, at least among vertebrates.

Sexual selection can be intrasexual or intersexual.

When individuals of the same sex compete directly for mates, this refers to the former and is usually found in species where the winning individual acquires a harem of mates. The latter is the more common type of sexual selection wherein individuals of one sex (usually females) are choosy in selecting mates.