Evolution : Unit 3

Natural Selection

Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. This theory, famously proposed by Charles Darwin, is the primary mechanism of evolution.

Artificial selection

Human choice and intervention. Specific desired traits (e.g., higher milk yield, colorful flowers). Can be very rapid (visible changes in just a few generations). Traits may be harmful to the organism (e.g., breathing issues in certain dog breeds).

Charles Darwin

Darwin provided the most comprehensive mechanism for evolution in his book On the Origin of Species (1859). During his travels on the HMS Beagle, he observed that finches on the Galápagos Islands had beak shapes specifically adapted to the food sources on their respective islands. This led to his theory of descent with modification. the foundational evolutionary concept that species change over time, inherit traits from common ancestors, and diverge into new species, driven primarily by natural selection.

Alfred Russel Wallace

Alfred Russel Wallace was a British naturalist and explorer who independently co-discovered the theory of evolution by natural selection.

Jean-Baptiste Lamarck

Though his specific mechanism was eventually proven wrong, Lamarck was a pioneer for suggesting that species change over time. He proposed the Inheritance of Acquired Characteristics, suggesting that if an animal used a body part frequently (like a giraffe stretching its neck), it would pass that stretched neck to its offspring.

Charles Lyell (Geology)

Lyell was a geologist who argued for Uniformitarianism—the idea that the Earth is shaped by slow-moving forces (like erosion) over vast periods of time. This was crucial for Darwin, as it provided the "deep time" necessary for slow biological evolution to occur.

Gregor Mendel (The "Missing Link")

Darwin knew traits were passed down, but he didn't know how. It wasn't until the work of Gregor Mendel on pea plants was rediscovered decades later that scientists understood genetics. This combined with Darwin's theories to create the Modern Synthesis, explaining how mutations and gene flow drive natural selection at a molecular level.

Variation

Variation is the raw material for natural selection

• there have to be differences within population

• some individuals must be more fit than others

Hardy-Weinburg formulas

p + q =1 (counting alleles)

p2 + 2pq + q2 = 1 (counting individuals)

Speciation

Formation of new species from existing species.

Pre-Zygotic Isolating barriers

Obstacle to mating or to fertilisation if mating occurs. The SIX types (geographic isolation, behavioural isolation, mechanical isolation, gametic isolation, temporal isolation, and ecological/habitat isolation)

Geographic isolation

Species occur in different areas (allopatric speciation and physical barrier)

Ecological/Habitat isolation (sympatric isolation)

species occur in the same region, but occupy different habitats so rarely encounter each other (reproductively isolated)

Temporal isolation

• species that breed during different times of day

• different seasons

• different years cannot mix gametes. [reproductive isolation and sympatric isolation]

Behavioural isolation

Behavioral isolation is a type of pre-zygotic barrier where two species do not mate because of differences in their courtship rituals, communication, or mating displays.

• identifies members of species

• attract mates of some species like courtship rituals or mating call

Mechanical isolation (sympatric isolation)

Mechanical isolation is a pre-zygotic barrier that occurs when two species are physically unable to mate because their reproductive structures are incompatible.

• Even if they live in the same place (no habitat isolation) and try to mate at the same time (no temporal isolation), the "parts" simply do not fit together. It is often described as a "lock and key" problem: if the key doesn't fit the lock, fertilization cannot happen.

Gametic isolation

• Sperm of one species may not be able to fertilise eggs of another species

• has a biochemical barrier so sperm cannot enter egg

• has a chemical incompatibility (sperm cannot survive in female reproductive tract)

Post Zygotic barriers

Prevent hybrid offspring from developing into a viable, fertile adult.

• reduced hybrid inviability

• reduced hybrid fertility (Hybrid Sterility)

• Hybrid breakdown

Reduced hybrid Viability

The hybrid zygote is formed, but the genetic instructions from the two parents are so different that they conflict.

• What happens: The embryo usually dies during development, or the offspring is born very weak and dies before it can reach reproductive age.

Reduced hybrid fertility

the capability of a hybrid organism—the offspring of two different varieties, species, or genera—to successfully reproduce

Hybrid breakdown

Hybrid breakdown is a post-zygotic barrier where the first generation of hybrids (f1) is healthy and fertile, but the following generations (f2 and beyond) are weak, sterile, or simply fail to develop properly.

Evolution

the process of genetic change in a population over time

• Individual organisms do not evolve, populations do.

Factors that change allele frequencies in Populations

1. Mutation

2. Gene flow

3. Non-Random Mating

4. Genetic Drift

5. Natural Selection

Mutation

• the origin of a genetic variation is mutation

• Mutation - any change in the DNA of an individual

• That inheritable mutation has the potential to affect an entire gene pool

Fossil Record

Fossils provide a chronological history of life on Earth. By studying different layers of sedimentary rock, scientists can track the gradual changes in species over millions of years.

• Transitional Fossils: These are "missing links" that show intermediate states between an ancestral form and its descendants (e.g., Archaeopteryx showing traits of both dinosaurs and birds).

Homologous Structures

Organs or skeletal elements that, by virtue of their similarity, suggest a connection to a common ancestor. For example, the pentadactyl limb (five-fingered) structure is found in humans, cats, whales, and bats, despite their different functions.

Analogous Structures

Features that perform similar functions but evolved independently (convergent evolution), such as the wings of a butterfly and the wings of a bird. They do not indicate common ancestry.

Vestigial Structures

Physical remnants that served a purpose in an ancestor but are no longer functional in the modern species (e.g., the pelvic bone in whales or the human tailbone).

Biogeography

The study of the distribution of species around the world. It shows that organisms are more closely related to those nearby than to those in similar environments elsewhere (e.g., marsupials in Australia).

Ring Species

a unique biological situation where a series of neighboring populations can interbreed with those living next to them,

Comparative Embryology

During early development, the embryos of different vertebrates look remarkably similar. All vertebrate embryos (including humans) possess gill slits and tails at some stage, suggesting they inherited a common developmental "blueprint" from a shared ancestor.

HOX Genes

These are a group of related genes that specify the "body plan" of an embryo along the head-to-tail axis. The fact that almost all animals—from flies to humans—use nearly identical HOX genes to build their bodies is profound evidence of shared ancestry.

Resistance

This is evolution in "real-time." When bacteria evolve antibiotic resistance or insects develop pesticide resistance, we are directly observing natural selection favoring individuals with traits that allow them to survive and reproduce.

Non-Random Mating

If individuals choose mates based on specific traits (like a peahen choosing a peacock with the brightest feathers) rather than mating randomly, the frequency of those traits will change in the next generation. This ensures that only certain genes are passed on.

Gene flow

When individuals immigrate (move in) or emigrate (move out) of an area, they take their genes with them. This movement changes the frequency of alleles in both the old and new populations.

Genetic drift

the change in the frequency of an existing gene variant (allele) in a population due to random chance. Unlike natural selection, which is "survival of the fittest," genetic drift is essentially "survival of the luckiest."

Bottleneck effect

The Bottleneck Effect is a specific type of genetic drift that occurs when a population’s size is reduced dramatically for at least one generation.

Allopatric speciation

Populations is split into two or more isolated groups by a geographical barrier

• eventually gene pool becomes so distinct that the two species cannot interbreed anymore

Ecological niche

the ecological role and physical distribution of a species in its environment.

Sympatric Speciation

• Populations live in the same geographic area, but become reproductively isolated

  • more common in plants 

Adaptive radiation

Diversification from a single species into a variety of differently adapted species (Allopatric)

Divergent Evolution

occurs when two or more species share a common ancestor but become increasingly different over time. This usually happens when populations are separated by a geographic barrier or move into different environments (niches).

Convergent Evolution

occurs when unrelated species evolve similar traits independently. This happens because the species live in similar environments or face similar selective pressures, so "nature" arrives at the same solution twice.

Gradualism

the theory that change occurs through the slow, steady accumulation of small genetic variations over very long periods of time.

Stabilizing Selection

the "average" or intermediate phenotype is favored, while the extremes are selected against. This reduces variation and keeps the population "stable" around a specific trait.

Directional Selection

occurs when environmental changes favor individuals at one extreme of the trait range. Over time, the entire population "shifts" in that direction.

Disruptive Selection

Disruptive selection (also known as diversifying selection) is a type of natural selection that favors individuals at both extremes of a trait's range, while selecting against the "average" individuals in the middle.

Founder effect

occurs when a small group of individuals breaks away from a larger population to establish (or "found") a new colony in a different location.

Sexual Selection

a specific type of natural selection where the "selective pressure" comes from the struggle to find a mate and reproduce, rather than just the struggle to survive.

Punctuated equilibrium

describes the "tempo" or speed at which new species evolve.

Adaptation

a specific physical trait or behavioral habit that improves an organism's ability to survive and reproduce in its environment. Adaptations are the "end result" of natural selection—the traits that "won" over many generations.

Extinct

when the very last living individual of that species dies.

Mimicry

type of adaptation where one species evolves to look, sound, or behave like another species (or even an inanimate object) to gain a survival advantage. This is usually done to trick predators.

Monoculture

the practice of growing or raising only one single species or variety of organism in a specific area at the same time.

Selective Advantage

a genetic advantage that improves an organism's chance of survival and reproduction in a specific environment. It is the "reason" why natural selection favors one individual over another.

Biotechnology

the use of living organisms, cells, or biological systems to develop products or technologies that improve human life and the health of the planet.

Selective Pressure

reduces the "fitness" of certain individuals in a population while favoring others. It is the "force" that drives natural selection. Without selective pressure, traits wouldn't change over time because there would be no reason for one version of a gene to be better than another.

Catastrophism

the theory that Earth’s geological features and the history of life were shaped by sudden, short-lived, violent events (catastrophes) rather than slow, gradual processes.

Paleontology

the scientific study of life that existed in prehistoric times, primarily through the examination of fossils.

Survival of the Fittest

Those best suited to their environment reproduce the most.