Biology 240

Directional Selection

This occurs when one extreme phenotype is favored over others, causing the entire population to shift in that direction. It happens when environmental changes or selective pressures make one trait more advantageous.

Stabilizing Selection

Type of selection favors the average phenotype while selecting against extreme variations. It reduces genetic diversity.

Disruptive/Dysfunctional Selection

This occurs when both extreme phenotypes are favored over the intermediate phenotype. It can lead to increased genetic diversity and, in some cases, speciation.

Directional Selection Example

The evolution of the peppered moth during the Industrial Revolution. Before industrialization, light-colored moths were more common because they blended into tree bark. When pollution darkened the trees, darker moths became more common because they had a survival advantage.

Stabilizing Selection Example

Human birth weight. Babies that are too small have difficulty surviving, and babies that are too large can cause complications during birth. As a result, intermediate birth weights are favored.

Disruptive/Dysfunctional Selection Example

Beak sizes in Darwin's finches. In an environment where only large and small seeds are available, birds with either large or small beaks have an advantage, while those with medium-sized beaks struggle to eat efficiently.

Adaptation

Traits that help an organism survive and reproduce.

Natural Selection

Process where beneficial traits become more common over generations.

Fitness

How well an organism can survive and reproduce in its environment

Trade-offs

Alternatives that must be given up when one is chosen rather than another

Mutations

Introduce new alleles

Gene Flow

Migration misses alleles between populations

Natural Selection

Certain traits increase survival and reproduction

What defines a species

can interbreed and produce viable offspring

Biological Species Concept

A species is a group that can reproduce and produce fertile offspring.

Limitations to Biological Species Concept

Doesn't apply to fossils or asexual organisms

Phylogenetic Species Concept

defines a species as the smallest group of individuals that share a common ancestor

Allopatric Speciation (Geographic Isolation)

The formation of new species in populations that are geographically isolated from one another.

Example of Allopatric Speciation

darwin finches, separated by sea

Sympatric Speciation (No physical barrier)

Species changes into a new species without a physical barrier

Assortative Mating

organisms prefer mates with similar traits

Resource Specialization

Different food sources cause genetic separations

Prezyotic barriers

prevent mating or fertilization between species

Postzygotic barriers

Barriers that prevent the hybrid zygote from becoming a fertile adult.

Evidence of Evolution

Fossil Record, Homology, Convergent Evolution, Vestigial Structure, Molecular Evidence (DNA)

Fossil Record

Shows changes in species over time.

Homology

similarity resulting from common ancestry

Example of Homology

Human arm, whale flipper, and bat wing bones.

Convergent Evolution

Process by which unrelated organisms independently evolve similarities when adapting to similar environments

Vestigial Structures

Leftover body parts with no function

Vestigial Structures Example

Human tailbone

Examples of Evolution

antibiotic resistance in bacteria, peppered moths

Sister Taxa

groups that share an immediate common ancestor

Monophyletic group (clade)

a common ancestor and all of its descendants

Paraphyletic Group

composed of some but not all members descending from a common ancestor

Principle of Parsimony

the belief that explanations of phenomena and events should remain simple until the simple explanations are no longer valid

Sexual Selection

when individuals select mates based on heritable traits

Intrasexual Selection

competition among individuals of one sex (often males) for mates of the opposite sex

Intersexual Selection

mate choice (mostly female)

Unequal Gamete Size

Females invest more into reproduction.

Asexual Reproduction

One parent produces clones

Cons of asexual reproduction

No genetic diversity, harder to adapt.

Pros of asexual reproduction

Fast no need to find a mate

Sexual Reproduction

involves two parents that combine their genetic material to produce a new organism.

Cons of Sexual Reproduction

Energy cost of finding mates

Pros of Sexual Reproduction

More genetic diversity —> better adaptations

Harmful genes passed down due to evolutionary trade-offs

Sickle cell anemia, Huntington disease, obesity genes

Genetic Drift

A change in the allele frequency of a population as a result of chance events rather than natural selection.

Genetic Drift (Bottle neck Effect)

a sudden change in the environment may drastically reduce the size of a population

Genetic Drift (Founder Effect)

A small group migrates and starts a new population, carrying only a fraction of the genetic diversity.

2 Muations

Neutral and Silent

Neutral Mutations

mutations that have no effect on organisms

Silent Mutations

DNA changes that do not change the protein (can be passed down)

Plascity

An individual changes its phenotype in response to the environment but cannot be passed down because its not genetic.

Plascity example

Tanning in humans - skin darkens in response to sunlight but isn't genetic change.

Genetic Adaptation

Permanent genetic changes in a population due to natural selection

Hybrid incompatibility

offspring of individuals from different populations or even species are worse off

Example of Sympatric Speciation

Different food sources, mating behavior differences

Synamoprhy

A shared, newly evolved trait

Symplesiomorphy

shared ancestral trait

Anisogamy

sexual reproduction by the fusion of dissimilar gametes

Sexual Dimorphism

Differences in physical characteristics between males and females of the same species.

Parental Investment

the time and energy parents expend for their offspring's benefit

Hermaphroditism

One organism produces both sperm and eggs.

Sequential hermaphroditism

Organism changes sex over time

Trioecy

rare sexual system characterized by the coexistence of males, females, and hermaphrodites

Where does sex occur in animals

Germ cells are set early in development.

Where does sex occur in plants

Meiosis happens in each flower.

Costs of Sexual Reproduction

Finding a mate takes time & energy.

Sexually transmitted diseases.

Risk of injury in competition

Why species go extinct

Natural and Human causes

Human causes of extinction

Deforestation, pollution, overfishing, invasive species.

Adaptive Radiation

An evolutionary pattern in which many species evolve from a single ancestral species

Genetic Constraints

genetic variation for a particular phenotype may not be present within a population

Environmental Constraints

Evolution is limited by the environment; selection acts on existing variation but cannot create perfect organisms.

Peripatric Speciation

A small population becomes isolated at the edge of a larger population (a form of allopatric speciation).

Polyploidy

condition in which an organism has extra sets of chromosomes

Transitional Fossils

Fossils that provide patterns of evolutionary change from the early ancestors to modern life forms.

Embryology

Similar developmental stages across species suggest common ancestry

Runaway Selection

When a trait becomes exaggerated due to female preference, leading to potential survival disadvantages

Sex Role Reversal

In species where males invest more in offspring, females compete for mates

Punctuated Equilibrium

Pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change

Classification order

Kingdom → Phylum → Class → Order → Family → Genus → Species

Tangled Bank Hypothesis

Sexual reproduction allows offspring to specialize in different environments, reducing competition.

Lottery Ticket hypothesis

The idea that genetic diversity from sexual reproduction increases the chance that some offspring will survive unpredictable environments.

The red queen hypothesis

The idea that species must constantly evolve to survive because their parasites, predators, and competitors are also evolving.

Examples where fitness is associated with reduced size/complexity

Cave Fish: Lose their eyes because eyes waste energy in total darkness. Parasites: Many lose unnecessary body parts (e.g., tapeworms have no digestive system).

Three Requirements for Natural Selection

Variation, heritability , and differential survival.

Directional Selection Example

Peppered moths during the Industrial Revolution.

Stabilizing Selection Example

Human birth weight (too small or too large is dangerous)

Disruptive Selection Example

Finch beak sizes in areas with only large or small seeds.

Fluctuating Selection Example

Snail shell thickness changes depending on predator levels.

Frequency-Dependent Selection

The rarer a trait, the more beneficial it becomes.

Frequency-Dependent Selection Example

Predators focus on common prey, giving an advantage to rare color variants.

Nodes (phylogenetic tree)

Speciation events (divergences).

Why is Altruism a Paradox?

Sacrificing for others should reduce reproductive success, yet altruism exists.

Kin Selection

Helping relatives increases the spread of shared alleles

Monogamous species

helping full siblings ensures more of their genes survive.

Haplodiploidy in Insects

In bees, ants, and wasps, females share more genes with sisters than offspring → more cooperation.