AP Biology Unit 7 Natural Selection

Evolution

Change in the genetic makeup of a population over time

Descent with Modification

Proposing that species change over generations by passing modified, inheritable traits from parents to offspring

Natural Selection

Individuals that have certain traits tend to survive + reproduce at higher rates than other individuals b/c of those traits

Fitness

The ability to survive and reproduce

Heritable Traits:

• Characteristics can be passed from parent to offspring

• Adaptations occur

Adaptations

Inherited characteristics of organisms that enhance their survival and reproduction

Artificial Selection

The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits

Natural Selection vs Artificial Selection

Natural Selection	Artificial Selection
Nature “selects” traits that are better suited for survival and reproduction	Humans select desirable traits Domestication of plants and animals

Relative Fitness

The reproductive success of a specific genotype or individual compared to the most successful genotype in a population

3 modes of Natural Selection

• Directional selection

• Stabilizing selection

• Disruptive selection

Directional Selection

Selection towards one extreme phenotype

Stabilizing Selection

Selection towards mean and against the extreme phenotypes

Disruptive Selection

Selection against the mean. Phenotypic extremes = highest relative fitness

S*xual Selection

A type of natural selection that explains why many species have unique traits

• Males often have useless structures simply because females choose that trait

• Can produce traits that = harm to survival (Ex: color feathers in peacocks make them easier to spot by predators)

Population

A group of individuals of the same species that live in the same area and interbreed to produce fertile offspring

Gene Pool

A population’s genetic makeup

• Consists of all copies of every type of allele

• If there is only one allele present for a particular locus in the population, it is fixed

• Many fixed alleles=less genetic diversity

• A population’s allele frequencies will change over time

Microevolution

Small-scale genetic changes in a population

Evolution driven by random occurrences like:

• Mutations

• Genetic Drift

• Migration/gene flow

• Natural selection

Mutation Results:

• Mutations result in genetic variation

• Can form new alleles (traits)

Genetic Drift

Chance events that cause a change in allele frequency from one generation to the next

Genetic Drift Characteristics:

• Significant to small populations

• Can lead to a loss of genetic variation

• Can cause harmful alleles to become fixed

Two types: Bottleneck effect & Founder effect

Bottleneck Effect

When a large population is drastically reduced by a non-selective disaster (famine, fires, natural disasters, hunting, etc.)

• Some alleles may become overrepresented, underrepresented, or absent

Founder Effect

When a few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population —> Loss of genetic diversity

Gene Flow

Transfer of alleles in or out of a population due to fertile individuals or gametes

• Alleles can be transferred between populations (Ex: pollen being blown to a new location)

Hardy-Weinberg

A model used to assess whether natural selection or other factors are causing evolution

Hardy-Weinberg Results:

If there are NO differences, then the population is NOT evolving, and if there ARE differences, then the population MAY BE evolving

5 conditions must be met to be in Hardy-Weinberg equilibrium:

• No mutations

• Random mating

• No natural selection

• Extremely large population size

• No gene flow

If any of these conditions are not met, then microevolution occurs

More genetic diversity equals what?

Better ability to respond to changes in environment

• More likely to be individuals who can withstand changes

• Species with low genetic diversity → risk of decline/extinction

• Ex: due to poaching and poisoning, a Condor was reduced to 27 individuals. It lowered the gene pool. Even though restoration efforts have increased, they have lost diversity.

Hardy-Weinberg Equations

q + p = 1 and q²+2qp+p²=1

• q = recessive allele

• p = dominant allele

• 2qp = heterozygous

• Squared = homozygous

Comparative Morphology

Analysis of the structures of living and extinct organisms

Homology

Characteristics in related species that have similarities, even if the functions differ

Embryonic Homology

Many species have similar embryonic development

Vestigial Structures

Characteristics that are similar in two species because they share a common ancestor

Convergent Evolution

Similar adaptations that have evolved in distantly related organisms due to similar environments

Analogous Structures

Structures that are similar but have separate evolutionary origins (Ex: wings in birds vs bats vs bees)

Biogeography

The distribution of animals and plants geographically

• Ex: Species on oceanic islands resemble mainland species

• Ex: Species on the same continent are similar + distinct from species on other continents

Systematics

Classification of organisms + determining their evolutionary relationships

Taxonomy

The naming and classifying species

Phylogenetics

Hypothesis of evolutionary history → Use phylogenetic trees to show evolution

Phylogenetic Trees

Diagrams that represent the evolutionary history of a group of organisms

• Similar to cladograms, but trees show amount of change over time measured by fossils

Cladograms

• A line = a Lineage (direct line of descent)

• A branch point = a node

• Node (Branch Point): represents common ancestors

• Clades: a group that includes a common ancestor. 

• Species in a clade have shared derived features

• Root: the common ancestor of all the species

• Sister Taxa: Two clades that emerge from the same node

• Basal Taxon: A lineage that evolved from the root and remains unbranched

Synapomorphy

A derived character shared by clade members

• Derived Characteristic: similarity inherited from the most recent common ancestor of an entire group

• Ancestral Characteristic: similarity that arose before the common ancestor

• Many cladograms and trees include an outgroup

• Outgroup: A lineage that is the least closely related to the rest of the organisms

Monophyletic Group

Includes the most recent common ancestor of the group and all of its descendants (clade)

Paraphyletic Group

Includes the most recent common ancestor of the group, but not all its descendants.

Polyphyletic Group

Does not include the most recent common ancestor of all members of the group

Species

A group able to interbreed and produce viable, fertile offspring

Speciation

Formation of new species → results in diversity of life forms; occurs due to Reproductive Isolation

Geography’s impact on speciation:

• Allopatric Speciation

• Sympatric Speciation

Allopatric Speciation

• A physical barrier divides the population, or a small population is separated from the main population

• Populations are geographically isolated. Prevents gene flow & is  often caused by natural disasters

Sympatric Speciation

• New species evolve while still inhabiting the same geographic region as the ancestral species

• Usually, due to the exploitation of a new niche

Reproductive Isolation

Occurs when populations cannot successfully reproduce with each other → prevents gene flow and can lead to speciation (formation of new species)

Two types of Reproductive Isolation

• Prezygotic Barriers

• Postzygotic Barriers

Prezygotic Barriers

Prevent mating or hinder fertilization

• Temporal Isolation: Species reproduce at different times

• Behavioral Isolation: Different courtship behaviors prevent mating

• Habitat Isolation: Species live in different habitats within the same area

• Gametic Isolation: Egg and sperm cannot fuse

Postzygotic Barriers

Prevent a hybrid zygote from developing into a viable, fertile adult

• Reduced Hybrid Viability: Genes of different parent species interact in ways that impair the hybrid’s development/survival

• Reduced Hybrid Fertility/Sterility: a hybrid can develop to a healthy adult, but it is sterile (no kids) → results due to differences in # of chromosomes between parents

• Hybrid Breakdown: The hybrid of the 1st generation may be fertile, but when they mate → offspring will be sterile (no reproduction)

Macroevolution

Large evolutionary patterns (Ex: adaptive radiation + mass extinction)

Stasis

No change over long periods of time

Punctuated Equilibrium

When evolution occurs rapidly after a long period of stasis

Gradualism

When evolution occurs slowly over hundreds, thousands, or millions of years

Divergent Evolution

Groups with the same common ancestor evolve and accumulate differences, resulting in the formation of a new species

Adaptive Radiation

If a new habitat or niche becomes available, species can diversify rapidly

Convergent Evolution

Two different species develop similar traits despite having different ancestors → analogous traits

Extinction

The termination of a species

• Extinctions have occurred throughout the Earth’s history (5 mass extinctions)

• Human activity affected extinction rates

• Anytime there is ecological stress, extinction rates can quicken

• Exit species → opens up a niche that a different species can exploit