AP Biology Unit 7: Natural Selection

Mutations

Generate genetic variations in a population.

Genetic Variations

They lead to different phenotypes in a population.

Evidence of Evolution

Can be found in living and extinct species. Some categories include: molecular evidence, morphology, fossils, vestigial structures, convergent evolution, biogeographical evidence, and observations of evolution in current species.

Molecular Evidence

Comparing DNA sequences and amino acid sequences in proteins from different organisms provides evidence of evolution. When comparing the DNA sequence of a gene that is shared by different organisms, the more recently the organisms share a common ancestor, the more similar their DNA sequences will be. This is considered very strong evidence since environmental factors do not usually change an organism’s DNA sequence.

Morphology

Homologous structures, which have common ancestry but different functions, also provide evidence of evolution.

Fossils

The existence of these from organisms that no longer live on Earth also provide evidence of evolution. Those that are transitional show intermediate states between ancestral and modern species. They can be dated by studying the age of the rock layers in which they are found or by using radioactive isotopes to date them.

Vestigial Structures

Some organisms contain anatomical features that no longer seem to have a purpose in the modern organism but may have had a function in an ancestral organism.

Convergent Evolution

Species that live in similar environments may evolve similar adaptations even though they may not have a recent common ancestor.

Biogeographical Evidence

Biogeography is the study of the distribution of species. Species on islands off the coast of South America are more similar to species found in South America than to species found in North America.

Observations of Evolution in Current Species

When repeatedly exposed to antibiotics, bacteria populations evolve resistance over time. Mosquito populations have evolved resistance to pesticides like DDT.

Charles Darwin

Proposed that individuals do not evolve, populations do in his theory of natural selection.

Darwin’s Theory of Natural Selection

Variations in populations lead to different phenotypes in members of a population.

Competition for limited resources, or predation, leads to some members of a population surviving while other members do not.

The environment determines which phenotypes are favorable. If the environment changes, different phenotypes may confer an advantage, and the changing environment can change the direction of evolution.

Individuals with phenotypes that give them a survival advantage are more likely to survive and reproduce (differential reproductive success)

Over time, favorable phenotypes will become more prevalent in a population as members of the population without those favorable phenotypes do not survive.

Directional Selection

Occurs when one end of the range of phenotypes is favored by natural selection, causing the frequency of that phenotype to increase over time.

Stabilizing Selection

Natural selection can lead to this, where the intermediate phenotype is favored and extreme phenotypes are selected against.

Disruptive Selection

Sometimes, natural selection can lead to this, where individuals on both extremes of the phenotypic range are more likely to survive and reproduce than individuals with an intermediate phenotype.

Artificial Selection

Individuals in a population can also experience differential reproductive success through this. In this, humans selectively breed domesticated plants and animals to produce populations with desired traits. Instead of the environment selecting for individuals with favorable phenotypes, humans select which individuals in a population survive and reproduce.

Sexual Selection

Occurs when individuals with certain characteristics are more likely to attract mates than other individuals. Over time, individuals with traits that are more likely to attract mates become more prevalent in the population.

Intersexual Selection

In this, individuals of one sex are particular in selecting mates from the other sex. Mate choice may be based on perceived fitness of the members of the other sex, with members who seem stronger or healthier being more likely to produce offspring that will survive.

Intrasexual Selection

In this, members of one sex compete for mates of the other sex. This may involve asserting dominance to ward off competitors and gain better access to mates.

Allele Frequency

The number of individual alleles of a certain type, divided by the total number of alleles of all types in a population. For a population to have a stable one of these, it must meet five conditions: large population size, random mating, no gene flow, no selection, and no mutations.

Population Genetics

The study of genetic variation within populations and the processes that can cause changes in allele frequencies within a population. The three major processes that drive changes in allele frequencies in a population are natural selection, gene flow, and genetic drift.

Gene Flow

The transfer of alleles from one population to another. It can be caused by the migration of individuals into a population. If these individuals carry different alleles than the receiving population, the allele frequency in the receiving population will change. In plants, this can occur through the transfer of pollen (by wind or animals) into new plant populations.

Genetic Drift

The random loss of alleles in a population. It is more likely to occur in smaller populations.

Genetic Drift Example

Assume an allele is found in 10% of a population. If there are 1000 individuals in the population, 100 have the allele; it would be likely that one of those 100 would survive and reproduce to pass on that allele. However, if a population had 10 individuals, only one would have that allele. If that individual failed to reproduce, that allele would be lost, decreasing the genetic diversity of the population.

Bottleneck Effect

It is one possible cause of genetic drift. A population bottleneck occurs when the size of a population is greatly reduced for one or more generations. Natural disasters like fires, floods, and volcanic eruptions and human-made events like over hunting or rapid habitat destruction can cause population bottlenecks. Because the population size is smaller after these bottlenecks, the surviving population is much less likely to posses all of the alleles the larger population had before the bottleneck and thus will likely have less genetic diversity.

Founder Effect

This is another cause of genetic drift. It occurs when a few members of a larger population start a new population. These few members of the larger population often have less genetic diversity compared to the larger population or may be a nonrandom sample of the larger population.

Large Population Size

Reduces the chances of genetic drift occurring. In small populations, a change in allele frequencies (caused by genetic drift) is much more likely.

Random Mating

Eliminates the possibility of changing allele frequencies caused by sexual selection.

No Selection

All phenotypes in the population need to have equal reproductive success in order to keep allele frequencies stable. If one phenotype has a survival advantage, the alleles in that phenotype will become more prevalent in the population.

No Mutations

Mutations are very rare and random occurrences. A mutation would change the allele frequencies in the population.

Hardy-Weinberg Equilibrium

Populations that meet all five conditions of having stable allele frequencies are said to be in this, making the following equations true:

p + q = 1 (Describes allele frequencies)

p^2 + 2pq + q^2 = 1 (Describes genotype frequencies)

p

Represents the frequency of the dominant allele (A).

q

Represents the frequency of the recessive allele (a).

p^2

Represents the frequency of the homozygous dominant genotype (AA).

2pq

Represents the frequency of the heterozygous genotype (Aa).

q^2

Represents the frequency of the homozygous recessive genotype (aa).

Phylogeny

The history of the evolution of a species or group. It shows lines of ancestry, common descent, and relationships among groups of organisms.

Phylogenetic Trees

A hypothesis about the history of evolution over time, with these indicating the approximate time of evolutionary events. They are created using morphological evidence from fossils showing traits that are gained, or lost, and time estimates from molecular clocks. They can show speciation events and extinction events.

Cladograms

A hypothesis about the history of evolution over time. They are created using morphological evidence from fossils and time estimates from molecular clocks. They can show speciation events and extinction events.

Molecular Clocks

Changes in DNA and protein sequences over time. Generally, evidence from these is considered more accurate than morphological characteristics because molecular data is less influenced by convergent evolution or external geological events.

Shared Characteristics

Traits that are present in more than one lineage.

Shared Derived Characteristics

Are found in a group of related organisms called a clade and set the clade apart from other organisms. They indicate homology among organisms in a clade and are evidence of their common ancestry.

Nodes

On phylogenetic trees, they represent common ancestors. The more recent the common ancestor of two species, the closer their degree of relatedness.

Outgroup

In a phylogenetic tree, this is the least closely related member of the tree.

Root

This part of a phylogenetic tree represents the common ancestor of all members of the tree.

Last Universal Common Ancestor (LUCA)

Organisms are linked by lines of descent, with a proposed common ancestor for all forms of life on Earth estimated to have existed about 3.5 billion years ago.

1st Major Theory about how Life on Earth Originated

Inorganic materials that were present in Earth’s early atmosphere combined to make the building blocks of biological molecules. This theory is supported by evidence from the Miller-Urey experiment, in which a model of Earth’s early atmosphere was constructed in a lab, and after a few weeks, amino acids and other components of biological molecules were found.

2nd Major Theory about how Life on Earth Originated

Meteorites may have transported organic molecules (that are needed for life) to Earth. It is thought that early Earth was bombarded with meteorites. Evidence for this theory includes the Murchison meteorite (found in Australia in 1969), which contained sugars and over 70 different amino acids.

Species

A group of organisms that are capable of interbreeding and producing viable and fertile offspring.

Speciation

The evolution of new species. It occurs when two populations are reproductively isolated from each other, preventing interbreeding, and as the environments where these isolated groups change, the evolution of new species may occur. Rates of it may vary.

Adaptive Radiation

Speciation can lead to this, which is the evolution of organisms into separate species that occupy different ecological niches.

Gradualism

If an environment is relatively stable, there will be less selective pressure on populations, and the rate of speciation will be slower. The slow and constant pace of speciation is called this.

Punctuated Equilibrium

If an environment rapidly changes, as it would after an asteroid strike, a volcanic eruption, or a rapid change in climate, rapid evolution may occur. A long period of stability in a species interrupted by periods of rapid evolution is called this.

Allopatric Speciation

In this, a larger population becomes geographically separated and the smaller subgroups diverge and become separate species over time.

Sympatric Speciation

Occurs in the same geographic area, but other factors lead to reproductive barriers between members of the groups.

Polyploidy

A mechanism of sympatric speciation, it is the replication of extra sets of chromosomes, which is a frequent method of sympatric speciation in plants. Plants that develop extra sets of chromosomes usually cannot interbreed with plants that maintained the original number of chromosomes and will thus become a separate species over time.

Sexual Selection

This in animals can lead to sympatric speciation.

Prezygotic Barriers

Prevent the formation of a zygote, or fertilized egg. They include: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.

Habitat Isolation

If organisms live in different habitats and do not come in contact with one another, they cannot mate and form zygotes and are thus reproductively isolated.

Temporal Isolation

Organisms can live in the same habitat, but if they are active at different times of the day or have breeding seasons during different times of the year, they will not interbreed.

Behavioral Isolation

Some species will interbreed only with others who perform compatible mating behaviors, such as mating calls or dances.

Mechanical Isolation

If the sexual organs of the organisms are incompatible and prevent the transfer of gametes, the species will remain reproductively isolated.

Gametic Isolation

Even if two organisms are able to successfully copulate, if their gametes are incompatible, no zygote will be produced and the organisms will be reproductively isolated.

Postzygotic Barriers

Occur after the zygote is formed, and they prevent the zygote from developing into a viable and fertile adult organism. They include: reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

Reduced Hybrid Viability

Two organisms that can form a zygote may still be reproductively isolated if that zygote does not survive to adulthood.

Reduced Hybrid Fertility

Even if the zygote survives until adulthood, if the adult hybrid is infertile, the two species that created the hybrid will remain reproductively isolated.

Hybrid Breakdown

In some plants, hybrids are viable and fertile, but with each subsequent generation, the hybrid becomes weaker and less robust and will cease to exist after a few generations, keeping some plant species reproductively isolated.

Extinction

The death of all members of a species, it has occurred throughout Earth’s history, as shown in evidence from the fossil record. The level of genetic variation in a population can affect the population’s ability to survive environmental changes. More genetically diverse populations have a greater ability to adapt to changing environments because they are more likely to contain some individuals who can withstand the changing environmental pressures.

Species Diversity

Depends on a balance between rates of speciation and extinction. If speciation rates are greater, this will increase. If extinction rates are greater, this will decrease.