Types of Selection

Evidence of Evolution

• populations evolves when some members of the population have greater fitness and reproductive success than others

• evidence can be found in extant (living) and extinct (non-living) species:

  • molecular evidence: comparison of dna and amino acid sequences (in proteins) from different organisms provides evidence of evolution

    • when comparing dna sequence of a gene that is shared by different organisms, the more similar the dna sequence is, the more closely related they are/ the more recently they share a common ancestor

    • considered very strong evidence because environmental factors usually do not change an organism’s dna sequence

  • morphology: homologous structures, which have common ancestry but different functions, provide evidence of evolution

    • structures between different organisms indicated common ancestry and the evidence of evolution

  • fossils: transitional fossils show intermediate states between ancestral and modern species

    • fossils can be dated by studying the age of rock layers in which they are found or by using radioactive isotopes

  • vestigial structures: some organisms have anatomical features that do not serve them any purpose modernly, but may have served the ancestral organism

    • humans have a tailbone, which is useless now but may have helped the ancestor of humans climb trees

  • convergent evolution: species that line in similar environments may evolve similar adaptations even though they don’t have a recent (or any) common ancestor

    • sharks and dolphins are similar in physical appearance, but have no recent common ancestor

  • biogeographical evidence: study of distribution of species contributes evidence to evolution

    • species found on islands off the coast of south america are more similar to species found in south america than to north america

  • observation of evolution in current species:

    • when repeatedly exposed to antibiotics, bacteria develop a resistance to it by evolving

    • tumor cells developing resistance to chemotherapy drugs

    • pesticide resistance in insects

    • evolution of previously unseen viruses and pathogens

    • genomic changes in an organism over time

Natural Selection

• individuals do not evolve; populations can evolve

• differential reproductive success: leads to changes in populations based on favored and unfavored phenotypes

• directional, stabilizing, and disruptive, selection all rely on mechanisms of natural selection

• natural selection: mechanism for evolution proposed by charles darwin; core theories of natural selection include:

  • variations in populations lead to different phenotypes in members of the population

  • competition for resources or predation lead to some members of the population surviving while others do not

  • the environment determines which phenotypes are favorable

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

  • favorable phenotypes will become more prevalent over time as members of the population without those favorable phenotypes do not survive

Examples of Evolution by Natural Selection

• antibiotic resistance: some bacteria in a population will have genotypes that cause them to be more or less sensitive to the antibiotics; those with less sensitivity to antibiotics have more fitness and reproductive success and therefore will survive and pass on the resistance to the next generation of bacteria

  • individual bacteria do not “learn” resistance; as a population, the more favorable trait (resistance) becomes more prevalent as the more sensitive bacteria dies off

• peppered moth in england: peppered moths had either dark wings or light wings; depending on their environment, wing color would make them harder or easier to see, making them less or more easy to be preyed upon; darker moths had the advantage to blend in with darker trees, so lighter moths died off and the darker phenotype became more prevalent in subsequent generations

• when the environment changes, different phenotypes may have the advantage and the changing environment can change the direction of the evolution of the species

Directional Selection

• 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

• example: peppered moth example from above

Stabilizing Selection

• stabilizing selection: where the intermediate phenotype is favored and the extreme phenotypes are selected against

• example: clutch size (number of eggs laid by birds during each reproductive cycle)

  • too many eggs: there are too many offspring to support and this leads to low reproductive success in the just born generation

  • too few eggs: the risk that none of the offspring survives increases

Disruptive Selection

• disruptive selection: where individuals on both extremes of the phenotypic range are favored more than the intermediate phenotype

• example: mice fur color in environment with both very light and very dark elements

  • light fur mice: can blend in with light elements

  • dark fur mice: can blend in with dark elements

  • medium fur mice: cannot blend in with either; become preyed upon more often

Artificial Selection

• artificial selection: requires human interference; when humans selectively breed certain plants or animals to make desirable traits more prevalent in a given population

• instead of the environment picking, humans select individuals with more favorable phenotypes and promote their rate of survival and reproduction

• example: wild cabbage breeding

  • farmers bred wild cabbage plants to achieve the perfect cabbage plant with desirable traits such as large in size, resistant to pesticide, etc.

• example: crossbreeding domesticated wolves

  • cross-breeding domesticated wolves has led to the majority of the domestic dog breeds we have today

Sexual Selection

• sexual selection: occurs when individuals of a given population have traits that are more likely to attract mates than others; over time individuals (and their characteristics) with the more attractive traits become more prevalent in the population

• mate choice: in nature, it can be determined by perceived fitness of an individual, coat color, mating call, etc.

• example: peacocks

  • peacocks feathers attract more mates if they are larger and more colorful, which increases the chance of larger and more colorful offspring being produced in a given population

• in intrasexual selection: members of one sex compete for mates from the other sex; this may involve asserting dominance to ward off competitors

Population Genetics

Population Genetics and Genetic Drift

• population genetics: the study of genetic variation within a given population and the processes that cause changes to the allele frequencies within the population

• three major processes that drive changes in allele frequencies:

  • natural selection: organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success

  • gene flow: the transfer of alleles from one population to another; can be caused by the migration individuals into a population

    • if the individuals carry different alleles than the receiving population, the allele frequency of the receiving population will change

    • in plants: gene flow can occur by transfer of pollen (by wind or animals) into new plant populations

  • genetic drift: the random loss of alleles in a population; more likely to occur in smaller populations; results in loss of genetic diversity

    • example: an allele is found in 10% of a population; in a population with 1000 people, it is more likely that at least 1 of the 100 individuals making up that 10% can pass the trait on; in a population with only 10 individuals, it is less likely that the 1 individual making up that 10% can pass the trait on

Bottleneck Effect

• bottleneck effect: possible cause of genetic drift; occurs when the size of a population is greatly reduced for one or more generations; can be caused by:

  • natural disasters: flooding, forest fires, volcanic eruptions, etc.

  • man-made events: overhunting, rapid habitat destruction, etc.

• population is smaller after bottleneck effect, leading to smaller chance of having genetic diversity in the population due to loss of alleles

Founder Effect

• founder effect: another cause of genetic drift; occurs when a few members of a large population start a new population (usually somewhere else as the populations wee separated)

• the few members of the larger population have less genetic diversity just amongst themselves or may be a non-random sample from the larger population

Hardy-Weinberg Equilibrium

• in populations with stable allele frequencies (are not evolving), they may live in an unchanging environment without the selective pressures that lead to evolution

• hardy and weinberg developed equations that describe the stable populations if they fit all of the given conditions:

  • large population size: reduces the chances of genetic drift occurring

  • random mating: eliminates the possibility of changing allele frequencies caused by sexual selection

  • no gene flow: for allele frequencies to be stable, the individuals must not be entering or leaving the population; introduction of new individuals or movement of individuals out the population could change allele frequencies

  • no selection: all phenotypes in the population must have the equal reproductive success to keep the allele frequencies stable (an advantageous phenotype would become more prevalent in the population)

  • no mutations: would change the allele frequency of the population, as mutations are rare and random occurrences

• if the conditions are met, then the follow equations can be used:

  • p + q = 1

    • used to describe allele frequencies

    • p: frequency of dominant allele (A)

    • q: frequency of recessive allele (a)

  • p^2 + 2pq + q^2 = 1

    • used to describe genotype frequencies

    • p^2: frequency of homozygous dominant type (AA)

    • 2pq: frequency of heterozygous genotype (Aa)

    • q^2: frequency of homozygous recessive type (aa)

  • if asked for allele frequency: find p or q

  • if asked for genotypic frequency or number of individuals: find p^2, 2pq, or q^2

Phylogeny, Speciation, and Extinction

Phylogeny and Common Ancestry

• phylogeny: history of the evolution of a species or group; shows lines of ancestry, common descent, and relationships between groups of organisms

  • phylogenetic trees and cladograms: visual representations of hypotheses about the history of evolutionary events; used to create morphological evidence from fossils and time estimates from molecular clocks

    • molecular clocks: changes in dna and protein sequences over time; information from them is generally considered more accurate than morphological characteristics because molecular data is less influenced by convergent evolution or external geographical events

• shared derived characteristics: found in a group of organisms (called a clade) that set them apart from other groups of organisms

  • they indicate homology between organisms in a clade and are evidence of common ancestry

• nodes: part of phylogenetic tree that represents common ancestry; the more recent the common ancestor, the more related any given two organisms are

  • outgroup: the least-related member of the tree

  • root: the common ancestor of all the tree members

• speciation: formation of new species

• extinction events: death of all members of a given species

• LUCA: last universal common ancestor; estimated to be from approx. 3.5 billion years ago

• theories of how life originated on earth:

  • inorganic materials that were present in earth’s early day atmosphere combined to make building blocks of macromolecules

    • supported by evidence from miller-urey experiment ( in which a model of earth’s early atmosphere was constructed in a lab; after a few weeks amino acids and other biological macromolecules were found)

  • meteorites may have transported organic molecules (that are needed for life) to earth

    • supported by evidence from murchison meteorite (found in australia 1969) which contained sugars and over 70 different amino acids

• evidence for common ancestry in all eukaryotes on earth:

  • membrane-bound organelles in all eukaryotes

  • linear chromosomes in all eukaryotes

  • all eukaryotes contain genes with introns

  • none of these characteristics are in prokaryotes, indicating common ancestry only between eukaryotes

Speciation

• species: group of organisms that are capable of interbreeding and producing viable and fertile offspring

• speciation: evolution of new species; occurs when two populations are reproductively isolated from each other; rates of speciation can vary

  • reproductive isolation prevents interbreeding, which results in evolution of new species (along with causes by environmental factors)

• adaptive radiation: sometimes caused by speciation; the evolution of organisms into separate species due to the occupation of different ecological niches

• gradualism: the low and constant pace of speciation; occurs when an environment is more stable with less selective pressures on the population

• punctuated equilibrium: long periods of stability in a species interrupted by periods of rapid evolution; occurs when rapid changes to the environment occur and lead to speciation

  • usually caused by natural disasters (ex. volcanic eruption, rapid climate change, asteroid, etc.)

• speciation can be allopatric or sympatric:

  • allopatric: a larger population becomes geographically separated; smaller subgroups diverge and become separate species over time

  • sympatric: occurs in the same geographic area, but other factors lead to reproductive barriers between members of the group

    • polyploidy: mechanism of sympatric speciation; replication of extra set of chromosomes (most often occurs in plants)

      • polyploidy plants can’t breed with regular plants, so they become a separate species over time

    • sexual selection in animals can lead to sympatric speciation

• reproductive barriers: lead to speciation by preventing interbreeding somehow; can be pre-zygotic or post-zygotic

• pre-zygotic barriers: pre-zygotic barriers: prevent formation of zygote (fertilized egg)

  • habitat isolation: organisms live in different habitats and do not come in contact with each other for interbreeding to occur

  • temporal isolation: organisms live in the same habitat, but have different breeding seasons that do not allow interbreeding to occur

  • behavioral isolation: some organisms only breed with species that exhibit compatible mating behaviors (ex. mating calls, dances, etc.); prevents interbreeding between less compatible individuals

  • mechanical isolation: the reproductive organs of organisms are located in incompatible spots on their bodies, preventing the mating process from occurring correctly

  • gametic isolation: despite organisms being able to mate, the gametes are still incompatible so no zygote is produced

• post-zygotic barriers: occur after zygote is formed; prevent the zygote from developing into viable and fertile adult organism

  • reduced hybrid viability: two organisms can produce a zygote, but it doesn’t live into adulthood, keeping them reproductively isolated

  • reduced hybrid fertility: two organisms can produce a zygote, but it is sterile and cannot continue another generation of the new species

  • hybrid breakdown: the a viable and fertile zygote can be formed in the first generation, but the subsequent generations produce weaker and less fit organisms that lead to the extinction of the species after a few generations

Extinction

• extinction: the death of all members of a species

• the level of genetic variation in a population can affect the population’s ability to survive in changing environmental conditions

  • more genetically diverse populations are more likely to withstand the environmental changes

• ecological stresses (created by human activity) increase extinction rates

  • examples: habitat destruction, overhunting, etc.

• species diversity depends on a balance between rates of speciation and extinction

  • if more species are going extinct than being formed, species diversity will decrease

• extinction can have negative consequences, but it can also clear available habitats and niches for other species