evolution - HL BIO

natural selection

process through which organisms better adapted to their environment tend to survive and reproduce more successfully than others, leading to evolutionary changes over time.

clade

a group of organisms that includes an ancestor and all its descendants, representing a branch on the tree of life.

heritable traits

• characteristics that can be passed from parent to offspring

  • heritable traits are coded for in DNA

  • in sexually producing gametes, only the DNA of gametes is passed to offspring

• traits must be inheritable for natural selection to affect them

acquired characteristics

traits that develop in an organism's lifetime and are not inherited by offspring. This concept is contrary to the principle of natural selection.

• many of these changes acquired do not involve a change to the base sequence of DNA

• typically somatic cell mutations that do not affect DNA that is passed onto gametes

  • skin cancer due to UV exposure

natural selection acts on variation

• Whereas mutation as a source of variation occurs at random, the probability of a variation being passed on to the next generation is not random if it impacts the survival and reproductive capabilities of that organism. 

  • Variations that promote survival and reproduction will be selected for and will increase in frequency in a population

  • Variations that reduce survival and reproduction will be selected against and will decrease in frequency in a population

variation within a species

genetic variation within a species can result from:

• mutation: the original source of variation, with new alleles produced by changes in the DNA

• gene flow: the movement of genes between different group of organisms

• meiosis: crossing over and independent assortment during the formation of egg and sperm which leads to the creation of new combinations of alleles

• sexual reproduction: random fertilization between egg and sperm of different parents

genetic variation

the differences in DNA among individuals within a species, which can arise from mutations, gene flow, and sexual reproduction.

example of potential to overproduce offspring in population

• leatherback sea turtles lay clutches of 80-100 eggs with only a few hatchlings survivings

• female elephant have average of six offspring in lifetime but if they all survived there would a 20 million elephant population in 750 years (much greater than real numbers)

evolutionary benefits of overproduction of offspring

1. increases the chance of offspring surviving into adulthood, required for the continuity of the species

2. ensures genetic variation, should there be a need to adapt to new environmental conditions

carrying capacity

the population size that can be supported by available resources.

• at carrying capacity, # of individuals added to the population equals the amount removed, keeping the population size stable

direct competition

when one individual directly affects another’s ability to obtain resources

• elk will use aggression demonstrate dominance and to prevent other males from mating with female

indirect competition

when individuals compete for resources such as light, territory or prey

• an individual may share a depleted resource, such as grizzly bear catching salmon so that salmon can no longer be eaten by bears at different points in the river

selection pressure

variables within the environment that allow for a certain phenotype to have a better chance of surviving and reproducing

biotic selection pressure examples

• predation

• competition

• availability for food

• interest in males

• disease

abiotiic selection pressures examples

• temperature/light availability

• oxygen and CO₂ availability

• pollutants

• natural disasters

• wind

• water availability

density-indpendent meaning in context

abiotic selection pressures are often density independent, meaning they act the same no matter the population density

major mechanisms of sexual reproduction

1. one biological sex chooses which individual of the opposite sex to mate with

2. competition with members of the same sex for access to members of individuals of opposite sex

fitness

the measure of an organism’s ability to survive past reproductive age, find a mate and produce offspring

adaptations

• forms that correspond to function

• different survival and reproduction results in a population that is adapted to its environment because individuals have adaptations that allow them to survive and reproduce

artificial selection

occurs when humans deliberately breed crop plants and domesticated animals with particular phenotypic traits. this includes for

• meat

• milk

• eggs

• tasks

• production of wool

• companians

• increasing yield

• fiber production/cotton

climate change as a selection pressure

1. access to resources: affects organism’s ability to access water, nutrients, mates, shelter, sunlight

2. phenotypic mismatches: historical climate conditions is not equal to current environment and thus is not maximizing phenotypic traits

3. phenology disruption: synchrony is affected by climate changes causing phenological events in the environment to be disrupted

selective pressures for and against guppies

for: the orange spotting on tail makes the appearance of the male more flashy for females

against: easier to spot for predators

john endler’s experiment summarized

• demonstrates how natural selection and sexual selection act as opposing evolutionary pressures on male colouration

gene pool

collection of all the genes and various alleles of those genes within a population is a gene pool

speciation

the process by which new species arise from existing species through evolutionary mechanisms such as genetic drift and natural selection.

allele frequency

the prevalence of a specific allele out of all alleles for a gene in a gene pool

differences in allele frequencies in isolated populations

1. genetic drift - when a population is altered due to random events, such as natural disasters. the surviving indivs. are a sample of the original population and their allele frequencies may be very different than other populations

2. founder effect - type of genetic drift when a small group of migrants — not typically representative of population from which they came establish in a new area

3. natural selection - diff populations have different selection pressures acting on the alleles within the gene pool, maintaining alleles that provide an advantage and selecting out alleles that negatively impact survival

processes that affect allele frequency

• gene flow

• genetic drift

• mutation

• selection

• non-random mating

natural selection affects allele frequency

if better adapted to the environment, more offspring will be produced compare to less adapted individual

• the selected allele will increase in frequency in the gene pool over time

stabilizing selection

a type of natural selection that favors intermediate phenotypes over extremes, helping to maintain the status quo of a population's traits.

• extreme varieties selected against

• highest fitness are those in the average

disruptive selection

a type of natural selection that favors extreme phenotypes at both ends of the range, leading to two or more contrasting phenotypes in the population.

• acquires two peaks

directional selection

a type of natural selection that favors one extreme phenotype over others, resulting in a shift in the population's trait distribution.

Hardy-Weinberg allele frequency

p + q = 1

• p represents the frequency of the dominant allele, A

• q represents the frequency of the recessive allele, a

Hardy-Weinberg genotype frequency

p^2 + 2pq + q^2 = 1

• p² represents the homozygous dominant genotype

• 2pq represents the heterozygous genotype

• q² represents the homozygous recessive genotype

needed to maintain Hardy-Weinberg equilibrium

• population size is large enough so minimize impact due to chance

• no gene flow (emigration/immigration)

• random mating between organisms

• natural selection is not favouring one over the other

• no mutations, so no new alleles are introduced

Biological Species Concept

predominant way to define many species

• species as a group of actually or potentially interbreeding populations which are reproductively isolated from such other groups

challenges with Biological Species concept

• naturally/artificially produced hybrid

• divergence during speciation

• variation between geographical ranges

• asexually reproducing organisms

• challenges of testing reproduction possibilities between groups

horizontal gene transfer

refers to the movement of DNA between individuals through mechanisms that are not normal movement of DNA from parent to offspring

speciation

the process by which a population of one species diverges to become two distinct species

• they can NO longer INTERBREED

speciation vs gradual evolution

speciation involves the formation of new species, while gradual evolution refers to the slow and continuous change in a species over time.

processes required for speciation to occur

1. reproductive isolation - populations must stop interbreeding with each other. if there is no gene flow and two populations are unable to interbreed, each population will have its own gene pool

2. differential selection - populations may experience different selection pressures

   1. more/less competition for resources

   2. different climate

   3. different predators

   4. different resources

allopatric speciation

process of speciation that occurs when populations are geographically separated, preventing gene flow. Over time, the isolated populations undergo evolutionary changes due to natural selection and genetic drift.

sympiatric speciation

process of speciation that occurs when populations are not geographically isolated, but instead become reproductively isolated within the same location, often through mechanisms like polyploidy, sexual selection, or habitat differentiation.

interspecies hybrid

A hybrid resulting from the mating of individuals from different species, often leading to unique traits or challenges in reproduction.

postzygotic barrier

barriers that occurs after fertilization, preventing the hybrid from developing into a viable, fertile adult

polyploidy

more than two complete sets of chromosomes in each cell

• tend to be bigger in size than their typical diploid relatives

• nucleus has to expand to accommodate the extra genetic material and increases cell size in turn increasing entire organisms size

abrupt speciation

a rapid evolution process resulting in formation of new species as a result of hybridization and polyploidy

autopolyploidy vs allopolyploidy

Autopolyploidy involves an increase in chromosome number within a single species, while allopolyploidy is the result of hybridization between two different species followed by chromosome doubling.

• in both cases, the tetraploid zygote is capable of developing into a fertile organism while the triploid is more likely to be sterile

adaptive radiation cause/consequence

cause: an event opens (mass extinctions or isolated islands) new niches or frees up previously occupied environmental niches, making way for evolution of new species that adapt to new living spaces

consequence: many species rapidly evolve from a common ancestor to fill vacant ecological niches

clade

a group of organisms evolved from a common ancestor, including those who have become extinct

cladogram

a branching tree diagram that shows the volutionary relationships among organisms

• point where lines branch represent a hypothetical or known ancestor which can be inferred to exhibit the traits shared amongst the liens that branch from it

root

the common ancestor of all organisms in a clade, depicted at the base of a cladogram.

homologous structures

structures inherited from a common ancestor; the species share common structures but may have evolved in diverse ways to become adapted for different functions

• anatomical structures

• embryological patterns

• metabolic reactions

• molecular sequences

• vestigial structures

pentadactyl limb

limb found in tetrapods with same general bone structure:

1. bone proximal to body

2. two bones distal to body

3. group of little bones

4. digit bones

divergent evolution

when organisms from a common ancestor accumulate differences over time as a result of exposure to different selective pressures in the environment, and eventually forming new species

• the pentadactyl bone has adapted to serve specialized locomotor functions to the niche of the enimal

preserved sequence

sequence of DNA that has remained largely unchanged across different species

• example: Hox genes code for proteins called transcription factros that regulate gene expresssion and control cell

hypotheses for conserved sequence

1. natural selection has favoured the maintenance of the sequence

2. low mutation rates compared to other parts of the genome

analogous structures

Structures that have similar functions in different species but evolved independently, not from a common ancestor. A result of convergent evolution since different species adapt to similar environmental challenges.

• example: wings in bats for birds