Forces of Evolution

Evolutionary forces

living single celled organism was present on earth, that had the potential to reproduce and make copies itself.

We can create phylogenies (family trees) to help determine relationship between organisms today.

modern synthesis

integrates darwin, mendel, and subsequent research into a unified theory of evolution.

darwin contributed -> survival of fittest, more produced than survives, variation can be inherited, helping variation trait gets passed down.

mendel contirbuted -> genetics and inheritance, law of segregation, random assortment, polymorphisms (multiple form of a trait)

what is a population?

defined as a group of individuals of the same species of are geographically near enough enough to one another that they can breed and produce new generations.

They are recognized by their genes, and are seen as gene pools

what is evolution?

a change in the allele frequencies in a population over time.

It requires -> 1. population of breeding individuals

2. genetic change occurring.

allele frequency -> ratio / percentage of one allele compared to others for gene within study population.

genotype frequency -> ratio / percentage of homozygous and heterozygous

forces of evolution

mutation, genetic drift, gene flow, natural selection

mutation

original source of genetic variation. a change into genetic code that new alleles were introduced into the population.

Occurs during cell division in which an error occurs in the genetic code. Mutations create distinct chromosomes, with that, cells become more complex.

Beneficial mutations -> changes in DNA providing an advantage to a population at a particular moment in time.

Mutations can occur in skin skills due to chemical changes (unless u have Xeroderma pigmentosum, in which the skin repair doesnt work), but does have repair mechanisms to detect and correct mutations(UV radiation, adjacent thymine bases bind to one another). only mutations found in gametes get passed down

spontaneous mutation

new mutation that pops up at random

inherited mutation

spontaneous passed down

point mutation

single nucleotide change in code, substitution for one base to another.

No change in protein (synonymous mutation).

change in protein (non synonymous mutation)

Insertions / Deletions

insertions -> addition of one or more nucleotides into DNA sequence.

Deletion -> removal of one or more nucleotides from DNA sequence.

Both typically occur during an error thru the replication process, entire codon sets can be removed or added.

Leads to frameshift mutations involves any # of nucleotides not a multiple of three.

Crossover events

dna swapped between homologous chromosomes while paired up in meiosis one. makes it possible for new combinations to exists on a chromosome (but does not introduce new alleles to population), allows for new combination of alleles to be found between siblings

non disjunction events

homologous chromosomes or sister chromatids fail to separate after pairing. can be fatal if at gamete level.

exception -> trisomy 21 (inherits three copies of chromosome 21). Others where children can be born but usually do not live after one year

chromosomal alternations

sex chromosomes trisomies & monosomies are known to occur (XYY, XXX, etc).

chromosomal translocation -> transfer of dna between non homologous chromosomes, could be large portions could be swapped.

Balanced translocations -> genes swapped but no genetic material are lost.

Unbalanced translocations -> unequal exchange of genetic material, duplicated or lost.

derivative chromosomes -> new chromosomal structures created from two different chromosomes. all are linked to cancers and infertility

genetic drift

drifting alleles, random changes in alleles frequencies from one gen. to the next.

random nature is crucial (happens when non of the variant alleles are advantageous).

it can increase or remove variation. so long as no advantage for an allele, they will remain random.

two types - bottleneck and founder effect

bottleneck effect

occurs when the number of individuals in a population drops dramatically due to some random event. The most obvious, familiar examples are natural disasters. the surviving population will be different compared to pre disaster population

founder effect

occur when members of a population leave the main or “parent” group and form a new population that no longer interbreeds with the other members of the original group. the newly founded population often has allele frequencies that are different from the original group. Alleles that may have been relatively rare in the parent population can end up being very common due to the founder effect. Likewise, recessive traits that were seldom seen in the parent population may be seen frequently in the descendants of the offshoot population.

gene flow

refers to the movement of alleles from one population to another. In most cases, gene flow can be considered synonymous with migration.

In humans, gene flow is described as admixture (becomes complicated for ancestry). In non humans its called hybridization (killer bees)

natural selection

occurs when certain phenotypes confer an advantage or disadvantage in survival and/or reproductive success.

The alleles associated with those phenotypes will change in frequency over time due to this selective pressure.

It’s also important to note that the advantageous allele may change over time (with environmental changes) and that an allele that had previously been benign may become advantageous or detrimental.  

natural selection acts on phenotypes rather than the alleles themselves, deleterious (disadvantageous) alleles can be retained by heterozygotes without any negative effects. ex - peppered moths

directional selection

the environmental pressures favor one phenotype over the other and cause the frequencies of the associated advantageous alleles (ruffled membranes, dark pigment) to gradually increase. In the case of the peppered moths, the direction shifted three times

balancing / stabilization selection

occurs when selection works against the extremes of a trait and favors the intermediate phenotype.

For example, humans maintain an average birth weight that balances the need for babies to be small enough not to cause complications during pregnancy and childbirth but big enough to maintain a safe body temperature after they are born.

and sickle cell anemia -> variations of hemoglobin gene = simple amino acid substitutions that is caused by a point mutation. heterozygous carriers are normal, recessive will die before reproduction

disruptive / diversifying selection

the opposite of balancing selection, occurs when both extremes of a trait are advantageous. Since individuals with traits in the mid-range are selected against, disruptive selection can eventually lead to the population evolving into two separate species. ex - finches in galapagos (large beaks to eat large seeds, small beaks for small seeds, no medium sized beaks bc no medium sized seeds)

Sexual Selection

sometimes considered 5th evolutionary force. an aspect of natural selection in which the selective pressure specifically affects reproductive success (the ability to successfully breed and raise offspring) rather than survival.

favours traits that will attract a mate. Sometimes these sexually appealing traits even carry greater risks in terms of survival.

Hardy Weinberg Equilibrium

a mathematical formula that allows estimation of the number and distribution of dominant and recessive alleles in a population.

This aids in determining whether allele frequencies are changing and, if so, how quickly over time, and in favor of which allele?

It’s important to note that the Hardy-Weinberg formula only gives us an estimate based on the data for a snapshot in time. (p2 +2pq +q2) = 1

Interpreting evolutionary change

Once we have detected change occurring in a population, we need to consider which evolutionary processes might be the cause of the change. It is important to watch for nonrandom mating patterns, to see if they can be included or excluded as possible sources of variation in allele frequencies.

1. non random mating 2. positive assortative mating 3. negative assortative mating

non random mating

(also known as assortative mating) occurs when mate choice within a population follows a nonrandom pattern.

positive assortative mating

patterns result from a tendency for individuals to mate with others who share similar phenotypes. This often happens based on body size.

negative assortative mating

patterns occur when individuals tend to select mates with qualities different from their own.

This is what is at work when humans choose partners whose pheromones indicate that they have different and complementary immune alleles, providing potential offspring with a better chance at a stronger immune system.

Species

organisms whose individuals are capable of breeding because they are biologically and behaviorally compatible to produce viable, fertile offspring.

viable offspring - offspring that are healthy enough to survive to adulthood.

fertile offspring - able to reproduce successfully, resulting in offspring of their own.

Both conditions must be met for individuals to be considered part of the same species. we must examine how much phenotypic variation is typically found within a comparable modern-day species; we can then determine whether the fossilized remains fall within the expected range of variation for a single species.

microevolution

refers to changes in allele frequencies within breeding populations—that is, within single species.

macroevolution

describes how the similarities and differences between species, as well as the phylogenetic relationships with other taxa, lead to changes that result in the emergence of new species.. Consider our example of the peppered moth that illustrated microevolution over time, via directional selection favouring the peppered allele when the trees were clean and the dark pigment allele when the trees were sooty.

speciation

When a single population divides into two or more separate species. two types - allopatric and sympatric speciation

allopatric speciation

caused by long term isolation. if subgroups of the population Something occurs in the environment—perhaps a river changes its course and splits the group, preventing them from breeding with members on the opposite riverbank. Over many generations, new mutations and adaptations to the different environments on each side of the river may drive the two subpopulations to change so much that they can no longer produce fertile, viable offspring, even if the barrier is someday removed.

sympatric speciation

occurs when the population splits into two or more separate species while remaining located together without a physical barrier.

This typically results from a new mutation that pops up among some members of the population that prevents them from successfully reproducing with anyone who does not carry the same mutation. This is seen particularly often in plants, as they have a higher frequency of chromosomal duplications.

human genome project

launched in 1990. Aim was to identify the entire genetic makeup of our species. the goal was to understand the genetic underpinning that makes humans uniquely humans.

it was unable to account for many factors that contribute to what it is to be human. Genetic determinism neglects other essential dimensions contributing to the development and evolution of human bodies (or the role culture plays)

richard Dawkins

published the selfish gene in 1976, popularized that the gene was the principal unit of selection in evolution, genes that contribute to greater reproductive success.

His complex of evolution at the molecular level undercuts complex. by 1980, biologists know that while genes construct bodies, genes, and bodies evolve at different rates with distinct patterns, evolutionary synthesis reduced organisms to their genotypes and gene pools. eventually proved, animal are reactive and adaptive beings, not passive and just genotypes

Stephen jay Gould

leading evolutionary biologist in late 20th century. created movement to identify and higher order of processes and features of evolution. not explained by population genetics.

Looked beyond reductive assumptions.

proposed there were 2 extinction events - 1. background extinction (balance of nature, species can be well adapted to niche but another species can come along and be better adapted and extinct the first).

2. Mass Extinction - disruption of nature (many species die off at the same time, result of ecological disaster, survival of luckiest,)

species selection

suggested by Gould. selection acting on entire species and not necessarily the individual.

rates of speciation & extinction, relationship between environmental context and variability within species and results in resistance to extinction.

Consistent bias of speciation rates could produce macroevlotionary diversity that cannot be understood by just the genes but have to understand it by the ecological perspective

punctuated equilibrium

emerged as a critique to phyletic gradualism (idea that species are unstable and constantly changing throughout time).

Species are stabile throughout time and only adapts during periods of instability and change. long periods of unchanged may see brief periods in which a species changes, then stabilize again as a new species

hominin brain size

historically, brain size was linked to body size, that mostly remained stable.

hypothesis for change of brain size was unpredictable climate, environment, social development, evolution of language. Fossil records suggests changes resulted from bio-cultural processes

evolutionary development biology

emerged as a theory for evolution of form. Intersection of evolutionary and developmental biology.

Focus is on how changes in form and shape arise. Embryo matures by stimulation of certain cells start to divide.

ex - hox gene - group of related genes that control the body plan of an embryo along the head tail axis (highly conservative, repeatedly turn on/off of basic genes guiding animal development, changes to them could be catastrophic).

allometry

differential growth of body parts. some genetic changes can affect the rate at which neighbouring groups of cell grow and divide.

can produce physical bumps or dents in a developing body.

when animal gets larger, it has to change shape to live at its size. ex - elephant -> needs longer and bigger bones, bone must be thicker as well to withstand increased mass