APBIO 22-24

Microevolution is a change in allele frequencies in a population over generations 

Macroevolution is the broad pattern of evolution above species level 

Speciation must have a population that lives in the same habitat, breeds, fertile offspring and does not with other groups 

A population that interbreeds producing fertile offspring but doesn’t interbreed with other populations. 

Fertile offspring 

When different species mate to have offspring.

Prezygotic prevent fertilization and postzygotic that occur after zygote formation

PREZYGOTIC Barriers

Explanation

Example

Habitat isolation

When two species live in different locations

A whale that lives in the ocean and a brown bear in the forest

Temporal isolation

Species that breed at different times so their gametes cannot mix 

The western spotted skunk and eastern spotted skunk both mate during different seasons.

Behavioral isolation

Courtship rituals that attract mates and other behaviors unique to species enable male recognition 

Blue-footed boobies mate only after courtship display unique to their species 

Mechanical isolation

When morphological differences prevent mating

The shells of two species of snails work in different ways not allowing them to mate

Gametic isolation

The sperm of one species is unable to fertilize the egg of another species 

Sea urchins release sperm  and eggs that only fertilize each other 

POSTZYGOTIC Barriers

Reduced hybrid viability 

Genes of different parent species may interact in ways that impair the hybrid’s development/survival in the environment 

Different salamanders hybridize but most don’t complete the development 

Reduced hybrid fertility

When hybrids are sterile, if two chromosomes in parents differ in number/structure then meiosis in hybrids might fail to produce normal gametes.

Mule and a hinny 

Hybrid breakdown

When first-generation  hybrids are viable and fertile but next generation are feeble or sterile 

Certain rice strains of the first gen of hybrids are great but the next are feeble. 

Allopatric speciation→ a population forms new species while geographically isolated from the parent population, for example, populations of mosquitofish colonized lakes that later became isolated. The now different species have different characteristics and no gene flow occurs between the two 

Sympatric speciation → subset of the population forms new species without geographic separation, for example, occurs if gene flow is reduced by polyploidy, sexual selection, or habitat differentiation (also promotes the other one) 

Allopatric speciation 

polyploidy, sexual selection, habitat differentiation 

Organisms have more than two sets of chromosomes from the same species. This happens due to an error in cell division that doubles chromosomes and then makes 2n plant that might produce 4n offspring that then if mated with 3n will reduce fertility so they generate reproductive isolation 

Organisms contain two or more sets of chromosomes from different species 

When a species of plants self propagates itself asexually so in subsequent generations, various mechanisms can change a sterile hybrid into a fertile polyploid and fertile when mating with each other 

A region where members of different species meet and mate, producing some offspring of mixed ancestry. 

Niles Eldredge and stephen jay gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change, punctuated pattern shows speciation occurred rapidly 

Punctuated speciation explains it as happening very quickly into two very distinct species while gradual shows how over time species can grow into separate species. 

Descent with modification 

Change in the genetic composition of a population from generation to generation

• Aristotle viewed species as fixed(unchanged), and concluded that life forms arranged on a ladder or scale of increasing complexity

• Linnaeus developed a binomial, formal for naming species, and adopted a nested classification system, grouping similar species in general categories 

The older the stratum, the more dissimilar its fossils were to current life forms, from one layer to the next, some species appeared while others disappeared, each stratum represented a sudden catastrophic event so when such regions later repopulated, different species immigrated. 

They proposed that Earth’s geologic features could be explained by gradual mechanisms, like valleys being formed by rivers. Lyell proposed the same geologic processes are operating today at the same rate. 

It give us more of an idea of the timeframe of Earth so that Darwin could use it to come up with the idea of evolution and other theories. 

He found two principles, use and disuse and inheritance of acquired characteristics while comparing living species with fossil forms 

1. The idea that parts of the body that are used become larger and stronger while those that are not used deteriorate(ex. Giraffe stretched its neck) 

2. Organism could pass these modifications to its offspring(giraffe with long neck pass trait to offspring so they all had long necks) 

It contributes to evolutionary change. 

Adaptations are inherited characteristics of organisms that enhance survival and reproduction in specific environments. Ex. the finches in the Galàpagos Islands where each group had different beaks and behaviors special to specific foods available on home islands

A process in which individuals have certain inherited traits tend to survive and reproduce at higher rates than others, because of those traits. 

No, the species evolves over many generations.

Observation

Cite and Example

1. Variations in traits exist.

The Asian ladybird beetles vary in color and spot pattern

2. These variations are heritable.

Asian ladybirds can get the spot patterns from the previous generation.

3. Species overproduce.

A single puffball fungus can produce billions of offspring but if all of these survived, they would carpet surrounding land and surface

4. There is competition for resources so not all offspring survive (there is editing).

If two species live in the same habitat and have the same food source, the species that is faster, stronger, and able to get food easier will be more likely to have more offspring than the other 

Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than others

The unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations. 

The species with characteristics that better equip them to live in that environment will live and produce more offspring 

Individuals do not evolve. Species evolve. 

When a new food source was introduced to the soapberry bugs, the bugs with shorter beaks who fed on the golden rain tree were able to produce more offspring and became more populated while bugs that fed on the balloon vine, the ones with longer beaks were much more populated. 

Methicillin-resistant S. aureus 

When penicillin was introduced, it saved millions of lives. However, in 1945, these bacteria were already resistant to penicillin (had enzymes that destroyed it), researchers decided to develop new antibiotics but some S. aureus developed resistance to these new ones within a few years. Then doctors used the powerful antibiotic methicillin but within two years the S. aureus bacteria were resistant. Different S. aureus populations had variations in how strongly their members were affected by the drug. So the multidrug-resistant strains of today emerged over time as MRSA. 

No, because the bacteria that wasn’t resistant to the antibiotics died, however, some bacteria did survive so they had mutations that developed enzymes that were resistant to the antibiotics. Then, these bacteria had offspring that also had enzyme resistance. 

Evidence for Evolution

Example

Direct Observation of Change 

Natural selection in response to introduced species. For example, the number of soapberry bugs with tall and short beaks changed because of new food sources. 

Homology (comparative anatomy)

All vertebrates have tails located posterior to the anus and throat arches. Descent from a common ancestor explains this. 

Fossil Record (paleontology)

Pelvic bone in fossil stickleback fish became greatly reduced in size over time in several different lakes, consistency of change suggests a reduction in the size of bone driven by natural selection.

Biogeography

Islands generally have species that are found nowhere else in the world, yet most island species are closely related to species from the nearest mainland/neighboring island. It also explains why two islands with similar environments have different species but those species are more related to those on the nearest mainland. 

Homologous structures→ structures in different species with the same structure but different functions (forelimbs in mammals) 

Vestigial structures→ structures in ancestors present but not used, ex. Hidden eye in blind fish 

Analogous structures→ structures in different species with different structures but same function(not related, butterfly and bird wings) 

A common ancestor 

3, they are both digit-bearing limbs but a big difference is that mammals are amnion. While birds have many different characteristics including feathers.

They are more related to birds because they share the most recent common ancestor compared to lizards and crocodiles whose ancestor is older. 

You might use DNA and homologs 

It is the independent evolution of similar features in different lineages. When two species adapt to similar environments in similar ways it gives rise to analogous features that share similar functions but not common ancestry. 

The sugar glider and flying squirrel evolved independently in two different places in the world. They both can glide through the air. 

1. Natural selection 

2. Genetic drift

3. Gene flow 

Natural selection 

In either-or characters, determined by a single gene locus, with different alleles producing distinct phenotypes. 

Characters along a continuum, variation results from the influence of two/more genes on a single phenotypic character. 

45

18; 1 

Many different sequences produce the same amino acid so if one was substituted, it could be a different letter but still produce the same amino acid. 

Mutation 

A point mutation is a change of as little as one base in a gene. No, they don’t.

Point mutations in noncoding regions result in this. 

During meiosis; gene duplications that don’t have severe effects can persist over generations, allowing mutations to accumulate to expand the genome with new genes that may take on new functions. If these functions suit the organisms better for their environment then this might cause changes in populations 

Independent assortment, random fertilization, crossing over

A group of individuals of the same species living in the same area, interbreeding, producing fertile offspring.

Consists of all copies of every type of allele at every locus in all members of the population. 

When only one allele exists for a particular locus in a population, all species homozygous 

If there are no differences, the population isn’t evolving. If there are differences, the population may be evolving, and then figure out why.

Condition for H/W equilibrium

Explanation

No mutations

The gene pool is modified if mutations alter alleles/entire genes deleted/duplicated

Random mating 

If individuals tend to inbreed, random mixing of gametes doesn’t occur, and genotype frequency changes

No natural selection

Differences in survival and reproductive success of individuals carrying different genotypes can alter allele frequencies 

Extremely large population size

The smaller the population, the more likely it is that allele frequencies will fluctuate by chance from one generation to the next(genetic drift)

No gene flow 

By moving alleles into/out of the populations, gene flow alters allele frequencies. 

Concept

Summary or explanation

Natural Selection

Those with traits best suited to their environment, tend to produce more offspring than those with traits not as well suited. 

Genetic Drift

The smaller the number of coin flips, the more likely it is that chance alone will cause a deviation from predicted results. 

Gene flow

The transfer of alleles into/out of a population due to movement of fertile individuals or gametes

Genetic drift

Gene flow 

Natural selection

When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population. For example, when a few members of the population are blown by a storm to a new island, genetic drift will occur because some individuals are transported but not others.

A sudden change in the environment like a natural disaster can dramatically reduce the size of the population. A severe drop in size is called the bottleneck effect. This population is affected by genetic drift until it becomes large enough, although many species are still affected by this. For example, a prairie chicken whose population decreased and now 50% of eggs hatched

Adaptive means an evolutionary process where organisms become more suited for their environments through mutations 

The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. 

It would be very low if it is unable to reproduce because it can’t contribute anything to the next generation. 

• Acts against both extreme phenotypes and favors intermediate variants

• Reduces variation and maintains status quo for the particular phenotypic character

• Example: babies are usually 6.6-8.8 pounds because those who are bigger or smaller have higher rates of mortality

• It occurs when conditions favor individuals exhibiting one extreme of a phenotypic range

• Shifts population’s frequency curve for phenotypic character in one direction or the other 

• Common when the environment changes or members migrate to a new habitat

• Example: An increase in the abundance of large seeds led to an increase in birds with deeper beaks

• When conditions favor individuals at both extremes of phenotypic range over those with intermediate

• Example: a population with very small seeds and very large seeds so birds with very small beaks and very large beaks have good relative fitness while those with medium beaks have low relative fitness

Sexual dimorphism is the difference in secondary sexual characteristics between males and females of the same species, including different sizes, colors, ornamentation, behavior 

Intrasexual selection is when one sex will compete with the other for someone of a different sex.

Intersexual selection is when one sex chooses a mate of the other sex which can depend on behavior or showiness 

In diploid organisms, many unfavorable recessive alleles persist because they are hidden behind the dominant allele, and selection itself can preserve variation at some loci 

They have greater fitness than do both kinds of homozygotes. The advantage can represent stabilizing or directional selection depending on the relationship between phenotype and genotype. For example, a heterozygote for the sickle-cell allele is protected against the most severe effects of malaria and the disease 

The fitness of a phenotype depends on how common it is in the population. For example, in fish that are either right or left-sided, they bite their prey the opposite way so from year to year the prey protect themselves depending on which side is more common to be bitten from which keeps the frequency of the phenotype at 50%

1. Selection can only act on existing variations, it only favors the fittest phenotypes among current populations which might not be ideal traits. New advantageous alleles don’t arise on demand

2. Evolution is limited by historical constraints, evolution doesn’t scrap ancestral anatomy and build each new complex structure from scratch, evolution co-opts existing structures and adapts them to new situations 

3. Adaptations are often compromises, in humans we have limbs that allow us to be athletic but they are more prone to sprains, torn ligaments, dislocations 

4. Chance, natural selection, and environment interact, environments can change year to year preventing adaptation to get a close match between species and habitat, species that migrate to a new place are random and might not be best suited, and the current species living there might not be either. 

It is the scientific study of geographic distributions of species. Continental drift predicts where fossils of different forums of organisms can be found while endemic species demonstrate how organisms from the mainland adapt to their new environment.