General Biology/Evolution of Life
Evolution: Any change in allele frequency in a population, often the result of natural selection.
Natural selection: differential reproduction of genotypes within a population; one genotype reproduces more successfully than another and donates more copies of itself to the next generation. This means that allele frequencies change within a population. Natural selection is the only mechanism known to produce complex adaptations in nature.
Natural selection can occur in any population that has heritable fitness differences.
Fitness: the ability of an individual to contribute its genes to the next generation. Differences in fitness are central to natural selection. Relative fitness: fitness of an individual compared to others in its species.
Hardy–Weinberg principle states that both allele and genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless specific disturbing influences are introduced.
Both Charles Darwin and Alfred Russell Wallace proposed natural selection. Wallace went to Darwin for help getting published and the result was that the two presented their papers together.
Natural selection is the result of violation of Hardy-Weinberg equilibrium, a state of stability in a population where allele frequencies do not change. A population stays in Hardy-Weinberg equilibrium when five assumptions (or prerequisites) are maintained:
- Large population size
- No differential immigration or emigration of alleles (no gene flow in or out of the population)
- No mutation
- No natural selection
- Random mating (with respect to genotype), that is, individuals in species do not choose mating partners based on their genotype
Results of Hardy-Weinberg equilibrium:
- allele frequencies remain unchanged
- genotype frequencies are in equilibrium
- equilibrium reached in one generation of random mating
Q: What happens when we violate one of the five assumptions of Hardy-Weinberg equilibrium? A: Allele frequencies change.
The Hardy-Weinberg principle is named after English mathematician G. H. Hardy and German physician G. Weinberg, who independently came to similar conclusions in 1908.
Note: In our lab computer simulation lab experiment, we relax the H-W restriction against natural selection, resulting in simulated evolution. When not all phenotype fitnesses are equal, selection occurs, allele frequencies change, and evolution results.
Q: Will a dominant allele take over a recessive allele in frequency? A: No. The Hardy-Weinberg principal shows that any allele that confers greater fitness will become more frequent in the population than an allele that confers lesser fitness, and disproves the easy assumption that a dominant allele would overtake a population over time. Allele frequency is based on fitness, not whether it is dominant or recessive.
- Allele one:
- Allele two:
- Sum of the alleles:
- Reduced fitness:
Examples of adaptation due to natural selection Skin color of the Australian Death adder: brilliant orange skin very unusual in an animal but quite similar to the local soil color. Likely that the snake moved towards this color over time as snakes with brownish and then orange coloring experienced higher survivorship than other-colored snakes.
Pesticide resistance in insects: Pesticide DDT (developed in ‘30s to combat malaria by knocking out its vector, malaria-carrying mosquitoes) had an initially high kill rate that diminished over time until it became essentially ineffective against insects. Hundreds of similar pesticide resistances have been developed in other insect species.
Antibiotic resistance in bacteria: disease-causing bacteria have followed a similar adaptive path to the insects. This is caused by incorrect use of antibiot-ics for disease treatment. An incomplete application of antibiotics kills most of the disease-carrying microbes in a patient’s body. The most antibiotic-resistant germs have then been selected. These multiply and can become increasingly uncontrollable by further antibiotic treatment. Public health officials fear that this process when repeated over time will create super-germs that will be re-sistant to all drugs. Tuberculosis (TB) is a disease currently following this path of increasing antibiotic resistance.
There are various mechanisms of acquired chemical resistance:
- Behavioral change
- Increased detection of chemical
- Decreased sensitivity to chemical at target site
- Degradation of the target
Industrial melanism in English pepper moth (Biston betularia). The moth has two naturally occurring morphs, or varieties: typical (light-colored) and carbonized (dark-colored). The more common morph has historically been the typical, but a change in its habitat led to an increase in frequency of the carbonized. (Air pollution killed off light-colored lichen on tree trunks, leaving moths exposed on dark bark instead of lighter lichen. Predatory birds acted as an agent against the more visible morph of the moth, which responded to the natural selection by increasing the frequency of the less visible morph.
Monkeys who kill others’ young: Observed by Sarah Hrdy. Langur monkeys live in social troops with one male presiding over several females and their young. Female young stay within the troop but male young are kicked out to roam the fringes of the troop and try to take over other troops. Incoming male leaders kill suckling young, as well as the babies of females with whom he hasn’t mated.
Q: Why? A: Because lactating females are in irregular estrus cycle and unavailable to become pregnant. Killing their young frees them up to become pregnant sooner with his babies. The counter adaptation in females: they mate with the new male even if already pregnant, as the new leader will not then not later kill their baby. (Similar situations have since been observed in other species, such as mice).
Selfless turkey problem: 1st generation male descendants of one female turkey set up a brotherhood, a tiny social hierarchy headed by the alpha male. The brotherhood courts females as a group. A female selects one brotherhood for mating, usually a larger one, then mates with the alpha male only.
Q: Did natural selection shape the behavior of these brother turkeys? A: No answer given, but an implied “yes”.
Q: Has natural selection tuned all characteristics of every organism? A: No. Example: The Indian rhino has 1 horn, and the African rhino has 2 horns. This difference is likely a historical accident, not an adaptation, as two horns don’t seem to give an advantage over one.
Five constraints on evolution:
- Historical constraints
- Formal constraints
- You-can’t-get-there-from-here constraints
- Time/variation constraints
- Pleiotropic constraints
Historical constraints: “present variation biases future possibilities”. Variation comes on top of past history.
Formal constraints: Variation can’t defy laws of physics. Ex: pigs don’t fly, and insects are limited in size by their exoskeletons.
You-can’t-get-there-from-here constraints: An advantageous end result must follow many tiny advantageous steps. Ex: There are no live-bearing birds, possibly because the thin eggshell necessary for gas transfer if incubated inside the bird is so disadvantageous for a egg development outside the bird. However, flight feathers evolved from reptilian scales because each tiny change to the scales over time was advantageous, and afterwards allowed the evolution of flight.
Time/variation constraints: new alleles formed thru random mutation, which needs time and is ultimately pushed by mathematic probabilities rather than pulled to a specific end. In limited time, limited results. Ex: Human heart disease the possible result of recent changes in human diet (last 100s of years as opposed to last 100,000s of years of human evolution).
Pleiotropic constraints: (pleiotropy: when one gene has multiple phenotypic expressions) one allele may have several effects, some good & some bad. If the sum of parts is positive, the allele is favored (even if it carries along some unfavorable baggage). Ex: Cystic fibrosis allele may have been advantageous against cholera in pre-industrial Europe.
Mutation: a random process that is the “ultimate” source for natural selection. Little mutation results in advantageous change, and so is very inefficient.
Q: Why does it work at all? A: Because:
- It has lots of time to work
- Each individual divides burden as a source of potential mutation; lots of chances for something to go well
Sampling error: the difference between a sample and the actual population. Greater for smaller sample sizes. Unrelated to population size, based on abso-lute size of sample.
Q: What happens when the population size is limited, restricting Hardy-Weinberg equilibrium? A: In small populations, chance events can dramatically change allele frequencies.
Example: “A” and “a” alleles are in equal frequencies in a population:
In a sample size of four organisms from this population we would expect to find:
However, 30% of the time we would have the following result, with a shift favoring one allele:
And about 8/1000 of the time (1%):
One allele is completely eliminated! (If the population size is 40, the chances of this are 1/500,000).
In this way, alleles can be eliminated from a real population.
Q: How can small sample sizes occur in real populations?
- Genetic bottleneck: population experiences a crash in numbers (this likely occurred recently in cheetahs, who have very little genetic variation)
- Founder effect: small group of “colonists” forms a new population
- Genetic drift: population lingers in low numbers, as in an endangered species.
Virtual small population size: a small proportion of individuals in population cover reproductive resposibilites. Usually occurs with males, sometimes females. Ex: Mammals such as seals, where one giant male guards over and mates with harem of many females. This is because the females go to an isolated spot to birth to avoid predators, then go into heat just days after giving birth. A large male fights other males for the privilege of guarding these females and their offspring as “beach master”. The females intermittently go off to feed, coming back to feed their calves. Result in males is that they are much larger than the females; they store lots of body fat so they can stay on the beach a long time (as soon as they leave another male will come take their spot). Effective breeding population for one generation is therefore much smaller than actual number of living members.
Another example: in wolves, only the alpha male and alpha female breed, and the others just help raise their young. So a wolf population of sixty may have a breeding population of just six.
Immigration / emigration: “gene flow” – individual moves in from a different area & brings new allele frequencies. “A little bit can do a lot”. Ex: interracial marriage.
Mutation: produces variation in gene pool. Rates are low and work very slowly: just 1/100,000 to 1/1,000,000 mutations occur per locus per gamete per generation. Mutation alone is not enough to drive an allele to a higher frequency. A general rule: any allele with a frequency of 1% or more of the total population was not driven there by mutation.
Non-random mating: individuals choose mates based on genotype. (Positive) assortative mating: choosing a mate with a genotype similar to your own, leading to homozygotic offspring. Dissortative mating: choosing for dissimilar genotype, leading to heterozygotes.
Inbreeding: mating with close relatives: a way to mate with your own genotype (and produce homozygotic offspring)
Note: the average human has about thirty lethal recessive allele loci in his total chromatin. Mating with close kin increases the possibility of an offspring with double recessive lethal alleles. Many species have outbreeding behaviors to discourage inbreeding. Ex: langur society: females born in a troop remain in their troop but males are booted out, discouraging brothers mating with sisters.
Non-random mating can change genotype frequencies but NOT allele frequencies by itself, therefore not responsible for evolution. But it can expose certain alleles to selection by making them homo or heterozygous in the genotype.
Aging senescence: decline in performance in the general body of an organism with increasing age. This is a selectable trait and is not present in all organisms. Tissue does not “need” to be senescent. Some protists basically “live forever”; fruit flies can be bred for longevity.
Q: Why is this? A: Gibson does not say why, but suggests that it is a selectively favored trait. After class the professors seem to say that senescence is not necessarily an evolutionarily favored trait as much as a byproduct of the processes that led to higher development. In order to achieve delayed reproduction, more resources were put towards early survivorship at the expense of later survivorship. A related example is the difference between salmon in the Atlantic and Pacific oceans. Atlantic salmon live through multiple spawning seasons but Pacific salmon have been selected to put all their energy into one, difficult spawning cycle, where they die right after.
Types of selection:
- Stabilizing selection
- Directional selection
- Disruptive selection
Stabilizing selection: Selection that moves organisms toward the center of their range of possible traits. Ex: human birth weight; especially large and small babies suffer greater infant mortality, favoring babies of intermediate weight. A stabilizing environment results in fossil records that are unchanged for millions of years, such as for the body plans of sharks and horseshoe crabs.
Directional selection: Selection that acts to eliminate an extreme from an array of phenotypes. Ex: Metals such as copper are usually almost lethal to some plants. A strain of copper-resistant grass has developed over many generations of growing in contaminated high-Cu soil.
Disruptive selection: Selection that tends to eliminate intermediate type. Ex: African seedcracker: has two bill sizes, one large and one small, each one best suited for a different kind of locally-abundant seed. Intermediate bill types are unfavored by selection because they are poorly suited for either kind of seed. Here, the homozygote that results in one bill type or the other is favored over the heterozygote, which produces the intermediate bill type.
Consider a population that has the following genotypic frequencies for a given locus having two alleles, “A” and “a”:
Q: What is the allele frequency for “A”? (i.e., what is p?) Q: Does the population appear to be in Hardy-Weinberg equilibrium?
Steps to solve:
- Add everything together to find total
- 77 + 14 + 49 = 140
- Divide each number by the total
|77 / 140||14 / 140||49 / 140|
- ( Note that: .55 + .10 + .35 = 1.0 )
- Find allele frequencies for “A” and “a”.
- A: .55 + (.10/2) = .60
- a: .35 + (.10/2) = .40
- ( Note again that .60 + .40 = 1.0 )
- Plug these derived genotype frequencies into the equation:
- .36 + .48 + .16
- ( Again, all total to 1.0 )
AA' = .36
|AA = .55||Aa = .10||aa = .35|
|Aa' = .48||aa' = .16|
We see that the genotype frequencies change from their initial values to their final vales. Therefore the population was not in Hardy-Weinberg equilibrium. Q: If the numbers are only somewhat dissimilar how do we tell if they should be considered similar or dissimilar? A: This is determined with statistical and mathematical analyses beyond the scope of this class.
Tests have been done on breeding many organisms for a certain trait by divid-ing successive generations into groups based on phenotype. Ex: Fruit flies (Drosophila) artificially selected into two groups: one with many abdominal bristles, one with few bristles. We find that neither of the two resulting groups even overlaps the bristle range found in the starting generation. This mimics the results of natural selection on a wild population under certain circumstances. This is similar to the process used for thousands of years by plant and animal breeders.
Heterozygote advantage: when heterozygote genotype confers greater fitness than a homozygote. A classic example of this is sickle-cell anemia. This human disease occurs when a person is homozygotic for the recessive allele. The recessive allele increases survivorship against malaria in the herozygotic carrier. Natural selection works in high malaria areas to increase the frequency of this allele in humans. The allele frequency is highest in central Africa, where malaria has been a big killer.
|HbN HbN||Normal, no sickle|
|HbN HbS||Single sickle, malaria-resistant|
|HbS HbS||Double sickle, lethal|
Q: How do we get a new species?
Q: What is a species? A: There are different definitions, but only one that we will go over in depth. Biological species concept (BSC): of Ernst Mayer. A species defined as a reproductively isolated population. If two organisms can interbreed, they are one species; if they cannot, they are not of the same species. Not a perfect definition as lions and tigers can interbreed in captivity, and this works only with sexually reproducing organisms (dandelions reproduce asexually). Phenotypic definition: species defined by phenotypic gaps in a population. Cladistic definition: populations or population groups which are members a single clade
Clade: a branch of the evolutionary tree.
Two ways to name a new species:
- Anagenesis: an arbitrary / convenient way to name a new species after it has undergone enough change through a single evolutionary line
- Cladogenesis: the creation of clades
Cladogenesis has three splitting models / hypotheses based on patra, “homeland”
- Allopatric speciation (allos: “other”; aka geographic isolation model, common) Allopatric species: don’t occur together.
- Sympatric speciation (sym: “together with”; common in plants)
- Parapatric speciation (para: “beside”; uncommon)
Parapatric speciation: perhaps the only example we have of this is found in the copper-resistant mine grass previously discussed. Selection strongly favors the genotype homozygous for the Cu-resistant allele. This has resulted in an adaptation in the pollination time to make it different than the pollination time for the surrounding grasses, avoiding cross-fertilization. Here is speciation without geographic isolation caused by the intense selection.
Sympatric speciation: (is this info right? Book seems to conflict with lecture) a common source of new species in plants but uncommon in animals, this occurs when gametes from two different species cross and create a viable new species. This believed to be the source of ½ of the flowering plants. Polyploidization: the most important model of sympatric speciation. Or, the doubling of chromosomes that can lead to sympatric speciation. Allopolyploid: polyploidization triggered by an interspecific hybridization event.
|Species A||Species B||Species AB|
The resulting zygote may be unviable or infertile. If viable, it may later develop fertility. This is due to the differences between cell divisions in mitosis and meiosis (and we thought that we had escaped). In mitosis, chromatids self-replicate. In meiosis, each chromatid has to “find” its homologue. If no homologue is present, the result is viable but infertile. However, with an abnormal mitotic event, cytokinesis (division of the cytoplasm after nuclear division) does not occur, resulting in a cell with double the normal number of chromosomes. If this cell enters meiosis, it can be fertile. (Many examples of this). Ex: Bread wheat is the result of two successive grass hybridization events. Grass went from having 14 > 28 > 42 chromosomes.
Geographic variation: Differences in a species based on geographic location, usually genetically based. Found in humans, where genotypes from one place are different than those from another. More common in less mobile populations as it is opposed by gene flow. Ex: yarrow, a plant which is found to be shorter at higher altitudes. Q: Is this variation due to genes or to environment? A: a “common garden experiment” finds the answer. Seeds of different origins are grown in the same environment. Result is that seeds from higher altitudes grow into shorter yarrow plants, revealing an underlying genetic factor. Ex: common garter snake, Thamnophis surtalis, common throughout the US and Canada, is divided into various local sub species with local colorations. As garter snakes are not highly mobile, the populations can become locally adapted. In mallard ducks this geographic variation does not occur as each generation a male hooks up with a female in the southern “win-tering grounds”, then follows her up to her northern home, which may be very distant from his other home. This results in a genetic shuffle each generation independent of location.
One population can be broken into different, isolated populations for a time, al-lowing two distinct evolutionary pathways. Ex: a glacier advances over North America and splits a population into SW and SE groups. When the barrier is removed (the glacier retreats) the populations can come into contact again, called secondary contact. Q: Then what happens? A: If inbreeding results in fertile offspring, they are still of the same species. If not, they are now new species.
Isolating mechanisms: anything that acts to prevent the production of viable zygotes between two organisms. Classified into two groups:
- post zygotic
The prezygotic mechanisms prevent the fertilization of an egg by the sperm, and hence the production of a zygote, commonly when male courtship is unrecognized by the female. Postzygotic isolating mechanisms result in a zygote which
- isn’t viable (dies), or
- cannot reproduce.
Ex: A cross between a donkey and a horse produces a mule, which is sterile.
Geographic separation can result in two quite morphologically different popula-tions that interbreed on secondary contact and are therefore one biological species. Like, they can look all different and stuff but still breed with each other. And sometimes, a seemingly slight difference prevents interbreeding. The point: the “amount” of differences that arise during the isolation of two populations doesn’t necessarily determine whether they will successfully interbreed upon secondary contact and therefore be considered one or two species.
Ex: Eastern and western meadow larks are almost identical in appearance (morphologically similar) but are distinct biological species. This is possibly the result changes which occurred during isolation caused by glacial separation. On secondary contact these two populations apparently did not respond to each other for mating, becoming two biological (reproductive) species. (There are various examples like this one).
Peripheral isolates: a relatively common version of geographic isolation. At the edge of a species range an organism lives at the limit of its survivability. When these areas become separated from each other, allopatric speciation can occur, high rates of selection can push allele frequencies.
Habitat islands: areas where a habitat favorable to a species is surrounded by an area where the species does not survive. Ex: a volcanic island in an ocean. In this case, founder populations arrive and find ecological vacuums, or different conditions, that influence adaptation. Adaptive radiation, a series of adaptation events, occurs as various species interact and adjust through evolution. Ex: the variation found in Darwin’s finches was caused when a founder population of finches adapted to the unique conditions of each island). Note: even greater variation is found amongst the finches of the Hawaiian Islands. Hawaii is also the home to countless species of Drosophila (fruit flies).
Binomial nomenclature: “two-named naming” (bi: two, nomin: name) originated by Carolus Linnaeus (Carl von Linné) of Sweden. Each species of organism has its own unique two-part name, made up of
- Genus name, which is capitalized, and denotes a group of species
- Species epithet, not capitalized, describes one species within the genus. (Epithet: “descriptor”, or according to www.m-w.com “a characterizing word or phrase accompanying or occurring in place of the name of a person or thing”.) Note that not all species epithets by themselves are unique to a species; in fact some are reused frequently, such as vulgaris, used in Pinguicula vulgaris and in Sturnus vulgaris, the scientific names for the common butterwort and the European starling (vulgaris: “common”). Species epithets commonly refer to such things as color.
Homo sapiens is the binomial name for the modern human. Homo is a genus of related primates (all but mankind are now extinct) and sapiens is a descrip-tor, “wise”. When typed, this two-part name is customarily italicized, and when handwritten, underlined.
The naming system is hierarchical, with succeeding levels dividing organisms into more specific taxonomic groups. The broadest division is that of kingdom (or domain, a more basic designation now accepted by many biologists. Here is the full list from most general to most specific. (Domain) >> Kingdom >> Phylum >> Class >> Order >> Family >> Genus >> Species.
There are subdivisions that are sometimes used between these levels (for example, subphylum) but we are not responsible for knowing them.
The higher the level, the greater the difference between groups, reflecting pro-gressively earlier speciation events.
Q: Why does Linnaus’ naming system neatly fit the evolutionary tree, as he invented it outside an evolutionary context? A: The organisms are related in a genealogical way with inherently hierarchical relationships.
Taxon: a given group of organisms at some given rank. Ex: the taxon “Canis”, a group of dogs at level genus, vs. the taxon “canidae”, at level family. (Taxonomy: orderly classification of plants and animals according to their presumed natural relationships.)
Monophyletic taxon: a species set, or grouping of species, made up of
- a common ancestor
- all its descended species.
Ex: great apes
Paraphyletic taxon: a grouping of species just like the monophyletic taxon except that it excludes species that have diverged farthest from the common ancestor. Ex: Great apes, minus humans.
Polyophyletic taxon: grouping of species that excludes the most recent common ancestor. Ex: A grouping of whales and sharks, without their common primitive ancestor (whatever that might be).
There are two taxonomic schools of thought:
- Traditional (also poorly called “evolutionary”, poorly because the alternate is evolutionarily based as well)
- Cladistic (or phylogenetic, or Hennigian, after founder Wm Hennig)
Both schools agree on how to draw the evolutionary tree, as well as that:
- the best way to group is in monophyletic groups, and
- the worst way to group is in polyphyletic groups.
However they disagree in the treatment of the middle ground, paraphyletic taxonomic groups. The traditionalists say: These groups are OK, as they reflect important evolutionary change. The cladists say, paraphyletic groups are NOT OK, as the identification of “important” changes is based on people’s opinions. Cladism prefers greater objectivity.
Q: How do we estimate evolutionary sequences? A: Derived homology, or transition from [ a -> a’ ], ex: a species’ loss of a digit thru time. Synapomorphy:
- a “shared derived similarity”
- derived character state, shared under two or more taxa under consideration
- used to infer common ancestry
Gibson says that synapomorphy gives the only real idea of branching sequence
- a “shared, ancestral similarity”
- an ancestral character state
- inherited from the common ancestor of the taxa
Evolutionary branching Q: How can we generate hypotheses about evolutionary branching? (How can we figure out which branching sequence is most likely the historical one?) A: There are various ways to do this. Generally, we start by considering certain characters, then arrange them into possible trees. Characters are found in either a more ancestral or more derived state, and sometimes the trait can disappear completely or even reappear. When a species goes from having one version of a trait to another version, it is called a trait change.
One way to determine the most likely branching sequences is by creating examples of all the possible sequences. Although this is a simple process, it is not feasible in many cases because the number of possible branching sequences is so great. For example, a group of just 10 species has over 34 million possible evolutionary trees. (Certain methods are used to eliminate many of these options and make the process simpler). The simplest sequence is the one that has the smallest number of trait changes, and is preferred. They are then used as a basis for further refinement that comes with more data.
Punctuated equilibrium: a hypothesis about the tempo and mode of evolution, presented by Stephen Gould and Niles Eldridge as a challenge to the traditional view known as phyletic gradualism.
|Phyletic gradualism||Slow and steady||Independent of speciation|
|Punctuated equilibrium||Long periods of no change (stasis) “punctuated” by rare events of rapid change||Largely restricted to speci-ation events|
Current thought sees evolutionary change as neither of these two extremes but rather somewhere in between, or sometimes more like the one and other times more like the other.
Evolutionary “novelties” Q: How can we explain macroevolution? Can the incremental changes of microevolution lead to the big changes of macroevolution, such as flight in birds? What other processes can lead to major changes in species?
- A structure that serves one function and then is co-opted to serve another function
- Ex: flight feathers from reptilian scales
- Ex: Mammals’ mammary glands from… modified sweat glands?
- A change in proportions which comes with increasing size, or different parts grow at different rates
- Selection for one body part may lead to development in another
- Ex: antler size in Irish elk, where huge antlers came as a result of oversize elk (antlers were still in proportion to normal deer antlers)
Paedomorhosis (paidos: child)
- retention of juvenile characteristics in adult, or in extreme, reproduction in larval state
- Ex: gills retained from juvenile aquatic salamanders to normally land-habiting adult, and reproduction at aquatic, juvenile stage
- Adult chimps have greater change in skull shape from infancy than humans experience: chimp and human babies have similar skull shape, which the human more closely maintains to adulthood than the chimp
Isometric growth: all parts grow at the same rate and stay in proportion, as in a growing salamander and in most organisms
Algometric growth: different parts grow at different rates, resulting in changing proportions thru growth cycle, as in human, and in the godwit, a bird whose beak grows exponentially faster than its head does
Analogous structures: two structures that serve a similar function but have different origins. Examples: wings of a butterfly and a bird, leglessness in snakes and in certain lizards, white eggs in various diverse bird species
Antennapedia: mutant fruitfly has small legs on head in place of normal antennae, caused by inappropriate development of cells into homologous part
Bithorax: ancestors to modern insects had two full pairs of wings but today some of these have evolved the second pair into small balancing structures called halteres. (Some extant insects, such as the dragonfly, retain all four wings, one pair on the second and third segments of the thorax). Insects with only two wings are called dipthera, meaning “two wings”. In the bithorax mutant, halteres develop into a second full set of wings.
Homeodomain: series of 60 amino acids (180 nucleotides) which encodes development and has remained similar throughout the evolution of very different organisms, spanning the animals and apparently extending into parts of the plant kingdom as well. Reveals the strong conservation of genes thru evolution.