Biology Chemistry Computer Engineering Electronics Mathematics Physics Science Home

UNIT 4:  EVOLUTION – Mechanisms of Evolution

 

Physical evidence, coupled with knowledge gained in the field of molecular biology, is used by modern scientists to make compelling arguments that support the theory of evolution and natural selection.  Genetic differences between entire species are made to help solve the puzzle of evolution.

 

A. Genetic Variation

 

·         genetic diversity between individuals of the same species exist and are obvious in the phenotype – observable traits of an organism that result from the interaction between genes and the environment

·         portions of the DNA molecule within a chromosome that serve as biological codes for particular polypeptide products are called genes

·         genes are located at specific places on the DNA strand called loci

·         most eukaryotic organisms are diploid – they have a set of paternal and a set of maternal chromosomes

·         this means that the organism possesses two forms of the same gene – these alternate forms are called alleles

·         organisms with identical forms of the same gene are said to be homozygous for the trait or phenotype that the genes code for

·         an individual that possesses two different alleles for the same gene is said to be heterozygous

·         all individuals of the same species possess the same genome – the complete set of genetic information and associated DNA

·         no two individuals (unless they are identical twins) have the same genotype – the combination of alleles at specific loci

·         natural selection acts on the phenotypes of individuals

·         using various technologies such as DNA sequencing and gel electrophoresis, geneticists are able to compare the genetic code of individuals, populations, and entire species

·         the amount of DNA found in different species varies as seen in Table 1, p. 545

·         some species are polyploidy – they have three or more complete sets of chromosomes

·         large genomes, have a greater potential for genetic diversity and a greater number of targets for mutation

·         however the size of the entire genome does not necessarily provide an accurate comparison of a species genetic diversity because large portions of any individual’s DNA are non-coding

·         most species possess large numbers of different genes – usually in the thousands

·         the greater the number of genes, the greater the extent of genetic variation there will be within a species and from individual to individual

·         this is because there exists a greater number of potential different combinations of alleles

·         sexual reproduction increases the genetic diversity among members of the same species that live in a particular area

·         for example, if an individual of a species had 10 000 genes in its genome, and it was heterozygous at only 10% of its loci, then it could produce 2¹⁰⁰⁰ different genetic recombinations in its gametes

·         this number would increase even more if you account for crossover during meiosis I of gamete formation, and mutations during DNA synthesis

·         this is why no two individuals (except for identical twins or clones) will ever have the same genotype

 

Homework:       1-5, p. 546

 

 

 

 

 

 

 

 

 

 

 

B.  Hardy-Weinberg Principle

 

·         one way to quantify the genetic information of an entire population is by measuring the proportion of gene copies in a population of a given allele

·         the relative abundance of gene copies of a given allele is called the allele frequency

·         if the frequency of an allele in any given population changes, then evolutionary changes can be detected

·         consider the example illustrated in Figure 1, p. 547:

 

In the population of 500 moths there are two alleles, A (dark brown wings – a dominant allele), and a (light brown wings – a recessive allele).  320 are homozygous dark-winged, 160 are heterozygous dark-winged, and the rest have light brown wings.  This means that 640 A alleles come from the homozygous dominant moths, and 160 A alleles come from the heterozygous moths, to total 800 A alleles.  160 a alleles come from the heterozygous moths, and 20 a alleles come from the homozygous recessive moths, to total 200 a alleles.  For this population of 500 moths, the allele frequency for A is 800/1000 = 80%, and the allele frequency for a is 200/1000 = 20%.

 

·         for the above example, there are two alleles for the same trait

·         if there were only one allele for a particular gene, that allele’s frequency would be 100% and is would be considered fixed

·         if one particular phenotype (i.e. dark brown wing colour) increased the reproductive success of a moth, then more moths would possess the A allele, and its frequency would increase in the population

·         two individuals, working independently, arrived at the same conclusions regarding allele frequencies – Hardy and Weinberg came up with the principle that allele frequencies will not change from generation to generation, as long as..

o        the population is very large

o        mating opportunities are equal

o        no mutations occur

o        no migrations occur

o        no natural selection occurs – all individuals have an equal reproductive success

·         if a gene has only two different alleles, the Hardy-Weinberg principle can be expressed using the equation… p + q = 1, where p is the frequency of one allele, and q is the frequency of the other allele

·         if you square both sides you get p² + 2pq + q² = 1, where p² is the frequency of the homozygous dominant genotype, 2pq is the frequency of the heterozygous genotype, and q² is the frequency of the homozygous recessive genotype

·         Figure 2, p. 548 shows how a typical Punnett square depicts the allele frequencies of offspring within the moth population in Figure 1

 

Homework:       1-3, p. 549

 

 

C. Random Change

 

·         by providing the set of conditions under which genetic change or evolution would not occur, the Hardy-Weinberg principle helps to identify key factors that can cause a change in gene and allele frequencies within a population or species

 

o        small populations are more vulnerable to changes in allele frequencies

o        nonrandom mating opportunities (i.e. mate preferences) result in only those “preferred” traits being passed onto future populations

o        new alleles may be created when mutations occur, thereby changing the frequencies of both new and original alleles

o        migration (immigration and emigration) cause changes in the relative abundance of alleles

o        natural selection takes place when individuals with certain alleles, thus traits, have greater reproductive success than others do – this means that the relative frequency of these “favoured” alleles increases in the next generation

 

·         natural populations are subjected to any or all of these factors – resulting in changes to allele frequencies

 

GENETIC DRIFT

 

·         small populations are affected by changes in allele frequencies a lot more than large ones due to chance alone

·         for example, if in a population of 100 endangered frogs there was only 1 frog that contained a particular allele, and half the population was wiped out due to a severe drought, then two results are possible for the frequency of this allele…

 

1. odds are that this frog would be one of the 50 frogs who died, thus the rare allele is eliminated from the gene pool
2. by some chance this frog survived and the allele frequency instantly doubled

 

·         extreme, sudden changes to allele frequencies as a result of chance (usually in small, isolated populations) is called genetic drift

·         Figure 2, p. 551 demonstrates how an allele frequency can become fixated at 100% when the population is small

·         the A allele becomes fixed at 100% in one populations of 25 stoneflies after only 5 generations – it is totally eliminated from the gene pool in one of the populations of 25 stoneflies after just 20 generations

·         when the population is 500, the A allele frequency remains relatively stable even after 50 generations

·         this means that in relatively small populations genetic drift can lead to fixation of alleles, thereby increasing the incidence of homozygous individuals within a population and reducing its genetic diversity

·         there are two kinds of genetic drift in nature:

 

1. when a severe event occurs that causes a drastic reduction in numbers so much so that genetic drift results, is called a bottleneck effect – Figure 3, p. 551 illustrates this effect
2. when a small number of individuals separate from their original population and find a new population – this kind of genetic drift is called the founder effect, and is very common with self-pollinating plants

 

GENE FLOW

 

·         when an individual migrates alleles move from one population to another causing gene flow to occur

·         the result of such an event affects both populations

·         gene flow reduces differences between populations, whereas genetic drift differentiates a population by creating a homogeny in genotype

 

MUTATIONS

 

·         mutations are the only source of new genetic material and new alleles

·         they may arise as a result of DNA synthesis mistakes, chromosome breakage and rejoining, or mutagens

·         most mutations occur in somatic cells and are therefore not passed on to offspring

·         if they occur in gametes then there is potential that they could be passed on to later generations

·         since mutations are random events, there is a greater chance that their effects are neutral or harmful rather than beneficial

·         neutral mutations have no immediate effect on an individual’s reproductive success

·         harmful mutations reduce an individual’s fitness usually by preventing a functioning protein from being made or adversely affecting meiosis and mitosis

·         beneficial mutations are inheritable changes in a cell’s DNA that results in an additional or enhanced gene product or regulatory function which ultimately improve the individual’s fitness, or reproductive success

Types of Mutations

 

·         point mutations – changes in single base-pairs along the DNA molecule by either substitution, deletion, or insertion

o        if they occur in non-coding regions then they will be neutral

o        if they occur in coding regions of DNA then a new phenotype is produced

o        such effects are rarely beneficial – most of the time they result in hindering the proper functioning of the final protein product , or sometimes the change may have no significant effect on the protein’s function

o        very rarely do these kinds of mutations improve or give a new function to a protein

o        insertions or deletions that occur in functioning genes almost always produce a nonfunctioning gene

o        these usually tend to be harmful and do not play a major role in evolution

 

·         gene duplication – when unequal crossing over takes place during meiosis resulting in an additional copy of one or more genes on one of the chromosomes, and a reduction of genes on the other chromosomes

o        the extra DNA may not have an immediate effect on an individual’s phenotype – however it is free to mutate and might eventually gain a new function

o        such duplication events can ultimately produce new gene “families” that can potentially give rise to new characteristics

o        gene duplication may also result in the production of pseudogenes – DNA sequences that are homologous to functioning genes but are not transcribed – kind of like “spare tire genes”

 

·         polyploidy – type of mutation that results in three or more sets of chromosomes in the offspring – i.e. if a haploid gamete combines with a diploid gamete to make an individual that is triploidy

o        this type of mutation provides an individual with an immediate doubling of genetic material

o        sometimes organisms can be tetraploid (4n) or even octoploid (8n)

o        most fern species are polyploidy – the species Ophioglossum reticulatum is actually 630n!!

o        polyploidy has played a major role in the evolution of plants

 

·         mutations occur once every 10 000 cell divisions in species with small genomes (like bacteria)

·         since they rarely result in obvious changes to phenotype, they are not immediately noticed

·         harmful mutations often make gametes nonfunctional, or even result in death of the individual before birth

 

Homework:       1, p. 554, and 1-10, p. 555

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

D. Patterns of Selection

 

MALARIA, SICKLE-CELL ANEMIA, AND NATURAL SELECTION

 

·         when genetic variation exists within competitive populations that live under different environmental conditions, natural selection can lead to a variety of outcomes

·         the specific pattern of natural selection that takes place is a result of three factors:  mutation, genetic variation, and environmental pressure

·         sickle-cell anemia (a serious blood disorder) is caused by a recessive allele HbS that is the result of a single base-pair mutation of the normal hemoglobin gene HbN

·         severe affliction takes place with individuals that are homozygous for the HbS allele – heterozygous individuals are only mildly affected by sickle-cell anemia, but are much more resistant to malaria than are people that are homozygous for the normal allele

·         malaria is a protozoan infection that kills one million people each year out of the 120 million new cases that arise each year

·         in regions where malaria is not common, individuals with the sickle cell allele are at a disadvantage and their phenotypes have a lower reproductive success – this means that these people are less likely to contribute alleles to the gene pool of future populations

·         in regions where malaria is common, heterozygous individuals at an advantage and have a greater fitness – they are more likely to contribute their genes to the next generation

·         basically the environment selects the best-suited phenotype, and in doing so, favours a particular set of alleles

·         Figure 1, p. 556 shows how the prevalence of malaria in Africa closely matches the distribution of the sickle-cell allele

·         recently, scientists have discovered a third allele for hemoglobin, which is another mutated form of the normal hemoglobin allele

·         the mutation occurs at the same location as the mutation for the HbS allele

·         the third form of this gene is called the HbC allele – it provides resistance to the most serious forms of malaria and does not affect the shape of hemoglobin enough to inflict the person with sickle-cell anemia

·         mutation and evolution are related in the following manner:

 

o        harmful mutations are more common but since they are selected against, the mutant alleles remain extremely rare

o        beneficial mutations are rare but since they are selected for, the mutant alleles accumulate over time

 

·         basically, particular alleles occur frequently – they are most likely passed on to future populations since they enhance the phenotype of an individual and  contributing to their over all reproductive success of that individual

 

TYPES OF SELECTION

 

Most species show very little change over periods of time as a result of natural selection.  Usually it takes over a thousand years for any change to become dramatic – especially in the appearance of the organism.  However, various types of selection, due to specific kinds of environmental conditions, do exist and influence how selection can operate on individual phenotypes and affect the evolutionary pathway that a population experiences.  These include…

 

1.        Stabilizing Selection – selection that is against individuals exhibiting variations in a trait or strategy that deviate from the current population optimum or average

 

ü       once a species becomes well adapted to their environment, unless it changes significantly, selection pressures will tend to prevent them from changing – in fact, more and more individuals of that species tend to have the exact same characteristic trait with less variation of the trait existing within the species

ü       this is the most common form of selection seen in nature

ü       the hummingbird bill length stabilizing selection seen in Figure 3, p. 557

ü       as long as the flower size does not change, the ideal (optimal) bill length will always be selected for – the environment will select against mutations that occur to produce birds with a bill length that differs from the best-adapted length

ü       the over all effect of this is a stabilization of allele frequency that codes for the ideal trait and less variation among the entire species of hummingbirds (see Figure 3 – not the narrowing of the “bell” curve over time)

 

 

 

 

 

 

2.        Directional Selection – selection that favors an increase or decrease in the value of a trait or strategy from the current population average or optimal condition

 

ü       if the environment favors a particular extreme variation of a trait, those individuals that possess the extreme traits will have a selective advantage or greater fitness

ü       for example, if the flowers that hummingbirds feed on become larger due to changes in abiotic conditions, those hummingbirds that inherited longer bills would now have a greater advantage over those that were best-adapted for medium-sized flowers

ü       longer-billed birds will feed more and have a greater reproductive success, putting forth more of their genes in future populations

ü       Figure 5, p. 558 demonstrates the effects of directional selection for this particular example

ü       another example is salmon (Figure 6, p. 558) – humans have created the directional selection force that lead to the decrease in salmon size over the last 25 years

 

3.     Disruptive Selection – selection that favors two or more variations, strategies, or forms of a trait that differ from the current population average

 

ü       individuals with extreme variations of a trait or strategy, existing at opposite ends of the distribution curve, are favoured over individuals with intermediate trait qualities

ü       in this case more than one phenotype is favoured

ü       in the black-bellied seedcracker finch bird population, two kinds of seed are available in the African forests for consumption – a hard seed and a soft seed

ü       finches with small bills (Figure 9a, p. 559) are extremely efficient at feeding on soft seeds, while finches with larger bills (Figure 9b, p. 559) are best-suited to eat hard seeds

ü       studies show that bill width (size) distribution curves exhibit disruptive selection curves

ü       quite often, more than one characteristic or trait exhibits this kind of selection in these two opposite variations and may in fact may lead to two distinct groups (species) that eventually become isolated breeding populations with separate gene pools

 

4.     Sexual Selection – selection that favours a particular strategy that increases the reproductive success of an individual, resulting in sexual dimorphism and mating and courtship behaviours

 

ü       when males and females of a particular species look different and have diverse reproductive strategies they are exhibiting sexual dimorphism

 

Essential Differences between Males and Females:

 

	MALES	FEMALES
GAMETES	motile       small	non-motile       large
MATING BEHAVIOUR	aggressive       non-selective       competitive	non-aggressive       selective       non-competitive
RISK TAKING	high	low
MISTAKES	more	less

 

ü       the above are general differences -- there are many exceptions!!

 

 

 

 

 

 

 

 

 

 

 

ü       why are males more competitive, and females “choosey”??

 

2 possible theories:             

 

1.     Bateman's Principle:  sperm is cheap, eggs are few and far between

 

§         much of the female's resources go into ovulation and the production of eggs

§         sperm is fairly inexpensive to produce

§         males put most of their resources into displays/increasing body mass/etc. to attract females

 

 

2.     Operational Sex Ratio:  # of fertilizable females to # of sexually active males  

 

§         although the primary sex ratio (number of adult males to the number of adult females - ignoring sexual status) is usually equal, there are fewer fertilizable females than there are "horny" males (i.e. fertilizable females are limiting – in short supply)

§         this is because in most species females invest more energy and time into rearing/feeding/lactating/parental care/etc.

§         this frees up males and ties up females

§         the result is more "horny" males than fertilizable females

 

MATE CHOICE:      any behaviour exhibited by an animal that aids in the selection of a suitable mate

 

ü       3 likely classes of traits females use in mate selection:

 

CLASS	IMMEDIATE SURVIVAL male                                     female	OVERALL FITNESS of offspring
RESOURCES*	decrease                             increase         the male can be killed by a predator; costly to catch/find resource       the female benefits by receiving the extra energy	increase         both benefit since their offspring will gain the benefit of the particular resource
FISHERIAN**	decrease         the male invests energy into evolving elaborate display mechanisms that can potentially attract predators	increase         the offspring result in "sexy" sons, and females who prefer them
GOOD GENES***	increase         the male is a good survivor and competitor for resources	increase         the offspring will have good survival genes

 

 

 

 

 

 

 

 

* RESOURCES:      The male brings/offers the female a resource (food, protection, good nesting site for offspring, etc.) in order to convince her that he is a good choice

 

                        example:  Scorpion fly

                       

§         hunts for prey item that it offers its mate

§         females mate with scorpion fly that catches largest prey

§         if males cannot find a prey they do one of 3 things:

 

                                                        i.   steal one from another male

                                                        ii.   secrete a sugary substance to offer female

                                                        iii.  force copulation (rape female)

 

§         however, there is a cost:  males fly around in search of prey and run into predatory webs

§         therefore, those aggressive males risk the chance of being captured and killed while their trying to attract mates

 

 

** FISHERIAN:        The male develops a trait/characteristic/physical feature that attracts the female ("he's sexy").

 

                                        example:  Widow Birds

 

§         females are attracted to the tails of male birds

§         the longer the tail, the more mates achieved by the male

 

                                        example:  Peacocks

 

§         the larger, more elaborate the display, the more attractive

§         cost:  more visible to predation

 

*** GOOD GENES:  The male displays characteristics that demonstrate that he is a "good survivor"/competitor/provider, and therefore a good choice to mate with.

 

                        example:  Soldier Wasps

                       

§         males prepare a nesting site for females to lay their eggs in

§         males aggressively defend their territory by attacking anything that invades -- including predatory insects

§         this aggressive behaviour is extremely risky

§         but the fact that it exists, suggests that the benefits outweigh the costs

 

                        example:  Stickleback Fish

 

§         studies show that female sticklebacks detect a scent to select males with the most diverse array of genes for fighting disease

§         they use their sense of smell to find a mate that will give them the most disease-resistant offspring

§         the elaborate courtship behaviours that precede mating demonstrate a sophisticated mechanism of communication between male and female, where each learn something about the other

 

PARENTAL CARE – Who looks after the baby?

 

1.     The sex that is more closely associated with the offspring

2.     The sex who invests more energy

3.     The sex who adds their gametes last

4.     The sex that is most certain that the offspring are theirs

 

EXAMPLE OF RESEARCH STUDY:  Acheta domesticus

 

5.     Selection Pressures due to Predation – staying alive and being able to find your food are strong pressures that influence the selection of a specific trait or strategy

 

Antipredatory Mechanisms:       

 

§         any adaptation that decreases the chances of being killed or injured by a predator

§         ”biological arms race"  -- "Red Queen Hypothesis"

§         constantly have to be evolving if you are preyed upon by one particular predator to try and continuously "outsmart" the other animal (i.e. must keep up with predator/prey)

 

2 LARGE GROUPS OF ANTIPREDATORY MECHANISMS:

 

1. PRIMARY DEFENSE:        mechanisms that act to decrease the probability of a prey encountering a predator or vice versa

§         encounter -  predator detects prey and recognizes it is edible

§         mechanisms act to avoid detection

 

a.     Anachoresis:  animal lives in holes or crevices -- animal is reclusive

 

b.     Crypsis:       camouflaged organisms -- animals are entirely visible but cannot be distinguished as being different than its

                            background

i.          Colouration  (counter-shading in fish)

ii.         body shape or outline  (thorn bugs, walking sticks)

 

c.     Aposematism:       advertisement by prey that they are dangerous -- (tomato frog, spotted/striped skunks) – bright

                                    colours, noisy, elaborate displays

 

d.     Meullerian Mimicry:      several different species of prey that are both dangerous/toxic/poisonous to eat and appear the

                                            same  (poisonous bugs)

 

e.     Batesian Mimicry:         one species is hazardous to eat and displays aposematism (model), and another species is

perfectly edible yet looks exactly like the model (mimic)  (viceroys - mimic, monarchs - model)

 

2.  SECONDARY DEFENSE:  mechanisms that act to decrease the chances of a prey being killed during an encounter

 

a.     Move to a Retreat::      move as fast as possible to a retreat (prairie dogs retreat to pre-dug holes when they see a hawk)

 

b.     Flight:           moving away from the predator in any way possible                                              

i.          prey runs away in a straight line - occurs when the prey is faster

ii.         protean defense - erratic path to try and "deak out" predator; sudden changes in direction -- occurs when the prey is slower

iii.        "flash" behaviour - prey goes from being conspicuously visible to instantly disappearing (grasshopper)

 

c.     Dematic Behaviour:     prey attempt to frighten the predator

                               

2 types:        i.   true warning - actual harm may result to the predator

                    ii.  bluff - prey is totally defenseless  (frilled lizard)

 

d.     Phanatosis:      playing dead -- effective since many predators don't eat meat that they themselves haven't killed

 

e.     Deflection of Attack:      prey orients the least important part of the body towards a predator  (moths)

 

f.      Retaliation:   prey simply fights back to stay alive -- last resort

 

ü       "BETTER TO LOSE YOUR LUNCH, THAN TO BE SOMEONE'S DINNER!!" -- the element of danger, where the organism is out in the open and more susceptible to predators, will affect the foraging behaviour of any animal and feeding strategy of an individual

 

Homework:       1-5, p. 564

E.  Cumulative Selection

 

·         complex structures, organs, features, body parts, elaborate displays have not evolved purely by chance

·         in 1802, a theologian William Paley argued that like a watch with complex gears and mechanisms, has a “maker”, so must a much more complex structure like an eye have a Creator

·         he questioned evolution because thought that it was based on probability – that complex organs and features have evolved by chance and circumstance

·         evolution does not rely on chance, but on natural selection – selection favours certain individuals over others, a process that is the opposite of chance

 

HOW DID COMPLEX STRUCTURES EVOLVE?

 

·         it is often a difficult for most people to understand how complex organs have evolved from an ancestor that lacks that organ

·         for example, how did something like an eye evolve from nothing?

·         the chance of a sudden production of a fully functional eye is very remote

·         genetically speaking, you would require many thousands of beneficial mutations to occur at the same time in one pair of gametes that bear no previous code for an eye

·         it is highly unlikely to get one beneficial mutation on a single gamete, let alone thousands

·         Figure 1, p. 566 illustrates a more probable explanation of how such complex structures evolved

·         in this model, a series of accumulated mutations that must have benefited the organism took place over a substantial amount of time

·         evolution of a complex structure is a cumulative process

·         beneficial mutations are rare, but when they do occur, natural selection favours them, and the adaptations they produce accumulate one at a time

·         adaptations are any trait that increases the individual’s ability to survive or reproduce

·         cumulative selection is the accumulation of many small evolutionary changes over long periods of time and many generations, resulting in a significant new adaptations relative to the ancestral species

 

HOW DID INSECT POLLINATION EVOLVE?

 

·         evolutionary relationships also occur between two different species that are intimately related

·         there are over 250 000 species of identified flowering plants to date

·         flowers have evolved for many reasons – one major one is for pollination by insects

·         it is believed that flowering plants have evolved from non-flowering seeds

·         the pollen on these ancestral plants was probably non-sticky so that wind could easily pick it up and carry it

·         a mutation that made pollen sticky would have meant a reduction in wind dispersal, but would have had an advantage of sticking to the legs and backs of insects that visited the plant for food

·         as the insects moved from plant to plant, they would inadvertently fertilize the next plant

·         plants with this new sticky pollen experienced greater reproductive success, and the insects also benefited because they helped increase their food supply

·         this unintentional pollination was naturally selected for, and the evolution of any feature that attracted insects, such as colourful leaves or petals, fragrances, and nectar, would have followed

·         Figures 5, 6, and 7, p. 568 show some highly specialized flower adaptations that have evolved to attract insects, birds, and other animals for the purpose of pollination

 

 

 

 

 

 

 

 

 

 

 

 

THE EVOLUTION OF ALTRUISTIC BEHAVIOUR

 

·         when an animal exhibits altruism behaviour its own fitness decreases while the fitness of the recipient of the behaviour increases

·         why should such behaviour exist, since it has no clear advantage to the assisting individual’s reproductive success?

·         animal behaviourists believe that by helping another animal, your own fitness increases indirectly?

·         examples of altruism are social insects, looking after a relative, sacrificing your life for your own child, etc.

·         9000 species of ants, 2000 species of termites, hundreds of species of bees and wasps all exhibit altruistic behaviour

·         non-reproducing sisters help raise their nieces and nephews

·         most Hymenoptera species possess a haplodiploidy genetic system – males are haploid and develop from unfertilized eggs and the females are diploid and develop from fertilized eggs

·         this means that females are more closely related to their sisters than to their own mother, father, or offspring – 50% of their genes are similar to their mother, father, and offspring, where 75% of their genes are shared with their sisters (of the same father)

·         it has been observed that females help their mother raise their sisters more than their own offspring

·         sister-pair nests are also common in wasp species – one lays and watches the eggs, the other looks for food

·         this behaviour or trait of one individual that enhances the success of closely related individuals, thereby increasing the assisting individual’s fitness indirectly is called kin selection

 

Homework:       1-7, p. 570

 

 

F.  Speciation:  The Formation of New Species

 

·         the selection for beneficial variations in nature – natural selection accounts for two things:

o        helps to explain why organisms possess various adaptations

o        accounts for speciation -- the formation of entirely new species, or the evolution of new groups of living organisms

·         there are two types of evolution – microevolution and macroevolution

·         macroevolution is large-scale evolutionary changes significant enough to warrant the classification of groups or lineages into distinct genera or even higher-level taxa

·         microevolution is the evolutionary changes that occur at the species level

 

WHAT IS A SPECIES?

 

• members of a species have similar structure and appearance and interbreed or have the ability to interbreed with each other under natural conditions
• if gene flow occurs between two groups or populations over many generations then the two groups belong to the same species
• reproductive isolation occurs between members of different species
• paleontology relies on similarities in structure and appearance to distinguish between thousands of different species
• many living examples have similar structure and appearance but are in fact a different species (Figure 1, p. 571) – the distinguishing feature of these two different species of frogs is their mating calls
• for sexually-reproducing organisms, behavioural, structural, or biochemical traits possesses by individual organisms, or environmental factors that prevent individuals of different species from reproducing successfully together are called reproductive isolating mechanisms
• there are two kinds of reproductive isolating mechanisms – prezygotic and postzygotic
• prezygotic mechanisms prevent interspecific mating or fertilization to occur
• mating is prevented via the following isolation mechanisms:
  • ecological – species occupying separate habitats
  • temporal – reproductive cycles occur at different times
  • behavioural – distinct mating rituals or displays that prevent the recognition of a mate
• fertilization is prevented via the following isolation mechanisms:
  • mechanical – incompatible sex organs or reproductive features
  • gametic – molecular markers on gametes that recognize each other as their counterparts
• Table 1, p. 572 lists some examples of each kind of isolating mechanism

 

• postzygotic mechanisms prevent maturation and reproduction of offspring from interspecies reproduction
• three things can happen when fertilization occurs successfully between closely related species:

 

• zygotic mortality – no fertilized zygotes or embryos develop to maturity
• hybrid inviability – hybrid offspring are unlikely to live long
• hybrid infertility – offspring of genetically dissimilar parents are likely to be strong but sterile

 

MODES OF SPECIATION

 

• reproductive isolation may lead to the formation of a new species
• when gene pools become isolated, any mutations and subsequent selection processes that occur in one population can make it substantially different than another population
• if the changes are significant they may result in the formation of an entirely new species
• if two populations become geographically separated from one another because of a physical barrier (i.e. water, mountain range), they experience allopatric speciation
• the evolution of populations within the same geographic area into separate species is called sympatric speciation
• imagine a mutation occurring that would produce a viable and healthy tetraploid organism from an original diploid form
• the phenotypes of the two would be similar, which is why they would be adapted to the same environment, but the newly established gene incompatibilities would result in immediate reproductive isolation of the two populations into two new species

 

 

Homework:       1-7, p. 576