IB Biology 11: Evolution - Topic 11

what does evolution concern?

evolution only concerns heritable characteristics, which are traits that are inherited by offspring from parents

evolution is change in the heritable characteristics of a population

what is the mechanism of evolution?

it is natural selection - evolution by natural selection is also called Darwinism

what are examples of characteristics of individual organisms changing during their lifetimes?

- trees can develop a very asymmetric form if they grow in a position exposed to wind

- birds are influenced by hearing their parents singing when they develop their song

- human tennis players develop stronger muscles and bones in the arm they use to hold the racket

- children learn the languages their parents speak

these are known as acquired characteristics

what was the leading theory for evolution before Darwin?

before Charles Darwin published On the Origin of Species in 1859, the leading theory for evolution was based on inheritance of acquired characteristics

inheritance of acquired characteristics is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use during its lifetime

Jean-Baptiste Lamarck was the inventor of this theory, so it is known as Lamarckism

how is Lamarckism falsified?

it is obvious that seeds from a tree growing asymmetrically will not grow into asymmetric offspring unless they are exposed to the same environment as the parent

similarly, a tennis player's children will not develop stronger bones in one arm than the other

no mechanism has been discovered for the environment causing specific adaptive changes to the base sequence of genes, or for it causing the creation of new genes

therefore, acquired characteristics are not inherited and do not lead to evolution

why can we expect to see changes in genes when evolution occurs?

because evolution is a change in the heritable characteristics of a population, meaning a change in the gene

- these changes will happen in the base sequence of DNA or RNA and in the amino acid sequences of proteins made using those base sequences

what is an example of changes in genes when evolution occurs?

COVID-19

- many base sequence changes occurred in the genes of this coronavirus, affecting the viral traits

- some new variants were more successful than earlier ones in spreading through the human population, and the virus evolved

where does evidence for evolution come from?

evidence for evolution also comes from comparing base sequences of the same gene in different species

the more closely related two species are, in their evolution and other traits, the fewer differences in base sequence there are.

- this trend convincingly explained by the theory that species develop over time, gradually diverging from a common ancestor as a result of differences in natural selection

what is a cladogram?

a diagram used in biological classification to show evolutionary relations (common descent) between groups of organisms

how are combinations of differences accounted for?

they are accounted for by repeated splitting of ancestral species by evolution

- this is why cladograms based on sequence differences usually match closely with classifications based on morphology and the likely sequence of splits between lineages

what is another place that evidence of evolution come from?

from gene families that occur across diverse groups of organisms

for example, the Hox gene family occurs widely in animal genomes

- genes in this family help to determine the body plan during development

- such similarities between Hox genes can only be explained by common ancestry, with duplication to give multiple copies of the gene and gradual modification for different functions in different lineages

in what species do Hox genes occur in?

Hox genes occur in aquatic invertebrates and in all animals with a clear head-to-tail axis, including worms, arthropods and vertebrates

- these species form a clade (group) known as the bilateria

what are examples of selective animal breeding done by humans?

humans have bred animals selectively over thousands of years for a range of purposes, including:

- meat and milk production, such as sheep

- transport, such as horses

- pets, such as cats

what are some examples of variation between modern and wild breeds?

- modern egg-laying hens and the junglefowl of Southern Asia from which they have been developed

- Belgian Blue cattle and the aurochs (now extinct) of Western Asia

there is also much variation between different breeds of domesticated livestock, as shown by the diversity of dog breeds

what are examples of selective plant breeding done by humans?

humans have selectively bred a range of plant species for various purposes, including:

- food for humans; for example, wheat

- fibres; for example, cotton

- cut flowers; for example, roses

as with livestock, crop plants resemble wild species of plant but are markedly different

- there are many different varieties of some crop plant species

what is artificial selection?

the only credible explanation for the fact that these species have not always existed in their current forms is that: changes have been achieved simply by repeatedly selecting and breeding the individuals most suited to human uses

- this aforementioned explanation is artificial selection

how does artificial selection show rapid evolution?

the considerable changes that have occurred in domesticated animals and crop plants over relatively short periods of time show that artificial selection can cause rapid evolution

if artificial selection achieved this over the twelve thousand years during which humans have grown crops and reared livestock, it seems reasonable to assume that natural selection could have caused major evolutionary changes over the billions of years of life on Earth

what are homologous structures?

features with similar anatomical position and structure despite differences in function

what are pentadactyl limbs and who discovered them?

Darwin found it curious that the forelimbs of a human, mole, horse, porpoise and bat were apparently so different, yet inside them are the same bones in the same relative positions

these limbs are pentadactyl, which means they have five digits (toes or fingers)

- pentadactyl limbs are an excellent example of homologous structures

what does the pentadactyl limb consist of?

a single bone in the proximal part - humerus (front) and femur (hind)

two bones in the distal part - radius and ulna (front), and the tibula and fibula (hind)

a group of wrist or ankle bones - carpals (front) and tarsals (hind)

a series of bones in each of the five digits - metacarpals (front) and metatarsals (hind), plus phalanges in both

what are examples of uses of petadactyl limbs across species?

amphibians, reptiles, birds and mammals, all have pentadactyl limbs

- crocodiles walk or crawl on land and use their webbed hind limbs for swimming

- penguins use their hind limbs for walking and their forelimbs as flippers for swimming

- echidnas use all four limbs for walking and also use their forelimbs for digging

- frogs use all four limbs for walking and their hind limbs for jumping

what is the explanation for homologous structures?

the explanation for homologous structures such as pentadactyl limbs is that they were inherited from a common ancestor but have evolved in diverse ways as they have become adapted for different functions

- the common ancestor of all tetrapods (four-legged vertebrates) had pentadactyl limbs, which it probably used for walking on land

all of its descendants retain the same basic arrangement of limb bones - this is Darwin's "unity of type"

what do homologous structures not explain?

they do not prove that organisms have evolved into their present forms or that groups of organisms had common ancestry, nor do they reveal anything about the mechanism of evolution

- however, they are difficult to explain without evolution

what are vestigial organs?

vestigial organs are reduced structures that serve no function

- these structures are easily explained by evolution: they no longer have a function so are being gradually lost

what are examples of vestigial organs?

- the beginnings of teeth found in embryo baleen whales, despite adults being toothless

- the small pelvis and thigh bones found in the body wall of whales and some snakes

- the appendix in humans

what are analogous structures?

organisms evolve structures that have similar (analogous) structures or functions in spite of their evolutionary ancestors being very dissimilar or unrelated

- unrelated species have similar or corresponding functions although they evolved from different evolutionary origins

what is convergent evolution?

it is the evolutionary explanation of analogous structures

analogous structures had different origins but became similar because they perform the same or a similar function

what is an example of an analogous structure?

- bird and insect wings

the human eye and the octopus eye are strikingly similar in some respects - however, the human eye has nerve fibres in front of the retina and there is a blind spot whereas in the octopus the nerve fibres are behind the retina and there is no blind spot

what is cladistics?

a method of classification of animals and plants according to the proportion of measurable characteristics that they have in common

- it is assumed that the higher the proportion of characteristics that two organisms share, the more recently they diverged from a common ancestor

how does one determine whether similar structures are homologous or analogous?

cladistics is increasingly used to deduce the evolutionary origins of organisms and their structures

how is the effectiveness of artificial selection shown?

the effectiveness of artificial selection is shown by the considerable changes that have occurred in domesticated animals over periods of time that are very short, in comparison to geological time

it shows that selection can cause evolution, but it does not prove that evolution of species has occurred naturally or that the mechanism for evolution is natural selection

under what circumstances can a species evolve in different ways? what is this process referred to as?

if two populations of a species become separated so they cannot interbreed and natural selection then acts differently on the two populations, they will evolve in different ways

- the characteristics of the two populations will gradually diverge, and after a time, they will be recognizably different

- if the populations subsequently merged and had the chance of interbreeding, but did not actually interbreed, it would be clear that they had evolved into separate species

this is known as speciation

what is explosive species diversification?

where speciation has happened many times in a group, leading to large numbers of species spread over a wide area

- this has occurred in Zosterops, a genus of birds called white-eyes

- there are now over a hundred species in this genus, from Africa though Asia to Australia and New Zealand

what is speciation?

speciation is the formation of a new species by the splitting of an existing species

what two processes must occur for speciation to happen?

two processes are required for this to happen - reproductive isolation of populations and differential selection

(speciation) what is reproductive isolation?

before two populations can split into separate species, they must stop interbreeding with each other - interbreeding causes a mixing of genes and therefore a blending of traits, whereas speciation depends on separation and divergence

for speciation to occur, there must be barriers preventing gene flow between the gene pools of the two populations - this can be achieved by any method of reproductive isolation

geographical separation is the most obvious and probably the most common cause of reproductive isolation - there may be gaps in the range of a species, which divide it into separate populations

- these gaps could be due to physical barriers that are difficult to cross, for example, a mountain range, a wide river or a stretch of ocean between two islands

such barriers prevent interbreeding between populations,so the gene pools are separated

geographical separation is usually associated with differences in selection pressures, which are also required for speciation

(speciation) what is differential selection?

natural selection can cause the traits of a population to change

- however, if it operates in the same way in two populations of a species, their traits will remain the same and they will not become separate species

where there are significant differences in selection, this is called differential or divergent selection

differential selection causes the traits of the populations to become more and more different; when this divergence is judged by taxonomists to be significant, the populations are classified as separate species

what factors can contribute to differential selection after reproductive isolation?

any or all of these factors might be different from the other parts of the species range:

climate - temperatures, rainfall and other aspects

predation - there might be different predators or even no predators in some areas

competition - there might be more or less competition for resources

what is an example of speciation?

the bonobo (shown foraging for insects in the river) and the chimpanzee are both primates from the genus Pan

bonobos are smaller and have markedly different behaviours from chimps

the range of the bonobo and the chimpanzee do not overlap as they are geographically separated by the Congo River which is renowned for being deep

it is thought that at one point in history the water level fell drastically for a time allowing chimpanzees to cross temporarily

it is thought that these migrants became geographically isolated from their ancestors when the water level of the Congo rose again

this founder population, being subject to different selection pressures, diverged from chimpanzees to become bonobos

what is allopatric speciation?

when populations in different geographical areas become separate species, allopatric speciation has occurred

what is sympatric speciation?

when a population of a species living together in one geographical area splits into two populations that do not interbreed

if they remain reproductively isolated, the populations could diverge to form separate species

what is the etymology of allopatric and sympatric speciation?

sympatric means "same homeland" and allopatric means "different homelands"

what may cause reproductive isolation in sympatric populations?

reproductive isolation in sympatric populations may be a consequence of behavioural differences in animals and temporal differences in animals or plants

- sympatric speciation is certainly much less common than allopatric speciation

- it is difficult to be sure whether closely related species living in the same geographical area are the product of true sympatric speciation, or allopatric speciation followed by migration

what is an example of reproductive isolation due to a behavioural difference?

two forms of a species of cichlid fish (astatotilapia calliptera) have been discovered in Lake Massoko, a seven hundred metre wide crater lake in Tanzania

one form prefers to feed near the shore (littoral) and the other in deeper water (benthic)

the two forms have adaptations corresponding to these preferences - body size and shape, structure of the jaw and teeth, coloration of breeding males (bluer or yellower) and sensitivity of retinal pigments to different wavelengths of light

genetic differences have been found between the two forms and experiments have shown that females tend to select a mate who is genetically similar to themselves

- this is an example of behavioural separation, which reduces the mixing of genes between the two forms

- over time, this may result in speciation

what is an example of reproductive isolation due to a temporal difference?

the winter pine processionary moth (thaumetopoea pityocampa) lives in countries around the Mediterranean

during summer and early autumn, they emerge and mate, and females then lay a hundred to two hundred fertilized eggs - the eggs hatch into larvae that feed during the autumn and winter on leaves of pine and cedar trees

in one area of Portugal, researchers have discovered a population of this species that has different timings for all stages in its life cycle - adults emerge in May or June and larvae feed and grow through the summer, rather than the winter

in the warm summer conditions, the larvae grow quickly and are ready to pupate by the end of September

the more common form of the moth, with winter larvae, also lives in this area of Portugal, therefore they are sympatric

the timing of the life cycle is a heritable trait so must be determined genetically - the two forms never mate with each other as there is temporal separation: the two or three days of adult life happen at different times of year

it seems reasonable to assume that the two forms will diverge, because different adaptations are needed by larvae active in summer or winter

- if the divergence becomes great enough, sympatric speciation will have occurred

what are adaptations?

characteristics that make an individual suited to its environment or way of life

- this term is used because the fit between structure and function is developed over time, by a process of modification

- the process of modification is "adaptation" and a trait developed by this process is "an adaptation"

how does a species extend its range?

species extend their range if a group of the population migrates to a new area

these individuals are the founders of a new population - if they cannot interbreed with other populations, the traits of the new population will tend to diverge from the rest of the species

- this is partly due to chance, often aided by the small initial number of founders

- it is also partly a result of adaptation to differences in the environment

what is rapid evolution?

when a species changes and evolves over a short time, or in fewer generations than normal

what is another factor that can cause rapid adaptation?

another factor that can cause rapid adaptation in a new population is the availability of an ecological niche that is not being fully exploited by other species

speciation and adaptation to new niches have happened repeatedly in some groups - this is called adaptive radiation, the word "radiation" meaning spreading out

in this case, the radiation is ecological, rather than geographic

what is adaptive radiation?

adaptive radiation is defined as a pattern of diversification in which species that have evolved from a common ancestor occupy a range of ecological roles

- it is a source of considerable biodiversity

- because of the diversity of ecological niches, adaptive radiation minimizes competition between species so they can coexist

even if the process of speciation is allopatric, migration can occur and closely related species can then live sympatrically

what is an example of adaptive radiation?

Galápagos finches are the best known example of adaptive radiation

- over the past 2.3 million years, fourteen species of finch have evolved from a common ancestor on the islands of the Galápagos archipelago

these finches have become adapted to different food sources: leaves, fruits, pollen, nectar, small soft seeds, large hard seeds, insects on leaves and insects under bark

- the beaks of the finches show particularly clear adaptations to the different types of food they eat

up to ten species of Galápagos finch have been found living together in one locality - it is unlikely that this would be possible without adaptive radiation, as there would be too much competition

what is an interspecific hybrid?

the offspring produced by cross-breeding members of different species (inter-species mating)

- the hybrids combine traits of the species that were crossed

what was likely the first hybrid?

the mule was probably the first hybrid, produced by cross-breeding a horse with a donkey (equus caballus × equus asinus)

mules combine useful traits of those two species and also have what is known as hybrid vigour, and for these reasons, they have been deliberately bred for five thousand years or more

horses have sixty-four chromosomes and donkeys have sixty-two, so a mule has sixty-three - this causes problems in meiosis

- for that reason and other genetic incompatibilities, mules are nearly always sterile (unable to produce children)

who was the first person to use interspecific hybridization to produce new varieties of plants?

the first person known to have done this was Thomas Fairchild who, in the early eighteenth century, crossed carnations with Sweet Williams (dianthus caryophyllus × dianthus barbatus)

- the hybrids showed traits of both parents and were nicknamed "Fairchild's Mule"

both parent species have thirty chromosomes, but even so Fairchild's Mule was sterile - this is very common in interspecific hybrids produced by breeders

how can interspecific hybridization happen naturally?

interspecific hybridization sometimes happens naturally if the ranges of closely related species overlap in an ecosystem

like artificial hybrids, natural interspecific hybrids are often totally or partially sterile so they cause little or no permanent mixing of alleles between the parent species

in evolutionary terms, the resources that a parent expends on producing a sterile hybrid are wasted

it is not surprising, therefore, that many species have evolved barriers to prevent the development of hybrid offspring, as a hybrid zygote may be produced but it is likely to die during development

what purpose does courtship behavior in animals serve?

an individual can check whether a potential partner is a member of its own species by looking for distinctive behavioural features via courtship

there are often several stages in courtship, with rejection at any stage if the characteristic behaviour pattern of the species is not displayed

to prevent interspecific hybridization, courtship behaviour needs to be distinctive

this explains the immense diversity, particularly among birds - birds of paradise in Papua New Guinea, for example

what types of overlap produces fertile interspecific hybrids?

in some cases, closely related species do have ranges that overlap and these species produce fertile interspecific hybrids

this can happen where geographical separation has allowed speciation but migration brings the newly separated species back together again

if barriers to hybridization have not developed, there may be mixing of alleles and speciation may be reversed

this can also happen if humans bring species together that would naturally have remained geographically separated

what does a polyploid organism have?

a polyploid organism has more than two sets of homologous chromosomes

what is polyploidy a consequence of?

polyploidy is a consequence of the duplication of chromosomes in a cell without a subsequent cell division, so it is whole-genome duplication

- genome sequencing studies show that it has happened many times in evolution

what is an autopolyploid?

when an organism has more than two sets of chromosomes from the same species

what is a tetraploid? how is a tetraploid cell created?

if whole genome duplication happens in a diploid cell, the result is four sets of homologous chromosomes, so the cell is tetraploid

what is an autotetraploid?

a cell with four sets of homologous chromosomes, where all the sets of chromosomes come from the same organism

what is autotetraploidy often associated with?

autotetraploidy is usually associated with low fertility, because there are four homologous chromosomes of each type and mis-pairing is very likely during meiosis

- over time, there can be genetic changes that overcome this problem, allowing autotetraploid populations to become established

what genetic changes must occur for autotetraploid populations to exist?

meiosis in an autotetraploid individual produces diploid cells and therefore diploid gametes - if these fuse with haploid gametes from an individual that is diploid, triploid offspring are produced

these may grow vigorously but they are very unlikely to perform meiosis successfully, so are sterile

what is an example of these autotetraploid populations?

sand rock-cress (arabidopsis arenosa) is an example of the establishment of autotetraploid populations

- diploid plants of this species (with 16 chromosomes) only grow in eastern and southeastern Europe

- part of this area, in the Balkan Peninsula and Western Carpathian mountains, has both diploids and autotetraploids

- it is thought that the autotetraploids originated here before spreading to western Europe and Scandinavia, where only autotetraploid plants have been found

what is an allopolyploid?

an organism that has more than two sets of chromosomes that originate from different species

what is an allotetraploid?

a cell with four sets of homologous chromosomes, where the four sets of chromosomes are from two different species

how does allopolyploidy originate? what is the process that occurs?

allopolyploidy is the result of a two-stage process

what is the first stage of allopolyploidy?

- individuals from different species cross-breed

- the resulting interspecific hybrid has two sets of chromosomes, with one set from each of the two different parent species

- unless these two species are very closely related, chromosomes will not form homologous pairs when meiosis is attempted, so the hybrids will be sterile

what is the second stage of allopolyploidy?

- if any cell in the sterile interspecific hybrid duplicates its chromosomes but does not then divide, the cell will have four sets of chromosomes

- it is an allotetraploid because the four sets of chromosomes are from two different species

- more of these allotetraploid cells can be produced by mitosis

- it is likely that these allotetraploid cells will be able to divide by meiosis because there are two homologous chromosomes of each type, which can reliably form pairs

- by becoming an allopolyploid, the interspecific hybrid will overcome its fertility problems

what can allotetraploids not breed with?

allotetraploids can interbreed with other allotetraploids, but not with either of the diploid parent species

- they are therefore a new species and, as they have a mixture of traits from both parent species, they are usually recognized and named as a distinct species

many species have been produced by this two stage process, especially in the plant kingdom - for example, in the genus Persicaria

how can the theory of evolution by natural selection be explained?

the theory of evolution by natural selection can be explained with the following series of statements:

- organisms produce more offspring than the environment can support

- among these offspring, there is variation

- some variants are better suited to the environment and have a higher chance of surviving to reproductive age, and less fit (less well adapted) variants have a higher risk of mortality from predation or other factors

- the features that aid survival are disproportionately inherited by successful offspring, and such features therefore increase in frequency in the population (heritable features of the population have changed, so it has evolved)

is evolution often gradual?

while evolution can occur rapidly in response to sudden changes in the environment, the process is often gradual

what observation did Darwin base the theory of evolution by natural selection on?

one of the observations on which Darwin based the theory of evolution by natural selection is variation

- variation in human populations is obvious in traits such as height, skin colour and blood group

in other species, the variation may not be so immediately obvious but careful observation shows that it is there

natural selection can only occur if...?

natural selection can only occur if there is variation within a population and some variants are removed while others survive and reproduce

what are causes of variation in a population?

mutation is the original source of variation - new alleles are produced by mutation, which enlarges the gene pool of a population

meiosis produces new combinations of alleles, by breaking up the existing combination in the diploid cells of a parent to produce haploid cells with only one allele of each gene

- because of crossing over and the independent orientation of bivalents, every cell produced by meiosis in an individual is likely to carry a different combination of alleles

sexual reproduction involves the fusion of male and female gametes - the gametes usually come from different parents, so offspring have a combination of alleles from two individuals

- this allows mutations that occurred in different individuals to be brought together

what is the only source of variation in species that do not carry out sexual reproduction?

in species that do not carry out sexual reproduction, the only source of variation is mutation

- it is generally assumed that such species will not generate enough variation to be able to evolve quickly enough for survival during times of environmental change

what is an example of living organisms varying in the number of offspring they produce?

an example of a species with a relatively slow breeding rate is the southern ground hornbill, Bucorvus leadbeateri

- it raises one fledgling every three years on average and needs the cooperation of at least two other adults to do this

- however, these birds can live for as long as seventy years so in their lifetime, a pair could theoretically raise twenty offspring

most species have a faster breeding rate - for example, the coconut palm Cocos nucifera, usually produces between twenty and sixty coconuts per year

what organism has the fastest breeding rate?

apart from bacteria, the fastest breeding rate of all may be in the fungus Calvatia gigantea

- it produces a huge fruiting body called a giant puffball in which there can be as many as seven trillion spores

what is the trend between the number of offspring produced and the resources available?

despite the huge variation in breeding rate, there is an overall trend that more offspring are produced than the amount able to be supported by the available resources such as food

- the population size that can be supported by the environment is known as the carrying capacity of the environment

- it tends to be determined by the limiting resource that is in shortest supply

for a plant species, this is likely to be water in a desert or light in a rainforest

what is a biotic factor?

living things within an ecosystem

- such as microorganisms, plants, and animals

what is an abotic factor?

non-living things within an ecosystem

- such as landforms or chemicals

what is a selection pressure?

external agents which affect an organism's ability to survive in a given environment

what does the overproduction of offspring result in?

Darwin pointed out that overproduction of offspring results in a struggle for existence within a population

there is competition for resources and not every individual will obtain enough to allow them to survive and reproduce

what is an example of a botic factor?

consider the example of a population of animals in which deaths are occurring due to shortage of food

competition for food is acting as a selection pressure - botic factor

- it is density-dependent, because as population density increases, competition for food becomes more intense

- a density-dependent factor causes either birth rate to fall with rising population density or death rate to increase with population density

what is an example of an abotic factor?

now consider the example of a population of plants experiencing a period of freezing weather that kills some of the plants but not others

low temperature is acting as a selection pressure - abotic factor

it is density-independent because the chance of an individual plant being killed by the freezing temperatures is the same whether the population density is high or low

differences in tolerance to cold, rather than competition between individuals, is the basis of the selection

- there are other examples of abiotic density-independent factors causing selection pressure on plant and animal populations, and they are often associated with catastrophe such as flooding, earthquakes, fire, pollution or extreme climate conditions such as heat waves or drought

what different adaptations can affect rates of survival and reproduction?

since Darwin published his theory, numerous cases have been found of differences in adaptation that affect rates of survival and reproduction

some traits are adaptations to abiotic factors in the environment and others help an individual in their response to biotic factors

what is fitness?

the term "fitness" means how well-adapted an individual is

- something that is suited to its purpose or role is said to be fit

fitness is therefore specific to purpose - in evolutionary biology, fitness results from having adaptations for a specific niche within an ecosystem

why do different species need different adaptations?

different species occupy different niches so they need different adaptations

- there is variation among individuals in a sexually reproducing population

what does fitness influence?

fitness influences whether or not an individual survives for long enough to be able to reproduce and, of those that survive, how many offspring they have

the fittest individuals tend to survive longest and have the most offspring, and they therefore make the largest contribution to the gene pool of the next generation

the next generation is more like them than the less fit individuals who produced few or no offspring

what is intraspecific competition?

competition between members of a species

what is fitness a form of?

fitness is a form of intraspecific competition, and differences in survival and reproduction are the basis of natural selection

what is a common misconception about evolution?

a common misconception about evolution is that adaptations acquired during the lifetime of an organism can be inherited by offspring

if this happened, a professional tennis player's children would be born with stronger bones and muscles in one arm, either the left or right, depending on which arm their parent used to hold the racket

do living organisms develop traits during their lifetime?

yes, living organisms do develop traits during their lifetime

- if these are caused by random mutations, they are not adaptive and are not heritable because base sequences of genes are unaltered

- acquired traits may be caused by the environment, for example, the loss of a leg in a spider

- they may also be caused by interaction between genes and environment

what is an example of a trait being caused by an interaction with the environment?

for example, a Himalayan rabbit kit (baby) that is exposed to as little as twenty minutes of cold develops darker fur

normally, black fur only grows at the extremities of the body, the ears and the feet, but coldness changes gene expression and causes skin cells responsible for hair growth to produce melanin

- no change in the base sequences of genes has occurred

furthermore, the skin cells are not used for gamete production so their genes cannot be passed on to offspring

the environment has not caused specific changes to the base sequences of genes in the sex cells, so the acquired trait is not heritable and it cannot contribute to evolutionary change

how can changes in gene expression caused by the environment be passed onto offspring?

to a limited extent, changes in gene expression caused by the environment can be passed onto offspring

this is achieved using epigenetic tags, which are chemical markers, reversibly added by a cell to its chromosomes, which establish a pattern of gene expression

a small proportion of these tags is passed on in gametes to offspring, but it is only a pattern of gene expression that has been passed on, not differences in the base sequence of genes

what will an animal that is well-adapted enough to survive until reproductive age do?

an animal that is well-adapted enough to survive until reproductive age will usually find a mate and reproduce sexually with them

- only half of the genes in a well-adapted animal's offspring are from them, and the other half are from the mate

what will happen if the mate is well-adapted or not well-adapted?

- if the mate is also well adapted, the offspring are likely to survive and reproduce, so the genes persist in the population and will contribute to evolution

if the mate is badly adapted, the offspring are unlikely to survive and reproduce, so the animal's genes do not persist in the long term

what is sexual selection?

the process of assessing accurately whether or not a potential mate is well-adapted

a binary choice must be made based on the overall fitness of the potential mate

- in many animal species, this happens during the courtship process

what is an example of sexual selection?

in some species, the criteria for selection are obvious, such as the ability of a male to overpower other males by fighting

in other species, the rationale is less obvious - some animals have anatomical features that seem to the human eye to be excessive, such as the plumage of a peacock

there are about forty species of birds of paradise living on New Guinea and nearby islands

the males have very showy plumage that are of no use in flying, whereas the females that build the nest, incubate the eggs, and care for the young, are relatively drab

males in many of the species have a complicated and eye-catching courtship dance that they use to try to attract females

- in some species, the males gather at a site called a lek and females select a mate from among the males displaying