A4.1 Evolution and speciation [DONE] Flashcards
3 types of evidence for evolution
- Molecular evidence from genetic data & amino acid sequences
- Experimental evidence from selective breeding of animals & plants
- Morphological evidence from homologous structures
Homologous vs Analogous structures
Homologous: features of organisms that reveal they come from a common ancestor
Analogous: features evolved from different body parts but serve the same purpose
Definitions
Evolution refers to the process of cumulative change in the heritable characteristics of a population
Cumulative change: changes are small and occur over multiple generations, leading to accumulation of traits that drive change over time
Heritable characteristics: traits that are derived from genes that can be inherited from one generation to another
Population: changes are only seen in a population of organisms, never within individuals
Evolution
Evolution is the cumulative change in population over time
A popln has variations amongst the individuals due to mutations, meiosis and sexual reproduction
- certain variations give an advantage to some organisms over others in certain environments
Poplns produce more offspring than the environment can support
- Indivs of the species compete for the same resource
- the better adapted organisms tend to survive and reproduce
- the less adapted organisms tend to die or reproduce fewer offspring
- Indivs that reproduce pass on their heritable genes to their offspring
Natural selection increases the frequency of heritable genes of the better-adapted organisms
eg evolution of antibiotic resistance in bacteria
Traditional evidence for evolution
Derived from observable (phenotypic) traits like fossil records
- many fossils link tgt existing organisms with their likely ancestors
- eg Equus (horse, zebra) are evolved from Hyracotherium (similar to rhinoceros)
Establish a sequence of smaller changes observed in a series of fossils that has been carbon-dated and arranged in chronological order
- the presence of subtle changes in bone length, shapes of bones can be used to determine directional changes that are used as evidence for evolution
Molecular evidence for evolution
Scientists look at changes in base sequences of genes in DNA
- trace sequence where mutation occured, leading to new alleles formed –> trace how organism has evolved
mRNA sequences (which are without introns) can be used in the same way
amino acid sequences in proteins have a genetic basis from DNA and hence can also be studied to find evidence for evolution
Selective breeding evidence for evolution (domesticated animals and crop plants)
- Variation between domesticated animals with their original wild species
- dogs are domesticated animals bred for traits like temperament, appearance, intelligence to be trained
- grey wolves are wild animals that are technically the same species but have v diff traits from dogs - Variation between domesticated plants with their original wild species
- eg corn vs wild corn - Variation between diff breeds of domesticated crop plant varieties as evidence for evolution
- selective breeding resulted in many diff varieties of grapes w diff taste profiles and uses
- evidence of evolution, showing that breeding can result in inheritable changes
*selective breeding can rapidly create new breeds of domesticated plants and animals which may take only a few generations before new varietals develop (bcs of the intervention of man to choose and selectively breed them, the rate of evolutionary change is much faster than in the wild)
Homologous structures evidence for evolution
Homologous structures are structures that show similarities in the underlying components (eg shape, size, arrangements), they do not necessarily have similar functions
- presence of homologous structures point towards the presence of a common ancestor that these structures descended from
Eg Pentadactyl Limb (MUST MEMO)
- pentadactyl means “five fingers”, found in amphibians, reptiles, birds and mammals
- BUT pentadactyl limb does not focus on presence of five fingers but is used to refer to the bone structure of limbs: 1 humerus bone, the radius and ulna attached to it (2 bones), and then multiple bones that make up the fingers
- the underlying similar bone structures found in pentadactyl limbs of animals point towards a common ancestor with the particular bone structure
- but in diff animals, it has diff uses (eg bat for flight, mole for digging) and this suggests evolutionary divergence
However, animals have evolved differences in the structures, adapted to the ways their limbs are used to walk, swim or fly.
THEREFORE, this is known as divergent evolution: evolution of traits that started from a common ancestor leading to similar underlying structures but different functions
This process is called adaptive radiation: the rapid evolutionary diversification of a single ancestry line (common ancestor)
- occurs when members of a single species occupy a variety of distinct niches w diff environmental conditions
- members evolve diff morphological features (adaptations) in response to the diff selection pressures
Summary of evidence for evolution
- Fossils give evidence of evolution as fossils show diff species existed in the past and how they changed over time
- Selective breeding of domesticated animals or crop plants give evidence for evolution, as selective breeding shows that artificial selection can cause rapid change resulting in evolution
- Homologous structures (eg vestigial organs) give evidence of evolution
- Homologous structures (eg pentadactyl limbs) also show common ancestry
- DNA sequences show common ancestry
Analogous structures
Analogous structures have the same function but different evolutionary origins
Analogous structures: structures that provide similar functions and external appearance, but do not have similarities in their underlying structures
–> implies that they have diff evolutionary origins from diff ancestors but due to the application of same selection pressure, structures w similar look and function evolved
Eg The human eye vs octopus eye (MUST MEMO)
- they are strikingly similar in some respects
- but 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
- the squid eye is v complex and similar to the mammalian eye, but they evolved independently from each other and hence they are the product of convergent evolution so they are analogous structures
Convergent evolution is the origin of analogous structures
Convergent evolution: evolution of traits that are very similar in function, but have different evolution origins
Eg a flat surface attached to muscles at the tail end of an animal is an efficient way to swim, as seen in penguins, dolphins and fish but they all have diff evolutionary origins and they evolved a tail to swim independent of each other
Homologous VS Analogous
- structures are similar in construction but may look superficially different and perform different functions VS structures which perform same function but diff in construction
- EVOLUTIONARY EXPLANATION: they have had the same ancestral origin and they have become diff bcs they perform diff functions –> adaptive radiation VS they have had diff ancestral origins and have become similar bcs they perform the same/ similar function –> convergent evolution
- eg forelimbs of humans, cats, whales and bats show the same arrangement of bones from the shoulder to the tips of the digits but they perform diff functions according to their habitat adaptation VS eg wings of bat and bird, both are for flying but bat wings consist of flaps of skin stretched between the bones of the fingers and arms while bird wings consist of feathers extending all along the arm. these structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor w wings
- eg in plants, all the stem modifications/ root modifications are homologous with each other, such structures serve diff functions like support, food storage etc VS eg leaf tendril and stem tendril perform the function of coiling/ support but one is of foliar origin and the other is stem
- similar internal structures eg pentadactyl limb due to common ancestry despite diff functions VS diff evolutionary origin as they do not share recent common ancestor despite similarities of function
- arise by adaptive radiation/ divergent evolution VS arise by convergent evolution
- classification based on homologous traits has predictive values as it matches evolutionary history VS artificial classification is based on analogous traits and it brings tgt dissimilar species
Speciation
Speciation is the creation of a new species that split off from the ancestral species
- new species can only arrive from the splitting of pre-existing species as new life cannot spontaneously occur
- due to differences in selection pressure, a popln may have accumulated sufficient differences in genes and traits due to natural selection that they are no longer able to mate with the original popln species and hence a new species is formed
Number of species on Earth
As speciation arises when a popln splits off from another existing popln species, this increases the total number of species on earth
BUT animals can go extinct and this reduces the total number of animals on earth
Gradual evolutionary change in a species is not speciation
Over the course of thousands/ millions of years, poplns may experience changes in the environment
- these poplns are exposed to selection pressures –> resulting in gradual evolutionary changes
BUT if the whole popln changes, there is no splitting off of a new species and hence no speciation has occurred
Species
- organisms can potentially interbreed to produce fertile offspring
- have same sequence of genes on chromosomes
- have similar phenotype
- have same chromosome number
Differential selection (and its role in speciation)
Speciation occurs when a popln is split into at least 2 groups, w each group exposed to a diff set of selection pressures
- with diff environments and conditions leading to diff selection, diff traits are selected for in diff groups
Therefore, speciation requires the presence of DIFFERENTIAL SELECTION in diff poplns of the same species
Reproductive isolation (and its role in speciation)
However, even if 2 poplns of the same species are exposed to diff selection pressures, if indivs from the 2 poplns can mate w each other, there will still be gene flow between the 2 poplns
- the introduction of alleles from other poplns will disrupt the effects of natural selection, leading to significant changes to allele frequencies and the selection of new traits
Therefore, for one popln to evolve into a new and distinct species from the other popln, REPRODUCTIVE ISOLATION is required
*can be sympatric/ allopatric
* polyploidy
Types of reproductive isolation
- Geographical isolation (MUST KNOW)
- natural barriers (eg mountain range, desert, wide rivers) can seperate two poplns
- various physical barriers (land/ water formation such as a river, mountain, or clearing in a forest) prevent males and females from finding each other so interbreeding becomes impossible
- gene pool of a popln is separated from that of another popln
- speciation may occur when this happens
- eg spotted owl subspecies living in diff geographic locations show some genetic and morphological differences –> consistent with idea that new species form through geographic isolation - Temporal isolation
- isolated in time: poplns may mate or flower at diff seasons/ diff times of the day –> prevents poplns/ gametes from encountering each other
Eg Malaysian orchid (dendrobium genus) is technically capable of pollinating w each other bcs they are not isolated geographically BUT the orchid flowers in response to changes in weather conditions (eg response to a sudden shower on a hot day)
- orchids will flower at dawn, and wither by nightfall
- there are 3 distinct variants of the orchid, and each differ from each other in the time after the stimulus before they flower
- one variant flowers 8 days after sudden shower, another 9 days and the third variant 11 days
Eg American toads mate in early summer but Fowler’s toad in late summer; preventing them from interbreeding even tho they live in same habitat - Behavioural isolation
- happens when one popln’s lifestyle and habits are not compatible with those of another popln
- when closely related indivs differ in courtship behaviour, they are often only successful in attracting members of their own popln
- hence little or no reproduction will occur between the members of 2 poplns because of behavioural differences
Eg satin bowerbird builds a channel between upright sticks and decorates with bright blue objects in order to woo females but macgregor’s bowerbird builds a tall tower of sticks and decorates with bits of charcoal
Example for geographical isolation (and behavioural?)
Separation of bonobos and common chimpanzees by Congo river:
- bonobos and chimpanzees are closely related species of monkeys that are believed to have diverged from their common ancestor about 1.8 million years ago
- chimpanzees have social structures that are male dominated and they are found north of Congo river
- bonobos have female dominated social structures and are found south of the Congo river
- chimpanzees are more aggressive & territorial but bonobos are more peaceful and normadic –> diff physical traits even if they look similar
- wide Congo river served as a geographical barrier leading to reproductive isolation of both poplns. leading to formation of two species
Sympatric speciation (AHL)
Sympatric speciation occurs without geographical isolation, and can be due to other forms of isolation like temporal/behavioural.
It can take place when there is an overlap in geographical location of two poplns, meaning that speciation can occur even if the two poplns are in the same location
Diff & similarities between sympatric & allopatric speciation (AHL)
Similarities:
1. both results in the divergence of a new species from another species
2. both involves some form of reproductive isolation & exposure to diff selection pressures
Differences:
1. allopatric speciation arises from geographic isolation while sympatric speciation arises from other forms of speciation other than geographic
Changes in gene pool (AHL)
Gene pool is all genes in an interbreeding population.
- gene pool splits during speciation due to reproductive isolation of groups within a species
- temporal, behavioural & geographic isolation causes reproductive isolation
- divergence of gene pools is due to:
- changes in allele frequencies due to differences in natural selection in isolated groups
- diff random mutations occur in isolated poplns
- speciation has occured when differences between populations prevent interbreeding
Adaptive radiation (AHL)
- as a source of biodiversity
Definition: Adaptive radiation refers to the evolution of diff populations/ species that arose from a single ancestral population
natural selection takes place w diff selection pressures arising from an environment where distinct ecological niches are available –> diff selection pressures select for certain traits, leading to diff poplns showing diff modified traits –> resulting in formation of diff species in the environment w diff species filling up the diff available ecological niches
Adaptive radiation allows for the closely related species to co-exist without competing, increasing biodiversity in ecosystems where there are vacant ecological niches
Interspecies hybrid (AHL)
An interspecies hybrid refers to the offspring formed when parents of two diff species mate
- these hybrids are sterile –> they cannot mate to produce offspring –> therefore they are not considered to be a new species
- the sterility of these hybrids are impt as they cannot reproduce, and this prevents mixing of alleles btw species
Example: Mules, horse & donkey pairing (MEMO)
1. the mule was produced by cross-breeding a female horse & male donkey
2. mules combine useful traits of those two species & have what is known as “hybrid vigour”
3. therefore they have been deliberately bred for 5000 years
4. horses have 64 chromosomes and donkeys have 62 so a mule has 63 –> causes problems in meiosis
5. for this reason & other genetic incompatibilities, mules are nearly always sterile
Barriers to prevent mixing of alleles between species (AHL)
- many species have specific mating calls & rituals and other behavioural factors that ensures that only animals of the same species will mate w each other
- in many species of plants, the pollen grains will only germinate if it lands on the stigma of the same species, based on recognition of specific chemicals on the stigma
Mixing of alleles in diff species (AHL)
it is believed that the mixing of allele btw 2 diff species is bad for the poplns
- it can result in the formation of intermediates which are less adapted to the environment
- in some cases, it can result in deleterious variation which reduces the reproductive fitness of the organisms
Barriers to hybridisation & sterility of interspecies hybrids prevent the mixing of alleles btw species
Polyploidy (AHL)
Definition: Polyploidy is defined as the condition when the organism contains more than 2 sets of homologous chromosomes, meaning that they are not diploid
*Indivs w 3 sets of chromosomes are known as triploid, 4 sets is tetraploid
Polyploidy usually arises from complete non-disjunction during meiosis, where homologous chromosomes do not separate at all, leaving gametes which are diploid
- after these diploid gametes fertilise haploid gametes, a triploid offspring forms
- can be lethal in most organisms but some plants can tolerate polyploidy very well
- some polyploid plants even have improved vigour, size, yield and disease resistance
Allopolyploidy (AHL)
Definition: allopolyploidy refers to the formation of hybrid species w more than two sets of chromosomes, arising from parents who have diff numbers of sets of chromosomes
Example: Knotweed/ Smartweed (MEMO)
- the plant genus persicaria consist of diff species that differ in the number of sets of chromosomes present
- persicaria foliosa is diploid
- persicaria japonica is tetraploid
- persicaria puritanorum is hexaploidy (6n), it is believed to have arisen when persicaria foliosa (2n) crosses w persicaria japonica (4n), & complete non-disjunction occured in both parental plants, leading to the fusion of a 2n gamete with a 4n gamete to form a 6n hexaploidy plant that is a new species
Abrupt speciation in plants by hybridisation & polyploidy (AHL)
Many plants can form new species through the process of hybridisation & polyploidy
- not all hybrids formed are sterile & some hybrids go on to reproduce and form new species
- more common in plants than animals
- polyploidy is another way that new species can form, esp in plants
Summary of speciation
- speciation is the splitting of a species into 2 species due to reproductive isolation
- isolation is due to geohraphy, reproductive timing, behaviour
- polyploidy can cause isolation as gene pools are separated
- disruptive selection cause gene pools to diverge
Summary on natural selection result in speciation
- variation is required for natural selection in species
- mutation, meiosis & sexual reproduction are sources of variation
- competition exist due to more offspring produced than the environment can support
- adaptations that make certain indivs more suited to their environment would lead to survival of these better adapted indices
- hence these better adapted will reproduce more & pass on genes to next gen
- speciation is formation of a new species
- can be due to reproductive isolation of separated poplns
- geographic, behavioural isolation & disruptive selection of populations can lead to speciation
- gradual divergence of poplns is due to natural selection
- changes in gene pools of separated poplns causing interbreeding to be impposible –> speciation
Analogous & Homologous structures (key points)
- the pentadactyl limb can be found in lizards, whales & bats, showing the diverse ways in which such a limb can be used, like walking, swimming & flying
- uniformity in bone structure & position within limbs indicates that all these organisms had a common ancestor
- analogous structures have evolved on diff branches of the tree of life but serve the same purpose (eg wings of birds & insects)
- wings allow flight in both groups of organisms, but they have not evolved from the same body parts
