Evolution and Speciation

A4.1 Evolution and Speciation 

A4.1.1: Evolution as change in the heritable characteristics of a population 

Theories of Evolution 

• Evolution is change in the heritable characteristics of a population over time 

Lamarck’s theory 

• Organisms acquire traits when they are alive 

- Acquired traits are beneficial for survival and can be passed onto offspring

- Causes Evolution over time 

- Theory is not supported by genetics 

- Acquired traits are NOT inherited/passed onto offspring 

Darwin’s (and Wallace's) theory 

• Variation exists within a population

• Nature selects individuals with traits the best adapted to survival and reproduction 

  • Favourable traits are passed onto offspring

  • Causes evolution of time 

• Theory IS supported by genetics-variation is present in a population due to the presence of alleles

Scientific Theories 

• A theory is an explanation of an aspect of the natural world which has been repeatedly tested through observation and experimentation

Evidence for evolution 

• Evidence for IB course includes DNA, RNA, protein sequences, homologous structures, and selective breeding 

A4.1.2: Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins 

Biomolecules

• The biomolecules DNA, RNA and proteins provide strong evidence for evolution 

  • Same genes are present in organisms which have evolved from a common ancestor

• Differences in base sequences of DNA (leads to RNA and proteins)-result of mutations

• Mutations accumulate gradually over long periods of time at a constant rate 

• Closely related species have very similar gene and protein sequences

  • Small number of mutations because they diverged from a common ancestor

A4.1.3: Evidence for evolution from selective breeding of domesticated animals and crop plants 

Selective Breeding 

• Selective breed is the process of humans choosing plants or animals with desirable traits to breed together and produce offspring with more of those desirable traits 

Breeding of dogs

• Humans have selectively bred dogs with desirable traits to create the wide variety of dog breeds available today 

Breeding of Brassica Oleracea

• Selective breeding results in changes to the heritable characteristics of organisms and is observable evolution 

• The selective breeding of Brassica oleracea demonstrates how rapidly evolution can occur 

• Kale, brussel sprouts, cauliflower, and cabbage are all varieties of a single plant species 

A4.1.4: Evidence for evolution from homologous structures 

Homologous structures 

• Homologous structure strongly suggest that organisms have evolved from a common ancestor 

• Homologous structure - similar physical features in organisms that share a common ancestor, but the feature has different functions

• Homo structures are similar in structure, but may have different functions

• Present in organism that have descended from a common ancestor as a result of evolution 

• Why did they differentiate?

  • Populations migrated to different locations=different challenges=different adaptations

  • Populations evolved over time and become better adapted to their environment-results in modified homologous structures which are adapted to their new environment

Pentadactyl Limbs are Homologous Structure 

• Pentadactyl limbs (limb with 5 digits) are an example of homologous structures. All organisms with pentadactyl limbs have evolved from a common ancestor

• The structure of the  limb is similar in all species but has evolved modifications for a variety of purposes such as carrying tools in humans, running dogs, flying in birds and swimming in whales

A4.1.5: Convergent evolution as the origin of analogous structures 

Convergent evolution

• Evolution of similar structures in species not related due to a recent common ancestor 

Analogous structures 

• Analogous structures have a common function but do not have a common structure 

• Analogous structures evolve by convergent evolution

  • Convergent evolution - distantly related organisms independently evolve similar traits to adapt to similar necessities 

  • Organisms with analogous structures do not share a common ancestor with the structure 

Wings 

• WIngs of birds, bats, and insects are analogous structures 

• Wings in all three groups have the same function (flight)

• None of the groups of organisms share a common ancestor with wings 

A4.1.6: Speciation by splitting of pre-existing species 

Speciation 

• Formation of new species through evolution 

• Occurs as pre-existing species evolve into new species over time 

Speciation and Extinction 

• Gradual evolutionary change within a species is not speciation 

  • Unless the original species evolves into a population of organisms which are no longer able to reproduce with the original population 

  • Extinction occurs when there are no living members of a species remaining 

  • Speciation increases the total number of species on earth but extinction reduces the total number of species on the planet

  • Many species are in danger of extinction due to human activities e

A4.1.7: Roles of reproductive isolation and differential selection in speciation 

Reproductive isolation 

• Reproductive isolation occurs when there is a barrier which prevents individuals from reproducing

• Speciation can only occur if populations of a species are reproductively isolated

Geographical  isolation

• Reproductive isolation is often a result of geographic isolation

• Geographical isolation occurs when two populations of the same species are prevented from reproducing because of geographical features such as rivers, mountains or being on different islands 

Evolution of CHimpanzees and Bonobos 

• CHimpanzees and bonobos are closely related apes-both are the closest relative to humans- but they have different characteristics 

A4.1.8:Differences and similarities  between sympatric and allopatric speciation 

Sympatric and Allopatric Speciation 

• Produce new species because of reproductive isolation 

Allopatric speciation 

• Occurs due to geographical isolation: the physical 

• separation of two populations of the same species 

  • 2 populations are not able to interbreed

  • Over time evolve into different species due to different selective pressures in the two locations

Sympatric Speciation 

• Occurs when a population divides into different species while inhabiting the same habitat. There is no geographical isolation 

• Reproductive isolation in sympatric speciation can occur due to behavioral isolation of temporal isolation 

Behavioural Isolation 

• Occurs when individuals are reproductively isolated due to behaviour 

• Over time can lead to sympatric speciation within a species 

• Mating behavior is an example 

• For instance, the geographical ranges of the western and eastern meadowlarks overlap and the birds are capable of reproducing but they do not reproduce with each other as they use different signs to attract mates 

Temporal Isolation 

• occurs due to organisms reproducing at different times, resulting in productive isolation 

• Occurs when populations differ in when they reproduce 

• Cicadas are an exmaple 

A4.1.9: Adaptive radiation as a source of biodiversity 

• Adaptive radiation–the evolution of a single ancestral species into several species, as the ancestral species spread out and become adapted to new environmental niches. 

results in divergent evolution.

• Occurs when a single species moves to and occupies a variety of different niches

Populations in the different niches evolve different features due to different selective pressures in each niche

AR increases Biodiversity 

• Adaptive radiation and divergent evolution increase the biodiversity of an ecosystem with vacant niches.

• A single species of finch evolved into 18 different species of finches on the Galapagos Islands due to adaptive radiation.

A4.1.10: Barriers to hybridization and sterility of interspecific hybrids as mechanisms for preventing the mixing of alleles between species

Hybrid

• A hybrid is the offspring of two different species.

• Mules are an example of hybrid organisms  as they are the offspring of a male donkey and a female horse.

• Hybrids are usually sterile, and are not considered a species.

• Hybrids are rarely formed between different species due to prezygotic or postzygotic barriers.

• The barriers prevent the mixing of alleles between different species.

• Prezygotic barriers include:

  • Behavioural isolation - courtship behaviour prevents the hybridization of closely related species.

  • Temporal isolation - different species are reproductively active at different times.

  • Ecological isolation - species cannot reproduce if they are in different habitats.

  • Mechanical isolation - occurs when physical differences between organisms prevent sexual intercourse.

• Postzygotic barriers include: 

  • Hybrid inviability - Offspring are produced, but do not survive to become sexually mature adults.

  • Hybrid infertility - Hybrid individuals are produced, but they are not capable of producing functioning gametes. The hybrid is sterile, due to the genetic incompatibility between the two species.

  • Hybrid breakdown - The first generation of hybrids are capable of reproducing, but their offspring are not able to reproduce. 

A1.2.11: Abrupt speciation in plants by hybridization and polyploidy

Polyploidy 

• Haploid nuclei have one set of chromosomes.

• Diploid nuclei have two sets of chromosomes.

• Polyploid organisms have more than two sets of chromosomes in their cells. 

• Polyploidy results from non-disjunction, an error during meiosis (while producing gametes).

• A triploid (3n) organism (three sets of chromosomes) is produced when a diploid gamete is fertilized by a haploid gamete.

• A tetraploid (4n) organism (four sets of chromosomes) is produced when a diploid gamete is fertilized by a diploid gamete.

Abrupt Speciation in Plants through Polyploidy 

• Polyploidy can produce immediate speciation, as an organism is produced with a different number of chromosomes.

• Polyploidy is common in many groups of plants including the genus Persicaria, which is commonly known as knotweeds. 

• Polyploid plants often produce bigger fruits.

• Polyploid plants with an even number of sets of chromosomes (4n, 6n, 8n …) are capable of reproducing sexually, as meiosis can occur with homologous pairs of chromosomes.

• Polyploid plants with an uneven number of sets of chromosomes (3n, 5n, 7n …) are sterile, as not all of the chromosomes can form homologous pairs during meiosis. Many of these plants can reproduce asexually.