A4.1 - Evolution and Speciation

Question 1

Theories of Evolution

Answer 1

• Lamarckism: suggests that organisms acquire traits when they are alive
• These acquired traits are beneficial for survival, and can be passed onto offspring, causing evolution over time
• Lamarck’s theory is not supported by genetics, as acquired traits are not inherited and passed on to future generations
• Darwinism: suggests that variation exists within a population and nature selects the individuals with the traits best adapted to survival and reproduction – natural selection
• Favourable traits are passed on to the offspring, causing evolution over time
• Darwin’s theory is supported by genetics, as variation is present in a population due to the presence of alleles

Question 2

Defining Evolution

Answer 2

• The cumulative change in the heritable characteristics of a population over
  time
• Cumulative change = a gradual accumulation of small changes over many
  generations
• Heritable characteristics = features encoded for by genes and transferred
  between generations (ie: alleles)
• Population = group of the same species living in the same area at the same time

Question 3

Types of Evolution

Answer 3

Divergent Evolution
When two species diverge from a common ancestor and develop different characteristics

Convergent Evolution
When organisms that do not share a common ancestor evolve similar features

Question 4

Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins

Answer 4

• Biomolecules like DNA, RNA and proteins provide strong evidence for evolution
• The same genes are present in organisms which have evolved from a common ancestor
• Differences in the base sequences of DNA (and therefore RNA and proteins) are the result of mutations, which accumulate gradually over long periods of time
• Closely related species that have recently diverged from a common ancestor have very similar gene and protein sequences, as there will be a smaller number of mutations

Question 5

Evidence for evolution from selective breeding of domesticated animals and crop plant

Answer 5

Evidence: Selective Breeding

• Selective breeding (aka artificial selection) is the process by which humans breed animals and plants based on desirable characteristics
• This accelerates the evolutionary process as changes become visible over a shorter time interval (ie: divergent evolution)
• Examples:
• Crops for increased yield and variety
• Cows for milk or meat production
• Dogs for herding, hunting or racing

Selective Breeding: Crops

• Humans often selectively breed crops to increase disease resistance and yield
• Selective breeding of plant crops has also allowed for the generation of new types of foods from the same ancestral plant source
• Ex: selective breeding of the wild mustard plant gave rise to many of the vegetables we eat today, like kale, cabbage and cauliflower

Question 6

Evidence for evolution from homologous structures

Answer 6

• Homologous structures are similar in
  structure, but may have different functions
• Homologous structures are present in
  organisms that have descended from a
  common ancestor
• They develop as a result of divergent
  evolution
• Populations of the common ancestor
  migrated to different locations with different
  challenges for survival
• Over time, populations evolved to be better adapted to their environments
• This resulted in modified homologous
  structures which are adapted to surviving in
  the new environments

Question 7

The Pentadactyl Limb is an evidnce of what

Answer 7

Evidence for evolution from homologous structures

• All tetrapod vertebrates share a similar arrangement of bones in their appendages, based on a five-digit limb
• This demonstrates common ancestry of these organisms
• The use of appendages in these organisms differs greatly:
• Human hands - tool manipulation
• Birds wings - flying
• Horse hooves - galloping
• Dolphin fins - swimming

Question 8

Analogous Structures

Answer 8

• Analogous structures are similar in function but have different structures
• Analogous structures develop as a result of convergent evolution
• Organisms with analogous structures do not share a common ancestor, but have developed similar structural features due to environmental pressures
• Example: wings of bats and butterflies – both are functionally used to fly, but structurally are very different (bats have bones, butterflies do not)

Question 9

Speciation

Answer 9

• Speciation is the process by which new
  species form
• It involves the splitting of one ancestral
  species into two or more descendent species which are genetically different from each other
• NOTE: gradual evolutionary change within a species is not speciation
• Speciation only occurs once the populations have become so genetically distinct that they are no longer able to reproduce fertile, viable offspring

Question 10

Speciation and Extinction difference

Answer 10

• Speciation is the formation of new species
• This process increases the total number of
  species on Earth
• Extinction occurs when there are no living
  members of a species remaining
• This process reduces the total number of
  species on the planet

Question 11

Reproductive Isolation

Answer 11

• Reproductive isolation refers to barriers that prevent populations of the same species from interbreeding and producing fertile offspring
• Barriers can be:
• Geographical
• Behavioural
• Temporal

Question 12

Geographical Barriers

Answer 12

• Geographical isolation occurs when two
  populations of the same species are
  prevented from reproducing because of a
  physical barrier
• Examples: formation of rivers, mountains,
  being on different islands, formation of roads/ bridges

Question 13

Speciation due to Geographical Barriers

Answer 13

• The formation of the geographical barrier divides an original population into two or more populations
• The barrier prevents members of the separated populations from interbreeding
• As the populations are exposed to slightly different environments, they will adapt and change over time
• This eventually leads to significant genetic differences between the populations
• The resulting behaviours and physiological changes prevent the populations from mating, even if the barrier is removed – speciation has occured

Question 14

Case Study: Chimpanzees and Bonobos, Example of speciation

Answer 14

• A common ancestor of chimpanzees and
  bonobos lived in central Africa around 2
  million years ago
• The ancestor population became
  geographically isolated into two distinct
  populations by the widening of the Congo
  river
• The ape populations became reproductively isolated
• The selection pressures were different on the two sides of the Congo river
• Different traits were selected in the apes on the two sides of the Congo river
• Over time, the two ape populations evolved into two separate species: aggressive chimpanzees and peaceful bonobos

Question 15

Allopatric Speciation

Answer 15

• Occurs due to geographical isolation – the physical separation of two populations of the same species
• The two populations can no longer interbreed, reducing gene flow
• Over time, each population will adapt to the selective pressures of their environments, leading to speciation

Question 16

Sympatric Speciation

Answer 16

• Occurs when a population divides into different species while inhabiting the same habitat – there is no geographical isolation
• Reproductive isolation occurs due to behavioural isolation or temporal isolation

Question 17

Sympatric: Behavioural Isolation

Answer 17

• Occurs when individuals within a population become reproductively isolated due to their behaviour
• Over time, the populations may become so
  isolated that sympatric speciation occurs
• Ex: courtship patterns like mating calls,
  physical differences like wing plumage,
  preventing mating or failure of gametes to
  fuse

Question 18

Sympatric: Temporal Isolation

Answer 18

• Occurs when individuals within a population become reproductively isolated due to differences in the timing of their reproduction cycles
• This prevents interbreeding even though the species share the same location
• Ex: different mating seasons, timing of
  gamete production

Question 19

Barriers to Hybridization

Answer 19

• Hybrids are rarely formed between different species due barriers preventing the mixing of alleles between different species
• Barriers can be:
• prezygotic: occurs before fertilisation
• postzygotic: occurs after fertilisation

Question 20

Prezygotic Barriers

Answer 20

• Prezygotic barriers prevent fertilization of the sperm and egg, so no offspring are produced
• Prezygotic barriers include:
• Behavioural isolation – differences in courtship patterns
• Temporal isolation – differences in reproductive timings
• Ecological isolation – difference in habitat
  locations
• Mechanical isolation – physical differences
  preventing intercourse

Question 21

Postzygotic barriers

Answer 21

• Postzygotic barriers occur after fertilization, as the offspring are either inviable or infertile
• Postzygotic barriers include:
• Hybrid inviability - offspring do not
  survive to reproductive age
• Hybrid infertility - offspring are not
  capable of producing functioning gametes
  (sterile)

Question 22

Polyploidy

Answer 22

• Haploid – organisms have one copy of
  each chromosome
• Diploid – organisms have two copies of
  each chromosome
• Polyploid – organisms have more than two
  copies of each chromosome

Question 23

Causes of Polyploidy

Answer 23

• Polyploidy is caused by non-disjunction,
  leading to gametes with extra chromosomes
• If a diploid gamete fuses with a haploid
  gamete, a triploid zygote forms (polyploid)
• If a diploid gamete fuses with another diploid gamete, a tetraploid zygote forms (polyploid)

Question 24

Abrupt Speciation due to Polyploidy

Answer 24

• Usually, speciation is a slow process due to the slow rate at which allele mutations accumulate
• However, in some plant species, speciation can happen within a single generation – known as abrupt speciation
• Abrupt speciation occurs because plant cells are able to remain viable even when they are polyploid
• Many polyploid plants have bigger cells and are bigger overall than their diploid relatives
• Polyploidy can lead to immediate speciation, as an organism is produced with a different number of chromosomes
• Due to differences in chromosome number, polyploid organisms are no longer able to breed with their original species
• This is common in many groups of plants,
  including the genus Persicaria (commonly
  known as knotweeds)

Question 25

Adaptive Radiation

Answer 25

* Adaptive radiation is the evolution of a single ancestral species into several species * This occurs as the ancestral species spreads out across different locations and each population becomes adapted to new environmental niches * Populations in the different niches evolve different features (ie: adaptations) in response to the selection pressures of their environment * This results in divergent evolution

Question 26

Adaptive Radiation and Biodiversity correlation

Answer 26

* Adaptive radiation and divergent evolution increases the biodiversity of an ecosystem with vacant niches * Despite being closely related, the species can coexist without competing for resources