5.4 Evolution

Question 1

Define evolution

Answer 1

Evolution is the cumulative change in the heritable characteristics of a population

Question 2

State what evidence supporting evolution requires

Answer 2

Evidence to support evolution must show a change in characteristics from an ancestral form.

Question 3

State three kinds of evidence for evolution

Answer 3

Fossils, selective breeding, homologous structures

Question 4

State the two kinds of fossils

Answer 4

• Direct (body fossils) - bones, teeth, shells, leaves, etc.

* Indirect (trace fossils) - footprints, tooth marks, tracks, burrows, etc.

Question 5

Outline four ways in which fossils can be formed

Answer 5

• Petrification - organic matter is replaced by mineral ions
  
  • Mould - organic matter decays, space left becomes a mould, filled by mineral layer
  • Trace - impression (footprint or leaf) hardens in the layers
  • Preservation - the organism is preserved, such as in amber or in anaerobic, acidic peat

Question 6

Define ‘the fossil record’

Answer 6

The fossil record is the totality of fossils (both discovered and undiscovered)

Question 7

Outline what we have learnt from the fossil record

Answer 7

• It shows that over time, changes have occurred in features of organisms living on the planet (evolution)
  
  • Different kinds of organisms do not occur randomly but are found in rocks of particular ages in a consistent order (law of fossil succession)

Question 8

Outline two limitations of the fossil record

Answer 8

• Fossilization requires a unusual combination of specific circumstances to occur, meaning there are many gaps in the fossil record
  
  • Only the hard parts of an organism are preserved and often only fragments of fossilized remains are discovered

Question 9

Outline selective breeding

Answer 9

Selective breeding of domesticated animals is an example of artificial selection, which occurs when man directly intervenes in the breeding of animals to produce desired traits in offspring.
As a result of many generations of selective breeding, domesticated breeds can show significant variation compared to the wild counterparts, demonstrating evolutionary changes in a much shorter time frame than might have occurred naturally.

Question 10

State two example of selective breeding

Answer 10

• Breeding horses for speed (race horses)

* Breeding cattle for increased meat production or milk

Question 11

Outline how homologous structures provide evidence for evolution

Answer 11

• Similar structures in distinct organisms shows traits derived from a common ancestor
  
  • Homologous structures are those that are similar in shape in different types of organisms despite being used in different ways
  • An example is the pentadactyl limb structure in vertebrates, whereby many animals show a common bone composition, despite the limb being used for different forms of locomotion (e.g. whale fin for swimming, bat wing for flying, human hand for manipulating tools, horse hoof for galloping, etc.)
  • The more similar the homologous structures between two species are, the more closely related they are likely to be

Question 12

Outline why populations tend to produce more offspring than they can support

Answer 12

• The Malthusian dilemma states that populations tend to multiply geometrically, while food sources multiply arithmetically
  
  • Hence populations tend to produce more offspring than the environment can support
  • By producing more offspring than the environment can support, the chances of survival are increased for the population as a whole

Question 13

Explain the consequence of overpopulation

Answer 13

• The ‘struggle for survival’ is a result of overpopulation, leading to competition for limited resources
  
  • Individuals with beneficial traits that increase their chances of survival will be selected for, and will be able to breed
  • Thus, these advantageous alleles are present in a higher frequency in the next generation as they pass on their genes
  • The consequent change in heritable characteristics is evolution

Question 14

List the two kinds of genetic variation

Answer 14

• Discontinuous variation: A type of variation usually controlled by a single gene, which leads to distinct classes (blood type in humans, eye colour)
  
  • Continuous variation: A type of variation controlled by many genes, which leads to a range of characteristics (skin pigmentation, height)

Question 15

List the three primary sources of variation within a given population

Answer 15

• Gene mutations - a permanent change to the genetic composition of an individual)
  
  • Gene flow - the movement of genes from one population to another via immigration and emigration
  • Sexual reproduction - the combination of genetic materials from two parental sources

Question 16

State the three main causes of variation from sexual reproduction

Answer 16

Independent assortment, crossing over, random fertilisation

Question 17

Explain how independent assortment creates variation

Answer 17

• During metaphase I, when homologous chromosomes line up at the equator, the paired chromosomes can randomly arrange themselves in one of two orientations (paternal left/maternal rightOR maternal left/paternal right)
  
  • When the chromosomes separate in anaphase I, the final gametes will differ depending on whether they got the maternal or paternal chromosome
  • Independent assortment of chromosomes creates 2ndifferent gamete combinations (n = haploid number of chromosomes)

Question 18

Explain how crossing over creates variation

Answer 18

• During prophase I, when homologous chromosomes pair up as bivalents, genetic information can be exchanged between non-sister chromatids
  
  • The further apart two genes are on a chromosome, the more likely they are to recombine
  • Crossing over greatly increases the number of potential gamete variations by creating new genetic combinations

Question 19

Explain how random fertilisation creates variation

Answer 19

• Fertilisation results from the fusion of gametes from a paternal and maternal source, resulting in offspring that have a combination of paternal and maternal traits
  
  • Because fertilisation is random, offspring will receive different combinations of traits every time, resulting in near infinite genetic variability

Question 20

Explain how natural selection leads to variation

Answer 20

• There is genetic variation within a population (which can be inherited)
  
  • There is competition for survival (populations tend to produce more offspring than the environment can support)
  • Environmental selective pressures lead to differential reproduction
  • Organisms with beneficial adaptations will be more suited to their environment and more likely to survive to reproduce and pass on their genes
  • Over generations there will be a change in allele frequency within a population (evolution)

Question 21

State the scientific name for Golden Staph

Answer 21

Staphylococcus aureus

Question 22

State the scientific name for Peppered Moth

Answer 22

Biston betularia

Question 23

Outline how antibiotic resistance in bacteria can arise in response to environmental change

Answer 23

• Alleles providing antibiotic resistance can be inherited, or passed from one bacteria to another by exchange of plasmids
• Some varieties of bacteria are more resistant than others
• Bacteria reproduce rapidly and have high mutation rates, meaning evolution can occur very rapidly
• Increased exposure to antibiotics, for example in hospitals, is the environmental change that selects for resistant varieties
• Bacteria without resistance die, while resistant bacteria survive and pass on their genes to next generation
• Over time, the genetic makeup of the population will change

Question 24

Explain the Peppered Moth as an example of evolution in response to environmental change

Answer 24

• Variation:Colouration (some moth have a light colour, while others are a darker melanic colour)
  
  • Environmental change: Pollution from industrial activities caused trees to blacken with soot during the Industrial Revolution
  • Response:Light coloured moths died from predation, whereas melanic moths were camouflaged and survived to pass on their genes
  • Evolution:Over time, the frequency of the melanic form increased