10. GENETICS AND EVOLUTION

Question 1

Outline how reproductive isolation can occur in an animal popluation. [3]

Answer 1

• temporal (populations reproduce at different times)
• behavioral (populations have different courtship behaviours)
• geographical (populations separated by i.e a river)
• due to polyploidy

Question 2

Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species.

Answer 2

Genes
- mutation of a gene results in the formation of a new allele
- alleles are alternative forms of a gene and they are responsible for different phenotypes
- allelles can be dominant or recessive
- in codominant inheritance both allelles influence the phenotype
- but genes may not always be expressed due to methylation patterns or environmental circumstainces

Chromosomes
- all genes occupy the same position on a chromosome in a species
- members of a species have the same number of chromosomes
- meiosis gives a new combination of chromosomes
- sex chromosomes X and Y determine sex

Question 3

Outline the process of speciation

Answer 3

• formation of a new species
• reproductive isolation
• temporal/behavioral/geographical or polyploidy
• leads to separation of a gene pool
• different selective pressure causes gene pools to diverge
• different mutations occur in two populations
• frequency of allelles change
• populations can no longer interbreed to produce fertile offspring
• speciation can occur via two models: gradualism is when speciation occurs gradually with differences accumulating over long periods of time
• punctuated equilibrium - species remain stable over long period before undergoing ubrupt change

Question 4

outline how homologous structures provide evidence for evolution

Answer 4

• homologous structures are different in function but similar in structure due to common ancestry
• i.e pentadactyl limb in a bat is used to fly whereas in humans it is used to grab, dolphins to swim
• point to adaptive radiation as the process of evolution

Question 5

Outline the criteria that should be used to assess whether a group of organisms is a species.

Answer 5

• could potentially interbreed to produce fertile offspring
• same number of chromosomes with the same sequence of genes on each chromosome

Question 6

Explain how polyploidy occurs and, using a named example, how polyploidy can lead to speciation.

Answer 6

• polyploidy is having more than two sets of chromosomes ✔
• non-disjunction of chromosome pairs during meiosis ✔
• diploid gamete «can lead to polyploidy» ✔
• fusion of diploid and haploid gamete produces triploid cells ( 3 sets of chromosomes) ✔
• fusion of two diploid gametes produces tetraploid/4n ✔
• tetraploids can form a new species because they can cross with each other
• meiosis fails in triploids because «homologous» chromosomes cannot pair up
• speciation by polyploidy is common in plants
• such as the genus Allium which has a diploid number 16 ✔
• polyploid individuals tend to be larger ✔
• polyploid individuals are reproductively isolated
• polyploidy leads to instant speciation