human bio variation and evoution (LESSON 2 MECHANISMS OF EVOLUTION)

5 ways variation occurs through, and explain all

1. mutations -most important, resulting in new/diff allele

2. random fertilisation - infinite combos of alleles in an offspring, due to different sperm fertilising different eggs

3. random assortment - random pairing of chromosomes during meiosis

4. chiasmata (crossing over) - exchange of genetic materials during meiosis

5. non dysjunction - one or more pairs of chromosomes fail to separate during meiosis

natural selection

• organisms better adapted to their environment tend to survive and pass on the more advantageous trait to their offspring (not random!)

• organisms with less desirable traits are less likely to survive and so the less advantageous traits are not passed on

• over many generations, this results in a change in the number and type of alleles in a gene pool

random genetic drift/sewell wright effect

random variation in gene frequency - populations of organisms are constantly changing and adapting to their environment. drastic changes in environmental conditions can sometimes cause drastic changes to the gene pool of a population

2 examples of random genetic drift

• a man steps on a group of beetles, randomly killing most of the green ones but leaving most of the brown ones alive, resulting in fewer green beetles being produced

• a random succession of births results in all other hair colours going extinct within a village full of redheads

the founder affect

the loss of genetic variation (smaller allele frequency) that occurs when a new population is established by a very small number of individuals from a larger population

example of the founder effect

northwest venezuala has an extraordinarily high frequency of Huntingtons disease. during the 1990s, approx 150 people in the area had this condition with a high risk of passing it on to offspring.

the emergence of this disease can be traced to maria concepcion soto, who moved into the area in the 19th century. she had 10 children who remained in the area and were therefore the ‘founders’ of what is now a population of about 20,000 people with a high risk of having this disease.

the bottleneck effect

the bottleneck effect occurs when some catastrophe, like an earthquake or a tsunami kills off most of a population at random and leaves only a handful of survivors

the bottleneck effect example

in 1775, a typhoon struck pingelap. only about 20 islanders survived the storm and the famine that followed. from this population and the geographical and cultural isolation, achromatopsia (colour blindness) increased in the pingelap population. today up to 5% have it and 30% are carriers

migration

affects the number and type of alleles in a gene pool (gene flow). immigrants bring alleles that are not currently in the population and so the frequency of these alleles will change

what do genetic diseases do to gene poool

they can change the frequency of alleles in a gene pool. alleles resulting in fatal diseases should be eliminated from a population, although some persist in certain populations.

sickle cell anaemia as an example of a genetic disease: what is it, what populations affected, whats the benefits of being a carrier

• what: crescent shaped RBCs, reducing oxygen carrying capacity. it also means the RBCs stick together and block blood vessels

• who: black africans in areas where malaria is present

• benefits: if your heterozygous it gives resistance to malaria