Human Bio Unit 4 - Evolution

allele frequency

the proportion of a particular allele in a population

gene pool

the sum of all alleles in a given population

what can affect allele frequency

gene flow, natural selection, random genetic drift, mutations

gene flow

the movement of genetic material from one population to another, facilitated by migration

how does gene flow change allele frequency

when individuals migrate and reproduce in a new population, they bring their alleles with them. this can increase the variation and change allele frequencies.

define barrier to gene flow

anything that prevents interbreeding between two populations

geographical barrier

barriers due to physical isolation and boundaries. e.g. mountains, oceans, deserts

sociocultural barrier

barriers due to religion, education, or economic status. e.g. some religions do not allow marriages outside the religion

how does gene flow affect the population on a small island?

gene flow is reduced to geographical barriers, limiting interbreeding. as a result, alleles not common in the original population may become more frequent

natural selection

the process where individuals with certain favourable traits have a higher chance of survival and reproduction, due to selective pressures in the environment  

evolution through natural selection (VOCSRG)

1. There is a variation of traits within a population.

2. overpopulation occurs as more individuals are produced then the environment can sustain.

3. Due to the excessive birth rate and limited resources, there is a struggle for existence - competition for survival.

4. The individuals with traits best suited to the environment have more chance of surviving and reproducing - survival of the fittest.

5. Individuals reproduce and favourable traits (those with survival value) are passed onto the next generation.

6. In the gene pool, the proportion of alleles that produce favourable traits gradually increases over several generation.

survival of the fittest

individuals with favourable traits survive, and individuals with unfavourable traits die before they have a chance to reproduce & pass on traits.

struggle for existence

there is more individuals in the population than the environment can sustain, thus there is competition between individuals for survival resources.

variation

the differences that exist within a population

random genetic drift

the random, non-directional change in allele frequency between generations

why does random genetic drift affect smaller populations more

because allele frequencies are more noticeable and the percentage change in variation is more drastic

how does genetic drift lead to allele lost

over generations, genetic drift can cause alleles to be lost during interbreeding or otherwise purely by chance.

founder effect

when a small group migrant away from the original population, founding a new area. The migrant group is small and their allele is not representative of the original population

bottleneck effect

occurs when there is a disaster of some sort that severely reduces the size of the population, and the new allele frequency is not representative of the original population. e.g. natural disaster, flood, war

tay sachs disease

a genetic disorder that results in the progessive destruction of the nervous system, caused by a mutation in the gene that codes for the production of enzyme Hex-a

TSD - symptoms & treatment

• TSD causes a build up of fatty proteins in the nerve cells, causing blindness, deafness and an inability to swallow

• there is no cure; people with this disease usually die at a young age.

TSD - who is affected

• tay sachs is a recessive trait

• people who are heterozygous for tay sachs disease have a survival/heterozygous advantage because they are not exposed to TB

• those who are homozugous for TSD or TB would die, but the carriers of TSD have immunity to both and survive to pass on traits

sickle cell anaemia

• a disease caused by a mutation on the gene that produces normal haemoglobin.

• the mutant allele is due to the substitution of the amino acid valine for glutamic acid

sickle cell anaemia - symptoms & treatment

• RBCs are shaped like sickles (rather then being biconcave) and are more rigid and sticky

• there is no cure; only treatments

sickle cell anaemia - who is affected

• sickle cell is a recessive trait

• people with stickle cell have a survival advantage because they are less exposed to malaria

• those who are homozygous for sickle cell, or those who have malaria would die, but the carriers of SCA have immunity to both and would survive