Evolution Flashcards
1
Q
What is evolution?
A
- It is the gradual change in the characteristics of a species over time
- It results in a species becoming better adpated to their environment as those with favourable characteristics survive and have more descendants with, eventually this trait becoming a feature of the whole population.
2
Q
Influences on the theory of evolution
A
- Lamark:
- He established a way of classifying plants and invertebrates
- He concluded that plants and animals change form to adapt to their environment and these changes are passed onto their offspring.
- E.g. Giraffe grew a longer neck to reach the leaves of tall trees
3
Q
Influences on the theory of evolution 2
A
- Lyell: came up with the theory that natural forces including temperature, running water and earth movements (constant change) have gradually moulded the earth’s surface over long periods of time.
- Wallace: Similar ideas as to Darwin and possibly worked together to form their conclusions.
- Darwin: Evolution through natural selection based on observations in Galapagos Islands, was influenced by the work of Lyell, Wallace and Lamarck.
4
Q
Darwin’s Theory of Natural Selection
A
- Darwin’s theory was based on three observations:
- Variation: Darwin noted that all members within a species vary and these variations can be passed from one generation to the next (heritable)
- High Birth Rate: Darwin realised that all living organisms reproduve at a rate far greater than that at which their available food supply and resources increase. Reproductive rates exceeded resources; there was a struggle for existence.
- Nature’s Balance: Although birth rates were very high, each species maintained its number at a relatively constant level.
- Darwin concluded that the struggle for existence (high birth rate vs low resources) ensured that those with the best characteristics survived therefore ensuring survival of the fittest (those with unfavourable traits die before passing genes onto offspring)
5
Q
Darwin and Genetics
A
- Darwin’s principle of ‘survival of the fittest’ is possible becuase there is variation in a population
- At the time, Darwin was unable to explain these variations as he knew nothing of genetics and inheritance
- We now know much of variation is due to the effects of meiosis and fertilisation
Mutations can be in two forms:
- Gene mutation - change in a single gene
- Chromosomal mutation - part or all of a chromosome is affected
- They can be somatic (body cells, mutation stays within the individual)
- They can be germline (reproductive cells and can be passed onto offspring)
6
Q
Darwin and Genetics 2
A
- With this knowledge of inheritance, scientists have built on the work of Darwin to explain the process of natural selection
- Natural Selection is the selection of favourable alleles in a population that will provide a greater survival advantage in a given environment
- The environemnt has an effect on the characteristic selected for
- Because these genes continue to be passed on to each successive generation over a long period of time, the characteristics of a population will change
7
Q
A summary of the principles of evolution through natural selection
A
- There is a variation of characteristics within a species
- More offpsring of a species are produced than can possibly survive to maturity
- Due to excessive birth rate, and limited resources, there is a struggle for existence - competition for survival
- The individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of the fittest
- Favourable characteristics (those with survival value) are passed on to the next generation
- In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases
8
Q
Natural Selection in Humans - Somatotypes
A
- E.g. Somatotypes (body type)
- Individuals with long bodies and short limbs (endomorph) have a smaller surface area in relation to body volume than those with short bodies and long limbs. Such individuals lose less heat in very cold environments and would have a survival advantage in such an environment.
- HUman populations native to cold climates, such as Inuit of Alaska, typically show endomorphic body type, that is short and stocky. Natural selection would explain this.
- When early humans first migrated from Africa to colder climates, the gene pool for the population would have a very high frequency of ectomorph alleles, and most people would show this phenotype. Over time, through evolution, there has been genetic change to endomorph alleles.
9
Q
Natural selection in humans - sickle cell anaemia
A
- E.g. sickle cell anaemia
- People heterozygous for sickle cell anaemia show no ill effects of the disease unless there is an oxygen deficiency. Some of their red blood cells are sickled, formed in a curve or sickle shape. They are still able to carry oxygen on their normal red blood cells. These people also do not get malaria. They have an immunity to it.
- People homozygous for sickle cell anaemia die young, as their red blood cells are all sickled and they have very reduced oxygen carrying capacity.
- People homozygous normal have normal blood, but can get malaria, which is fatal.
- Why does the sickle cell gene exist commonly in populations from Africa, India and Asia where there is a significant mosquito problem?
= sickle cell allele has a survival advantage in areas where malaria is prevalent
= the presence of malaria acts as a selective agent for the sickle-cell allele
10
Q
Population studies and gene pools
A
- Gene pool = sum of all alleles (available genotypes) in a population
- Allele frequency = how often an allele occurs in the gene pool for a population (%)
- The favourable trait alleles will be of a higher frequency than a dud trait
- Populations that differ in characteristics are likely to be different in frequencies of various alleles
- e.g. Scandanavians commonly have blue eyes, African brown eyes. Frequency of blue allel would be higher in the Scandanavian gene pool
11
Q
Changes to allele frequencies
A
- Overtime, the frequency of alleles in a population may change, changing the gene pool composition
- Such changes may be due to chance such as mutation, or natural means such as changes in the environment.
- Factors that can alter allele frequencies:
- Mutations
- Natural Selection
- Random genetic drift
- The founder effect
- The bottle-neck effect
- Migration
- Barriers to gene flow
- Genetic diseases
12
Q
Random Genetic Drift
A
- It is the random, non-direcitonal changes in allele frequency over time based purely on chance occurence
- It has a dramatic impact on small populations e.g. an allele which is rare in a large population may, purely by chance, become frequent in a small population.
- Can be a result of differential reproductive success or accidental (premature) deaths of individuals
- If the effects of a genetic drift are strong enough, the allele may be lost (0%) from the gene pool of fixed (100%)
- The reduces the amount of variation in a population’s gene pool.
13
Q
How does random genetic drift work?
A
- When there is a small population, there may be statistical anomalies, which can lead to random changes in allele frequencies. This is called random genetic drift.
- There is no selective pressure that leads to this, it is purely a chance occurence
- The degree of allele frequency change increases as the population size decreases
14
Q
How does random genetic drift work - scenario 1
A
- Consider an isolated human population of 50, living on an island paradise
- They are typical of any European population with a mix of brown and black hair
- One day, seven black haired brothers go fishing to catch a huge fish for the feast to celebrate their father’s birthday. THe boat that they go in capsizes and they all drown.
- What would you expect to happen to the allele frequencies for brown and black hair in this population? Black hair frequency would decrease and brown hair frequency would increase.
15
Q
Genetic Drift in Human Population - The ‘Dunkers’ in Pennsylvania, U.S.
A
- In human populations, religious isolates offer one of the best opportunities to witness genetic drift
- The ‘Dunkers’ of eastern Pennsylvania are descended from Old German Baptist Breathren Hesses Germany, who came to the U.S. in the early 18th century. They have remained genetically isolated by rigid marriage customs.
- Bentley Glass compated certain genetic traits of Dunkers to the surrounding American population, and to their parent population in Hesses
- His group looked at: ABO blood groups, MN blood types, Rh Blood types, Ear lobes, Handedness, Mid-digital Hair, Hitch-Hikers thumb
- Results = the Dunkers varied in allele frequency for most traits from both populations, even though the American shared the same environment.
- Genetic drift within this small population allowed certain alleles to become more frequent purely by chance
