7.3 - Evolution may lead to speciation Flashcards
Give the genetic factors that cause phenotypic variation within species.
- Mutation of alleles
- Random fertilisation by gametes
- Random assortment of genetic material during meiosis.
Other than genetic factors, why else may phenotype vary within a species?
Environmental influences
Why does natural selection occur?
- Predation
- Disease
- Competition
All resulting in differential survival and reproduction.
How does natural selection cause a change in a population’s gene pool over generations?
Organisms with advatageous characteristics are more likely to survive and pass their favourable alleles to offspring. Frequeny of unfavourable alleles decreases.
What is stabilising selection?
Occurs when environmental conditions stay the same. Individuals closesnt to the mean are favoured, and any new characteristics are selected against. This results in low diversity.
What is directional selection?
Occurs when environmental conditions change. Individuals with phenotypes suited to the new conditons will survive and pass on their genes. Over time the mean of the population will move towards these characteristics.
What is disruptive selection?
The opposite of stabilising selection, in that both extremes of the normal distribution are favoured over the mean. Over time, the population becomes phenotypically divided.
Define speciation.
Where poulation is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes to the extent the two groups can not longer interbreed, they have become separate species.
What is meant by allopatric speciation?
Speciation resulting from a physical barrier e.g. river, mountain range. The environments occupied by the two groups are different and therefore different alleles are favoured.
What is meant by sympatric speciation?
Speciation resulting from a non-physical barrier e.g. a mutation that no longer allows two organisms to produce fertile offspring. Any chances in anatomy or behaviour may also preveny breeding.
Define genetic drift.
A change in population’s allele frequencies that occurs due to chance rather than selective pressures. In other words, it is cause by ‘sampling error’ during reproduction.
Why does genetic drift affect small populations more than large ones?
The gene pool is smaller, so there are alleles available and any change in frequency becomes pronounced very quickly.
