Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 3 of 4 TOPICS
Evolution may lead to speciation:
Individuals within a population of a species may show a wide variation in the phenotype. This is due to genetic and environmental factors. Meiosis and random fertilization of gametes during sexual reproduction produces further genetic variation:
- Meiosis: Independent segregation is when any combination of homologous chromosomes can align any way on the equator of the cell so that each daughter cell can inherit a different allele. Crossing over means that when two homologous pairs of chromosomes are aligned on the equator of the cell, the alleles may get swapped over from one chromosome to the other in the same pair.
- Random fertilization: This means daughter cells inherit new alleles.
Natural selection can occur when predation, disease or competition occurs. In each of these circumstances organisms with the best alleles survive and are more likely to reproduce than organisms with alleles that are at a disadvantage. The best alleles are passed on to offspring and the allele frequency increases. This is natural selection.
Those organisms with phenotypes providing a selective advantage are likely to produce offspring and pass on their favourable alleles to the next generation. The gene pool decreases because the numbers of different alleles are decreasing as a result of the favourable alleles increasing and the less favourable alleles decreasing. Natural selection can have different effects on the gene pool as to which alleles survive and pass on.
Stabilising selection is where organisms with phenotypes in the middle are more favourable than the two extremes:
EXAMPLE: Mass of babies at birth can be small, large or in the middle. The middle ones are more favourable. See what happens to the graph below:
Directional selection is where organisms at the higher phenotype are more likely to survive:
EXAMPLE: Cheetahs are the fastest animals on land. It is likely that this characteristic was inherited by directional selection as individuals that are the fastest are more likely to catch their prey. See what happens to the graph below:
Disruptive selection is a new type of selection that you will need to know for A2 as well as the above two which you have learnt about at AS. Disruptive selection is where the two extreme phenotypes are favoured more than the middle alleles:
EXAMPLE: Birds’ beaks can either be big or small depending on the appetite they have. Birds that have beaks that are in the middle cannot really eat anything as their beaks will always have a problem. See what happens to the graph below:
A change of allele’s frequency overtime is known as evolution where one method is natural selection as explained above. Another way is genetic drift which is as follows:
- Individuals within a population show variation in their genotypes e.g. genotype H and I
- By chance, the allele for one genotype (I) is passed on the offspring more than others
- So the number of individuals with this allele increases
- Change in the allele frequency in two isolated populations could lead to reproductive isolation and speciation (development of new species from existing species).
The diversity of life on Earth today is a result of the continuous speciation and evolutionary change over millions and millions of years. It is still continuing today.
Reproductive separation of the two populations can result in the accumulation of difference in their gene pools. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. In this way new species arise from existing species. There are two types of isolation in which this can happen:
- Allopatric speciation: This is geographical isolation where the population is split into two by a physical barrier made by a natural disaster etc. This causes the two populations to experience different climates and conditions in their new habitat. Different selection pressures are applied i.e. different alleles are favoured more than others in each population. This will cause different allele frequencies and different gene pools. Eventually the two groups become so different to each other that they no longer can breed with one another to produce fertile offspring. This means the two groups are no longer the same species.
- Sympatric speciation: This is when the population is reproductively isolated and does not geographical influences. There are two types of sympatric speciation:
- Pre-zygotic isolation mechanisms: These are things that stop two individuals coming together to mate. There are six types of pre-zygotic isolation mechanisms:
- Temporal isolation: This is where organisms mate at different times of the year or at different time of the day.
- Gametic isolation: The gametes of each population are different so that they cannot fuse.
- Behavioural isolation: Difference in the mating rituals leads to different courtships or reproduction.
- Ecological isolation: Different habitats mean less interaction.
- Polyploidy: Some organisms may have more than two copies of chromosomes (called polyploid instead of diploid). This means that organisms that are polyploid cannot mate with organisms that are diploid. This is more common in plants.
- Mechanical isolation: Different anatomy of the genitalia means cross-mating cannot occur.
- Post-zygotic isolation mechanisms: This when the zygote is prevented from further reproduction after it has formed. There are two types:
- Inviable zygote: Two organisms are able to produce a zygote but it develops incorrectly. The offspring dies later.
- Hybrid sterility: An offspring is produced but is sterile meaning that it cannot produce fertile offspring.