Evolution: rate of evolution CfE Advanced Higher Biology Unit 2: Organisms and Evolution

SQA mandatory key information Where selection pressures are high, the rate of evolution can be rapid. The rate of evolution can be increased by factors such as shorter generation times, warmer environments, the sharing of beneficial DNA sequences between different lineages through sexual reproduction and horizontal gene transfer.

Selection pressure What is meant by selection pressure in an evolutionary context? Selection pressure can be defined as any factor in the internal or external environment that reduces the reproductive success of a population and influences the frequency of traits in a population. It is the driving force behind natural selection.

Rate of evolution What can affect the rate of evolution? When selection pressures are high, the rate of evolution can be rapid. Factors which can increase the rate of evolution include:  Shorter generation times  Warmer environments  Sharing of beneficial DNA sequences between different lineages through sexual reproduction and horizontal gene transfer. We will now go on to look at each of these factors which can increase the rate of evolution in more detail.....

The effect of shorter generation times on the rate of evolution Shorter generation times = more generations in a set period of time. Each cycle of reproduction requires DNA replication. Mutations occur during meiosis and DNA replication. This would lead to an increased rate of mutation which in turn would speed up the rate of evolution. e.g. MRSA antibiotic resistance in bacteria e.g. Insect resistance to use of pesticides.

Warmer Climates The mean global surface temperature has increased by about 0.5° to 0.7°C since the late 19th century, and by about 0.3° to 0.4°C over the last 40 years.

Warmer climates continued... The warming occurred largely during two periods, between 1910 and 1940 and since the mid-1970s. Warming is evident in both sea surface and land-based surface air temperatures.

What effect do warmer climates have on the rate of evolution? Increasing global temperatures exerts a selection pressure on all organisms. Clearly, if the temperature increases, those heat tolerant individuals will have an advantage and will leave more offspring - and those offspring will also carry the genes for heat tolerance. Over many generations, this process produces a population with adaptations well-suited for the hotter environment. So long as the population has different genetic variants in it, some better able to survive and reproduce in particular situations than others, the population has the capacity to evolve when faced with a changing environment.

An example of increased rate of evolution as a result of warmer climates.... European great tits are evolving different breeding times. Birds that are able to adjust egg-laying to earlier in the spring can time hatching so that it coincides with greater caterpillar abundance. With recent climatic changes, the caterpillars have been maturing earlier in the spring. Birds with genes for more flexible egg-laying times are more successful than birds with less flexibility in their egg-laying.

Another example of increased rate of evolution as a result of warmer climates.... We know that species have actually evolved, experienced a change in gene frequency in the population, in response to global warming. Interestingly, in those cases, the species are not necessarily becoming more heat tolerant, but are adapting to changes in seasonal timing. Field mustard (Brassica rapa) has evolved in response to an extreme drought in southern California, which some sources have linked to global warming. These plants flower and produce seeds near the end of the rainy season, but when the rainy season is cut short by a drought, late blooming plants may wither and die before they can produce seeds. This form of natural selection favours early bloomers. Is just four years enough time to see the results of this evolutionary shift? Researchers compared plants grown from wild seeds collected before and after the drought and found that post-drought plants had evolved to flower much earlier ‒ sometimes by as much as 10 days!

Sharing of beneficial DNA sequences The sharing of beneficial DNA sequences between different lineages through sexual reproduction creates genetic variation. This increase in variation in a population allows them to adapt to ever changing environmental conditions. It gives sexually reproducing organisms the advantage over asexually reproducing genetic clones. Horizontal gene transfer has played a major role in the evolution of bacteria. Horizontal gene transfer has several mechanisms but it always involves the transfer of genetic material (DNA/RNA) between organisms. It often involves the use of plasmids. Barth F. Smets, Ph.D., with permission

Mechanisms of horizontal gene transfer There are 3 main mechanisms:  Transformation  Transduction  Conjugation.

Transformation This involves the genetic alteration of a prokaryotic cell due to the uptake and successful expression of foreign DNA or RNA. The foreign allele that is taken up by the cell replaces the cell’s existing allele for a particular characteristic. Many bacteria have cell-surface proteins that recognise DNA from closely related species and transport it into the cell. Once inside the cell, the foreign DNA can be incorporated into the genome by homologous DNA exchange.

Transduction 1.Bacteriophage infects bacterial cell. 2.Host cell DNA is fragmented, phage DNA and proteins are made, this is the donor cell. 3.Bacterial DNA fragment ( blue) may be packaged in a phage capsid instead of phage DNA (purple). 4.Phage with bacterial DNA infects a recipient bacterial cell. 5.Recombination occurs between donor DNA and recipient DNA. 6.The genotype of the recombinant cell is different to the donor cell and the original recipient,

Conjugation Bacterial conjugation involves the transfer of genetic material through cell- cell contact. DNA transfer is one-way only. 1.The donor cell uses sex pili (singular: pilus) to attach to the recipient. 2. The donor cell, containing an F plasmid, forms a bridge with the recipient cell. 3.A single strand of the donor F- plasmid moves into the recipient cell. DNA replication then begins. 4.DNA replication takes place in both the donor and the recipient cells. The plasmid in the recipient cell forms into a circle and both the donor and the recipient cell now contain a complete F-plasmid. Genetic transfer has taken place and the recipient cell is now a recombinant.

Summary Where selection pressures are high, the rate of evolution can be rapid. The rate of evolution can be increased by factors such as: – shorter generation times – warmer environments – the sharing of beneficial DNA sequences between different lineages through sexual reproduction and horizontal gene transfer.