Biology revision 2012 - patterns of evolution
Genes, inheritance, and selection
Each individual in a population is the carrier of its own particular combination of genetic material. In sexually reproducing organisms, different combinations of genes arise because of the shuffling of the chromosomes during gamete formation. New allele combinations also occur as a result of mate selection and the chance meeting of different gametes from each of the two parents. Some combinations are well suited to the prevailing environment while others less so. Those organisms with well suited allele combinations will have greater reproductive success (fitness) than those with less favourable allele combinations and consequently, their genes (alleles) will be represented in greater proportion in subsequent generations. For asexual species, offspring are essentially clones, but new alleles can arise through mutation and some of these may infer a selective advantage.
Gene pools and evolution
Populations can gain alleles when they are introduced from other gene pools. Immigration is on aspect of gene flow
Genes may be lost to other gene pools
Spontaneous mutations can develop that alter the allele frequencies of the gene pool, and even create new alleles. Mutation is very important to evolution, because it is the original source of genetic variation that provides new material for natural selection
· Natural selection
Selection pressure against certain allele combinations may reduce reproductive success or lead to death. Natural selection sorts genetic variability, and accumulates and maintains favourable genotypes in a population. It tends to reduce genetic diversity within the gene pool and increase differences between populations.
The term deme describes a local population that is genetically isolated from other populations in the species. Demes usually have some clearly definable genetic or other character that sets them apart from other populations
· Gene flow
Genes are exchanged with other gene pools as individuals move between them. Gene flow is a source of new genetic variation and tends to reduce differences between populations that have accumulated because of natural selection or genetic drift.
· Genetic drift
Chance events can cause the allele frequencies of small populations to drift/ change randomly from generation to generation. Genetic drift can play a significant role in the microevolution of very small population. The two situations most often leading to populations small enough for genetic drift to be significant are the bottleneck effect where the population size is dramatically reduced by catastrophic event) and the founder effect where a small number of individuals colonise a new area.
Stages in species development
As time progresses the amount of genetic variation increases and each group becomes increasingly isolated from the other. The mechanisms that operate to keep the two gene pools isolated from one another may begin with geographical barriers. This may be followed by prezygotic mechanisms which protect the gene pool from unwanted dilution by genes from other pools. A longer period of isolation may lead to postzygotic mechanisms. As the two…