POPULATION GENETICS In-build mechanisms for adaptation (or evolutionary mechanisms) Biology Stage 3 Text: Chapter 23 Human Biology Stage 3 Text: Chapter 15

Key ideas Population genetics Observable facts Natural selection Mutations Biological fitness Genetic variation Genetic drift Speciation We will be looking at population genetics. These concepts are not related to evolution, although in your textbook they are automatically linked. The processes involved in population genetics are directly observable and can be studied scientifically.

The theory of evolution The theory of evolution as proposed by Darwin: The proposition that different kinds of living organisms have developed and diversified from an earlier form(s) through the process of natural selection and gene mutations. The earlier form is referred to as a common ancestor Example: The Australopithecus is considered to be the common ancestor to the great apes (chimpanzees, gorillas, organgutangs) and humans.

The theory of evolution The theory of evolution is based on an atheistic worldview It assumes that life arose from non-life and that this process has taken billions of years. It is closely linked to the Big Bang theory, being a continuum in the spectrum of the ‘evolving’ universe. The picture below represents the idea of evolution

What is evolution and what is not? Concepts grounded in experimental science: Natural selection Mutations Genetic variation Genetic drift & gene flow Genetic bottlenecks Genetic barriers Speciation Concepts of evolution (historical science dependent on worldview): The evolution of all species on Earth (both past and present) from a single common ancestor New genetic information comes into existence through mutations Concepts of creation (historical science dependent on worldview): The creation of all species on Earth (both past and present) by God All genetic information has already been created. Any variation is a result of in-built processes.

Definitions Species The basic category or group in the naming system of Linnaeus. Organisms that are grouped into the species usually closely resemble each other and interbreed. Population A group of organisms of the same species living together in a particular place at a particular time Gene pool The sum of all alleles in a given population Allele / variation Variation in a gene – one gene may have 3 or 4 ‘varieties’ or alleles, but each individual can only inherit 2 of those. Allele (gene) frequencies How often a particular allele occurs in a population

Natural selection Definition Natural selection occurs when environmental conditions favour one allele at the expense of other alleles The result is that the process of natural selection enhances the chances of survival in particular environments Despite this definition, natural selection is mostly not a strong enough process to favour single genes. Rather, environmental conditions act to favour multiple characteristics that enhance survival. Example The difference between the Inuit and Masai (pg. 251)

Population size and allele frequency In large populations: Individuals make only a small contribution to the gene pool. Allele frequencies are relatively stable and change slowly over time These patterns depend on: ◦ Dominance ◦ Co-dominance ◦ Monogenic inheritance ◦ Polygenic inheritance In small populations: Individuals make a relatively large contribution to the gene pool. Change in allele frequency can occur rapidly. Many changes are due to chance events.

Population bottlenecks A bottleneck is caused by events that create a sudden drop in population size (e.g. wars, natural disasters or migration) or prevent individuals from reproducing. This reduces mating possibilities and can cause a genetic bottleneck. Example Noah’s flood

In-breeding Reduced mating opportunities often lead to an increase in in-breeding. In-breeding results in reduced genetic diversity in the population’s gene pool. The Pharaoh and Inca nobility practised consanguinity to preserve the 'purity' of their gene line.

In-breeding the Old Order Amish community of Lancaster County in Pennsylvania, USA is a fundamentalist religious sect who do not marry out or use modern technology. The Amish community numbers around people and shares only eight family names. Within the community there is an above average incidence of inherited illnesses including a rare form of microcephally (small brain), albinism, dwarfism, cretinism, webbed fingers and limb girdle muscular dystrophy.

Genetic drift Genetic drift is the random fluctuation of allele frequencies in a population from one generation to the next. Example The frequency of a particular trait could, for no obvious reason, drift from 2% in generation A, to 11% in generation B, to 5% in generation C Genetic drift is often a consequence of a genetic bottleneck It results from in-breeding brought about by the limited mating possibilities in a small community.

The effects of genetic drift The effects of genetic drift can be amplified by differences in the number of children raised by couples or individuals dying prematurely. Genetic drift can result in:  traits being lost from small populations.  unusual traits, not commonly found in the parent population, and that are often non- adaptive, becoming established.

Genetic drift

The founder effect Founder effect is an example of a genetic bottleneck. Founder effect occurs when a small number of people migrate and settle in a new area. The founding population carry only a small fraction of the original population's genetic variation. As a result, they may differ both genetically and in appearance, compared with the parent population.

Gene flow Migration is describes as gene flow from one population to another This brings in new alleles into the population The allele frequencies for that gene will be altered over time Example Chinese only had Rh+ allele until Europeans came and brought in Rh- allele

Barriers to gene flow Populations are often kept apart by physical or social barriers These may result in populations becoming less alike over time. Isolation due to barriers results in separate gene pools Barriers include: Geographical barriers Sociocultural barriers

Speciation The development of different species over time Steps to speciation: Variation A single population exists with a common gene pool Isolation A barrier forms and divides the population into 2 Selection Different selection pressures exist for these 2 populations Speciation Over time, the allele frequencies in each of the populations change When the two populations no longer interbreed, a new species is said to have ‘evolved’

Mutations Mutations A change in the DNA sequence. Can result from DNA copying mistakes, exposure to radiation, exposure to chemicals or infection by viruses. Mostly have deleterious effects Gene mutations Changes in the DNA of a single gene ◦ Example: Phenylketonuria Chromosome mutations All or part of the chromosome is affected ◦ Example: Down’s syndrome

Mutations Somatic mutations A mutation in the body cells Germinal mutations A mutations in the gametes

Three types of mutations National Human Genome Research Institute - NIH

Evolutionary mechanisms Charles Darwin and Alfred Wallace proposed that natural selection and mutations are the two main forces that drive evolutionary mechanisms. The theory requires that new genetic material is created over time by random natural forces. Alfred Wallace Charles Darwin

In-built mechanisms for adaptation Creation scientists propose that God created living organisms with an immensely complex yet flexible genetic code that enables them to adapt and change to survive in different environments. This theory proposes that existing DNA is modified according to natural selection, genetic drift and population bottlenecks, rather than new genetic material being created.