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Chapter 12: Processes of Evolution. Part 2. Condition 5: No Natural Selection. Balanced polymorphism occurs when natural selection favors heterozygous individuals in a population. Heterozygotes are selected for by the environment.
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Condition 5: No Natural Selection • Balanced polymorphism occurs when natural selection favors heterozygous individuals in a population. • Heterozygotes are selected for by the environment. • A good example of this is the gene that codes for the beta globin chain of the hemoglobin molecule in our red blood cells.
Condition 5: No Natural Selection • HbA is the normal allele. • HbS is a mutated allele that is recessive and causes sickle cell anemia. • In tropical and subtropical regions of Asia and Africa: • HbS/HbS (homozygous recessive individuals) die prematurely due to sickle cell anemia. • HbA/HbA (homozygous dominant individuals) are very susceptible to malaria and have a high mortality rate. • HbA/HbS (heterozygous individuals) are more likely to survive to reproduce. • Why?
Condition 5: No Natural Selection • Heterozygous individuals produce enough normal hemoglobin to survive (due to their HbA –normal- gene). • However, when their red blood cells become infected with the malaria parasite, they become sickle-shaped (due to their HbS- mutated- gene). • Their immune system recognizes the sickle-shaped red blood cells and destroys them, destroying the malaria along with them. • Homozygous dominant individuals’ red blood cells do not sickle when infected with the malaria parasite and so malaria is able to hide from and escape the immune system in the red blood cells of these individuals.
SmallCondition 1: Large population = No Genetic Drift • Genetic drift is a random change in allele frequency over time due to chance alone • We must consider probability in this condition. • If one population has 10 individuals (Population 1) and the other has 100 individuals (Population 2), a change in the frequency of an allele has a much greater effect in the small population than in the larger population.
SmallCondition 1: Largepopulation = No Genetic Drift • For example, imagine that 10% of each population has the genotype bb, while all others are BB. • That would be 1 individual in Population 1 and 10 individuals in Population 2 who are bb. • In Population 1, if that bb individual dies, the b allele has been totally lost from that population. • In Population 2, for the b allele to be totally lost from the population, all 10 bb individuals would have to die before reproducing. • The chances (or probability) that 1 individual will die before reproducing ( in population 1) are much greater than that 10 individuals will die before reproducing (in Population 2).
SmallCondition 1: Largepopulation = No Genetic Drift • The bottleneck effect is seen when there is a large decrease in the size of a population caused by some extreme environmental pressure such as a disease, natural disaster, or human activity. • The genetic drift that is seen with the bottleneck effect is dramatic. • An example would be elephant seals. • In the 1890’s, there were only 20 known surviving elephant seals due to overhunting. • Since then, hunting restrictions have been implemented so that there are now at least 170,000 elephant seals. • Since all of these 170,000 arose from the original population of 20, all 170,000 seals are homozygous for every gene that has been analyzed to date. • Why?
SmallCondition 1: Largepopulation = No Genetic Drift • A specific type of bottleneck, called the founder effect, occurs when a small group of individuals separate from the rest of their population and go off on their own to found a new population. • This small group of individuals is not representative of the entire original population in terms of allele frequencies, so the population that results will not be representative of the original population either. • In fact, the smaller the founding group, the more reduced the genetic diversity of the new population will be.
SmallCondition 1: Largepopulation = No Genetic Drift • Genetic drift especially effects populations that inbreed. • Inbreeding is also an example of nonrandom mating. • Inbreeding lowers the genetic diversity of a population. • This loss of genetic diversity tends to be bad since more individuals end up homozygous for recessive alleles with damaging effects. • This is the reason incest is taboo and illegal.
SmallCondition 1: Largepopulation = No Genetic Drift • An example of the founder effect would be the Amish in Lancaster County, Pennsylvania. • A group of 400 Amish individuals immigrated to the U.S. in the 1700’s. • Of these 400 individuals, one man and his carried a recessive allele for Ellis-van Creveld syndrome. • This syndrome results in dwarfism, polydactyly, and heart defects. • Also, in the Amish community, intermarriage with other groups is not permitted and no outsiders are permitted to join the community. • Therefore, the Amish marry only within their community, leading to inbreeding. • Due to the founder effect and inbreeding, about 1 in 8 people in this Amish population in Lancaster County are now heterozygous for this allele and one in 200 are homozygous, meaning they have Ellis-van Creveld Syndrome.
Condition 2: No Immigration/Emigration (No Gene Flow) • Gene flow is the movement of alleles between populations when individuals immigrate (enter) into a population or emigrate (leave) from a population. • Gene flow tends to counteract the effects of mutations, natural selection, and genetic drift, keeping allele frequencies in a population relatively stable by increasing genetic diversity. • Gene flow is a fear of opponents of GM crops. • And it is already occurring, since scientists have already found genetically engineered genes such as the bt gene and herbicide resistant genes in wild populations such as weeds and crops that have not been genetically engineered.
Speciation • Speciation is the evolutionary process by which new species form. • Speciation occurs when mutations, natural selection, and genetic drift cause gene frequencies to vary far from each other in different populations. • Genetic differences in the separate populations accumulate independently of one another, leading to genetic divergences and forming of new species. • Speciation is often triggered by a physical barrier that arises and cuts off gene flow between populations. • No matter how it happens, reproductive isolation is always part of the speciation process.
Reproductive Isolation • Several different things can result in reproductive isolation and, then, the formation of new species: • Physical barrier (finches in Galapagos) • Timing of reproduction (cicadas) • Size or shape of reproductive parts (sages) • Adaptations that allow certain individuals to survive in different microenvironments (manzanita plants) • Behavioral differences (bird courtship displays)
Reproductive Isolation • Even if the sex cells of two different species do somehow meet up and fertilization occurs, generally these hybrids will either die prematurely or have reduced fitness if born. (ex, tigons and ligers) • Other interspecies hybrids are sterile, so no more hybrids can be produced • (female horse + male donkey = mule =infertile) • (64 chromosomes + 62 chromosomes = 63 chromosomes)
Allopatric Speciation • Allopatric speciation occurs when a physical barrier separates two populations and ends gene flow between them. • This results in reproductive isolation mechanisms developing so that, even if individuals met up again, they would not be able to interbreed with one another. • Physical barriers can include man-made structures (Great Wall of China) or natural occurrences (usually caused by plate tectonics-formation of a land bridge separating one body of water into two, formation of islands, etc.)
Sympatric Speciation • In sympatric speciation, populations inhabiting the same geographic area speciate even though there is no physical barrier between them. • Can occur instantly with a change in chromosome number (caused by nondisjunction)
Sympatric Speciation • However, sympatric speciation can occur without a change in chromosome number. • In the waters of shallow Lake Victoria in Africa, there exists more than 500 species of cichlid fish though there has been no change in chromosome number and no physical barriers. • Scientists believe all of these species have arisen due to mutations in genes that affect color perception. • The ability to perceive color affects choice of mates and habitats in this shallow lake. • The colors that a female cichild sees best are the same colors displayed by males of her species. • It is believed that the ability to see certain colors caused certain females to mate only with certain colored males, resulting in reproductive isolation based upon color vision and leading to speciation.
Homework • Of all of the processes of evolution discussed in this presentation, pick one that you think has the potential to cause the greatest change in the gene frequency of a population over time (evolution) and explain why.
