Kiki Coffman and Lorah Norman D2: Species & Speciation
D.2.1. Define allele frequency and gene pool. Allele frequency – a measure of the proportion of a specific variation of a gene in a population. It is expressed as a proportion or a percent. Gene pool – all of the genetic information present in the reproducing members of a population at a given time. A large gene pool exists in a population that shows substantial variety in its traits.
D.2.2. State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations. • After many generations of natural selection, some alleles prove to be advantageous and tend to be more frequent. • Some alleles are disadvantageous and are not passed on to as many offspring. • Several generations later, proportions may not be the same. • When allele frequency changes, we know some sort of evolution has occurred. • No change means no evolution has occurred.
D.2.3. Discuss the definition of the term species. • A species is the basic unit for classifying organisms. • Made up of organisms which: • Have similar physiological and morphological characteristics • Have the ability to interbreed to produce fertile offspring • Are genetically distinct from other species • Have a common phylogeny
D.2.3. Continued… • Challenges to the definition: • What about two populations that have the potential to interbreed but do not because they live in different niches or are separated by distance? • How should we classify organisms that do not interbreed because they reproduce asexually? • What about infertile individuals?
D.2.4. Describe three examples of barriers between gene pools. • Geographical isolation- happens when physical barriers such as land or water prevent males and females from mating • Example: tree snails in Hawaii. One population lives on the flank of a volcano and another population is on the other flank. They never come in contact with each other.
D.2.4. Continued… • Temporal isolation- incompatible time frames which prevent populations and their gametes from encountering each other • Example: if the female parts of the flowers of one population of plants reach maturity at a different time from the release of pollen of another population, they will have a difficult time producing offspring. • Example: One population of mammals is still hibernating or has not returned from a migration when another population of the same species is ready to mate.
D.2.4. Continued… • Behavioral isolation- happens when one population’s lifestyle or habits are not compatible with those of another population • Example: many species of birds rely of a courtship display for one sex to mate with the other. If one population’s courtship display is different from another population, they may not consider each other to be a mate.
D.2.4. Continued… • Hybrids • The vast majority of animal and plant hybrids are infertile • Even if one generation of hybrids is produced, a second generation is unlikely. • This presents a genetic barrier.
D.2.5. Explain how polyploidy can contribute to speciation. • Polyploidy is when a cell contains 3 or more sets of chromosomes. • It can result when cell division does not completely separate the copies of chromosomes into distinct nuclei so theyend up in the same cell. • Speciation is the process of an evolving population changing significantly enough so that the production of offspring with the original population becomes impossible. • A new species has evolved from an old one.
D.2.6. Compare allopatric and sympatric speciation. • Allopatric speciation – when a new species forms from an existing species because it is separated by a physical barrier. • Example: Might occur in a land-dwelling species when sea levels rise. This would cut off land-dwelling populations from each other and they could evolve separately on opposite sides of the water. If sea levels dropped again, and the populations came in contact with each other, they could be so different they wouldn’t be able to interbreed.
D.2.6. Continued… • Sympatric speciation – when a new species forms from an existing species while living in the same geographical area. • Example: moths produce chemical pheromones that play an important role in attracting/finding a mate. If, due to a mutation, a member of a moth population produced a different pheromone from others, it may be repulsive to some, but irresistible to others. Over time, the group producing this new pheromone might interbreed with only those producing the same pheromone. After a while, these combinations may produce a new species of moth. Both populations live in the same location no longer breed together.
D.2.7. Outline the process of adaptive radiation. • Adaptive radiation – occurs when many similar but distinct species evolve relatively rapidly from a single species or from a small number of species. • Happens as variations in the population allow certain members to exploit a different niche in a more successful way. • By natural selection and presence of one or more barriers, a new species evolves.
D.2.8. Compare convergent and divergent evolution. • Convergent – organisms not closely related independently evolve similar traits as a result of having to adapt to similar environmental or ecological niches. • Example: development of flight among different taxa • Divergent– related species evolve different traits. • Example: human foot and monkey foot. • In both types it is the process of natural selection that allowed the organism to adapt to their environment.
D.2.9. Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium. • Gradualism – the changes are small, continuous and slow. • The process of speciation is steady and ongoing • Punctuated equilibrium – changes are relatively quick and followed by periods of little or no change. • Happens often in response to changes in environment (ex. volcanic eruption) • Species are often destroyed and others adapt to new surroundings • The rest of the time, species live with little or no change
D.2.10. Describe one example of transient polymorphism. • Transient polymorphism means the changes are only temporary. • The peppered moth (Bistonbetularia) can have a grey or black form. • Grey is well camouflaged on tree trunks under normal conditions. Therefore, they are usually more numerous in population because black moths are more likely to be eaten by birds. • During the Industrial Revolution in the UK, factories produced large black clouds of smoke that stuck to tree trunks. Therefore, the black moth became more numerous than the grey because they were now better camouflaged. • This is called industrial melanism. • This example illustrates that there are no alleles or traits that are intrinsically “good” or “bad”, but rather “fit” or “unfit.”
D.2.11. Describe sickle-cell anemia as an example of balanced polymorphism. • Balanced polymorphism – when two or more alleles within a population are not transient and changing but are stabilized by natural selection. • An example is seen in the allele for normal red blood cells, and the allele for sickle cells. • Sickle cells are caused by a recessive allele. • Those who have sickle cell are very resistant to malaria infection. • Most people are homozygous for normal red blood cells and are susceptible to malaria. • People who are heterozygous have the sickle cell trait – they have normal and sickle shaped cells. They do not suffer from anemia in most cases. • People who are homozygous for sickle-shaped cells suffer from anemia but are highly resistant to malaria. • Balance is in the fact that the heterozygotes tend to be more fit for survival in zones with malaria, but do not suffer anemia.