Advanced Topics in Evolutionary Theory For natural selection to operate, there must be variation among individuals in a population. Variation arises from: Mutations Sexual Reproduction Balanced Polymorphism Mutations are random changes in the DNA of an individual. Mutations can introduce new alleles into a population. Mutations provide the raw material for variation in a population. Mutations in fruit flies include changes in eye color and wing shape This dog has more muscle due to a muatation. Good or bad—what do you think? Sexual reproduction produces individuals with new combinations of alleles. 3 Events lead to genetic variation: 1. Crossing over 2. Independent Assortment of homologs 3. Random joining of gametes in fertilization Balanced polymorphism is the maintenance of different phenotypes in a population. Often, a single phenotype provides the best adaptation and becomes more common in the population. However, examples of polymorphism exist in many populations. The Gouldian Finch possesses a genetic color polymorphism in the form of three genetically determined head-colors (yellow, black and red) that coexist in the same population. Some characters are fixed in a population—this means that all individuals in a species have these traits. Other characters are polymorphic, meaning there are two or more variants. All tulips grow from bulbs—a trait that is fixed in their species. However, there are numerous varieties of colors in their flowers. Sickle Cell Heterozygote is Advantage: occurs when Anemia caused by a being heterozygous for a homozygous recessive trait has a greater gene -- ss selective advantage. Ex: Sickle Cell Anemia— individuals who are Ss have an advantage because they will not get either Sickle Cell Disease or Malaria People who are homozygous dominant for this gene—SS are suseptible to malaria. Hybrid Vigor: describes the superior quality of some hybrid offspring. This comes from a reduced number of deleterious recessive conditions and heterozygous advantage Ex: Hybrid Corn The 2 corn plants on the left are the original parents. The corn plant on the right is a hybrid of the two. Frequency-dependent selection (or minority advantage) occurs when the least common phenotype have a selective advantage. Common phenotypes are selected against! However, since rare phenotypes have a selective advantage, they soon increase in frequency and become common. Once they become common—they are selected against! Some predators form a “search image” or standard representation of their prey. By standardizing on the most common form of its prey, the predator optimizes its search effort. The prey that is rare, however, escapes predation. Not all variation has selective advantage. Instead, much of the variation observed, especially at the molecular level in DNA and proteins, is neutral variation. In many cases, the environment determines whether a variation is neutral or not. Every human has slightly different fingerprints—even identical twins. However, this is an example of neutral variation. Natural selection is not the only way that evolution occurs. Factors that can change allele frequencies are: Natural selection Mutations Gene Flow Genetic Drift Nonrandom Mating If there is no natural selection, gene flow, mutation, genetic drift and there is random mating, then Evolution Cannot Occur! Mutations—introduce new alleles that may provide a selective advantage (most, however, are harmful) Gene Flow—describes the addition or removal of alleles from the population by emigration or immigration Genetic Drift is a random increase or decrease of alleles. Some alleles may increase or decrease for no other reason than just by chance. When populations are small (usually less than 100 individuals), the effect of genetic drift can be very strong and can dramatically affect evolution. How will this random event affect the evolution of this beetle population? The Founder Effect— occurs when allele frequencies in a group of migrating individuals are, by chance, not the same as that of their population of origin. Go to: http://evolution.berkel ey.edu/evolibrary/artic le/_0/speciationmodes _03 A Bottleneck—occurs when the population undergoes a dramatic decrease in size. Regardless of the cause of the bottleneck (natural disaster, etc.), the small population that results becomes severely vulnerable to genetic drift. The Founder Effect: One of the founding members of the small group of Germans that began the Amish community in Pennsylvania possessed an allele for polydactylism. After 200 years of reproductive isolation, the number of cases of this trait among the 8,000 Amish exceeds the number of cases occurring in the remaining world’s population Nonrandom mating occurs when individuals choose mates based upon their particular traits. They may always choose mates with traits similar to their own (or the opposite!) Nonrandom mating also occurs when mates choose only nearby individuals. When the allele frequencies in a population remain constant from generation to generation, the population is said to be in genetic equilibrium, or Hardy-Weinberg Equilibrium. There is No Evolution! 1. All traits are selectively neutral (no natural selection) 2. Mutations do not occur 3. The population must be isolated from other populations (no gene flow) 4. The population is large (no genetic drift) 5. Mating is random Can This Ever Occur in Real Populations???? Genetic equilibrium is determined by evaluating the following values: 1. Allele frequencies for each allele (p, q) 2. Frequency of homozygotes (p2, q2) 3. Frequency of heterozygotes (pq + qp = 2pq) Also, the following two equations: 1. p + q = 1 (all alleles sum to 100%) 2. p2 + 2pq + q2 = 1 (all individuals sum to 100%) African Cichlids have evolved different feeding mechanisms and anatomy to explore different food niches. These are just a few, including insect-eaters, snail eaters, algaeeaters, plankton feeders, fish-eaters, etc. A species is a group of interbreeding organisms. Speciation is the process by which new species evolve Two forms of speciation: Allopatric and Sympatric Allopatric (“other country”) speciation occurs when some sort of barrier separates a single population into two The two populations evolve independently, and if they change enough, then even if the barrier is removed, they cannot interbreed. An example of sympatric speciation could occur if a new food source became available. Only certain organisms would exploit this new resource , eventually leading to a reduced gene flow between them and the original. Sympatric (“same country”) speciation is the formation of new species without the presence of a physical barrier. This may happen when a new species originates while still living in the same area as the parent species. Balanced polymorphism— When variations in a population diverge so much that the two variants can no longer interbreed. Polyploidy—A condition in which an individual has more than the normal number of sets of chromosomes (more common in plants); these will not be able to mate with the original organisms. The plant in the middle is a polyploid individual while the others are diploid. Because it is polyploid, it cannot mate with its diploid “cousins” and thus becomes its own species. Adaptive radiation is a rapid series of speciation events that occur when one or more ancestral species invades a new environment. If there are many ecological niches, several species will evolve because each can fill a different niche. An example of adaptive radiation is the evolution of finches on the Galapagos Islands. Each evolved to take advantage of the special food available on its island. We define different species as organisms who cannot (or will not) reproduce with each other. Interbreeding between two different species cannot produce viable, fertile offspring. These two species are very closely related, yet one prefers to rest on open, sunny areas and the other rests on shady branches of trees. They have evolved different behaviors and have become different species. If species are not physically separated by a geographic barrier, various mechanisms commonly exist to maintain reproductive isolation and prevent gene flow. Remember: a zygote is a fertilized egg—it is a single cell made from the union of an egg and a sperm Prezygotic isolating mechanisms, then, will prevent fertilization in some way: Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Habitat isolation—species do not encounter each other Temporal isolation—species mate or flower during different seasons or at different times of the day Behavioral isolation—when a species does not recognize another species as a mating partner because it doesn’t perform the correct courtship rituals, sing the correct songs, or release the correct chemicals (scents). Mechanical isolation—when male and female genitalia are structurally incompatible or when flower structure select for different pollinators Gametic isolation—when male gametes do not survive in the environment of the female gamete (such as in internal fertilization) or when female gametes do not recognize male gametes Gametic isolation is particularly important in aquatic environments because many aquatic animals release their gametes into the water, where fertilization takes place Postzygotic isolating mechanisms prevent the formation of fertile progeny. So, the egg has been fertilized but the offspring doesn’t survive (or it survives, but can’t reproduce) One example: Hybrid inviability—when the zygote fails to develop properly and aborts (dies) before reaching reproductive maturity. Papilio glaucus (the Eastern Tiger Swallowtail) and Papilio canadensis (the Canadian Tiger Swallowtail) are two species of North American butterflies. Although similar in appearance, they are adapted to different climates, and they show mild hybrid incompatibility Hybrid sterility occurs when hybrids become functional adults, but are reproductively sterile (eggs or sperm are nonexistent or dysfunctional). Hybrid breakdown occurs when hybrids produce offspring that have reduced viability or fertility. Divergent Evolution—describes two or more species that originate from a common ancestor and become increasingly different over time. This may happen as a result of allopatric or sympatric speciation or by adaptive radiation. Convergent Evolution—describes two unrelated species that share similar traits. Their similarities arise, not because they share a common ancestor, but because each species has independently adapted to similar ecological conditions. These species are not closely related, nor did they come from a common ancestor, yet all evolved wings for flight Parallel Evolution— describes two related species or two related lineages that have made similar evolutionary changes after their divergence from a common ancestor. This is one of several species of rainforest moths that have evolved coloration patterns similar to those of butterflies. Coevolution is the titfor-tat evolution of one species in response to new adaptations that appear in another species. Coevolution occurs between predator and prey, plants and planteating insects, and flowering plants and pollinators. The Madgascar Orchid, shown here, has a flower with a petal that it 11” long with a tiny drop of nectar at the very tip. The fly shown with it has a proboscis that is the exact length needed to get the nectar and in the process, pollinate the flower.