NIS - BIOLOGY Lecture 67 - Lecture 68 - Lecture 69 - Lecture 70 Shaping Evolutionary Theory Ozgur Unal

Mechanisms of Evolution Other than natural selection, how else might species change over time? Evolution can be the result of many different environmental and genetic circumstances. In this section you will learn about the five principle mechanisms of evolution: natural selection, gene flow, genetic drift, mutation and nonrandom mating.

Hardy-Weinberg Principle At the beginning of the 20th century, genes had not been discovered  the allele was understood to be one form of an inherited trait In 1908, Godfrey Hardy and Wilhelm Weinberg independently showed that evolution will not occur in a population unless allelic frequencies are acted upon forces that cause change. According to Hardy-Weinberg principle, when allelic frequencies remain constant, a population is in genetic equilibrium. Check out Figure 15.4!!

Hardy-Weinberg Principle Example: Consider a population of 100 humans. 40 people  homozygous dominant (EE) 40 people  heterozygous (Ee) 20 people  homozygous recessive (ee) What are the allele frequencies? p: the E allele frequency = 0.6 q: the e allele frequency = 0.4 q + p = 1  p2 + 2qp + q2 = 1 p2: equilibrium frequency for homozygous dominant q2: equilibrium frequency for homozygous recessive 2qp: equilibrium frequency for heterozygous For our example: p2: 0.36 q2: 0.16 2qp: 0.48

Hardy-Weinberg Principle Example: In a population of dogs, an allele for short ears (E) is dominant, while the allele for long ears (e) is recessive. For every 100 dogs in the population, 16 have long ears. Use the Hardy-Weinberg equation to calculate the frequencies of the E allele (frequency p) and the e allele (frequency q). http://bcs.whfreeman.com/thelifewire/content/chp00/00020.html

Hardy-Weinberg Principle According to Hardy-Weinberg principle, a population in genetic equilibrium must meet conditions. There must be no genetic drift, no gene flow, no mutation, mating must be random and there must be no natural selection. If a population is not in genetic equilibrium, at least one of the five conditions has been violated. Check out Table 15.3!! What could cause a population to shrink in size? What might cause an organism to emigrate from an area?

Mechanisms of Evolution According to Hardy-Weinberg principle, a population is in genetic equilibrium if the following 5 conditions are met: No genetic drift No gene flow No mutation Random mating No natural selection Since genetic equilibrium means no change in allele frequency, it also implies no evolution. Therefore, we can say that genetic drift, gene flow, mutation, non-random mating and natural selection are the mechanisms of evolution.

Mechanisms of Evolution Genetic Drift: Any change in the allelic frequencies in a population that is due to chance is called genetic drift. Remember law of independent assortment  only one of a parent’s two alleles passes to the offspring (randomly) In a large population, enough alleles ”drift” to ensure that the allelic frequencies of the entire population remain constant. In small populations, genetic drift becomes more pronounced. There are two examples of genetic drift: Founder effect Bottleneck

Mechanisms of Evolution Genetic Drift: The founder effect can occur when a small sample of a population settles in a location separated from the rest of the population. Alleles that were uncommon in the original population might be common in the new population and the offspring in the new population will carry those alleles. Example: Amish commmunity in the US Bottleneck occurs when a population declines to a very low number and then rebounds. The gene pool of the rebound population often is genetically similar to that of the population at its lowest level, that is it has reduced diversity. Example: Cheetahs in Africa

Mechanisms of Evolution Gene Flow: A population in genetic equilibrium experiences no gene flow. It is a closed system  no new genes entering and leaving the population  no immigration or emigration In reality, few populations are isolated (closed). Non-random Mating: Rarely is mating completely random in a population. Usually, organisms mate with individuals in a close proximity. Mutation: Mutation is a random change in genetic material due to mutagens. The cumulative effect of mutations in a population might cause a change in allelic frequencies. Many mutations a lethal, but some provide advantage to an organism.

Mechanisms of Evolution Natural Selection: The Hardy-Weinberg principle requires that all individuals in a population be equally adapted to their environment and thus contribute equally to the next generation. However, natural selection acts to select the individuals that are best adapted for survival and reproduction. Natural selection acts on an organism’s phenotype and change allelic frequencies. There are four ways that natural selection alters phenotypes: Stabilizing selection Directional selection Disruptive selection Sexual selection

Mechanisms of Evolution Natural Selection: Most common form of natural selection is stabilizing selection. It operates to eliminate the extreme expressions of a trait when the average expression leads to a higher fitness. Example: Human babies with above normal and below normal weights have lower chances of survival than babies born with normal weights  Figure 15.16!! If an extreme version of a trait makes an organism more fit, directional selection might occur. Example: The peppered moth in industrial England. Check out Figure 15.17!!

Mechanisms of Evolution Natural Selection: Disruptive selection is a process that splits the population into two groups. It tends to remove individuas with average traits, but retains individuals with extreme traits. Example: Northern water snakes In sexual selection a change in frequency of a trait is based on the ability to attract a mate. This type of selection often operates in populations where males and females differ significantly in appearance. Example: Peacock

Reproductive Isolation Remember the 5 mechanisms of evolution. To what extent each mechanism contributes to the origin of new species is a major topic of debate. Today, we will consider two types of isolating mechanisms that prevent gene flow among populations. Prezygotic isolating mechanisms Postzygotic isolating mechanisms Prezygotic isolating mechanisms: These operate before fertilization occurs. These mechanisms prevent genotypes from entering a population’s gene pool through geographic, ecological, behavioral or other differences. Example: Eastern and western meadowlarks. Check out Figure 15.20!!

Reproductive Isolation Postzygotic isolation mechanisms: These mechanisms operate after fertilization occurs. When fertilization has occured but a hybrid offspring cannot develop or reproduce, postzygotic isolation has occured. These mechanisms prevent offspring survival and reproduction. Example: Lion + Tiger  Liger (sterile)

Speciation Speciation is the process whereby some members of a sexually reproducing population change so much that they can no longer produce fertile offspring with members of the original population There are two types of speciation: Allopatric Speciation Sympatric Speciation Allopatric Speciation: In allopatric speciation, a physical barrier (such as islands, rivers, lava flows etc) divides one population into two or more populations. The separate populations eventualy will contain organisms that, if enough time has passed, will no longer be able to breed with one another. Example: Kaibab and Albert Squirrels

Speciation Sympatric Speciation: In sympatric speciation, a species evolves into a new species without a physical barrier. The ancestor species and the new species live side by side during the speciation process. Example: Several insect species  appear to be diverging based on the type of fruit they eat Suppose some members of a tree frog population begin mating a month earlier then the other members of the same population. What type of isolation will occur? What type of speciation does reproductive isolation cause?

Speciation Allopatric Specication vs Sympatric Speciation

Patterns of Evolution Speciation is a long process compared to the human life span. Many details of the speciation process remain unresolved. However, evidence of speciation is visible in patterns of evolution. Adaptive radiation (divergent evolution) Coevolution Convergent evolution Adaptive Radiation: More than 300 species of cichlid fish once lived in Africa’s Lake Victoria. Data shows that these species diverged from a single ancestor within the last 14,000 years. Adaptive radiation (divergent evolution) can occur in a relatively short time when one species gives rise to many species in response to the creation of new habitat. Adaptive radiation often follows large-scale extinctions.

Patterns of Evolution Coevolution: Many species evolve in close relationship with other species. This relationship might be so close that the evolution of one species affects the evolution of other species  coevolution Mutualism is one form of coevolution. Example: Comet orchids and moths In another form of coevolution, one species can evolve a parasitic dependency on another species  coevolutionary arms race Example: Plants and insect pathogen

Patterns of Evolution Convergent Evolution: Sometimes unrelated species evolve similar traits even though they live in different parts of the world Convergent evolution occurs in environments that are geographically far apart but that have similar ecology and climate. Example: Mara and rabbit Check out Table 15.4!!

Patterns of Evolution Rate of Speciation: Evolution is a dynamic process. In some cases traits might change rapidly. In other cases, traits might remain unchanged for millions of years. Most scientists think that evolution proceeds in small, gradual steps  gradualism However, the fossil record contains instances of abrupt transitions  Example: certain species of fossil snails The theory of punctuated equilibrium attempts of explain such abrupt transitions in the fossil record. Check out Figure 15.25!!