Mechanisms of evolution Lesson 5

Darwin’s Theory Darwin summarized natural selection in these words. “can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind?”

Source of Variation Darwin admitted that a weakness in his theory was that he could offer no explanation for the source of variation. Six years after Darwin, Gregor Mendel presented his work on garden peas. Now we understand mutation and recombination is the source of new inheritable variations.

Key Terms Natural Selection: the way in which nature favours the reproductive success of some individuals with in a population over others It is the survival of the fittest – the organisms that are best able to adapt to the environment will survival and reproduce. Artificial Selection – directed breeding of animals/plants that exhibit a particular trait

Mechanisms of Evolution Gene pool is the complete set of all alleles contained within a species or population Not all evolutionary changes are the result of natural selection Evolution can occur due to catastrophic events Mutation is the ultimate source of variation in an individual’s gene pool

Random Change Evolution defined in genetic terms as any change in gene (and allele) frequencies within a population or species.

Key factors in a Change to a Gene Pool 1.Genetic Drift; chance fluctuations cause change 2.Non-random mating opportunities; preferred individuals pass on their alleles in greater numbers than others 3.Genetic mutation; new alleles are created or alleles are changed 4.Migration - removes alleles from the population 5.Natural Selection; individuals with certain alleles have greater reproductive success than others increasing the relative frequency of their alleles

Genetic Drift When populations are small – chance can play a significant role in altering allele frequencies Change in the genetic makeup of a population resulting from chance is called genetic drift Genetic drift can lead to a fixation of alleles

Genetic Drift in Small Populations

Genetic Drift – Coin Flip analogy Take a loonie. And write a capital letter for a dominant allele and a lower case letter for the recessive allele What should be the ratio of T : t? Perform 5 flips and record the results Perform 25 flips and record the results

Bottleneck Effect When a severe event results in a drastic reduction in numbers, a population may experience a bottleneck effect A very small sample of alleles survive to establish a new population Relative frequencies may differ from original population

Bottleneck Effect

Founder Effect When a few individuals from a large population leave to establish a new population The allele frequencies of the new population will not be the same as the original population

Polydactyly Dominant trait Philadelphia Pennsylvania Amish communities were founded in in 1700s by few families

Gene Flow When organism migrate, leaving one population and joining another, they alter the allele frequencies of both In this way, genetic information is shared between populations. Unlike genetic drift, gene flow tends to reduce between populations.

Natural Selection Small populations that result from a bottleneck or founder effect are also subject to the effects of genetic drift This increases the chances that their gene pool will differ from that of the original population Although genetic drift and bottlenecks can be important in some cases, natural selection is usually the major driver behind changes that result in the evolution of a significant adaptation Natural selection is the only mechanism known that is able to shape a species to its environment

Stabilizing Selection -Favours an ____________ phenotype - Acts against __________ variants of the phenotype The most common phenotype (the intermediate phenotype) is made more common in the population by removing the extreme forms. This type of selection: - Reduces ______________ - Improves adaptation of the population to aspects of the environment that remain constant

Directional Selection - Favours the phenotypes at one ________________ over the other - Common during times of environmental__________ or when a population ___________ to a new habitat that has different environmental conditions and niches to exploit Examples: - The changes in coloration of peppered moths - Antibiotic resistance in infection- causing bacteria

Disruptive Selection - Takes place when the __________ of a range of phenotypes are favoured over intermediate phenotypes - Thus, intermediate phenotypes can be eliminated from the population Example: The extreme size differences of mature male coho salmon. The smaller phenotype averages 500g, and the larger phenotype averages 4500g. This difference in size reflects the means by which each phenotype gains access to females. The smaller specialize in a “sneaking” technique to fertilize eggs. The larger specialize in fighting for access to eggs.

Practice Problem: Identify the Type of Selection The graph below shows a representation of the changes that have occurred in a population of bentgrass over a period of time. Use your understanding of the three types of natural selection (stabilizing, directional, disruptive) to identify the type of selection that is acting on this bentgrass population.

Sexual Selection The different phenotypes of the male coho salmon are also a specific example of natural selection referred to as sexual selection. In general, sexual selection involves: -Competition between males through combat or visual displays -Females choosing their mates

Sexual Selection Males and females of many animal species often have very different __________ characteristics, such as colourful plumage in male birds and antlers in male deer. This difference between males and females is called ________ _____________

Doesn’t he look fit?

Is mate choice in humans based on smell?