- Slides: 32
Evolution of Populations Chapter 17 How can populations evolve to form new species?
Genes & Variation Section 17 -1 How do genes make evolution possible? To be added at a later date….
Evolution as Genetic Change in Populations Section 17 -2 How does natural selection affect singlegene and polygenic traits?
Evolution of insecticide resistance in insect populations
Evolution of Populations, not individuals, evolve Individuals are selected (natural selection), populations evolve – Population = a group of individuals of the same species that live in the same area
Natural Selection on Polygenic Traits Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce three types of selection: – Directional – Diversifying – Stabilizing
Modes of Natural Selection Directional Selection – Shifts the overall makeup of the population by favoring one variant or extreme Diversifying Selection – Favors variants of two opposite extremes Stabilizing Selection – Acts on extreme phenotypes and favors the more common intermediate
Figure 23. 12 Modes of selection
Besides natural selection, is there anything else that may influence allele frequencies? Yes, chance occurrences in a small population cause an allele to become more or less common in a population
Genetic Drift Related to small population sizes that may not be representative of the original population Two examples of genetic drift – Bottleneck Effect natural disaster drastically reduces the size of a population Potential loss of individual variation and adaptability – Founder Effect A few individuals from a population colonize a new habitat
Figure 23. 5 The bottleneck effect: an analogy
Can populations remain stable and not evolve? The answer is yes, some populations may be so stable they are considered to be in equilibrium (which means no net change) To describe a population that is in equilibrium and test to see whether things have changed over time, we use the Hardy -Weinberg Equilibrium Theory
Hardy Weinberg Equilibrium Theory Describes the genetics of a nonevolving population States that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination Remember! Meiosis and random fertilization do not change the overall genetic structure of a population
Hardy Weinberg Equilibrium. Theory To maintain equilibrium the following conditions must be met: – Very large population size – No migration – No net mutations – Random mating (= NO sexual selection!) – No natural selection
Hardy Weinberg Equilibrium, cont’d p+q=1 p 2 + 2 pq + q 2 = 1 Allows us to calculate the frequencies of alleles if we know the frequencies of genotypes and vice versa Note that p and q represent the frequencies of two different alleles and that p 2 and q 2 represent homozygous individuals for each of the alleles and pq represents the heterozygous individuals
Figure 23. 3 a The Hardy-Weinberg theorem
Practice Problems, cont’d Q. In a population that is in Hardy-Weinberg equilibrium, 16% of the individuals show the recessive trait. What is the frequency of the dominant allele in the population? A. q 2 =. 16 so q = 0. 4 therefore, p = 0. 6
The Process of Speciation Section 17 -3 How do new species form?
How do new species form?
What defines a species? Biological Species Concept – Defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to provide viable, fertile offspring
How do new species form? In order for a new species to form, some type of reproductive isolation must occur, in other words, there is a failure to produce viable, fertile offspring Reproductive Isolation can develop in a variety of ways: – Behavioral Isolation – Geographic/Habitat Isolation – Temporal Isolation
Reproductive Isolation cont’d Geographic/Habitat Isolation – Live in different habitats, encounter each other rarely – Ex: two species of garter snakes, one predominantly aquatic, one living predominantly terrestrial Kaibab & Abert’s Squirrels
Reproductive Isolation cont’d Important note Although geographic isolation often leads to reproductive isolation and the development of new species, it is not a guarantee. It may be possible for geographically separated populations to be reunited and still interbreed and produce fertile offspring. Should this occur, no new species developed during the period of separation.
Reproductive Isolation cont’d Temporal Isolation – Two species breed at different times of the day or year – Ex: skunks late spring vs late summer or flowers bloom at different times of the year Behavioral Isolation – Signals that attract mates are very different between species – Ex: songs of birds, firefly lighting pattern, courtship dances
Speciation in Darwin’s Finches What is the current hypothesis about the development of the various finch species on the Galapagos Islands?
Speciation in Darwin’s Finches The steps proposed include: – Founders Arrive – Geographic Isolation – Changes in the Gene Pool – Reproductive Isolation – Ecological Competition – Continued Evolution
Adaptive Radiation A proposed mechanism by which different finch species may have evolved.
Molecular Evolution Section 17 -4 What are molecular clocks? Where do new genes come from? How may Hox genes play a role in evolution?
What are molecular clocks? Molecular clocks use mutation rates in DNA to estimate the time that two species have been evolving independently
What are molecular clocks? Neutral Mutations as Ticks – Neutral mutations must be used for the clock because they are not selected for or against. They simply mark time. – The more similar the series neutral mutations, the more recently two species are thought to have shared a common ancestor – Distantly related organisms are expected to have very few similarities in their accumulated neutral mutations
Where do new genes come from? One way in which new genes evolve is through the duplication and modification of existing genes Once genes are duplicated, they may accumulate different mutations over time, thereby changing their function Several different “gene families” are present in our genomes as a result of duplication/mutation cycles – Ex: the globin gene family (makes hemoglobin), ribosomal RNA gene family, etc.
How may Hox genes be involved in evolutionary change? Hox genes are involved in the development of body plans – Ex: How many legs do you have? Where are they placed on your body? How long or short will they be? Small changes in Hox gene activity during embryonic development can produce large differences in adult animals.