Evolution - Unifying Theme of Biology Microevolution Chapters 13 &14

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1 Evolution - Unifying Theme of Biology Microevolution Chapters 13 &14 New Synthesis Natural Selection Unequal Reproductive Success Examples and Selective Forces Types of Natural Selection Speciation

2 The New Synthesis Ernst Mayr ( ), is pictured here on the right during an ornithologic expedition in New Guinea in 1928, with his Malay assistant.

3 Natural Selection and Adaptation Evolutionary biologist Ernst Mayr helped develop the modern evolutionary synthesis of Mendelian genetics and Darwinian evolution He dissected the logic of Darwin s theory into three inferences based on five observations

4 Observation #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully

5 Observation #2: Populations tend to be stable in size, except for seasonal fluctuations

6 Observation #3: Resources are limited Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

7 Observation #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike

8 Observation #5: Much of this variation is heritable Inference #2: Survival depends in part on inherited traits; individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

9 Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: Stabilizing Disruptive Directional

10 Stabilizing Selection When the average individuals in the population have higher fitness and are favored.

11 Directional Selection When individuals at one end of the curve have higher fitness and are favored more than individuals in the middle or at the other end. Communities spray DDT pesticide on Mosquitoes to stop the spread of malaria and the West Nile virus. Those mosquitoes with a high resistance to DDT will be favored over those who have no or little resistance to DDT.

12 Disruptive Selection When individuals at both of the extremes (upper and lower ends) of a trait are both more fit and are favored over the average individuals in the population.

13 Frequency of individuals LE Original population Original population Evolved population Phenotypes (fur color) Directional selection Disruptive selection Stabilizing selection

14 Speciation can occur in two ways: Allopatric speciation Sympatric speciation

15 Allopatric and Sympatric Speciation In allopatric speciation, a new species forms while geographically isolated from its parent population In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species

16 LE 24-5 Allopatric speciation Sympatric speciation

17 Allopatric ( Other Country ) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations One or both populations may undergo evolutionary change during the period of separation

18 LE 24-6 A. harrisi A. leucurus To determine if allopatric speciation has occurred, reproductive isolation must have been established

19 LE 24-7a Reproductive Barriers may Evolve as Populations Diverge Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Mating experiments after several generations Some flies raised on maltose medium

20 Maltose Male Male Different populations Starch Same population LE 24-7b Note: Mating behavior has changed!! Female Female Same Different Starch Maltose populationpopulations Mating frequencies in experimental group Mating frequencies in control group

21 Sympatric ( Same Country ) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations May occur when mating and resulting gene flow between populations are reduced by factors such as polyploidy, habitat differentiation, or sexual selection.

22 Habitat Differentiation and Sexual Selection Sympatric speciation can also result from the appearance of new ecological niches In cichlid fish, sympatric speciation has resulted from nonrandom mating due to sexual selection

23 LE P. pundamilia P. nyererei Normal light Monochromatic orange light

24 Allopatric and Sympatric Speciation: A Summary In allopatric speciation, a new species forms while geographically isolated from its parent population In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species

25 Adaptive Radiation Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities

26 The evolution of many diverselyadapted species from a common ancestor is called an adaptive radiation. Fig Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

27 LE 22-6 Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp. Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground. Insect eater. The green warbler finch (Certhidea olivacea) used its narrow, pointed beak to grasp insects.

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29 Heterozygote Advantage The case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. This selection favoring the heterozygote is one of the mechanisms that maintain polymorphism and help to explain some kinds of genetic variability. There are several cases in which the heterozygote conveys certain advantages and some disadvantages while both versions of homozygotes are only at disadvantages. A well-established case of heterozygote advantage is that of the gene involved in sickle cell anaemia.

30 A well-established case of heterozygote advantage is that of the gene involved in sickle cell anemia. Homozygous recessive sicklers (ss) have resistance to falciparum malaria. But this is usually fatal, About 100,000 people around the world succumb to sickle-cell related health problems every year. However, that is far fewer than the1,500,000 who die from malaria. But the heterozygote (Ss) provides some protection from malaria.

31 Polyploidy Polyploidy is presence of extra sets of chromosomes due to accidents during cell division It has caused the evolution of some plant species An autopolyploid is an individual with more than two chromosome sets, derived from one species

32 LE 24-8 Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. Gametes produced by flowers on this tetraploid branch are diploid. Offspring with tetraploid karyotypes may be viable and fertile a new biological species. 2n = 6 4n = 12 2n 4n

33 LE 24-9 An allopolyploid is a species with multiple sets of chromosomes derived from different species Species A 2n = 4 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Viable fertile hybrid (allopolyploid) 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species B 2n = 6

34 The End