The Evolution of Populations Ch. 23 Lecture Objectives Microevolution & Natural Selection Genetic Drift Founder & Bottleneck Effects Sexual Selection

Recall from our lecture on natural selection….. Charles Darwin presented evidence to support Descent with Modification (aka evolution) Natural Selection (driving force behind D w/ M) Finch Population on Galapagos Islands Daphne Major Island During a drought, large-beaked birds were more likely to crack large seeds and survive The finch population evolved by natural selection  Of 1200 birds 180 survived

1976 (similar to the prior 3 years) Figure 23.2 10 9 Average beak depth (mm) 8 Figure 23.2 Evidence of selection by food source 1976 (similar to the prior 3 years) 1978 (after drought)

Microevolution Change in gene frequencies in a population over generations Population is a localized group of individuals capable of interbreeding & producing fertile offspring Three mechanisms cause frequency changes Natural selection Genetic drift Gene flow

Variation in coat color (PHENOTYPE) is influenced by genes Figure 23.3 Variation in coat color (PHENOTYPE) is influenced by genes Figure 23.3 Phenotypic variation in horses

Figure 23.5 Not all traits are heritable (although we will focus mainly on heredity (a) (b) Figure 23.5 Nonheritable variation

1. Natural Selection Individuals in a population exhibit variations in their heritable traits  best suited traits tend to produce more offspring. Adaptive Evolution: consistently favoring some traits over others (not coincidental) improvement in the match between organisms and their environment

Natural Selection

Directional, Disruptive, and Stabilizing Selection There are three modes of selection Directional selection favors individuals at one extreme end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes

Frequency of individuals Figure 23.13 Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Figure 23.13 Modes of selection (a) Directional selection (b) Disruptive selection (c) Stabilizing selection

2. Genetic Drift Describes how gene frequencies fluctuate unpredictably from one generation to the next Tends to reduce genetic variation through losses of genes The smaller a sample, the greater the chance of random deviation from a predicted result

Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Figure 23.9–3 5 plants leave offspring 2 plants leave offspring CRCR CRCW CRCR CWCW CRCR CRCW CRCR CRCR CRCR CRCR CRCR CRCW CRCR CRCR CWCW CWCW CRCR CRCW CWCW CRCR CRCR CRCR CRCW CRCW CRCR CRCR CRCR CRCW CRCW CRCR Figure 23.9-3 Genetic drift (step 3) Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0

Ex. Of Genetic Drift

The Founder Effect (under umbrella of genetic drift) Occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population can be different from those in the larger parent population

The Bottleneck Effect (under umbrella of genetic drift) A sudden reduction in population size due to a change in the environment Resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift Understanding the bottleneck effect can increase understanding of how human activity affects other species

Original population Bottlenecking event Surviving population Figure 23.10–3 Figure 23.10-3 The bottleneck effect (step 3) Original population Bottlenecking event Surviving population

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Figure 23.11 Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken (a) Number of alleles per locus Percentage of eggs hatched Population size Location Illinois 1930–1960s 1993 1,000–25,000 <50 5.2 3.7 93 <50 Figure 23.11 Genetic drift and loss of genetic variation Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 (b)

Effects of Genetic Drift: A Summary Genetic drift is significant in small populations Genetic drift can cause allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful genes to become fixed

Gene Flow – AKA Migration Movement of genes among populations Genes can be transferred through the movement of fertile individuals or gametes (for example, pollen) Tends to reduce variation between populations over time Can increase or decrease the fitness of a population

Gene Flow

To summarize.. Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between a species and its environment increases Because the environment can change, adaptive evolution is a continuous process Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

Sexual Selection Natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics 1. Intersexual Selection: Members of the competitive sex show off for mates and the opposite sex chooses the best display. Some examples include dancing, singing, or showing bright colors.

Figure 23.15 Figure 23.15 Sexual dimorphism and sexual selection http://dragonflyissuesinevolution13.wikia.com/wiki/Intrasexual_Selection_vs._Intersexual_Selection?file=The_Mating_Dance

2. Intrasexual Selection: Members of the competitive sex fight amongst themselves and the key event determines reproductive success