Lecture 1 – Evolution of Populations

Lecture 1 – Evolution of Populations The Smallest Unit of Evolution • One misconception is that organisms evolve during their lifetimes • Natural selection acts on individuals, but only populations evolve Genetic Variation: • Variation in heritable traits is a prerequisite for evolution • Mendel’s work on pea plants provided evidence of discrete heritable units (genes) • Genetic variation among individuals is caused by differences in genes or other DNA segments • Phenotype is the product of inherited genotype and environmental influences • Natural selection can only act on variation with a genetic component Population: Localized group of organisms which belong to the same species. Species: Groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. Gene pool: The total aggregate of genes in a population at any one time. - Consists of all the alleles at all gene loci in all individuals of a population. Alleles from this pool will be combined to produce the next generation. The frequency of an allele in a population can be calculated - For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2 - The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles - By convention, if there are 2 alleles at a locus, p andq are used to represent their frequencies - The frequency of all alleles in a population will add up to p+ 1 q( = 1) The Hardy-Weinberg Theorem states that the frequencies of alleles in the gene pool will remain constant unless acted upon by other agents. His theorem describes a nonevolving population. - For Hardy-Weinberg equilibrium to apply these five criteria must be met: 1. No mutations 2. Random mating 3. No natural selection (equal reproductive success) 4. Extremely large population size 5. No gene flow (be totally isolated) Microevolution: A change in allele or genotype frequencies in a population, can occur when the conditions required for Hardy-Weinberg equilibrium are not met. Microevolution can be caused genetic by drift, gene flow, mutation, nonrandom mating, andnatural selection. Nonrandom mating: increases the number of homozygous loci in a population, but does not in itself alter frequencies of alleles in a population’s gene pool. There are two kinds of nonrandom mating: inbreeding andassortative mating. Inbreeding: Individuals of a population usually mate with close neighbors rather than with more distant members of a population, especially if the members of the population do not disperse widely. - Self-fertilization, which is common in plants, is the most extreme example of inbreeding. Assortative mating: is another type of nonrandom mating which results when individuals mate with partners that are like themselves in certain phenotypic characters. For example: - Snow geese occur in a blue variety and a white variety, with blue color allele being dominant. Birds prefer to mate with those of their own color; this results in a lower frequency of heterozygotes than predicted by Hardy-Weinberg. Genetic drift: Changes in the gene pool of a small population due to chance. - The larger the population, the less important is the effect of genetic drift. - Two situations which result in populations small enough for genetic drift to be important are thebottleneck effect and thefounder effect. Bottleneck effect: When large segments of a population are destroyed by disasters. - In the small remaining population, some alleles may be over represented, some under-represented, and some alleles may be totally absent. Founder effect: When a few individuals colonize a new habitat, genetic drift is also likely to occur. Genetic drift in a new colony is called the founder effect. - The smaller the founding population, the less likely its gene pool will be representative of the original population’s genetic makeup. - The most extreme example would be when a single seed or pregnant female moves into a new habitat. Gene flow: The migration of fertile individuals, or the transfer of gametes, between populations. - Gene flow tends to reduce between-population differences which have accumulated by natural selection or genetic drift. Therefore depending on a population, gene flow can reduce or increase fitness in a population. Mutation: is a change in nucleotide sequence of DNA. A new mutation which is transmitted in gametes immediately changes the gene pool of a population by substituting one allele for another. - Mutation itself has little quantitative effect on large populations in a single generation, since mutation at any given locus is very rare. - Mutation is however, important to evolution since it is the original source of genetic variation, which is the raw material for natural selection. Natural Selection: • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions • For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture • Evolution by natural selection involves both change and “sorting” • New genetic variations arise by chance • Beneficial alleles are “sorted” and favored by natural selection • Only natural selection consistently results in adaptive evolution The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals. Reproductive success is generally more subtle and depends on many factors. • Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Selection favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection: • Directional selection favors individuals at one 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 Striking adaptation have arisen by natural selection • For example, cuttlefish can change color rapidly for camouflage • For example, the jaws of snakes allow them to swallow prey larger than their heads • • • • Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between an organism 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 Why Natural Selection Cannot Fashion Perfect Organisms: 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact