THE MODERN SYNTHESIS: DARWINISM MEETS GENETICS

           The modern synthesis is the fusion of

           genetics with

           evolutionary biology.

 

Populations as the Units of Evolution

           A population is

           a group of individuals of the same species, living in the same place at the same time and

           the smallest biological unit that can evolve.

           The total collection of alleles in a population at any one time is the gene pool.

           When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale.

 

Genetic Variation in Populations

           Individual variation abounds in all species.

           Not all variation in a population is heritable.

           Only the genetic component of variation is relevant to natural selection.

           Variable traits in a population may be

           polygenic, resulting from the combined effects of several genes, or

           determined by a single gene.

           Polygenic traits tend to produce phenotypes that vary more or less continuously.

           Single-gene traits tend to produce only a few distinct phenotypes.

 

Sources of Genetic Variation

           Genetic variation results from processes that both involve randomness:

           mutations, changes in the nucleotide sequence of DNA, and

           sexual recombination, the shuffling of alleles during meiosis.

 

Sources of Genetic Variation

           For any given gene locus, mutation alone has little effect on a large population in a single generation.

           Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutation as the only source of genetic variation.

 

Analyzing Gene Pools

           A gene pool

           consists of all the alleles in a population at any one time and

           is a reservoir from which the next generation draws its alleles.

           Alleles in a gene pool occur in certain frequencies.

           Alleles can be symbolized by

           p for the relative frequency of the dominant allele in the population,

           q for the frequency of the recessive allele in the population, and

           p + q = 1.

           Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.

           Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving).

           The Hardy-Weinberg formula

           p2 + 2pq + q2 = 1

           can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles.

 

Population Genetics and Health Science

           The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease.

 

Population Genetics and Health Science

           PKU

            is a recessive allele that prevents the breakdown of the amino acid phenylalanine and

            occurs in about one out of every 10,000 babies born in the United States.

           People with PKU must strictly regulate their dietary intake of the amino acid phenylalanine.

 

Microevolution as Change in a Gene Pool

           How can we tell if a population is evolving?

           A non-evolving population is in genetic equilibrium, also known as Hardy-Weinberg equilibrium, meaning the populations gene pool is constant over time.

           From a genetic perspective, evolution can be defined as a generation-to-generation change in a populations frequencies of alleles, sometimes called microevolution.

 

MECHANISMS OF EVOLUTION

           The main causes of evolutionary change are

           genetic drift,

           gene flow, and

           natural selection.

           Natural selection is the most important, because it is the only process that promotes adaptation.

 

Genetic Drift

           Genetic drift is a change in the gene pool of a small population due to chance.

 

The Bottleneck Effect

           The bottleneck effect

           is an example of genetic drift and

           results from a drastic reduction in population size.

           Passing through a bottleneck, a severe reduction in population size,

           decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and

           results in a loss of individual variation and hence adaptability.

 

           Cheetahs appear to have experienced at least two genetic bottlenecks:

           during the last ice age, about 10,000 years ago, and

           during the 1800s, when farmers hunted the animals to near extinction.

           With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges.

 

The Founder Effect

           The founder effect is likely when a few individuals colonize an isolated habitat.

           This represents genetic drift in a new colony.

           The founder effect explains the relatively high frequency of certain inherited disorders in some small human populations.

 

Gene Flow

           Gene flow

           is another source of evolutionary change,

           is separate from genetic drift,

           is genetic exchange with another population,

           may result in the gain or loss of alleles, and

           tends to reduce genetic differences between populations.

 

Natural Selection: A Closer Look

           Of all causes of microevolution, only natural selection promotes adaptation.

           Evolutionary adaptation results from

           chance, in the random generation of genetic variability, and

           sorting, in the unequal reproductive success among the varying individuals.

 

Evolutionary Fitness

            Relative fitness is

           the contribution an individual makes to the gene pool of the next generation

           relative to the contributions of other individuals.

 

Three General Outcomes of Natural Selection

           If we graph the coat color of a population of mice, we get a bell-shaped curve.

           If natural selection favors certain fur-color phenotypes,

           the populations of mice will change over the generations and

           three general outcomes are possible.

           Directional selection shifts the overall makeup of a population by selecting in favor of one extreme phenotype.

           Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population by shifting the overall makeup of a population towards both extremes.

           Stabilizing selection favors intermediate phenotypes, occurs in relatively stable environments, and is the most common, and shifts the overall makeup of a population by selecting in favor of the most central phenotypic form.

 

Sexual Selection

           Sexual selection is a form of natural selection in which individuals with certain traits are more likely than other individuals to obtain mates.

           Sexual dimorphism is a distinction in appearance between males and females not directly associated with reproduction or survival.