Evolution of populations Chapter 17 Evolution of populations

17.1 Genes and Variation 1. An organism’s genotype is the combination of alleles it carries.

2. An organism’s phenotype is the individual’s genotype in combination with its environmental condition. The phenotype is the physical, physiological, and behavioral characteristics of an organism.

3. Natural selection acts on an organism’s phenotype, not genotype because it does not act on a single gene but rather the entire organism.

4. gene pool (483) – all the genes, including all the different alleles for each gene, that are present in a population at any one time.

5. allele frequency (483) is the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene. Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur

6. Evolution is a change in the frequency of alleles in a population over time.

7. Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

8. Mutation – change in the genetic material of a cell 8. Mutation – change in the genetic material of a cell. Some mutations affect a single gene, others affect a whole chromosome.

9. Genetic recombination - The process or act of exchanges of genes between chromosomes, resulting in a different genetic combination and ultimately to the formation of unique gametes with chromosomes that are different from those in parents.

10. Lateral gene transfer – is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring.

11. The number of phenotypes produced for a trait depends on how many genes control the trait.

12. single-gene trait (485) – a trait controlled by only one gene; may only have 2 or 3 phenotypes

13. In populations, the phenotypic ratios are determined by the frequency of alleles in the population as well as whether the alleles are dominants or recessive. Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur

14. polygenic trait (486) traits controlled by two or more genes; results in many different phenotypes.

17.2 Evolution as Genetic Change in Populations 1. Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.

2. Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selection: directional selection, stabilizing selection, or disruptive selection.

3. directional selection (489) form of natural selection in which individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve.

EXAMPLE OF DIRECTIONAL SELECTION Beak size varies in a population Birds with bigger beaks can feed more easily on harder, thicker shelled seeds. Suppose a food shortage causes small and medium size seeds to run low. Birds with bigger beaks would be selected for and increase in numbers in population. http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg

Ex. Peppered Moths during Industrial Revolution Before 1850’s Moths had light colored Wings After industrial revolution the moths with dark colored wings outnumbered the light colored.

4. stabilizing selection (489) form of natural selection in which individuals near the center of a distribution curve have higher fitness than individuals at either end of the curve.

Example: human babies born with below normal and above normal birth weights have lower chances of survival than babies born with average weights. Result: Birth weight varies Little in human Populations.

5. disruptive selection (489) natural selection in which individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle of the curve.

6. In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

7. Genetic diversity – sum total of all the different forms of genetic information carried by a particular species, or by all organisms on Earth.

8. genetic drift (490) random change in the allele frequency of a population Two types: bottleneck effect and founder’s effect

9. bottleneck effect (490) a change in the allele frequency following a dramatic reduction in the size of a population.

The other type of genetic drift 10. founder effect (490) change in allele frequencies as a result of the migration of a small subgroup of a population

11. genetic equilibrium (491) situation in which allele frequencies in a population remain the same in the gene pool.

Sexual reproducing organisms in a population can remain in genetic equilibrium even though genes are shuffled during meiosis and fertilization.

12. Hardy-Weinberg principle (491) principle that states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

13. The Hardy-Weinberg principle predicts that five conditions can disturb genetic equilibrium and cause evolution to occur: (1) nonrandom mating; (2) small population size; and (3) immigration or emigration; (4) mutations; or (5) natural selection.

14. Nonrandom mating occurs when the probability that two individuals in a population will mate is not the same for all possible pairs of individuals. Random mating would be every organism has the same opportunity to mate with each other.

15. sexual selection (492) when individuals in a population select mates based on heritable traits

SEXUAL SELECTION Stabilizing Selection Key Section 16-2 Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Brightness of Feather Color Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

16. immigration – an organism moving into a population 17. emigration- an organism moving out of a population

18. gene flow - is the transfer of alleles or genes from one population to another. Migration into or out of a population may be responsible for a marked change in allele frequencies

19. One or more of these conditions usually holds for real populations 19. One or more of these conditions usually holds for real populations. So, most of the time, in most species, evolution happens.

17.3 The Process of Speciation 1. species (494) a population or group of populations whose members can interbreed and produce fertile offspring.

2. speciation (494) the formation of a new species

3. reproductive isolation (494) – separation of a species or population so that they no longer interbreed and evolve into two separate species

4. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation.

5. behavioral isolation (495) form of reproductive isolation in which two populations develop differences in courtship rituals or other behaviors that prevent them from breeding.

6. Geographic isolation (495) form of reproductive isolation in which two populations are separated by geographic barriers such as rivers, mountains, or bodies of water, leading to the formation of two separate subspecies.

7. temporal isolation (495) form of reproductive isolation in which two or more species reproduces at different times.

8. Speciation in Galápagos finches most likely occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, behavioral isolation, and ecological competition.

17.4 Molecular Evolution A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.

2. Researchers can compare such DNA sequences in two species and determine how many mutations have occurred.

3. The more differences between the DNA sequences of the two species, the more time has elapsed since the two species shared a common ancestor.

4. One way in which new genes evolve is through the duplication, and then modification, of existing genes.

5. Gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions.

6. Hox genes determine which parts of an embryo develop arms, legs, or wings.

7. Small changes in Hox gene activity during embryological development can produce large changes in adult animals.

Change in a Hox Gene Insects and crustaceans are descended from a common ancestor that had many pairs of legs.   Crustaceans (such as brine shrimp) still have lots of legs. Insects, however, have only three pairs of legs.

Change in a Hox Gene Recent studies have shown that in insects, a mutation in a single Hox gene, called Ubx, “turns off” the growth of some pairs of legs.   Because of mutations in a single Hox gene millions of years ago, modern insects have fewer legs than modern crustaceans. A variant of the same Hox gene directs the development of the legs of both animals.