natural selection

natural selection and similar environmental pressures produce similar adaptations in organisms from different evolutionary lineages.

it can be quantified as the average percentage of loci that are heterozygous

differences among individuals in the composition of their genes or other sequences

they can occur on an “either-or” basis which are determined by a single gene locus; or they can vary in gradations along a continuum and be influenced by two or more genes on a single phenotypic character.

at the molecular level of DNA which would be called nucleotide variability

most nucleotide changes are silent mutations that do not alter the amino acid sequence of proteins, which are crucial for determining phenotypic traits.

mutations in cells that produce gametes can be passed on to offspring

differences in nucleotide base-pair sites between individuals of a population

mutations

natural selection, genetic drift, gene flow

changes in the allele frequencies of a population from generation to generation

duplication of small pieces of DNA increase the gene pool, and duplicating genes can take on new functions by further mutation.

reshuffling of alleles into new combinations in each individual

crossing over and independent assortment during meiosis as well as the random combination of gametes in fertilization

the allele frequencies of a population should remain constant from one generation to the next if the population is large and only sexual reproduction is involved

1. no mutations

2. random mating

3. no natural selection

4. extremely large population to offset chance fluctuations (genetic drift)

5. no movement of alleles in or out of the population (gene flow)

adaptive evolution

fluctuations in a population’s allele frequencies from one generation to the next as a result of chance events

small populations

the migration of individuals or transfer of gametes between populations which ends up causing a change in allele frequencies

reduced

genetic drift that occurs when only a few individuals colonize a new area is known as the founder effect

genetic drift that occurs when some disaster or other factor reduces population size dramatically and the few surviving individuals are unlikely to represent the genetic makeup of the original population

natural selection that favors intermediate phenotypes over extreme (human baby birth weight)

natural selection that favors both extreme phenotypes (Darwin’s finches and their beaks)

natural selection that favors one extreme phenotype (dark peppered moths)

match; increases

the selection for characteristics that enhance an individual’s chances of obtaining mates

sexual dimorphism

the distinction between males and females on the basis of secondary sexual characteristics

competition among individuals of the same sex for mates of the opposite sex

individuals of the same sex act choosy when selecting a mate of the opposite sex

frequency of a phenotype declines if it becomes too common in a population

geographic isolation interrupts gene flow between subpopulations

arises from natural selection, genetic drift, mutations

reproductive barriers prevent gene flow between populations that share the same area

autopolyploid or allopolyploid chromosomal changes

new species is formed by chromosome number doubling

2 species produce a hybrid from the chromosome numbers adding up.

direct observations, homology, fossil record, biogeography

the geographic distribution of a species; species in nearby geographic areas resemble each other

similar anatomy from common ancestors

structures with little or no use

share similar functions but not common ancestry

independent evolution of similar features in different lineages; analogous structures occur in this

the migration of individuals or transfer of gametes cause changes in allele frequencies

fluctuations in a population’s allele frequencies per generation due to chance

the frequencies of alleles and genotypes remain constant

barriers that prevent members of a different species from producing hybrids

prevents fertilization and mating

prevents offspring from being fertile

an area where two different species interbreed and produce hybrid offspring. This can occur when the ranges of two species overlap, leading to genetic mixing between them.

reinforcement, fusion, stability

hybrids don’t form because reproductive barriers are strengthened

2 species fuse because reproductive barriers weaken

continued production of hybrids