Ch. 17 Part 1

Genetic Variation PHENOTYPIC VARIATION Provides raw material for natural selection to occur Variation in population  some members have features that give them advantage over other members Causes of Genetic Variation: Independent assortment Chromosomes and alleles Crossing over Between chromatids of homologous chromosomes Random Mating Random fertilization of gametes Mutation Does NOT reshuffle alleles Produces completely new alleles Mistakes during DNA replication Gene mutations unpredictable New alleles, usually recessive Appearance in population is many generations after initial mutation Must occur in GAMETES to affect offspring (not somatic cells) Processes 1-4 involve RESHUFFLING of alleles in a population…leads to PHENOTYPIC VARIATION (also caused by ENVIRONMENT…NOT passed off to offspring) Mutation to somatic cells in ovaries/testes CAN lead to mutation to gametes Review gametogenesis and germ layer cells

Continuous vs. Discontinuous Variation Qualitative Phenotypic Differences Continuous vs. Discontinuous Variation Quantitative Phenotypic Differences Blood types Clearly distinguishable categories No intermediates Discontinuous variation Different alleles at a single gene locus have large effects on phenotype Different genes have different effects on phenotypes Examples: Inheritance of sickle cell anemia or hemophilia Flower color Stem color Feather color Dominance & Gene interaction  reduce phenotypic variation Height Mass Differences are hard to distinguish Rages between extremes are created Continuous variation Different alleles at a single gene locus have small effects on phenotypes Different genes have same (sometimes additive) effect on phenotype Polygenes (polygenetic traits) Large number of genes have combined effect on phenotypes

Discontinuous Variation

Environmental Effects on Phenotype Environment affects full genetic potential of genes Examples Two plants can have same genetic make up but one gets more light intensity than another, it will grow to its full potential Allele that controls dark pigment formation ONLY at low temperatures Selective breeding when humans purposefully apply selection pressures to populations Important to know how much of phenotypic variation is genetic and how much is environmental

Natural Selection All organisms trying to survive and reproduce Not all organisms within a population survive to adulthood to reproduce Would lead to exponential population growth (review logistic and exponential growth) Influenced by environmental factors Abiotic vs. Biotic Factors Abiotic factors caused by non-living components Water availability Nutrients in soil Climate Biotic factors  caused by other living organisms Pathogens Predation Competition Carrying Capacity The largest number of individuals that a given environment can support The part of the logistic graph after the exponential growth…the flattening out The point at which this flat line reaches the y-axis is the size of the population when the growth rate reaches zero This doesn’t mean the population stops growing Many factors slow the growth of plants and animals…

Fitness Capacity of an organism to survive and transmit a genotype to its offspring Refers to “extent to which an organism has adapted to their environment” The ability of an organism to survive, and make copies of its alleles that are represented in the next generation Organisms that produce more surviving offspring are more fit, those that produce fewer are less fit

Carrying Capacity The largest number of individuals that a given environment can support The part of the logistic graph after the exponential growth…the flattening out The point at which this flat line reaches the y-axis is the size of the population when the growth rate reaches zero This doesn’t mean the population stops growing Many factors slow the growth of plants and animals…

Limiting Factor Condition that can restrict a population’s growth..create the CARRYING CAPCITY Could be: Space Disease Availability of food Abiotic and Biotic

Limiting Factors Density-dependent Factors Density-Independent Factors Factor that limits population as population density increases Competition Predation Disease Parasitism Crowding and Stress Density-Independent Factors Factors that affect population but are unrelated to population density; affect population regardless of size Insects vulnerable to this Weather /Natural disasters Human activities Fires deforestation

How Environment Influences Population Pressure of environmental factors is sufficiently great  population size will decrease Selection Pressure environmental factors which may reduce reproductive success in a population and thus contribute to evolutionary change or extinction through the process of natural selection competition predation disease parasitism land clearance climate change pollutants Selection Pressure INCREASES the chances of some alleles being passed onto the next generation and DECREASES the chances of others Natural Selection effects of selection pressures on the frequency of alleles in a population Raises frequency of alleles that lead to an advantage Reduces frequency of alleles leading to a disadvantage