Evolution of Populations

Remember… An individual cannot evolve….a POPULATION evolves! Certain individuals within the population survive to reproduce…they pass on their genes…this could lead to change over time…this is EVOLUTION

Variation Differences within a population Due to inheritance of different alleles Gene pool – all the genes (including different alleles) that are present in a population

Sources of Variation Mutations  Random  Result of radiation or chemicals  Point, frame-shift, chromosomal Gene shuffling  Gamete production (meiosis)  Independent assortment  Crossing over

Microevolution Small changes in allele frequencies over time due to natural selection, genetic drift, or gene flow If the trait is controlled by a single gene:  May see an increase or decrease in dominant allele or recessive allele if a particular trait is favored

If the trait is polygenic with a spectrum of phenotypes:  Directional Selection – individuals at one end have a higher fitness (one extreme)  Stabilizing Selection – individuals that fall in the middle have higher fitness (intermediate)  Disruptive Selection – individuals at either end have higher fitness (both extremes)

Genetic Equilibrium Situation in which allele frequencies remain constant  Hardy-Weinberg principle – allele frequencies will remain constant unless one or more factors cause them to change  Five conditions to maintain equilibrium Random mating, large population, no genetic drift/gene flow, no mutations, and no natural selection Neither likely nor realistic!!!  However, if a population does not maintain equilibrium, they are evolving!  Is a way to show that change is occurring

Genetic Drift Random change in allele frequencies from one generation to the next  More likely in small populations Examples:  Founder effect – a small group becomes isolated and establish a new population with a different gene pool  Bottleneck effect – a sudden change drastically reduces the population and the few survivors’ gene pool does not reflect the original populations

Gene Flow Occurs when a population gains or loses alleles by additions to or subtractions from the population (immigration and emigration)  Tends to reduce genetic differences and make populations more similar

Macroevolution Large-scale evolutionary patterns and processes that occur over long periods of time Includes or caused by  Speciation  Extinction  Adaptive Radiation  Convergent Evolution  Coevolution  Punctuated Equilibrium  Changes in developmental genes

Speciation Changes can lead to formation of a new species Divergent evolution – members of the same species become so different it results in new species Occurs when the two groups (old and new) no longer interbreed due to reproductive isolation

Reproductive Barriers Habitat/Geographic isolation – separated by barriers such as rivers, mountains, or bodies of water Behavioral isolation – populations have different mating habits or mating signals/rituals Temporal isolation – species breed at different times of the day, seasons, or years Mechanical isolation – anatomically incompatible Gametic isolation – gametes are unable to fuse to form zygotes

Extinction More than 99% of all species that have ever lived are now extinct Earth has had several mass extinctions which wiped out entire populations and ecosystems  Example: extinction of dinosaurs  Probably due to change in environment, volcanic eruptions, changing sea levels, movement of continents, etc Extinctions lead to evolution of remaining species when new niches/resources become available

Adaptive Radiation A single species or group evolves into many diverse forms (one gives rise to many) Examples: Darwin’s finches – more than a dozen new species from one original species Dinosaurs Mammals

One gives rise to many……..

Convergent Evolution Process by which unrelated organisms come to resemble each other (look alike) due to similar functions or environmental pressures  Streamlined bodies of swimming animals such as sharks, dolphins, seals and penguins  Leads to analogous structures

Coevolution Process by which two species evolve in response to changes in the other  Flowering plants and insects that pollinate or plants and plant-eating insects  Often occurs in symbiotic relationships They evolve “together”  As the predator changes, so does the prey  As the host changes, so does the parasite

Rate of Change Gradualism – change is slow and steady Punctuated Equilibrium – pattern of long, stable periods interrupted by brief periods of rapid change  Could occur when small populations become isolated or migrate  Organisms evolve rapidly to fill available niches  Rapid change occurs after mass extinctions

Developmental Genes and Body Plans Transformations in body shape and size could occur due to changes in genes for growth and development  Hox genes or “master control genes” guide the development of major body structures in animals  Mutations in these control genes could change animal structure

Evolutionary Life History Earth is estimated to be about 4.6 billion years old First life estimated about 3.5 billion years ago  Prokaryotes Endosymbiotic theory – eukaryotes formed from symbiotic prokaryotes that lived together  Evidence in mitochondria and chloroplasts – double membranes, DNA, ribosomes, enzymes

General Order Earth formed (very hot and no life) Earth cooled – water planet First life – anaerobic prokaryotes Followed by photosynthetic prokaryotes  Put oxygen into the atmosphere  Allowed for cell respiration Followed by the first eukaryotes Continued to evolve from simple unicellular to complex multicellular eukaryotes