Evolution of Populations Chapter 11

Genetic Variation Within Populations Variation and gene pools Gene pool – all genes present in a population Allele Frequency – the number of times an allele occurs in the gene pool, compared to other alleles Genetic variation is often studied in populations rather than in individuals. Genetic variation in a population increases the likely hood of some members surviving A population is a group of individuals of the same species that interbreed, not inbreed. Because of the interbreeding, the members of the group share a common group of genes among them called the gene pool Allele frequency = Relative frequency is often expressed as a percentage and has nothing to do with whether the allele is dominant or recessive For example one study showed that the incidence of polydactyly is 35%

Genetic Variation Within Populations In genetic terms, evolution is any change in the relative frequency of alleles in a population. This change can be an increase or decrease in the frequency of an allele.

Genetic Variation Within Populations Scientists now recognize two main sources of genetic variation Mutations Recombination

Genetic Variation Within Populations Mutations Any change in a sequence of DNA Occurs Because: Problems with replication Exposure to radiation or chemicals Mutations do not always affect the phenotype Changing from GGA to GGU will still code for the amino acid glycine Some mutations can affect an organisms fitness or its ability to survive and reproduce

Genetic Variation Within Populations Recombination Parents alleles rearrange when forming gametes The 23 pairs of chromosomes can produce 8.4 million different combinations! During meiosis, each chromosome of a homologous pair moves independently.

Genetic Variation Within Populations Crossing Over Occurs during meiosis Further increases genetic variation

Natural Selection In Populations Normal distribution Highest frequency in middle and lowest at the extremes Also representative for allele freqencies

Natural Selection In Populations Microevolution Observable changes in allelic frequencies Occurs in a single population 3 ways Directional Stabilizing Disruptive

Natural Selection In Populations Directional Selection Shift in phenotype to one extreme Mean also shifts Rise of drug resistant bacteria In 1940’s some bacteria had a resistance to antibiotics, but it was of no use. After the creation of antibiotics the resistance became an advantage This has increased the number of bacteria that are resistant to antibiotics to some degree

Natural Selection In Populations Stabilizing Selection Norm is selected for and becomes more common Ex. Gall Fly and Goldenrod Gall flies lay eggs in goldenrod plants Downy woodpeckers attack large galls and certain wasps attack and lay eggs in small galls Therefore, the median value is the best Humans – select against heavy and light infants

Natural Selection In Populations Disruptive Selection Selection for the extremes Ex. Peppered moths

Natural Selection In Populations Single-gene and polygenic traits Widows peak is a single gene trait – a single gene with two alleles Polygenic Traits are controlled by two or more genes This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

16-2 Evolution as Genetic Change Natural Selection on Single-Gene Traits A few things to keep in mind: Natural selection does not affect single genes, rather it affects entire organisms. Natural selection determines if the organism survives and reproduces or dies. If an organism dies, then it does not pass on its genes to the next generation. Genes can influence the gene pool, and this in turn can challenge natural selection. Individual organisms can not evolve, only populations. Natural selection on single gene traits can lead to changes in allele frequencies and thus cause evolution.

Other Mechanisms of Evolution Gene Flow Some animals move once they are able When the move into new population, their alleles become part of gene pool Occurs with movement between populations Increases variation If less movement, the more different two species become Bald eagles have been found over 2500 km from their birthing nest when they can fly

Other Mechanisms of Evolution Genetic Drift – Random change in allele frequency Bottleneck Effect Founder effect – when allele frequencies change because of migration Natural selection is not the only source of evolutionary change With a small population, an allele can become more common just by chance You can use the laws of probability to predict the overall results for genetic crosses in large populations. This however, does not work for small populations and the results may be farther from what the probabilities predict. This is called genetic drift. Genetic drift is random change, natural selection is change due to adaptations This may happen when a new species colonizes a new habitat because they may carry a certain allele at a higher relative frequency. Natural selection does not cause the change in frequency, just chance. Bottleneck effect – genetic drift that occurs after some event greatly reduces the population size – once the genes are gone, the are gone. If two species happen to migrate then you can get what is known as the founder effect

Other Mechanisms of Evolution Sexual Selection Females the important factor in reproduction This makes females “picky” Intrasexual selection Intersexual selection Intrasexual selection – involves competition between males – rams butting heads Intersexual selection – males display certain traits that attract females - peacocks

Hardy-Weinberg Equilibrium Evolution vs. Genetic Equilibrium Hardy-Weinberg principle – genotype frequencies will stay constant unless some factor changes the frequency Genetic Equilibrium – when allele frequencies remain constant Scientists sometimes find it helpful to compare data to a model. This model is the Hardy-Weinberg principle. This leads to genetic equilibrium where the allele frequency remains the same and evolution does not occur. There are certain conditions that have to be met

Hardy-Weinberg Equilibrium Five Conditions to Maintain Equilibrium Random Mating Large Population No Movement into or out of the population No Mutations No Natural Selection Since these conditions can’t be met equilibrium is rarely met. However, biologists can compare real data to what the model indicates

Hardy-Weinberg Equilibrium Biologists can use an equation for comparison purposes p2 + 2pq + q2 = 1 p = dominant alleles q = recessive alleles Population = 3000 Attached earlobes = 2200 (dominant) Unattached earlobes = 800 (recessive) Q2 = 800/3000 = 0.27 Q = 0.51 P= 1-q = 0.49 P2 = 0.24 2pq = 0.49

Hardy-Weinberg Equilibrium Five factors leading to evolution Genetic drift Gene flow Sexual selection Natural selection Mutations Genetic drift = change due to chance Gene flow – movement of alleles from one population changes allele frequencies in that population Sexual selection – certain traits may improve mating success Natural selection – certain traits may be of an advantage for survival

Speciation Through Isolation Reproductive Isolation – members of populations can no longer mate Mules or Hinnies Speciation - when two species come from one existing species This could be due to not being able to physically mate with one another or the offspring do not survive Mules or hinnies are the result of a horse and an ass or a donkey and a mare- differences in chromosome numbers make these animals sterile

Speciation Through Isolation Behavioral Isolation Two populations will not breed because of differences in courtship Fireflies If gene flow stops between two populations, then the populations are said to be isolated – over time the differences between two populations increases Over 2000 different species of fireflies that differ in blinking patterns

Speciation Through Isolation Geographical Isolation Rivers, mountains, or bodies of water separate two populations Snapping Shrimp and placental vs. marsupial mammals Snapping shrimp became isolated by a geographical barrier in the atlantic ocean – now when the two separate species are placed together males and females snap at each other When Australia seperated, the marsupials became seperated – because the rest of the world was more condusive to surviving as a placental mammal they displaced them marsupials everywhere but Australia

Speciation Through Isolation Temporal Isolation Different species mate at different times Times of day Times of year Trees and plants

Patterns in Evolution Convergent Evolution – evolution towards similar characteristics in unrelated species Analogous structures Bird and bat wings Evolution occurs because animals are subjected to similar pressures

Patterns in Evolution Divergent Evolution – closely related species in different directions Results from adapting to different environments Kit and Red Fox Kit fox lives in the desert Red fox in the forest

Patterns in Evolution Coevolution – two or more species evolve in response to changes in each other Benefit to both species Plants and bees

Patterns in Evolution Coevolution can also drive predator-prey relationships Cheetahs and gazelles

Patterns in Evolution Extinctions – species are eliminated Background extinctions – occur continously, but at very low rates Mass extinctions – rare, more intense, involve many species

Patterns in Evolution Punctuated equilibrium – bursts of evolutionary activity followed by periods of inactivity Adaptive radiation – diversification of multiple species from one ancestral species Mammals