Evolution as Genetic Change

Nazca Booby Clip

Natural selection affects which individuals survive and reproduce and which do not. Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, can evolve over time.

Q: How does natural selection affect single gene traits?

Organisms of one color may produce fewer offspring than organisms of other colors. For example, a brown lizard population has mutations that lead to the production black forms. Black lizards warm up faster on cold days given them more energy to get away from predators. Black lizards produce more offspring. The allele from black color will increase in relative frequency. Natural Selection on Single Gene Traits

A: Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. 20%

Q: How does natural selection affect polygenic traits?

Natural Selection on Polygenic Traits Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

A: Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection

Sexual Selection Sexual selection is a “special case” of natural selection. Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate.

Giraffe Clips camelopardalis/video-09e.html camelopardalis/video-09a.html

Sexual Selection: Giraffe Males use the flehmen (urine test) sequence to determine which females are in estrus According to Pratt (1985), female giraffes prefer older, more dominant males. When approached for urine-testing, Pratt found that females urinated more often for dominant males than subordinate males. Thus, dominant males have more success in determining which females are receptive. This advantageous for the female because it allows her to get the best genes for her offspring: the most dominant male is the one who ultimately fertilizes her eggs

Sexual selection Lion’s Mane: Females are attracted to males with larger, dark manes Correlation with higher testosterone levels –better nutrition & health –more muscle & aggression –better sperm count / fertility –more successful young But imposes a cost to male –HOT! Is it worth it??

Sexual selection Acting on reproductive success –Attractiveness of potential mate –Fertility of gametes –Successful rearing of offspring Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Sexual selection Peacocks: /6/l_016_09.html

Sexual Selection – Blue Footed Boobies

Humans /6/l_016_08.html

Genetic Drift In small populations, an allele can become more or less common just by chance. The smaller the population is, the farther the results maybe from what the laws of probability predict. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. A random change in allele frequency is called genetic drift.

Genetic Drift NOT adaptation to environmental conditions NOT natural selection

Genetic Drift: Bottleneck Effect a disaster reduces population to small number & then population recovers & expands again but from a limited gene pool who survives disaster may be random Loss of variation of alleles from gene pool due to a chance event (famine, natural disaster, loss of habitat, etc.)

Genetic Drift: Founder Effect Genetic drift may occur when a small group moves to a new habitat. When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect. The change of allele frequency is not the result of natural selection but simply chance—the chance that particular alleles were in one or more of the founding individuals.

Descendants of founding populations

Are there conditions in which evolution will not occur? The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium.

The Five Conditions Required for Genetic Equilibrium 1.there must be random mating (equal opportunity to reproduce) 2.the population must be very large (less affected by genetic drift) 3.there can be no movement into or out of the population (no mixing of gene pools) 4.there can be no mutations (no changes in alleles) 5.there can be no natural selection (no phenotypes with an advantage) If these conditions are not met, the population will evolve.